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Manufacturing profile-free copper foil using laser shock flattening
International Journal of Machine Tools & Manufacture 152 (2020) 103542
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Manufacturing profile-free copper foil using laser shock flattening Yang Haifeng a, b, Xiong Fei a, *, Wang Yan a, Jia Le a, Liu Hao a, Hao Jingbin a a b
School of Mechatronic Engineering, China University of Mining and Technology, XuZhou, 221116, China Jiangsu Key Laboratory of Mine Mechanical and Electrical Equipment, China University of Mining and Technology, XuZhou, 221116, China
A R T I C L E I N F O
A B S T R A C T
Keywords: Laser shock flattening Profile-free copper foil Flattened deformation mechanism Microstructures Mechanical properties Corrosion resistance
Copper foil is a key material of printed circuit boards and plays an important role in the conductance of electric circuits and interconnection of electronic components. When high-frequency signals were transmitted in rough copper foil wires, the conductor resistance, wire loss, and signal loss increased because of the skin effect. To reduce the negative influence of the skin effect and improve the quality of the copper foil, a laser shock flattening (LSF) method was proposed to manufacture profile-free copper foil with high performance. It was concluded that the better flattening effect for large-area profile-free copper foil could be achieved at a pulse energy of 0.25 J and an overlap rate of 25%, and its surface roughness decreased by 67.0% from 52.1 nm to 17.2 nm. Subsequently, to determine the mechanism for the flattened deformation of copper foil induced by LSF, the microstructures of the copper foil before and after flattening were characterised using transmission electron microscopy. A higher dislocation density and a few deformation twins were found in the profile-free copper foil. Ultimately, nanoindentation, micro-tensile, and electrochemical corrosion tests indicated that the mechanical properties and corrosion resistance of the copper foil were significantly improved by LSF. This technique would enable the successful fabrication of large-area profile-free copper foil with high performance for the emerging applications of ultra-high-frequency signal communication and printed circuit board manufacture.
1. Introduction Copper foil is widely used in many fields such as micro-electromechanical systems (MEMSs), flexible copper clad laminates (FCCLs), industrial communication devices (ICDs), and printed circuit boards (PCBs) because of its excellent electrical conductivity, thermal conduc tivity, and ductility [1,2]. With the rapid development of science and technology and the sharp increase in social demand, various industries have set higher requirements for copper foil. This demand is particularly prominent in the development of 5G and subsequent terahertz wireless communication techniques [3,4]. The skin effect refers to a phenomenon wherein the current distri bution inside a conductor is uneven when there is an alternating current or alternating electromagnetic field in the conductor. The current is concentrated in a thin layer on the conductor surface, the current den sity on the conductor surface increases, and the actual interior current decreases. It is well-known that the frequency of the millimeter wave band used in ultra-high-frequency signal wireless communication techniques is as high as tens or hundreds of gigahertz [5]. The skin effect of the copper foil in a PCB is more obvious at such high frequencies. The
current is concentrated and transmitted in a skin-depth area under the surface of the copper foil, and this skin-depth gradually decreases with an increase in the signal frequency [6,7]. Therefore, a high-frequency signal must travel along the surface of the rough copper foil during the transmission process [8,9]. This significantly increases the trans mission path length, conductor resistance, and wire loss, as well as decreasing the signal integrity and causing the delay or attenuation of the signal [10,11]. In addition, the wear resistance, fatigue strength, and corrosion resistance of the copper foil are increased with a decrease in the surface roughness [12,13]. In order to reduce the influence of the skin effect on the transmission process for a high-frequency signal and improve the performance of copper foil, it is imperative to reduce its surface roughness. An examination of the current polishing methods shows that tradi tional mechanical polishing is obviously not suitable for metal foils, while electrochemical polishing, chemical polishing, chemical me chanical polishing, ion beam cleaning, ultrasonic vibration-assisted polishing, and other methods still have some limitations when used for large-area metal foils [14,15]. However, an advanced laser shock processing technique has attracted extensive attention in recent years
* Corresponding author. School of Mechatronic Engineering, China University of Mining and Technology, No.1 Daxue Road, XuZhou, 221116, China.; E-mail address: [email protected] (X. Fei). https://doi.org/10.1016/j.ijmachtools.2020.103542 Received 18 December 2019; Received in revised form 29 February 2020; Accepted 2 March 2020 Available online 5 March 2020 0890-6955/© 2020 Elsevier Ltd. All rights reserved.
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because it has advantages that include high efficiency, energy saving, green environmental protection, a wide application range, easy control, and no direct contact with the processing parts. Thus, some researchers have studied the surface roughness values of metal materials processed by laser shock. For instance, Dai FZ [16] studied the influence of laser shock peening (LSP) on the roughness of a target material under different overlap modes, and the results showed that the surface roughness values of the isosceles-right-triangle style and equilateral-triangle style attained their minimum values at X-axis overlap ratios of 29.3% and 42.3%, respec tively. Lu GX [17] found that increasing the overlap rate helped to improve the surface quality of low-strength metals. Subsequently, Dai FZ [18] also proposed a new technique of elastic contact LSP, which could effectively reduce the target material roughness compared with tradi tional LSP, but it was difficult to control the surface roughness of metal foil subjected to the LSP process. Yang HF [19,20] also found that the laser shock imprinting (LSI) technique could reduce the surface rough ness of micro-formed parts imprinted from aluminium foil, but there were 3D micro-structures with large fluctuation on the surface of the metal foil, and the surface roughness values at different positions on the surfaces of micro-formed parts were different. Therefore, this laser shock processing technique still has some limitations when used to smooth the surface of metal foil, which are disadvantages for the manufacture of profile-free copper foil. In addition, a laser shock processing technique can also strengthen the properties of the target material, which is particularly significant with the LSP technique [21,22]. LSP was first used to improve the fa tigue resistance of materials, because this technique can induce a re sidual stress layer with a large depth [23,24]. Moreover, LSP can refine the target grain, and even produce a nano-crystalline layer and intro duce a large number of dislocations in the metal material [12,25,26]. Therefore, this technique is also widely used to modify the character istics of metal materials such as their hardness [21], strength [27], wear resistance [28], and corrosion resistance [29].
In order to reduce the surface roughness and surface micro-defects of copper foil, and improve its performances, this paper proposes a laser shock flattening (LSF) method to manufacture profile-free copper foil with high performance. LSF experiments were performed using different pulse energy values and overlap ratios, and the optimum processing parameters were obtained by evaluating the macro-morphology and micro-roughness of the flattened copper foil. Furthermore, the changes in the microstructures of the copper foil before and after flattening were characterised, and the mechanism for the flattened deformation of the copper foil was revealed. Further, the degree that the mechanical properties and corrosion resistance of copper foil flattened by LSF were strengthened was evaluated using nano-indentation, micro-tensile, and electrochemical corrosion tests. 2. Experiments 2.1. Experimental principle and equipment Fig. 1(a) shows a schematic diagram of the LSF technique. Its prin ciple and experimental equipment were very similar to those of the LSP process. The only difference was that the experimental equipment used for LSF had to contain a backing plate with a sufficiently smooth surface. In this experiment, a laser pulse (with a pulse duration of 7 ns, wave length of 1064 nm, and pulse frequency of 3 Hz) was provided by a pulsed laser (Hercules-1000-TH), and the focused spot diameter (D, 1/ e2) was 1.2 mm. The laser beam (round with a Gaussian distribution) penetrated the confinement layer and irradiated the absorption layer. The laser energy was absorbed, and the top surface of the aluminium foil was instantaneously vaporised into a high-temperature and highpressure plasma. The plasma continued to absorb laser energy and rapidly expanded. Then, a high-pressure shock wave was generated because of the confinement effect of the confinement layer on the plasma [19]. The peak pressure of the shock wave was usually as high as several gigapascals, which was far beyond the dynamic yield strength of
Fig. 1. LSF process: (a) schematic diagram of LSF, (b) non-flattening area and flattening area on copper foil, (c) surface morphology of glass backing plate, (d) height distribution of micro-bulges (peaks) on surface of glass backing plate, (e) overlap effect and moving path of laser spot. 2
