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Weldability modification of conductive silver adhesion for piezoelectric composite material by co-doping of metal material and oxide material
Chemical Physics 517 (2019) 237–246
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Weldability modification of conductive silver adhesion for piezoelectric composite material by co-doping of metal material and oxide material
T
Xingli Zhou, Likun Wang, Qingwei Liao , Lei Qin ⁎
R
esearch Center of Sensor Technology, Beijing Information Science & Technology University, Beijing 100192, China
ARTICLE INFO
ABSTRACT
Keywords: Adhesion Weldability Piezoelectric composites
The weldability is essential for the practical applications of conductive adhesion. In this paper we focused on the weldability through microstructure, bonding strength, and shear strength of welding spot with co-doping of metal material Al and oxide material ZnO. When Al and ZnO doped separately, it can be seen from the results that Al-doping would increase the volume resistivity and adhesion strength for welding spot, and a proper amount of ZnO-doping would decrease the volume resistivity and improve adhesion strength for welding spot. Co-doping with Al and ZnO could decrease volume resistivity and enhance adhesion strength of welding spot concurrently, and the typical properties were 1.119 × 10−4 Ω cm for volume resistivity, 34.84 MPa for shear strength, and 17.00 MPa for bonding strength with addition of 4 wt% Al and 7 wt% ZnO.
1. Introduction Piezoelectric composite material which composed of organic polymer and piezoelectric ceramics is the core functional materials of underwater acoustic transducers [1,2]. It was insulating material, and only could be used for electrical testing and practical application after the conductive treatment of their surface. The conductive silver adhesion held both characteristic of electrical conductivity and adhesiveness and had a simple coating process and excellent electrical conductivity, making it an ideal choice for the surface metallization of piezoelectric composites. However, at present, most researches on conductive silver adhesion focused on following aspects: conductive properties, mechanical properties, and curing mechanisms [3–11]. There was hardly to see researches on the strength of conductive silver welding spot adhesion, and piezoelectric composite materials can only be practically applied with welding leads. In addition, due to the deformation and cracking of the piezoelectric composite material at high temperatures, the wire bonding treatment must be performed under the temperature lower than 100 °C. Our group focused on ultra-low temperature curable adhesion in recent years and many interest and useful results were obtained [12]. But in the later practical applications, the weldability became a one kind of scientific point should be studied due to lack of weldability for common conductive silver adhesion with most our results or commerce products. ZnO nanomaterials have quantum size effects, surface effects, and tunneling properties, and were widely used in electronics, chemicals, and ceramics [13–25]. Nano powders have
⁎
Corresponding author. E-mail address: [email protected] (Q. Liao).
https://doi.org/10.1016/j.chemphys.2018.10.021 Received 15 August 2018; Accepted 29 October 2018 Available online 30 October 2018 0301-0104/ © 2018 Published by Elsevier B.V.
high surface activity and are easily adhered to silver grains, inhibit silver grain growth, and change the microstructure of conductive silver adhesion [26–30]. Doping them in the conductive silver adhesion would change the colloidal conductivity by changing the carrier concentration and the carrier mobility and the colloidal welding joint strength would be enhanced by changing the degree of refinement of the colloidal structure [25–29]. Therefore, in this paper, the weldability of conductive silver adhesion for piezoelectric composite material promoted via co-doping of nano metal material Al and nano oxide material ZnO was studied. The weldability through microstructure, bonding strength, and shear strength of welding spot was discussed. 2. Experimental section The Al/ZnO co-doped conductive silver adhesion was prepared by two-step method. The first step was to prepare resin matrix solution: Heating the E51 epoxy in 120 °C for 30 min to remove water, after it cooled down, added curing agent (Ethylenediamine), accelerator (Triethanolamine), plasticizer (Dibutyl Phthalate, C16H22O4, Analytical Pure), flexibilizer (Carboxylated- terminated liquid acrylonitrile rubber, Analytical Pure), coupling agent (Silane Coupling Agent, Analytical Pure) in proportion of 100:50:10:10:10:10, Stiring it thoroughly with stirrer, then placed it in vacuum oven (DZF-6090, Bluepard instruments) for vacuum deaeration, finally obtained the matrix resin. The second step was to prepare conductive silver adhesion: Thoroughly mixed the matrix resin, nano silver powder, nano-aluminum powders
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(100 nm), nano-zinc oxide (100 nm), acetone (Analytical Pure) in proportion of 30: (40–75): (0–9): (0–9):10, stir well until obtain the final conductive silver adhesion. The volume resistivity was test with the precision LCR digital bridge (TH2821B, measuring range is 0.0001 Ω–9.999 MΩ, Tonghui, Changzhou). The welding spot bonding strength and the welding spot shear strength was test with the microcomputer control electron universal testing machines (20 kN specification, 0.5 precision grade, Reger, Shenzhen). The surface morphological was observed using a Scanning Electron Microscope (SEM: EVO18, ZEISS, Germany). The thermal analysis was carried out by thermogravimetric analysis (SDT Q600, TA, USA) and differential scanning calorimetry (DSC: SDT Q600, TA, USA).
excess silver particles cannot be fully bonded. The number of conductive channels reached the largest value and would no longer increase significantly, so that the volume resistivity would not show a significant downward trend. Considered the cost and colloidal concentration after doping, the silver content was determined to be 65 wt% in subsequent experiments. Fig. 2 shows the SEM photographs of conductive silver paste with different silver content and the corresponding pictures processed by color selection. It can be seen that the average size of the silver flake after curing was between 1 and 2 μm, and a small part of the silver flake reached 3 μm. In addition, the degree of densification of the conductive adhesion system increased with the increasing of silver content, and the silver flake become closely connected by partial contact. When the content of silver powder less than 50 wt%, the gap between the silver flake was became larger, and most of them are filled with resin and cannot contact each other, leaded to the resin play a barrier role in the formation of the conductive channel, and the large gap between the silver sheets made the current unable to pass. Therefore, the conductive adhesion has a large insulation resistance and poor electrical conductivity. When the content of silver powder increased to 50 wt%, the density of the silver flake increased significantly, the gap between the silver flake shrunk, the tunnel resistance decreased sharply, and the conductive adhesion initially formed a better conductive network. When the content of silver powder was 60 wt%, the conductive adhesion silver flakes were pressed against each other, and directly contacted, hence, the conductive network gradually completed. When the content of silver powder reached 65 wt%, the density of the silver flakes in the resin matrix increased, and the continuity of the conductive network became better. Fig. 2(e) shows an SEM image of a conductive silver adhesion processed by color selection which could simply type–token distinct between silver flakes and insulated gap. The color range selection processing is performed on the SEM photograph of the conductive silver adhesion. The sampling color was a silver color, and the color tolerance was 50 wt%. After processing, the silver area is a black portion, and the white line is a silver film connecting edge that was, an insulated gap portion between the silver flakes. In this way, the degree of compactness of the silver flake connection could be more intuitively seen. It could be that when the silver powder content increased, the white line became smaller, the silver filling area became larger, and the more conductive paths formed in the unit volume. These results in agreement with the volume resistivity data of conductive silver paste with different silver content. Fig. 3 shows the conductive mechanism of the conductive silver paste. The point of conductive mechanism of the conductive silver paste is close related to the degree of contact between the conductive filler silver powder. In the curing process, when the amount of silver powder is added to a certain extent, the internal stress generated by the contraction of the polymer matrix causes decreasing of the distance between the silver particles, hence, the insulating gap became smaller, and part of the silver particles grew together, as a result, the direct contacted forming the conductive path. The electrical current also could generate between the non-direct contacts which had the gap smaller than 10 nm. On the contrary, when the gap between silver particles larger than 10 nm, the conduct current was not going to happen, and the adhesion were in its non-conducting state. Therefore, the total resistance of the conductive silver adhesion was mainly composed of silver sheet insulation resistance (R1), intrinsic resistance (R1′), tunnel resistance (R2) and contact resistance (R2). The resistivity of silver (1.62 × 10−6 Ω cm) was much smaller than the insulation resistance, tunnel resistance and contact resistance. When the silver powder is nanoscale, the silver particles cannot be directly contacted. When the amount of silver powder was small and the spacing between the silver flakes was large (> 10 nm), there was no current conduction between the particles, and the conductivity was determined by the insulation resistance, and the whole adhesion was almost non-conductive. When the distance between the silver plates was small enough