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the metal layer. Thus, plastic deformation could be induced in the metal foil, and its surface became smooth, as shown in Fig. 1(b). The ablative layer could increase the peak pressure of the shock wave induced by the pulse laser, prevent the ablation of the target material surface, and ensure the integrity of the target material. In this study, if the ablative layer was too thin, it could easily be completely ablated during the LSF process, resulting in the laser ablation of the upper sur face of the copper foil, which was not conducive to improving its surface quality. In addition, aluminium foil is one of the most commonly used ablative layers in laser shock processing, and it has a uniform thickness, which makes it more convenient than black paint and graphite. There fore, 100 μm thick aluminium foil was selected as the ablative layer. In addition, a clamping device was used in the LSF experiments to ensure that the confinement layer, absorption layer, metal layer, and backing plate were closely combined. In these experiments, the pressure induced by clamping force (only tens of newtons, with a contact area of more than 15 mm � 15 mm and a pressure of less than 1 MPa) was constant, with a value that was far smaller than the peak pressure of the laser shock wave and the yield strength of the copper foil. Therefore, the clamping force could not cause deformation of the copper foil and could be ignored. The backing plate under the copper foil had to be sufficiently smooth to ensure that the surface roughness of the copper foil treated by LSF was significantly reduced. A 2 mm thick piece of glass was selected as the backing plate, and its surface roughness (Sa) was only 6.3 nm, as shown in Fig. 1(c). Fig. 1(d) shows the height distribution of the micro-bulges (peaks) on the glass surface. The heights of the micro-bulges were mainly concentrated in the 230–290 nm range, with other height ranges (171–230 nm and 290–500 nm) only accounting for very small per centages. This indicated that the surface roughness of the glass was caused by a few relatively high micro-bulges, and the glass surface was extremely flat. In this work, the 36-μm-thick copper foil was used as the experimental material (metal layer). In order to clearly understand the mechanism of the deformation of the copper foil induced by laser shock, the copper foil was annealed at 723 K for 1 h in a vacuum environment to reduce the original defects before flattening. During single-spot LSF experiments, the laser pulse energy was var ied from 0.05 J to 1.0 J in 0.05 J steps. The overlap was realised using a 3-D high-precision mobile platform in large-area LSF experiments, and the overlap effect and moving path of the laser spot are shown in Fig. 1 (e). The overlap ratios in the transverse (RT) and longitudinal (RL) di rections were the same and could be calculated using formulas (1) and (2), respectively. In other words, d1 was always equal to d2. In addition, the overlap ratios ranged from 0% to 75%, in 12.5% steps. RT ¼ ðD
d1 Þ=D � 100%
(1)
RL ¼ ðD
d2 Þ=D � 100%
(2)
filtering type was the three-level low-pass filtering (finite-impulseresponse). Besides, in this study, the software of “CSPM imager 4.60” was used to process the roughness data. Optical microscopy (OM) (LEICA DM4 M) and high-resolution field emission scanning electron microscopy (FSEM) (MAIA3 LMH) were used to observe the surface morphologies and cross-sectional thicknesses. Subsequently, the grains of the cross sections of epoxy resin embedded samples were observed using OM, and an FeCl3 solution (5 g FeCl3⋅6H2O þ 5 ml HCl þ 100 ml H2O) was selected as the etching agent. Then, the microstructures of the copper foil were characterised using TEM (JEM2100). Furthermore, nano-indentation tests with a standard Berkovich diamond indenter (Agilent U9820A, Nano Indenter G200) were used to determine the load–displacement curves, nano-hardnesses, and elastic modulus of the copper foil, where the indentation depth was fixed at 500 nm. A microtensile test bench (MTS-450) was used to measure the tensile strengths and elongations, where the stretching speed was set at 10 μm/s. Each sample was measured three times, and the average value of three measurements was used to characterise the performance of the copper foil. An electrochemical workstation (CHI660D) was selected to research the corrosion resistances in 3.5 wt% NaCl solution, and the surface morphologies after corrosion were characterised using OM. 3. Results and discussions 3.1. Single-spot LSF experiments Single-spot LSF experiments were performed on the copper foil to study the effect of pulse energy on its flattening effect. The annealed copper foil became smoother after flattening, and the visible microcracks and micro-defects on the surface of the copper foil close to the glass backing plate were significantly reduced. This phenomenon could be clearly observed by comparing the surface morphologies of the annealed copper foil (Fig. 2(a)) and flattened copper foil (Fig. 2(b)) and attributed to the extrusion deformation of copper foil under the ultrahigh peak pressure produced by the laser shock wave as well as the re striction effect of the smooth glass backing plate. Fig. 2(c) shows the surface morphology of the copper foil flattened at 0.25 J, the boundary between the non-flattening and flattening areas of the copper foil was obvious, and this boundary was free of cracks and defects. However, the laser shock adversely affected the flattening of the copper foil when the pulse energy was large enough. On one hand, the surface of the flattened copper foil appeared to be a transition area, and there were some longer micro-cracks in this transition area, which reduced the surface quality of the copper foil. On the other hand, a large amount of pulse energy could cause large plastic deformation of the copper foil and form a circular concave shape on its surface, with a piledup region formed at the edge of this circular structure, as shown in the illustration (OM image of a single-spot flattened copper foil) of Fig. 2(d). At this time, the peak pressure of the shock wave was as high as several GPa [20,30], which resulted in the serious plastic deformation of the copper foil. Because of the obstruction of the glass backing plate, copper metal firstly flowed toward the edge of the laser spot irradiation region (flattening area) under the laser shock wave with a high peak pressure. Then when the shock wave was transmitted to the contact surface be tween backing plate and copper foil, partial shock wave was reflected, which resulted in secondary deformation of the copper foil, the copper metal flowed again during the extrusion process, and thus a piled-up region was formed at the edge of the flattening area, as shown in Fig. 2(e). Fig. 3(a) shows the relationship between the laser pulse energy and the surface roughness (Sa) found in the single-spot LSF experiments. Each sample was scanned three times using AFM, and its surface roughness value was the average of these three measurements. The scanning positions were the three points (A, B, and C) shown in Fig. 3(a), and the surface roughness of the annealed copper foil was 52.1 nm. With an increase in the laser pulse energy, the surface roughness gradually
where D is the focused laser spot diameter; and d1 and d2 are the dis tances between two adjacent laser spot centers in the transverse and longitudinal directions, respectively. The optimum processing parameters for LSF were obtained by adjusting the pulse energy and overlap rate. After the flattening exper iment, the annealed copper foil and flattened copper foil (profile-free copper foil, treated by LSF using the optimum processing parameters) were selected as experimental materials to perform a thinning test, transmission electron microscopy (TEM) test, nano-indentation test, micro-tensile test, and electrochemical corrosion test, respectively. 2.2. Characterisation methods The surface roughness values of the copper foil were determined using atomic force microscope (AFM) (CSPM5500). Among them, the working mode was contact mode; the scanning area was 80 μm � 80 μm; the scanning frequency was 0.5 Hz; the scanning angle was 0� ; and the 3
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Fig. 2. Surface morphologies: (a) annealed copper foil, (b) flattened copper foil, (c) copper foil flattened at 0.25 J, (d) copper foil flattened at 1.0 J, (e) schematic diagram of piled-up region formation and metal flow.
decreased by 86.2%, reaching a minimum (7.2 nm) at 0.4 J. Subse quently, the surface roughness tended to become stable and hovered at 10 nm with a continuous increase in pulse energy. Fig. 3(b) shows a statistical chart for the height distribution of the micro-bulges on the surfaces of the annealed copper foil and flattened copper foil at 0.4 J. The peak height (the height of the largest proportion of all the microbulge heights) of the copper foil surface decreased by 84.1%, from 540 nm to 86 nm, and the height concentration range (the percentage greater than 0.2%) decreased from 434–639 nm to 54–91 nm. Therefore, the fundamental reason for the reduction in the surface roughness was the production of extrusion deformation of the micro-bulges under the ultra-high pressure induced by laser shock. However, when the laser pulse energy exceeded 0.25 J, the flattening area of the copper foil formed a circular concave structure with the same size as the laser spot, which had a negative effect on the reduction of the surface roughness. With an increase in the pulse energy, the macroscopic deformation and piled-up region became increasingly obvious, and the surface quality of the transition area decreased, which greatly aggra vated the macroscopic unevenness of the copper foil surface. Therefore, considering the surface quality, micro-roughness, and macroscopic
unevenness of the copper foil, the better flattening effect was found with a laser pulse energy of 0.25 J, and the surface roughness (12.2 nm) decreased by approximately 76.6%. 3.2. Large-area LSF experiments During the large-area LSF experiments, the laser pulse energy of 0.25 J was used to study the effect of overlap ratio on the flattening effect of copper foil. From Fig. 4(a), we can conclude that surface roughness first decreased and then increased with an increase in the overlap ratio. Each sample was measured nine times to eliminate the potential error to the greatest extent, and the average value of these nine measurements was used to characterise the surface roughness of the flattened copper foil, with the nine measurement positions distributed at a square array. The surface roughness reached a minimum of 17.2 nm at an overlap ratio of 25%，and the height concentration range of the micro-bulges was 105–178 nm, as shown in Fig. 4(b). Compared with the surface rough ness of the annealed copper foil, the surface roughness of the large-area flattened copper foil at a 25% overlap ratio was reduced by 67.0%. Fig. 4 (c) and (d) show the surface morphologies of the large-area flattened 4
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Fig. 3. (a) Laser pulse energy vs. surface roughness, (b) statistics for height distribution of micro-bulges on surfaces of copper foil.
Fig. 4. (a) Overlap ratio vs. surface roughness, (b) morphology and micro-bulge height distribution of flattened copper foil at 25% overlap ratio, (c) morphology of flattened copper foil at 0% overlap ratio, (d) morphology of flattened copper foil at 25% overlap ratio, (e) influences of overlap ratio on flattening area, overlap area, and non-flattening area, (f) confinement layer after ablating and sputtering.
copper foil with overlap ratios of 0% and 25%, respectively. As seen, the surface of the large-area flattened copper foil with the overlap ratio of 25% was more uniform and its surface roughness was more consistent. It is well known that the overlap ratio affects the size and proportion of the flattening area, overlap area, and non-flattening area on the copper foil surface, thus affecting its surface roughness, as shown in Fig. 4(e). And the area proportion (η) of the non-flattening area can be calculated by formula (3) as follows: � � pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi πD2 1 θ 45 2Dx x2 2R R2 η¼1 ¼1 2 1 R D x 4ðD xÞ � � π arccosð1 RÞ 1 2 45 4ð1 RÞ
where x is the overlap width between two adjacent laser spots, and x ¼ D – d1 ¼ D – d2, θ is a self-defined angle, as shown in Fig. 4(e), and R is the overlap ratio. From formula (3), the area proportion of the non-flattening area was only related to the overlap ratio. With an increase in the overlap ratio, the area proportion of the non-flattening area gradually decreased. Thus, the area proportion of the non-flattening area was extremely close to zero (η ¼ 0.42%) at a 25% overlap ratio, and the surface roughness gradually decreased. However, with a further increase in the overlap ratio, the overlap area became larger, and the surface roughness increased. This may have been because the confinement layer was destroyed during the laser shock, and the transparency of the confine ment layer decreased as a result of the ablating and sputtering of the ablative layer (as shown in Fig. 4(f)), which reduced the transmittance of the next laser pulse and hindered its transmission. In addition, the area
(3)
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of the confinement layer damaged by a single laser pulse was certain. When the overlap ratio increased, the distance between the front and back laser pulses radiated on the surface of confinement layer became closer, the area of the next laser pulse radiated on the destroyed confinement layer (damaged by the previous laser pulse) was larger, and the hindrance of the destroyed confinement layer to its transmission was greater. In other words, the transmittance of the second laser pulse through the constraint layer was smaller. Therefore, the peak pressure of the shock wave decreased, which resulted in a decrease in the defor mation of the micro-bulges on the copper foil surface and an increase in the surface roughness. Fig. 5 shows cross sections of epoxy-resin-embedded samples of the annealed copper foil and large-area flattened copper foil at a 25% overlap ratio. The fluctuation degree of the cross-sectional profile of the annealed copper foil was obviously greater than that of the flattened copper foil, which also indicated that laser shock could improve the surface smoothing of the copper foil. In addition, the measured values of the annealed copper foil conformed to the actual thickness. The copper foil was thinned after flattening, with its thickness decreasing from 35.9 μm (Fig. 5(a)) to 28.1 μm (Fig. 5 (b)), which was a thinning rate of approximately 21.7%. This phenomenon could be attributed to the plastic deformation of the copper foil under the ultra-high pressure induced by the laser shock.
Fig. 6. Fig. 6 also shows the microstructures of the dislocation cell (DC) (Fig. 6(b) and (c)), dislocation wall (DW) (Fig. 6(c)), and dislocation tangles (DTs) (Fig. 6(d)) formed by the accumulation, movement, and interaction of dislocations in the grain. Yea YX et al. [30] also found a large number of dislocation structures with various morphologies in a study of the plastic deformation mechanism of polycrystalline copper shocked using a femtosecond laser. At this time, the DWs and DCs had not evolved into the subgrain boundaries and subgrains, respectively, which could be confirmed by the selected area electron diffraction (SAED) pattern, which showed the characteristics of discrete diffraction points in the coarse grain, as shown in the illustration of Fig. 6(c). In addition, the DC diameter in the grain of the annealed copper foil was approximately 1500 nm, as shown by the circular marked area in Fig. 6 (c). Fig. 7 shows the microstructures of the copper foil after LSF. The average grain diameter of the flattened copper foil did not show obvious refinement, as shown in Fig. 7(a). By studying the metal strengthening that occurred during LSP processing, Umapathi A [33] and Nie XF [24] also found that a metal target processed by laser shock didn’t experience grain refinement. However, a large number of DCs (Fig. 7(b) and (c)), DWs (Fig. 7(d)), and DTs (Fig. 7(d)), along with a small number of dislocation loops (DLs) (Fig. 7(d)) and stacking faults (SFs) (Fig. 7(e)), were formed in the coarse grains. These dislocation structures greatly increased the density of the crystal defects. In addition, the coarse grains still exhibited the characteristics of discrete diffraction points. Thus, the DCs and DWs still had not evolved into subgrains and subgrain bound aries after laser shock, respectively, as shown in the illustration in Fig. 7 (c). However, the migration and sliding of the original DWs were induced by the shock wave with the ultra-high peak pressure, and the DCs were cracked and multiplied, which resulted in the formation of a greater quantity of DC structures with a smaller size (with a diameter of approximately 600 nm), as shown in Fig. 7(c). This may have been due to the shorter interaction time of the shock wave induced by the laser pulse in the copper foil. Fig. 7(f) shows the corresponding inverse fast Fourier transform (IFFT) images of the high-resolution TEM (HRTEM) images of the flattened copper foil and dislocation, where the lattice constant of the flattened copper foil was 0.2083 nm at the indices of the (111) crystal face. A small number of DLs and SFs were found in the microstructures of the flattened copper foil, while these were not found in the annealed copper foil, as shown in Fig. 7(d) and (e), respectively. A dislocation ring is a type of linear defect in a crystal, which belongs to one of the many
3.3. Deformation mechanism of copper foil induced by LSF Laser shock processing is an efficient processing method that can achieve the super-high strain rate deformation of a metal material. The strain rate is usually as high as 106–107 s 1, which is different from the traditional processing method [30,31]. During the laser shock process, the metal can be strengthened, which is mainly due to the plastic deformation caused by the ultra-high pressure (up to several giga pascals) shock wave induced by the pulsed laser. Existing studies have shown that different metals have different plastic deformation mecha nisms [26,32]. However, the most significant point is that laser shock can increase the grain defects in metals and strengthen their compre hensive properties [31,32]. Based on this, the deformation mechanism of the copper foil flattened by LSF was analysed based on the evolution of the internal microstructure of the copper foil. The copper foil was annealed to reduce the crystal defects and re sidual stress, and a uniform microstructure with a grain diameter of 10–30 μm was acquired in the annealed copper foil, as illustrated in
Fig. 5. Cross-sectional morphologies and thicknesses: (a) annealed copper foil, (b) flattened copper foil. 6
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Fig. 6. Microstructures in annealed copper foil: (a) grains of cross section, (b) TEM image of grain boundaries (GBs), DC, and dislocations, (c) TEM image and size of DC, and SAED pattern, (d) TEM image of dislocations and DTs.
dislocation morphologies. It is mainly caused by the dislocation multi plication or vacancy defects and has a certain strengthening effect on the copper foil. An SF is a kind of surface defect, which occurs to adapt to the plastic deformation under an ultra-high strain rate. Further, twins were found in the copper foil based on their typical morphological characteristics and SAED pattern, as shown in Fig. 8. Fig. 8(a) shows the annealing twins in the annealed copper foil. Fig. 8(b) and (c) show the deformation twins in the flattened copper foil, and Fig. 8(d) shows the HRTEM image and corresponding IFFT image of the twin boundary (TB). Current studies have found that an ultra-high strain rate, a large strain, and high pressure are beneficial in producing the deformation twins by enhancing the local stress to the critical twinning stress [30,34]. Furthermore, the dislocations in the copper foil did not have enough time to multiply, migrate, and slide during the laser shock, and the DWs did not evolve into the subgrain boundaries after laser shock. Therefore, a shock wave with an ultra-high peak pressure pro vided enough driving energy for twinning at the local area inside the copper foil, which eventually led to the formation of deformation twins. Typically, there are two kinds of modes for the plastic deformation of
copper: the dislocation slip and deformation twinning [32]. In this study, the dislocation structures in the flattened copper foil obviously increased, while the number of deformation twins was very small. Thus, the dislocation slip was the main mode of plastic deformation for the copper foil during the laser shock process. At the beginning stage of deformation, high-speed shock wave could trigger most of the disloca tion sources in the coarse grains because of the ultra-high strain rate and pressure and could concurrently excite dislocation avalanches in mul tiple slip systems. Numerous dislocations exist in coarse grains with various morphologies, including dislocation lines, DLs, DTs, DWs, and DCs. With an increase in the deformation degree of the copper foil, a small number of deformation twins were formed when the shock wave pressure exceeded its local critical twinning stress, and these deforma tion twins existed as a supplemental plastic deformation mode for the copper foil. In addition, an increase in the density of crystal defects and the formation of deformation twins has an obvious strengthening effect on metal materials. In other words, the plastic deformation caused by the laser shock can improve the mechanical properties and corrosion resistance of metal materials. 7
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Fig. 7. Microstructures in flattened copper foil: (a) grains of cross section, (b) TEM image of GB and DCs, (c) TEM image and size of DCs, along with SAED pattern, (d) TEM image of dislocations with various morphologies, (e) TEM image of SF, (f) corresponding IFFT images of HRTEM images of flattened copper foil and dislocation.