3. Results and discussions Fig. 1 shows the relationship between volume resistivity and the silver content. The volume resistivity testing of conductive silver adhesion referred to the national military standard GJB548a-1996. It can be seen from the Fig. 1 that the volume resistivity decreased as the silver content increasing. When the content of silver powder was below 40 wt%, the resistance tended to be infinite, the system was in an insulated state. When the amount of silver powder was increased from 40 wt% to 45 wt%, the volume resistivity decreased from 87.2 × 10−4 Ω cm to 39.7 × 10−4 Ω cm, and the electrical conductivity was poor. With the silver filling amount increased to 50 wt%, the volume resistivity of the conductive adhesion dropped sharply to 3.586 × 10−4 Ω cm. It indicated that the amount of silver powder has exceeded the percolation threshold and the resistance has dropped sharply to meet the basic electrical properties. When the silver filling amount is 60 wt%, the volume resistivity was reduced to 1.557 × 10−4 Ω cm, which was 56.6% lower than the 50 wt% filling amount, and the conductivity is obviously improved. However, when the silver filling amount continued to increase to 70%, the volume resistivity decreased to 1.248 × 10−4 Ω cm, which was only 19.8% lower than the 60 wt% filling amount, and the decreasing speed became slower. When the content of silver powder reaches 75 wt%, the volume resistivity was 1.155 × 10−4 Ω cm, and the value seemed stable. When the amount of silver powder was lower than the percolation threshold, the spacing between the silver sheets was large, making the charge transfer is difficult, and a complete conductive network cannot be formed. When the silver filling amount reaches 60 wt%, the formation of the colloidal conductive network tends to be completed, and the volume resistivity was significantly reduced. However, when the silver powder reached to 70 wt%, the density of the silver powder in the polymer matrix has reached a relatively large value, and the silver flakes are in close contact. Even if the silver content kept increasing, the
Fig. 1. The relationship between volume resistivity and the silver content. 238
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Fig. 2. The SEM image of conductive silver paste with different silver content and the corresponding pictures processed by color selection.
(< 10 nm), the electrical conductivity was mainly determined by the tunnel resistance, and the tunnel resistance was determined by the distance between the silver plates. The smaller the distance between the silver sheets, the easier the charge transfer and the smaller the tunnel resistance. However, when the nano-silver powder was cured and sintered and connected in micrometers, some of the silver flakes were in direct contact. At this time, the conductivity of the conductive silver paste was mainly determined by the contact resistance. The contact resistance was determined by the contact area of the contact point. The smaller the contact area, the greater the contact resistance [31–35]. ZnO was easily adsorbed by silver grains due to the high surface activity of it when an appropriate amount of ZnO was doped. As the number of ZnO adsorbed on the surface of silver increases, the surface energy of grains decreases, and the grain growth is inhibited. At the same time, the adsorption of ZnO will hinder the diffusion of Ag, reducing the diffusion flux of Ag particles, making the spacing between grains smaller, hence, the tunnel resistance and contact resistance were reduced, and the conductivity of the conductive adhesion was increased. However, when ZnO content increasing, ZnO tended to agglomerate at the interface between Ag and ZnO, forming a ZnO-rich region, making the conductive adhesion too viscous, impeding the spread and
dispersion of Ag particles, and deteriorating the conductive and mechanical properties of the conductive silver paste. When ZnO and Al are co-doped, since the ionic radius of Al3+ (0.0535 nm) is smaller than that of Zn2+ (0.074 nm), the difference in ionic radius is -20.5%, and Al3+ easily replaces Zn2+, reducing the grain size of ZnO and simultaneously lowering ZnO. The possibility of particle agglomeration increases the wettability between Ag and ZnO, reduces the defects and holes between Ag and ZnO, and increases the carrier concentration, the density and conductivity of metal particles of the conductive silver adhesion. Inversely, when the doping amount was not enough, excess Al3+ ions cannot effectively replace Zn2+, the carrier concentration would not increase any more, and Al3+ would enriched on the ZnO crystal grain surface or grain boundary as Al2O3 amorphous phase or cluster [36], becoming the scattering center at the time of carrier migration, reducing the carrier mobility and increasing the resistivity of the conductive paste. Fig. 4(a) shows the welding spot bonding strength and welding spot shear strength with different Ag content. It can be seen that the solder joints of conductive silver paste without added dopants had very low shear strength and bond strength, which made the solder joints were easily dropped and also difficult to solder during soldering. The 239
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Fig. 3. The conductive mechanism of the conductive silver paste. (a) Conductive path diagram; (b) the total resistance; (c) doping mechanism diagram; (d) the atomic number, ion radius and electronegativity of doping element.