3.4. Measurement and analysis of mechanical properties and corrosion resistance
load–displacement curves of the annealed copper foil and flattened copper foil (formed using a pulse energy of 0.25 J and overlap ratio of 25%) were investigated, and their indentation morphologies were scanned using AFM, as shown in Fig. 9(a). During the loading process, the indentation load required by the flattened copper foil was always greater than that of the annealed copper foil at the same indentation depth. In addition, the maximum indentation load of the flattened copper foil (11.3 mN) was significantly higher than that of the annealed copper foil (9.1 mN) at a fixed indentation depth of 500 nm. This
3.4.1. Study on the mechanical properties To investigate the influences of the LSF on the mechanical properties and corrosion resistance of the copper foil, nano-indentation, microtensile, and electrochemical corrosion tests were performed to charac terise its performances, where the nano-indentation test positions were cross sections of the epoxy-resin-embedded samples. The 8
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Fig. 8. Twins in copper foil: (a) TEM image and SAED pattern of annealing twins in annealed copper foil, (b) TEM image and SAED pattern of deformation twins in flattened copper foil, (c) TEM image of deformation twins, (d) HRTEM image and corresponding IFFT image of twin boundary.
indicated that LSF could improve the deformation resistance of the copper foil. Fig. 9(b) shows the nano-hardness and elastic modulus values of the annealed copper foil and flattened copper foil, where an average of three measured values was used to characterise each of these parameters. The nano-hardness and elastic modulus of the flattened copper foil were improved compared with the annealed copper foil. After LSF, the nanohardness of the copper foil increased from 1.5 GPa to 2.3 GPa, which was an increase of 53.3%. The elastic modulus increased from 91.6 GPa to 123.0 GPa, which was an increase of 34.3%. In this study, the
improvements in the mechanical properties of copper foil were mainly attributed to the work hardening from the plastic deformation caused by the laser shock. In addition, a large number of dislocation structures with various morphologies (as shown in Fig. 7) were formed during the deformation process, and the dislocation strengthening was an impor tant strengthening mechanism of laser shock processing, which caused the metal to obviously be strengthened [25,26]. Fig. 10 shows the stress–strain curves of the annealed copper foil and flattened copper foil, where these curves are divided into three stages: elastic deformation stage I, plastic deformation and strengthening stage
Fig. 9. Nano-indentation test results for copper foil: (a) load–displacement curves, (b) nano-hardness and elastic modulus. 9
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II, and necking stage III. Dog-bone specimens were used in this experi ment, and the tensile strength and elongation of the annealed copper foil were 173.9 MPa and 11.4%, respectively, as illustrated in Fig. 10(a). Compared with the annealed copper foil, the linear slope was larger at elastic deformation stage I. In other words, the LSF technique improved the elastic modulus of the copper foil, which was consistent with the results of the nano-indentation test. In addition, after the LSF, the tensile strength (295.6 MPa) of the flattened copper foil increased by 70.0%, and the elongation (4.4%) decreased by 61.4%, which also indicated that the LSF technique could strengthen the copper foil. The main reason for the above phenomenon was the work hardening of the copper foil caused by the LSF. In addition, the introduction of residual compressive stress and the increase in the dislocation density helped to improve the tensile strength. Furthermore, the improvement of the nano-hardness, elastic modulus, and tensile strength indicated that the LSF technique could effectively improve the deformation resistance of the copper foil, but this processing also reduced its plas ticity. Therefore, the elongation, as an important index to describe the plasticity of the material, also decreased.
effectively reduce the corrosion rate and improve the corrosion resis tance of the copper foil. Vcorr ¼ Hcorr ¼
M � Icorr M ¼ 373:11 � Icorr n n�F
(4)
Vcorr
(5)
ρ
¼ 3:27 � 103 �
M Icorr nρ
where Icorr is the current density; M ¼ 63.55 g/mol is the molar mass of the metal (Cu); n ¼ 2 is the valence of the metal; F ¼ 96485.33 C/mol is the faraday constant; ρ ¼ 8.96 g/cm3 is the density of the copper. The improvement of the corrosion resistance could be attributed to the following three aspects: 1) after LSF, the decrease of surface microcracks and surface roughness reduced the contact area between the copper foil and the NaCl solution, which reduced the corrosion ten dency; 2) laser shock increased the dislocation density in the copper foil, a large number of dislocation structures was induced, and they were beneficial to the improvement of the corrosion resistance; 3) laser shock processing could introduce larger residual compressive stress on the target surface, which had universal significance and had been confirmed by many scholars in the same field [36,37]. The existence of residual compressive stress could effectively inhibit the propagation of corrosion micro-cracks and avoid the defects caused by stress imbalance. In addition, it was worth noting that the theoretical proportion of non-flattening area (η ¼ 0.42%) wasn’t zero at the overlap ratio of 25%, but the surface of the flattened copper foil was fully strengthened because of the existence of the deformation affected zone. In other words, the deformation zone and strengthening zone on the copper foil surface were larger than the laser spot diameter, and the area of the non-strengthening zone was zero.
3.4.2. Study on the corrosion resistance Fig. 11 shows the potentiodynamic polarization curves in Tafel re gion of the copper foil and their surface morphologies after corrosion. The linear fitting results of the anode area and cathode area of the annealed copper foil and flattened copper foil were showed in Fig. 11(b) and (c), respectively. Fig. 11(d) shows the surface morphology of the annealed copper foil after corrosion, the corrosion pits were large, and their distributions were not uniform, it might be developed from the growth of micro-cracks. On the contrary, owing to the reduction of the micro-cracks and micro-defects on the surface after flattening, the flat tened copper foil had smaller corrosion pits after corrosion, the corro sion pits were uniformly distributed, which showed that the flattened copper foil had a better corrosion resistance, as illustrated in Fig. 11(e). In this study, Tafel extrapolation method was used to calculate the anode area slope (Ka), cathode area slope (Kc), and self-corrosion po tential (Ecorr), and these results were listed in Table 1. The Ecorr of the annealed copper foil was 0.23 V, the Ecorr of the flattened copper foil increased to 0.21 V. Thus, the Ecorr of copper foil shifted positively after flattening. Existing research had shown that the Ecorr can judge the corrosion difficulty degree from the thermodynamic view, and material showed a better corrosion resistance if the Ecorr shifted positively [35]. Therefore, LSF could improve the corrosion resistance of the copper foil from the thermodynamic analysis view. Furthermore, the self-corrosion current density (Icorr), corrosion rate (Vcorr), and corrosion depth (Hcorr) of the flattened copper foil were lower than those of the annealed copper foil, these parameters could be calculated by formulas (4) and (5), and these calculation results were also listed in Table 1. After LSF, the Vcorr of the annealed copper foil decreased by approximately 36.2%, which indicated that LSF could
4. Conclusions In this paper, the LSF technique was proposed to manufacture the profile-free copper foil with high performance. The single-spot LSF ex periments, large-area LSF experiments, microstructures characterisa tion, and performance tests were respectively carried out on the copper foil. The main conclusions were as follows: (1) LSF could effectively reduce the surface roughness (Sa) of copper foil. During single-spot LSF experiments, considering the microroughness and macroscopic unevenness of the copper foil, its surface had a better flattening quality when using a laser pulse energy of 0.25 J, and its surface roughness (12.2 nm) decreased by approximately 76.6%. (2) In large-area LSF experiments, the flattening effect on the largearea profile-free copper foil was better at a pulse energy of 0.25 J and an overlap rate of 25%, and its surface roughness decreased from 52.1 nm to 17.2 nm, which was a 67.0% decrease.
Fig. 10. Stress–strain curves: (a) annealed copper foil, (b) flattened copper foil. 10
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International Journal of Machine Tools and Manufacture 152 (2020) 103542
Fig. 11. (a) Potentiodynamic polarization curves in Tafle region, (b) potentiodynamic polarization curves and linear fitting results of the annealed copper foil, (c) potentiodynamic polarization curves and linear fitting results of the flattened copper foil, (d) surface morphology of the annealed copper foil after corrosion, (e) surface morphology of the flattened copper foil after corrosion.
CRediT authorship contribution statement
Table 1 Test and calculation results of the electrochemical corrosion tests. Samples
Ka
Kc
Ecorr (V)
Annealed copper foil Flattened copper foil
4.74
3.77
0.23
4.33
7.57
0.21
Icorr (A/ cm2)
Vcorr (g/ m2h)
Hcorr (mm/ year)
1.16 � 10 5 7.43 � 10 6
0.138
0.134
0.088
0.086
Yang Haifeng: Conceptualization, Supervision, Writing - review & editing. Xiong Fei: Methodology, Formal analysis, Investigation, Writing - original draft. Wang Yan: Validation, Resources. Jia Le: Validation, Resources. Liu Hao: Supervision, Funding acquisition. Hao Jingbin: Project administration. Acknowledgements This work was supported by the Fundamental Research Funds for the Central Universities [2020ZDPYMS21]; a project funded by the Priority Academic Program Development of Jiangsu Higher Education In stitutions [PAPD]. We would like to acknowledge the assistance of FSEM (MAIA3 LMH) test provided by the Advanced Analysis & Computation Center of CUMT.
Meanwhile, the profile-free copper foil had a thinning rate of 21.7% when using the optimum flattening parameters. (3) After LSF, a large number of dislocation structures with different morphologies were formed in the copper foil, the dislocation density was significantly increased, and a few deformation twins were found. The flattened deformation mechanism for the copper foil was mainly the dislocation slip, and a few deformation twins existed in the copper foil as a supplementary mechanism of plastic deformation. (4) LSF had an obvious strengthening effect on the copper foil and improved its mechanical properties. After flattening, the nanohardness and elastic modulus of the copper foil were increased by 53.3% and 34.3%, respectively. The tensile strength increased by 70.0%, the elongation decreased by 61.4%, and the corrosion rate was reduced by 36.2%.
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Manufacturing profile-free copper foil using laser shock flattening Yang Haifeng a, b, Xiong Fei a, *, Wang Yan a, Jia Le a, Liu Hao a, Hao Jingbin a a b
School of Mechatronic Engineering, China University of Mining and Technology, XuZhou, 221116, China Jiangsu Key Laboratory of Mine Mechanical and Electrical Equipment, China University of Mining and Technology, XuZhou, 221116, China
A R T I C L E I N F O
A B S T R A C T
Keywords: Laser shock flattening Profile-free copper foil Flattened deformation mechanism Microstructures Mechanical properties Corrosion resistance
Copper foil is a key material of printed circuit boards and plays an important role in the conductance of electric circuits and interconnection of electronic components. When high-frequency signals were transmitted in rough copper foil wires, the conductor resistance, wire loss, and signal loss increased because of the skin effect. To reduce the negative influence of the skin effect and improve the quality of the copper foil, a laser shock flattening (LSF) method was proposed to manufacture profile-free copper foil with high performance. It was concluded that the better flattening effect for large-area profile-free copper foil could be achieved at a pulse energy of 0.25 J and an overlap rate of 25%, and its surface roughness decreased by 67.0% from 52.1 nm to 17.2 nm. Subsequently, to determine the mechanism for the flattened deformation of copper foil induced by LSF, the microstructures of the copper foil before and after flattening were characterised using transmission electron microscopy. A higher dislocation density and a few deformation twins were found in the profile-free copper foil. Ultimately, nanoindentation, micro-tensile, and electrochemical corrosion tests indicated that the mechanical properties and corrosion resistance of the copper foil were significantly improved by LSF. This technique would enable the successful fabrication of large-area profile-free copper foil with high performance for the emerging applications of ultra-high-frequency signal communication and printed circuit board manufacture.
1. Introduction Copper foil is widely used in many fields such as micro-electromechanical systems (MEMSs), flexible copper clad laminates (FCCLs), industrial communication devices (ICDs), and printed circuit boards (PCBs) because of its excellent electrical conductivity, thermal conduc tivity, and ductility [1,2]. With the rapid development of science and technology and the sharp increase in social demand, various industries have set higher requirements for copper foil. This demand is particularly prominent in the development of 5G and subsequent terahertz wireless communication techniques [3,4]. The skin effect refers to a phenomenon wherein the current distri bution inside a conductor is uneven when there is an alternating current or alternating electromagnetic field in the conductor. The current is concentrated in a thin layer on the conductor surface, the current den sity on the conductor surface increases, and the actual interior current decreases. It is well-known that the frequency of the millimeter wave band used in ultra-high-frequency signal wireless communication techniques is as high as tens or hundreds of gigahertz [5]. The skin effect of the copper foil in a PCB is more obvious at such high frequencies. The
current is concentrated and transmitted in a skin-depth area under the surface of the copper foil, and this skin-depth gradually decreases with an increase in the signal frequency [6,7]. Therefore, a high-frequency signal must travel along the surface of the rough copper foil during the transmission process [8,9]. This significantly increases the trans mission path length, conductor resistance, and wire loss, as well as decreasing the signal integrity and causing the delay or attenuation of the signal [10,11]. In addition, the wear resistance, fatigue strength, and corrosion resistance of the copper foil are increased with a decrease in the surface roughness [12,13]. In order to reduce the influence of the skin effect on the transmission process for a high-frequency signal and improve the performance of copper foil, it is imperative to reduce its surface roughness. An examination of the current polishing methods shows that tradi tional mechanical polishing is obviously not suitable for metal foils, while electrochemical polishing, chemical polishing, chemical me chanical polishing, ion beam cleaning, ultrasonic vibration-assisted polishing, and other methods still have some limitations when used for large-area metal foils [14,15]. However, an advanced laser shock processing technique has attracted extensive attention in recent years
* Corresponding author. School of Mechatronic Engineering, China University of Mining and Technology, No.1 Daxue Road, XuZhou, 221116, China.; E-mail address: [email protected] (X. Fei). https://doi.org/10.1016/j.ijmachtools.2020.103542 Received 18 December 2019; Received in revised form 29 February 2020; Accepted 2 March 2020 Available online 5 March 2020 0890-6955/© 2020 Elsevier Ltd. All rights reserved.
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because it has advantages that include high efficiency, energy saving, green environmental protection, a wide application range, easy control, and no direct contact with the processing parts. Thus, some researchers have studied the surface roughness values of metal materials processed by laser shock. For instance, Dai FZ [16] studied the influence of laser shock peening (LSP) on the roughness of a target material under different overlap modes, and the results showed that the surface roughness values of the isosceles-right-triangle style and equilateral-triangle style attained their minimum values at X-axis overlap ratios of 29.3% and 42.3%, respec tively. Lu GX [17] found that increasing the overlap rate helped to improve the surface quality of low-strength metals. Subsequently, Dai FZ [18] also proposed a new technique of elastic contact LSP, which could effectively reduce the target material roughness compared with tradi tional LSP, but it was difficult to control the surface roughness of metal foil subjected to the LSP process. Yang HF [19,20] also found that the laser shock imprinting (LSI) technique could reduce the surface rough ness of micro-formed parts imprinted from aluminium foil, but there were 3D micro-structures with large fluctuation on the surface of the metal foil, and the surface roughness values at different positions on the surfaces of micro-formed parts were different. Therefore, this laser shock processing technique still has some limitations when used to smooth the surface of metal foil, which are disadvantages for the manufacture of profile-free copper foil. In addition, a laser shock processing technique can also strengthen the properties of the target material, which is particularly significant with the LSP technique [21,22]. LSP was first used to improve the fa tigue resistance of materials, because this technique can induce a re sidual stress layer with a large depth [23,24]. Moreover, LSP can refine the target grain, and even produce a nano-crystalline layer and intro duce a large number of dislocations in the metal material [12,25,26]. Therefore, this technique is also widely used to modify the character istics of metal materials such as their hardness [21], strength [27], wear resistance [28], and corrosion resistance [29].
In order to reduce the surface roughness and surface micro-defects of copper foil, and improve its performances, this paper proposes a laser shock flattening (LSF) method to manufacture profile-free copper foil with high performance. LSF experiments were performed using different pulse energy values and overlap ratios, and the optimum processing parameters were obtained by evaluating the macro-morphology and micro-roughness of the flattened copper foil. Furthermore, the changes in the microstructures of the copper foil before and after flattening were characterised, and the mechanism for the flattened deformation of the copper foil was revealed. Further, the degree that the mechanical properties and corrosion resistance of copper foil flattened by LSF were strengthened was evaluated using nano-indentation, micro-tensile, and electrochemical corrosion tests. 2. Experiments 2.1. Experimental principle and equipment Fig. 1(a) shows a schematic diagram of the LSF technique. Its prin ciple and experimental equipment were very similar to those of the LSP process. The only difference was that the experimental equipment used for LSF had to contain a backing plate with a sufficiently smooth surface. In this experiment, a laser pulse (with a pulse duration of 7 ns, wave length of 1064 nm, and pulse frequency of 3 Hz) was provided by a pulsed laser (Hercules-1000-TH), and the focused spot diameter (D, 1/ e2) was 1.2 mm. The laser beam (round with a Gaussian distribution) penetrated the confinement layer and irradiated the absorption layer. The laser energy was absorbed, and the top surface of the aluminium foil was instantaneously vaporised into a high-temperature and highpressure plasma. The plasma continued to absorb laser energy and rapidly expanded. Then, a high-pressure shock wave was generated because of the confinement effect of the confinement layer on the plasma [19]. The peak pressure of the shock wave was usually as high as several gigapascals, which was far beyond the dynamic yield strength of
Fig. 1. LSF process: (a) schematic diagram of LSF, (b) non-flattening area and flattening area on copper foil, (c) surface morphology of glass backing plate, (d) height distribution of micro-bulges (peaks) on surface of glass backing plate, (e) overlap effect and moving path of laser spot. 2