maximum shear strength was only 2.935 MPa, and the maximum bond strength was 1.249 MPa. When there was an external force, the solder joint and silver layer can easily separate leaving a layer of gray and black organic impurities, which was shown in Fig. 4(b). The measurement shows that the drop layer resistance of the solder joint reached MΩ level, which was almost non-conductive. In addition, it can be seen from the Fig. 4(a) that the bond strength of solder joints was less than the shear strength. Fig. 5 shows the microscopic mechanism of weld spot for piezoelectric composite. Fig. 5(a) is a schematic diagram of bonding process and shear stretching direction. During the welding process, the welding iron head cannot directly contact the surface of the conductive silver adhesion so as not to damage the conductive surface due to the conductive silver adhesion layer is thin and the organic content cannot bear the high temperature. Therefore, solder needed to melt on the welding
iron first and then cool on the conductive silver adhesion surface after touching the solidified solder and solder the wire to the silver layer gently. The solder joint shear direction is parallel to the silver layer and evenly distributed at the joint of the entire solder joint. When wires and solder joints were subjected to tensile force, the crystal grains shifted, and stress deformation occurred at the solder joints. The stress deformation continuously accumulates at the position of the solder joints. When the stress deforming reached a certain level, microscopic cracks would appear within the solder joints, and the cracks continued to expand, eventually leading to the dropping of welding spot. The solder joint was oriented perpendicular to the silver layer, and the stress mainly concentrated on the junction between the wire and the junction of solder joint and silver layer, thus, the stress was not uniform. To sum up, the stress from the shear direction of the solder joint was evener than that where from the bonding direction, therefore, the shear
Fig. 4. (a) The welding spot bonding strength and welding spot shear strength with different Ag content; (b) the photo of welding spot fallen area and it’s the conductive behavior. 240
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Fig. 5. The microscopic mechanism of weld spot for piezoelectric composite. (a) bonding process and shear stretching direction; (b) diffusion schematic of welding layer; (c) SEM photograph of interface layer between welding spot and conductive adhesion.
strength of the solder joint was much greater than that of the solder joint. Fig. 5(b) shows the schematic diagram of the diffusion layer of the solder. The principle of welding was the phenomenon of mutual diffusion between atoms. When the solder melted at a high temperature and solidified on the conductive silver layer at room temperature, tin atoms and silver atoms crossed each other and entered the lattice of each other. Finally, Ag3Sn, Ag6Sn5 and Sn63Pb37 compounds were formed in the intermediate layer [37]. Fig. 5(c) shows an SEM photograph of the interface between the solder joint and the silver layer. It can be seen that there was a clear compound layer at the interface between the solder joint and the silver layer. Fig. 6(a) shows the structure of the epoxy resin AG-80. AG-80 had low viscosity, strong adhesion, good mechanical properties, with the main functional group of epoxy. The two carbon atoms of the epoxy group and the one oxygen atom were in the same plane, making the epoxy group resonant. The epoxy CeC bond (0.147 nm) is longer than the CeO bond (0.145 nm) and the ∠COC (61°24′) is greater than ∠OCC (59°18′). The epoxy group had a large tilt with chemically active. Since the electronegativity of oxygen was larger than that of carbon, resulting in electrostatic polarization, the density of electron clouds near oxygen atoms increased, and the density of electron clouds near carbon atoms decreased. Therefore, positive and negative charged active centers were
formed around carbon atoms and oxygen atoms, respectively. After adding the curing agent ethylenediamine, the amino group attacked the carbon atoms and reacted rapidly, causing CeO bond cleavage, ringopening polymerization of the epoxy group, and finally generating a huge network structure molecule. In order to rapidly cure the conductive silver paste at a low temperature, 10 wt% of triethanolamine was added at the same time, and the main functional group of triethanolamine was a tertiary amine, which accelerated the curing reaction of the conductive silver adhesion as a catalyst. Fig. 6(b) shows the thermogravimetric analysis of a conductive adhesion with a silver content of 65 wt% and no dopant. The total weight loss was 13.6%. Fig. 6(c) shows the thermogravimetric analysis of a conductive adhesion doped with 3 wt% of Al with a silver content of 65 wt%. The total weight loss rate was 13.2%. Fig. 6(d) shows the thermogravimetric analysis of conductive adhesion doped with 7 wt% of Al with a silver content of 65 wt%, and the total weight loss rate was12.4%. It can be found that when adding the dopant, the weight loss of the conductive silver paste reduced, which indicated that the addition of the Al and the ZnO could enhance the colloidal stability to some extent. Fig. 7 shows the volume resistivity of conductive silver adhesion with different content of Al and ZnO for the silver powder content of 65 wt%. It can be seen that the volume resistivity became larger when Fig. 6. (a) The structure of the epoxy resin AG-80; (b) the thermogravimetric analysis of a conductive adhesion with a silver content of 65 wt% and no dopant; (c) the thermogravimetric analysis of a conductive adhesion doped with 3 wt% of Al with a silver content of 65 wt%; (d) the thermogravimetric analysis of conductive adhesion doped with 7 wt% of Al with a silver content of 65 wt%.
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material with a wide bandgap (3.37 eV) and high exciton binding energy (60 meV)[37]. Adding it to the conductive silver adhesion could reduce the contact resistance between the silver flakes and increase the colloidal carrier concentration. However, when the amount of ZnO is too large, the specific surface area and the specific surface energy of ZnO were increasing, and the surface activity was in the high lever. Therefore, it is easy to agglomerate together to increase the viscosity of the conductive adhesion, resulting in local zinc-rich area, which is not conducive to the diffusion filling of silver flakes, making silver flakes are easily stacked together and forming a large number of defects. As a result, the conductivity of conductive adhesion deteriorated. In a word, the best conductivity could be obtained when the content of silver powder was 65 wt%, and the ZnO content is 6 wt%. Fig. 8 shows the SEM of conductive silver adhesion doped with different Al contents (5 wt%, 9 wt%) and different ZnO contents (5 wt% and 9 wt%). It can be seen that the aluminum powder adhered to the silver flakes and the silver flakes was wrapped with the Al doping, which hindered the growth of the silver flakes. There were many holes and obvious gaps between the silver flakes. The matrix resin filled in and the tunnel resistance was very high. When the ZnO powder is added by 6 wt%, the silver flakes were surrounded by ZnO, and showed relatively uniform in particle size, dense and non-porous, and could form good conductive paths. But when ZnO powder is added up to 9 wt%, ZnO dispersed unevenly, agglomeration formed locally, the silver flakes formed a stack, the delamination was obvious, and the conductivity was poor. It was consistent with the data in Fig. 7. In order to enhance the bonding strength and shear strength of welding spot, ZnO and Al were adopted to dope into silver adhesion respectively. Firstly, the schematic diagram of ZnO pinning effect is shown in Fig. 9(a). After being doped with nano-ZnO powder, ZnO pinned at the grain boundary to hinder the grain boundary sliping and inhibit the diffusion of silver atoms and tin. Fig. 9(b) shows the comparison picture of doping and un-doping with ZnO welding spot. Fig. 9(c) and (d) show the shear strength and bonding strength of conductive silver adhesion with different ZnO content and different Al content, respectively. It can be seen that when ZnO or Al was doped, both the bonding strength and the shear strength of the conductive silver adhesion increased first and then decreased. The peak point of 24.31 MPa for shear strength and 12.44 MPa for bonding strength, respectively, was obtained as the content of ZnO was 7 wt%. It was
Fig. 7. The volume resistivity of conductive silver adhesion with different content of Al and ZnO.