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the metal layer. Thus, plastic deformation could be induced in the metal foil, and its surface became smooth, as shown in Fig. 1(b). The ablative layer could increase the peak pressure of the shock wave induced by the pulse laser, prevent the ablation of the target material surface, and ensure the integrity of the target material. In this study, if the ablative layer was too thin, it could easily be completely ablated during the LSF process, resulting in the laser ablation of the upper sur face of the copper foil, which was not conducive to improving its surface quality. In addition, aluminium foil is one of the most commonly used ablative layers in laser shock processing, and it has a uniform thickness, which makes it more convenient than black paint and graphite. There fore, 100 μm thick aluminium foil was selected as the ablative layer. In addition, a clamping device was used in the LSF experiments to ensure that the confinement layer, absorption layer, metal layer, and backing plate were closely combined. In these experiments, the pressure induced by clamping force (only tens of newtons, with a contact area of more than 15 mm � 15 mm and a pressure of less than 1 MPa) was constant, with a value that was far smaller than the peak pressure of the laser shock wave and the yield strength of the copper foil. Therefore, the clamping force could not cause deformation of the copper foil and could be ignored. The backing plate under the copper foil had to be sufficiently smooth to ensure that the surface roughness of the copper foil treated by LSF was significantly reduced. A 2 mm thick piece of glass was selected as the backing plate, and its surface roughness (Sa) was only 6.3 nm, as shown in Fig. 1(c). Fig. 1(d) shows the height distribution of the micro-bulges (peaks) on the glass surface. The heights of the micro-bulges were mainly concentrated in the 230–290 nm range, with other height ranges (171–230 nm and 290–500 nm) only accounting for very small per centages. This indicated that the surface roughness of the glass was caused by a few relatively high micro-bulges, and the glass surface was extremely flat. In this work, the 36-μm-thick copper foil was used as the experimental material (metal layer). In order to clearly understand the mechanism of the deformation of the copper foil induced by laser shock, the copper foil was annealed at 723 K for 1 h in a vacuum environment to reduce the original defects before flattening. During single-spot LSF experiments, the laser pulse energy was var ied from 0.05 J to 1.0 J in 0.05 J steps. The overlap was realised using a 3-D high-precision mobile platform in large-area LSF experiments, and the overlap effect and moving path of the laser spot are shown in Fig. 1 (e). The overlap ratios in the transverse (RT) and longitudinal (RL) di rections were the same and could be calculated using formulas (1) and (2), respectively. In other words, d1 was always equal to d2. In addition, the overlap ratios ranged from 0% to 75%, in 12.5% steps. RT ¼ ðD
d1 Þ=D � 100%
(1)
RL ¼ ðD
d2 Þ=D � 100%
(2)
filtering type was the three-level low-pass filtering (finite-impulseresponse). Besides, in this study, the software of “CSPM imager 4.60” was used to process the roughness data. Optical microscopy (OM) (LEICA DM4 M) and high-resolution field emission scanning electron microscopy (FSEM) (MAIA3 LMH) were used to observe the surface morphologies and cross-sectional thicknesses. Subsequently, the grains of the cross sections of epoxy resin embedded samples were observed using OM, and an FeCl3 solution (5 g FeCl3⋅6H2O þ 5 ml HCl þ 100 ml H2O) was selected as the etching agent. Then, the microstructures of the copper foil were characterised using TEM (JEM2100). Furthermore, nano-indentation tests with a standard Berkovich diamond indenter (Agilent U9820A, Nano Indenter G200) were used to determine the load–displacement curves, nano-hardnesses, and elastic modulus of the copper foil, where the indentation depth was fixed at 500 nm. A microtensile test bench (MTS-450) was used to measure the tensile strengths and elongations, where the stretching speed was set at 10 μm/s. Each sample was measured three times, and the average value of three measurements was used to characterise the performance of the copper foil. An electrochemical workstation (CHI660D) was selected to research the corrosion resistances in 3.5 wt% NaCl solution, and the surface morphologies after corrosion were characterised using OM. 3. Results and discussions 3.1. Single-spot LSF experiments Single-spot LSF experiments were performed on the copper foil to study the effect of pulse energy on its flattening effect. The annealed copper foil became smoother after flattening, and the visible microcracks and micro-defects on the surface of the copper foil close to the glass backing plate were significantly reduced. This phenomenon could be clearly observed by comparing the surface morphologies of the annealed copper foil (Fig. 2(a)) and flattened copper foil (Fig. 2(b)) and attributed to the extrusion deformation of copper foil under the ultrahigh peak pressure produced by the laser shock wave as well as the re striction effect of the smooth glass backing plate. Fig. 2(c) shows the surface morphology of the copper foil flattened at 0.25 J, the boundary between the non-flattening and flattening areas of the copper foil was obvious, and this boundary was free of cracks and defects. However, the laser shock adversely affected the flattening of the copper foil when the pulse energy was large enough. On one hand, the surface of the flattened copper foil appeared to be a transition area, and there were some longer micro-cracks in this transition area, which reduced the surface quality of the copper foil. On the other hand, a large amount of pulse energy could cause large plastic deformation of the copper foil and form a circular concave shape on its surface, with a piledup region formed at the edge of this circular structure, as shown in the illustration (OM image of a single-spot flattened copper foil) of Fig. 2(d). At this time, the peak pressure of the shock wave was as high as several GPa [20,30], which resulted in the serious plastic deformation of the copper foil. Because of the obstruction of the glass backing plate, copper metal firstly flowed toward the edge of the laser spot irradiation region (flattening area) under the laser shock wave with a high peak pressure. Then when the shock wave was transmitted to the contact surface be tween backing plate and copper foil, partial shock wave was reflected, which resulted in secondary deformation of the copper foil, the copper metal flowed again during the extrusion process, and thus a piled-up region was formed at the edge of the flattening area, as shown in Fig. 2(e). Fig. 3(a) shows the relationship between the laser pulse energy and the surface roughness (Sa) found in the single-spot LSF experiments. Each sample was scanned three times using AFM, and its surface roughness value was the average of these three measurements. The scanning positions were the three points (A, B, and C) shown in Fig. 3(a), and the surface roughness of the annealed copper foil was 52.1 nm. With an increase in the laser pulse energy, the surface roughness gradually
where D is the focused laser spot diameter; and d1 and d2 are the dis tances between two adjacent laser spot centers in the transverse and longitudinal directions, respectively. The optimum processing parameters for LSF were obtained by adjusting the pulse energy and overlap rate. After the flattening exper iment, the annealed copper foil and flattened copper foil (profile-free copper foil, treated by LSF using the optimum processing parameters) were selected as experimental materials to perform a thinning test, transmission electron microscopy (TEM) test, nano-indentation test, micro-tensile test, and electrochemical corrosion test, respectively. 2.2. Characterisation methods The surface roughness values of the copper foil were determined using atomic force microscope (AFM) (CSPM5500). Among them, the working mode was contact mode; the scanning area was 80 μm � 80 μm; the scanning frequency was 0.5 Hz; the scanning angle was 0� ; and the 3
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Fig. 2. Surface morphologies: (a) annealed copper foil, (b) flattened copper foil, (c) copper foil flattened at 0.25 J, (d) copper foil flattened at 1.0 J, (e) schematic diagram of piled-up region formation and metal flow.
decreased by 86.2%, reaching a minimum (7.2 nm) at 0.4 J. Subse quently, the surface roughness tended to become stable and hovered at 10 nm with a continuous increase in pulse energy. Fig. 3(b) shows a statistical chart for the height distribution of the micro-bulges on the surfaces of the annealed copper foil and flattened copper foil at 0.4 J. The peak height (the height of the largest proportion of all the microbulge heights) of the copper foil surface decreased by 84.1%, from 540 nm to 86 nm, and the height concentration range (the percentage greater than 0.2%) decreased from 434–639 nm to 54–91 nm. Therefore, the fundamental reason for the reduction in the surface roughness was the production of extrusion deformation of the micro-bulges under the ultra-high pressure induced by laser shock. However, when the laser pulse energy exceeded 0.25 J, the flattening area of the copper foil formed a circular concave structure with the same size as the laser spot, which had a negative effect on the reduction of the surface roughness. With an increase in the pulse energy, the macroscopic deformation and piled-up region became increasingly obvious, and the surface quality of the transition area decreased, which greatly aggra vated the macroscopic unevenness of the copper foil surface. Therefore, considering the surface quality, micro-roughness, and macroscopic
unevenness of the copper foil, the better flattening effect was found with a laser pulse energy of 0.25 J, and the surface roughness (12.2 nm) decreased by approximately 76.6%. 3.2. Large-area LSF experiments During the large-area LSF experiments, the laser pulse energy of 0.25 J was used to study the effect of overlap ratio on the flattening effect of copper foil. From Fig. 4(a), we can conclude that surface roughness first decreased and then increased with an increase in the overlap ratio. Each sample was measured nine times to eliminate the potential error to the greatest extent, and the average value of these nine measurements was used to characterise the surface roughness of the flattened copper foil, with the nine measurement positions distributed at a square array. The surface roughness reached a minimum of 17.2 nm at an overlap ratio of 25%，and the height concentration range of the micro-bulges was 105–178 nm, as shown in Fig. 4(b). Compared with the surface rough ness of the annealed copper foil, the surface roughness of the large-area flattened copper foil at a 25% overlap ratio was reduced by 67.0%. Fig. 4 (c) and (d) show the surface morphologies of the large-area flattened 4
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Fig. 3. (a) Laser pulse energy vs. surface roughness, (b) statistics for height distribution of micro-bulges on surfaces of copper foil.
Fig. 4. (a) Overlap ratio vs. surface roughness, (b) morphology and micro-bulge height distribution of flattened copper foil at 25% overlap ratio, (c) morphology of flattened copper foil at 0% overlap ratio, (d) morphology of flattened copper foil at 25% overlap ratio, (e) influences of overlap ratio on flattening area, overlap area, and non-flattening area, (f) confinement layer after ablating and sputtering.
copper foil with overlap ratios of 0% and 25%, respectively. As seen, the surface of the large-area flattened copper foil with the overlap ratio of 25% was more uniform and its surface roughness was more consistent. It is well known that the overlap ratio affects the size and proportion of the flattening area, overlap area, and non-flattening area on the copper foil surface, thus affecting its surface roughness, as shown in Fig. 4(e). And the area proportion (η) of the non-flattening area can be calculated by formula (3) as follows: � � pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi πD2 1 θ 45 2Dx x2 2R R2 η¼1 ¼1 2 1 R D x 4ðD xÞ � � π arccosð1 RÞ 1 2 45 4ð1 RÞ
where x is the overlap width between two adjacent laser spots, and x ¼ D – d1 ¼ D – d2, θ is a self-defined angle, as shown in Fig. 4(e), and R is the overlap ratio. From formula (3), the area proportion of the non-flattening area was only related to the overlap ratio. With an increase in the overlap ratio, the area proportion of the non-flattening area gradually decreased. Thus, the area proportion of the non-flattening area was extremely close to zero (η ¼ 0.42%) at a 25% overlap ratio, and the surface roughness gradually decreased. However, with a further increase in the overlap ratio, the overlap area became larger, and the surface roughness increased. This may have been because the confinement layer was destroyed during the laser shock, and the transparency of the confine ment layer decreased as a result of the ablating and sputtering of the ablative layer (as shown in Fig. 4(f)), which reduced the transmittance of the next laser pulse and hindered its transmission. In addition, the area
(3)
5
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of the confinement layer damaged by a single laser pulse was certain. When the overlap ratio increased, the distance between the front and back laser pulses radiated on the surface of confinement layer became closer, the area of the next laser pulse radiated on the destroyed confinement layer (damaged by the previous laser pulse) was larger, and the hindrance of the destroyed confinement layer to its transmission was greater. In other words, the transmittance of the second laser pulse through the constraint layer was smaller. Therefore, the peak pressure of the shock wave decreased, which resulted in a decrease in the defor mation of the micro-bulges on the copper foil surface and an increase in the surface roughness. Fig. 5 shows cross sections of epoxy-resin-embedded samples of the annealed copper foil and large-area flattened copper foil at a 25% overlap ratio. The fluctuation degree of the cross-sectional profile of the annealed copper foil was obviously greater than that of the flattened copper foil, which also indicated that laser shock could improve the surface smoothing of the copper foil. In addition, the measured values of the annealed copper foil conformed to the actual thickness. The copper foil was thinned after flattening, with its thickness decreasing from 35.9 μm (Fig. 5(a)) to 28.1 μm (Fig. 5 (b)), which was a thinning rate of approximately 21.7%. This phenomenon could be attributed to the plastic deformation of the copper foil under the ultra-high pressure induced by the laser shock.
Fig. 6. Fig. 6 also shows the microstructures of the dislocation cell (DC) (Fig. 6(b) and (c)), dislocation wall (DW) (Fig. 6(c)), and dislocation tangles (DTs) (Fig. 6(d)) formed by the accumulation, movement, and interaction of dislocations in the grain. Yea YX et al. [30] also found a large number of dislocation structures with various morphologies in a study of the plastic deformation mechanism of polycrystalline copper shocked using a femtosecond laser. At this time, the DWs and DCs had not evolved into the subgrain boundaries and subgrains, respectively, which could be confirmed by the selected area electron diffraction (SAED) pattern, which showed the characteristics of discrete diffraction points in the coarse grain, as shown in the illustration of Fig. 6(c). In addition, the DC diameter in the grain of the annealed copper foil was approximately 1500 nm, as shown by the circular marked area in Fig. 6 (c). Fig. 7 shows the microstructures of the copper foil after LSF. The average grain diameter of the flattened copper foil did not show obvious refinement, as shown in Fig. 7(a). By studying the metal strengthening that occurred during LSP processing, Umapathi A [33] and Nie XF [24] also found that a metal target processed by laser shock didn’t experience grain refinement. However, a large number of DCs (Fig. 7(b) and (c)), DWs (Fig. 7(d)), and DTs (Fig. 7(d)), along with a small number of dislocation loops (DLs) (Fig. 7(d)) and stacking faults (SFs) (Fig. 7(e)), were formed in the coarse grains. These dislocation structures greatly increased the density of the crystal defects. In addition, the coarse grains still exhibited the characteristics of discrete diffraction points. Thus, the DCs and DWs still had not evolved into subgrains and subgrain bound aries after laser shock, respectively, as shown in the illustration in Fig. 7 (c). However, the migration and sliding of the original DWs were induced by the shock wave with the ultra-high peak pressure, and the DCs were cracked and multiplied, which resulted in the formation of a greater quantity of DC structures with a smaller size (with a diameter of approximately 600 nm), as shown in Fig. 7(c). This may have been due to the shorter interaction time of the shock wave induced by the laser pulse in the copper foil. Fig. 7(f) shows the corresponding inverse fast Fourier transform (IFFT) images of the high-resolution TEM (HRTEM) images of the flattened copper foil and dislocation, where the lattice constant of the flattened copper foil was 0.2083 nm at the indices of the (111) crystal face. A small number of DLs and SFs were found in the microstructures of the flattened copper foil, while these were not found in the annealed copper foil, as shown in Fig. 7(d) and (e), respectively. A dislocation ring is a type of linear defect in a crystal, which belongs to one of the many
3.3. Deformation mechanism of copper foil induced by LSF Laser shock processing is an efficient processing method that can achieve the super-high strain rate deformation of a metal material. The strain rate is usually as high as 106–107 s 1, which is different from the traditional processing method [30,31]. During the laser shock process, the metal can be strengthened, which is mainly due to the plastic deformation caused by the ultra-high pressure (up to several giga pascals) shock wave induced by the pulsed laser. Existing studies have shown that different metals have different plastic deformation mecha nisms [26,32]. However, the most significant point is that laser shock can increase the grain defects in metals and strengthen their compre hensive properties [31,32]. Based on this, the deformation mechanism of the copper foil flattened by LSF was analysed based on the evolution of the internal microstructure of the copper foil. The copper foil was annealed to reduce the crystal defects and re sidual stress, and a uniform microstructure with a grain diameter of 10–30 μm was acquired in the annealed copper foil, as illustrated in
Fig. 5. Cross-sectional morphologies and thicknesses: (a) annealed copper foil, (b) flattened copper foil. 6
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Fig. 6. Microstructures in annealed copper foil: (a) grains of cross section, (b) TEM image of grain boundaries (GBs), DC, and dislocations, (c) TEM image and size of DC, and SAED pattern, (d) TEM image of dislocations and DTs.
dislocation morphologies. It is mainly caused by the dislocation multi plication or vacancy defects and has a certain strengthening effect on the copper foil. An SF is a kind of surface defect, which occurs to adapt to the plastic deformation under an ultra-high strain rate. Further, twins were found in the copper foil based on their typical morphological characteristics and SAED pattern, as shown in Fig. 8. Fig. 8(a) shows the annealing twins in the annealed copper foil. Fig. 8(b) and (c) show the deformation twins in the flattened copper foil, and Fig. 8(d) shows the HRTEM image and corresponding IFFT image of the twin boundary (TB). Current studies have found that an ultra-high strain rate, a large strain, and high pressure are beneficial in producing the deformation twins by enhancing the local stress to the critical twinning stress [30,34]. Furthermore, the dislocations in the copper foil did not have enough time to multiply, migrate, and slide during the laser shock, and the DWs did not evolve into the subgrain boundaries after laser shock. Therefore, a shock wave with an ultra-high peak pressure pro vided enough driving energy for twinning at the local area inside the copper foil, which eventually led to the formation of deformation twins. Typically, there are two kinds of modes for the plastic deformation of
copper: the dislocation slip and deformation twinning [32]. In this study, the dislocation structures in the flattened copper foil obviously increased, while the number of deformation twins was very small. Thus, the dislocation slip was the main mode of plastic deformation for the copper foil during the laser shock process. At the beginning stage of deformation, high-speed shock wave could trigger most of the disloca tion sources in the coarse grains because of the ultra-high strain rate and pressure and could concurrently excite dislocation avalanches in mul tiple slip systems. Numerous dislocations exist in coarse grains with various morphologies, including dislocation lines, DLs, DTs, DWs, and DCs. With an increase in the deformation degree of the copper foil, a small number of deformation twins were formed when the shock wave pressure exceeded its local critical twinning stress, and these deforma tion twins existed as a supplemental plastic deformation mode for the copper foil. In addition, an increase in the density of crystal defects and the formation of deformation twins has an obvious strengthening effect on metal materials. In other words, the plastic deformation caused by the laser shock can improve the mechanical properties and corrosion resistance of metal materials. 7
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Fig. 7. Microstructures in flattened copper foil: (a) grains of cross section, (b) TEM image of GB and DCs, (c) TEM image and size of DCs, along with SAED pattern, (d) TEM image of dislocations with various morphologies, (e) TEM image of SF, (f) corresponding IFFT images of HRTEM images of flattened copper foil and dislocation.