added Al compared to non-doping sample, and the electrical conductivity deteriorates. On the one hand, the chemical properties of aluminum are very active, aluminum powder particles are small, and the specific surface area is very large, when it was exposed to air, Al3+ and O2− in the air are mutually dispersed, forming a dense Al2O3 film which is insulated on the surface of Al powder. This layer of Al2O3 film structure defected, making it had a very high surface energy, resulting in the aluminum powder surface could adsorb a lot of gas and oxidation temperature was reduced, thus, it could be oxidized at room temperature. On the other hand, since the doped aluminum powder is a nanomaterial which shows insulated because of the quantum size effect. Therefore, after the addition of aluminum powder, the conductivity of the conductive silver paste decreased [38]. When only ZnO was added, the volume resistivity first decreased and then increased with the increase of ZnO content. The volume resistivity reached a minimum value of 0.845 × 10−4 Ω cm with doping 6 wt% ZnO. Compared with the undoped one, the volume resistivity is reduced by 45.7%. When the ZnO content higher than 6 wt%, the volume resistivity of the conductive paste increased. This is because ZnO is a semiconductor
Fig. 8. The SEM photograph of Al-doping (5 wt%, 9 wt%) and ZnO-doping conductive silver adhesion. 242
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Fig. 9. (a) The schematic diagram of ZnO pinning effect; (b) the comparison picture of doping and un-doping with ZnO welding spot; (c) the shear strength of welding spot with different ZnO content and different Al content; (d) bonding strength of welding spot with different ZnO content and different Al content.
because that the stability and elastic modulus of ZnO were very high. When adding it to the conductive silver colloid, ZnO particles were dispersed in the matrix and pinned at the grain boundary to act as a diffusion barrier to inhibit the diffusion of silver atoms and tin atoms, which could prevent the grain boundary slippage, increase the dislocation density, inhibit dislocation movement, and at the same time reduce the surface energy and surface tension of the conductive silver adhesion, and also improve the wettability of the conductive adhesion, thereby improving the shear strength and bond strength of the solder joints [39,40]. However, excessive ZnO content would increase colloidal viscosity, hinder colloidal spreading, weaken wettability, and decrease bonding strength and shear strength [41]. The peak point of Al-doping sample was 19.8 MPa for shear strength and 12.44 MPa for bonding strength when the Al content was 8 wt%. It was because that when Al was added in an appropriate amount, on the one hand, the density of Al was low and it would accumulate in the interface region of the solder joint. The distribution of the nano-Al powder at the interface played a certain role in strengthening the stability of the solder joint; on the other hand, Al was an inert reinforcing phase that did not react with the solder or silver layer, thus, it could maintain its own stable morphology, and have a good reinforcing effect on the strength of solder joints. In addition, the interface compounds between Ag and Sn were
mostly brittle phases which would have an adverse effect on the stability of solder joints. The addition of Al powder could reduce the formation of compounds, inhibit the coarsening of compounds in the structure, and improve the stability of solder joints. However, when the amount of Al powder was excessive, Al tended to form Al-rich phase, and there were many internal structural defects in aluminum powder. A large amount of Al powder aggregated to adsorb a large amount of gas on the surface and increased the degree of surface oxidation, hence, made solder joints difficult to solder. Fig. 10 shows the thermogravimetric analysis of conductive adhesion co-doped with 3 wt% Al and 7 wt% ZnO, and the total weight loss was 13.4%. At 81 °C, the maximum weight loss rate was reached, and the curing reaction proceeded fastest. After 108 °C, the weight loss tended to be stable, and the post-curing reaction was mainly carried out at this time, and the reaction tended to the end and the performance was stable. Thus, it can be found out that when the Al powder and the ZnO powder are co-doped, the weight loss rate was not very different from that in Fig. 6, which indicated that the dopant properties were relatively stable and the weight loss in the curing reaction was small. In addition, it can also be proved that the addition of dopants had little effect on the curing reaction of the conductive silver adhesion and did not change the conductive silver adhesion curing system. Fig. 11 shows the SEM image of co-doped conductive silver adhesion with different contents of Al and 7 wt% ZnO. It can be seen that when 5 wt% of Al powder was added, the silver sheet showed the best spreadability, the crystal grains were fine, and the structure was dense. The ZnO nanoparticles and the Al powder were uniformly filled in the gaps of the silver sheets and were connected to form a good conductive network. For non-Al-doping sample, ZnO agglomerated, unevenly distributed between the silver flakes, and the silver particle size was relatively large. With the increase of Al content, the growth of silver flakes was significantly suppressed, the particle size decreased, and the spacing between silver flakes decreased. When the Al powder filling amount reached to 9 wt%, the dopant content saturated and overflowed, and the dopant adhered to the silver plate. Excessive Al3+ and O2− combined to form Al2O3 molecules, which hindered the conductive path and degraded the conductivity. The undoped ZnO was N-type semiconductor, and its conductivity was not high. Al dopant as a group III element, would provide donor levels, the number of carriers would increase, and the conductivity would increase. Therefore, different content of Al powder was added to the conductive silver adhesion with 65 wt% silver content and 7 wt%
Fig. 10. The thermogravimetric analysis of conductive adhesion co-doped with 3 wt% Al and 7 wt% ZnO. 243
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Fig. 11. The SEM photograph of Al and ZnO co-doping conductive silver adhesion.
ZnO content, and the volume resistivity was tested. Fig. 12 shows the volume resistivity of conductive silver adhesion of 7 wt% ZnO-doping with different Al-doping. It can be seen that the volume resistivity of the conductive silver adhesion first decreased with increasing Al content. When the Al doping up to 5 wt%, the volume resistivity of the conductive adhesion reached the lowest value, which was 0.947 × 10−4 Ω cm. This was because that when a small amount of Al powder was added, the lattice position of Al3+ instead of Zn2+ achieved donor doping and could be expressed as:
ZnO + xAl3 +
ZnO
formation of silver-tin compounds and increase bonding strength. Fig. 12(b) and (d) show the SEM photographs of solder joint fallen area in the shear direction and the bonding direction, respectively. It can be seen that when the solder joint was subjected to the force from the shear direction, the silver sheet slipped and partially stretched. Al and ZnO were pinned between the silver sheets in the conductive silver adhesion, preventing the grain boundary from shifting and increasing the friction. When the tensile force exceeded the maximum capacity of the silver sheet, ductile fracture between solder joint and substrate occurred. When the solder joint was subjected to the force along the bonding direction, the ZnO and Al were almost completely dropped because the bonding direction was perpendicular to the substrate, and the transverse friction force was not hindered during the stretching. The direction of the silver strip falling was the z-axis direction and the silver flakes fell. The area was much larger than the shear drop area, which was also the reason why the bonding strength of the solder joint was far less than the shear strength of the solder joint. Fig. 13(a) and (c) show the shear strength and bonding strength of welding spot for Al and ZnO co-doping conductive silver adhesion, respectively. It can be seen that as the Al content increasing, the shear
Zn12 +x (Al3 +·e )x O + xZn2+
Zn12 +x (Zn2 +·2e) x O12
x
+
x O2 2
Al3+ had one more positive charge than Zn2+, so each Al3+ provided one electron for the substitution of Zn2+, and each oxygen hole provided two electrons. To maintain electrical neutrality, Al3+ weakly binds a valence electron. This binding force was much smaller than that of the normal lattice for valence electrons. Therefore, valence electrons easily escaped from the bound center, and the oxygen ion vacancy concentration and carrier concentration increased. On the other hand, the ionic radius of Al3+ is smaller than that of Zn2+ and Ag+, thus, Al3+ easily entered lattice, filled gaps or defects between crystal grains, improved Hall mobility of carriers, thereby, conductivity had been improved. However, when Al was added overmuch, the Al atoms would escape from the position of the replacement Zn, occupying the gap position, suppressing the growth of crystal grains, increasing the interfacial energy at the grain boundary, and forming Al2O3 at the grain boundary to hinder the carrier migration. As a result, the resistivity increased and the conductivity deteriorated [42]. Fig. 12(a) and (c) show the shear strength and bonding strength of welding spot for Al and ZnO co-doping conductive silver adhesion, respectively. It can be seen that as the Al content increasing, the shear strength and bonding strength first increased and then decreased. The peak shear strength with value of 34.84 MPa and peak bond strength with value of 17.00 MPa both for the 4 wt% Al sample. It was because that Ag atoms and Al atoms had large difference in electronegativity and strong affinity. Al atoms can be filled in holes and defects between ZnO and Ag, and the density of metal atoms in the conductive silver adhesion increased, the wet force increased, and wetting time reduced. At the same time, addition of trace amounts of Al could reduce the
Fig. 12. The volume resistivity of conductive silver adhesion of 7 wt% ZnOdoping with different Al-doping. 244
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Fig. 13. (a) The shear strength of welding spot for Al and ZnO co-doping conductive silver adhesion; (b) The SEM photograph of solder joint fallen area in the shear direction; (c) The bonding strength of welding spot for Al and ZnO co-doping conductive silver adhesion; (d) The SEM photograph of solder joint fallen area in the bonding direction.