3.4. Measurement and analysis of mechanical properties and corrosion resistance
load–displacement curves of the annealed copper foil and flattened copper foil (formed using a pulse energy of 0.25 J and overlap ratio of 25%) were investigated, and their indentation morphologies were scanned using AFM, as shown in Fig. 9(a). During the loading process, the indentation load required by the flattened copper foil was always greater than that of the annealed copper foil at the same indentation depth. In addition, the maximum indentation load of the flattened copper foil (11.3 mN) was significantly higher than that of the annealed copper foil (9.1 mN) at a fixed indentation depth of 500 nm. This
3.4.1. Study on the mechanical properties To investigate the influences of the LSF on the mechanical properties and corrosion resistance of the copper foil, nano-indentation, microtensile, and electrochemical corrosion tests were performed to charac terise its performances, where the nano-indentation test positions were cross sections of the epoxy-resin-embedded samples. The 8
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Fig. 8. Twins in copper foil: (a) TEM image and SAED pattern of annealing twins in annealed copper foil, (b) TEM image and SAED pattern of deformation twins in flattened copper foil, (c) TEM image of deformation twins, (d) HRTEM image and corresponding IFFT image of twin boundary.
indicated that LSF could improve the deformation resistance of the copper foil. Fig. 9(b) shows the nano-hardness and elastic modulus values of the annealed copper foil and flattened copper foil, where an average of three measured values was used to characterise each of these parameters. The nano-hardness and elastic modulus of the flattened copper foil were improved compared with the annealed copper foil. After LSF, the nanohardness of the copper foil increased from 1.5 GPa to 2.3 GPa, which was an increase of 53.3%. The elastic modulus increased from 91.6 GPa to 123.0 GPa, which was an increase of 34.3%. In this study, the
improvements in the mechanical properties of copper foil were mainly attributed to the work hardening from the plastic deformation caused by the laser shock. In addition, a large number of dislocation structures with various morphologies (as shown in Fig. 7) were formed during the deformation process, and the dislocation strengthening was an impor tant strengthening mechanism of laser shock processing, which caused the metal to obviously be strengthened [25,26]. Fig. 10 shows the stress–strain curves of the annealed copper foil and flattened copper foil, where these curves are divided into three stages: elastic deformation stage I, plastic deformation and strengthening stage
Fig. 9. Nano-indentation test results for copper foil: (a) load–displacement curves, (b) nano-hardness and elastic modulus. 9
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II, and necking stage III. Dog-bone specimens were used in this experi ment, and the tensile strength and elongation of the annealed copper foil were 173.9 MPa and 11.4%, respectively, as illustrated in Fig. 10(a). Compared with the annealed copper foil, the linear slope was larger at elastic deformation stage I. In other words, the LSF technique improved the elastic modulus of the copper foil, which was consistent with the results of the nano-indentation test. In addition, after the LSF, the tensile strength (295.6 MPa) of the flattened copper foil increased by 70.0%, and the elongation (4.4%) decreased by 61.4%, which also indicated that the LSF technique could strengthen the copper foil. The main reason for the above phenomenon was the work hardening of the copper foil caused by the LSF. In addition, the introduction of residual compressive stress and the increase in the dislocation density helped to improve the tensile strength. Furthermore, the improvement of the nano-hardness, elastic modulus, and tensile strength indicated that the LSF technique could effectively improve the deformation resistance of the copper foil, but this processing also reduced its plas ticity. Therefore, the elongation, as an important index to describe the plasticity of the material, also decreased.
effectively reduce the corrosion rate and improve the corrosion resis tance of the copper foil. Vcorr ¼ Hcorr ¼
M � Icorr M ¼ 373:11 � Icorr n n�F
(4)
Vcorr
(5)
ρ
¼ 3:27 � 103 �
M Icorr nρ
where Icorr is the current density; M ¼ 63.55 g/mol is the molar mass of the metal (Cu); n ¼ 2 is the valence of the metal; F ¼ 96485.33 C/mol is the faraday constant; ρ ¼ 8.96 g/cm3 is the density of the copper. The improvement of the corrosion resistance could be attributed to the following three aspects: 1) after LSF, the decrease of surface microcracks and surface roughness reduced the contact area between the copper foil and the NaCl solution, which reduced the corrosion ten dency; 2) laser shock increased the dislocation density in the copper foil, a large number of dislocation structures was induced, and they were beneficial to the improvement of the corrosion resistance; 3) laser shock processing could introduce larger residual compressive stress on the target surface, which had universal significance and had been confirmed by many scholars in the same field [36,37]. The existence of residual compressive stress could effectively inhibit the propagation of corrosion micro-cracks and avoid the defects caused by stress imbalance. In addition, it was worth noting that the theoretical proportion of non-flattening area (η ¼ 0.42%) wasn’t zero at the overlap ratio of 25%, but the surface of the flattened copper foil was fully strengthened because of the existence of the deformation affected zone. In other words, the deformation zone and strengthening zone on the copper foil surface were larger than the laser spot diameter, and the area of the non-strengthening zone was zero.
3.4.2. Study on the corrosion resistance Fig. 11 shows the potentiodynamic polarization curves in Tafel re gion of the copper foil and their surface morphologies after corrosion. The linear fitting results of the anode area and cathode area of the annealed copper foil and flattened copper foil were showed in Fig. 11(b) and (c), respectively. Fig. 11(d) shows the surface morphology of the annealed copper foil after corrosion, the corrosion pits were large, and their distributions were not uniform, it might be developed from the growth of micro-cracks. On the contrary, owing to the reduction of the micro-cracks and micro-defects on the surface after flattening, the flat tened copper foil had smaller corrosion pits after corrosion, the corro sion pits were uniformly distributed, which showed that the flattened copper foil had a better corrosion resistance, as illustrated in Fig. 11(e). In this study, Tafel extrapolation method was used to calculate the anode area slope (Ka), cathode area slope (Kc), and self-corrosion po tential (Ecorr), and these results were listed in Table 1. The Ecorr of the annealed copper foil was 0.23 V, the Ecorr of the flattened copper foil increased to 0.21 V. Thus, the Ecorr of copper foil shifted positively after flattening. Existing research had shown that the Ecorr can judge the corrosion difficulty degree from the thermodynamic view, and material showed a better corrosion resistance if the Ecorr shifted positively [35]. Therefore, LSF could improve the corrosion resistance of the copper foil from the thermodynamic analysis view. Furthermore, the self-corrosion current density (Icorr), corrosion rate (Vcorr), and corrosion depth (Hcorr) of the flattened copper foil were lower than those of the annealed copper foil, these parameters could be calculated by formulas (4) and (5), and these calculation results were also listed in Table 1. After LSF, the Vcorr of the annealed copper foil decreased by approximately 36.2%, which indicated that LSF could
4. Conclusions In this paper, the LSF technique was proposed to manufacture the profile-free copper foil with high performance. The single-spot LSF ex periments, large-area LSF experiments, microstructures characterisa tion, and performance tests were respectively carried out on the copper foil. The main conclusions were as follows: (1) LSF could effectively reduce the surface roughness (Sa) of copper foil. During single-spot LSF experiments, considering the microroughness and macroscopic unevenness of the copper foil, its surface had a better flattening quality when using a laser pulse energy of 0.25 J, and its surface roughness (12.2 nm) decreased by approximately 76.6%. (2) In large-area LSF experiments, the flattening effect on the largearea profile-free copper foil was better at a pulse energy of 0.25 J and an overlap rate of 25%, and its surface roughness decreased from 52.1 nm to 17.2 nm, which was a 67.0% decrease.
Fig. 10. Stress–strain curves: (a) annealed copper foil, (b) flattened copper foil. 10
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Fig. 11. (a) Potentiodynamic polarization curves in Tafle region, (b) potentiodynamic polarization curves and linear fitting results of the annealed copper foil, (c) potentiodynamic polarization curves and linear fitting results of the flattened copper foil, (d) surface morphology of the annealed copper foil after corrosion, (e) surface morphology of the flattened copper foil after corrosion.
CRediT authorship contribution statement
Table 1 Test and calculation results of the electrochemical corrosion tests. Samples
Ka
Kc
Ecorr (V)
Annealed copper foil Flattened copper foil
4.74
3.77
0.23
4.33
7.57
0.21
Icorr (A/ cm2)
Vcorr (g/ m2h)
Hcorr (mm/ year)
1.16 � 10 5 7.43 � 10 6
0.138
0.134
0.088
0.086
Yang Haifeng: Conceptualization, Supervision, Writing - review & editing. Xiong Fei: Methodology, Formal analysis, Investigation, Writing - original draft. Wang Yan: Validation, Resources. Jia Le: Validation, Resources. Liu Hao: Supervision, Funding acquisition. Hao Jingbin: Project administration. Acknowledgements This work was supported by the Fundamental Research Funds for the Central Universities [2020ZDPYMS21]; a project funded by the Priority Academic Program Development of Jiangsu Higher Education In stitutions [PAPD]. We would like to acknowledge the assistance of FSEM (MAIA3 LMH) test provided by the Advanced Analysis & Computation Center of CUMT.
Meanwhile, the profile-free copper foil had a thinning rate of 21.7% when using the optimum flattening parameters. (3) After LSF, a large number of dislocation structures with different morphologies were formed in the copper foil, the dislocation density was significantly increased, and a few deformation twins were found. The flattened deformation mechanism for the copper foil was mainly the dislocation slip, and a few deformation twins existed in the copper foil as a supplementary mechanism of plastic deformation. (4) LSF had an obvious strengthening effect on the copper foil and improved its mechanical properties. After flattening, the nanohardness and elastic modulus of the copper foil were increased by 53.3% and 34.3%, respectively. The tensile strength increased by 70.0%, the elongation decreased by 61.4%, and the corrosion rate was reduced by 36.2%.
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Profile-free copper foil with high-performance is indispensable in fine-line, high-frequency, and high-speed PCB. Therefore, the LSF technique is of great significance to reduce the loss of high-frequency signals during its transmission process and promote the development of electronic information industry. 11
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[7]
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