strength and bonding strength first increased and then decreased. The peak shear strength with value of 34.84 MPa and peak bond strength with value of 17.00 MPa both for the 4 wt% Al sample. It was because that Ag atoms and Al atoms had large difference in electronegativity and strong affinity. Al atoms can be filled in holes and defects between ZnO and Ag, and the density of metal atoms in the conductive silver adhesion increased, the wet force increased, and wetting time reduced. At the same time, addition of trace amounts of Al could reduce the formation of silver-tin compounds and increase bonding strength. Fig. 13(b) and (d) show the SEM photographs of solder joint fallen area in the shear direction and the bonding direction, respectively. It can be seen that when the solder joint was subjected to the force from the shear direction, the silver sheet slipped and partially stretched. Al and ZnO were pinned between the silver sheets in the conductive silver adhesion, preventing the grain boundary from shifting and increasing the friction. When the tensile force exceeded the maximum capacity of the silver sheet, ductile fracture between solder joint and substrate occurred. When the solder joint was subjected to the force along the bonding direction, the ZnO and Al were almost completely dropped because the bonding direction was perpendicular to the substrate, and the transverse friction force was not hindered during the stretching. The direction of the silver strip falling was the z-axis direction and the silver flakes fell. The area was much larger than the shear drop area, which was also the reason why the bonding strength of the solder joint was far less than the shear strength of the solder joint.
strength, and 17.00 MPa for bonding strength with addition of 4 wt% Al and 7 wt% ZnO. Acknowledgements This work was supported by the National Natural Science Foundation of China (No. 11604363, 61471047, 61671068), and the Beijing College Innovation Capability Promotion Plan of Beijing Municipal Institutions (No. TJSHG201510772015). The project is also supported by The Importation and Development of High-Caliber Talents Project of Beijing Municipal Institutions (No. CIT& TCD201504053) and Beijing Municipal Education Commission Science and Technology Plan Project (No. KM201811232021). References [1] V.K. Varadan, Y. Ma, V.V. Varadan, Scattering of acoustic waves by piezoelectric composites, J. Acoust. Soc. Am. 80 (Suppl. 1) (1986) S74, https://doi.org/10.1007/ 1-4020-4131-4_19. [2] L. Qin, L.K. Wang, Y. Lu, Decoupling in multi-elements composite for transducer array application, Curr. Appl Phys. 11 (3) (2011) S368–S370, https://doi.org/10. 1016/j.cap.2011.03.048. [3] D. Klosterman, L. Li, J.E. Morris, Materials characterization, conduction development, and curing-effects on reliability of isotropically conductive adhesives, IEEE Trans. Compon. Packag. Manuf. Technol. Part A 21 (1) (1996) 23–31, https://doi. org/10.1109/95.679028. [4] J.M. Lin, W.N. Chen, C.Y. Lin, C.F. Lin, J.C. Chang, A novel highly electrically conductive silver paste, Int. Conf. Electron. Packag. IEEE (2016) 615–618, https:// doi.org/10.1109/ICEP.2016.7486902. [5] H. Ma, X. Tian, S.C. Yan, Z. Li, X.H. Guo, Y.Q. Ma, L. Ma, Silver flakes and silver dendrites for hybrid electrically conductive adhesives with enhanced conductivity, J. Electron. Mater. 47 (5) (2018) 2929–2939, https://doi.org/10.1007/s11664-0186145-5. [6] X. Peng, F. Tan, W. Wang, X. Qiu, F. Sun, X.L. Qiao, J.G. Chen, Conductivity improvement of silver flakes filled electrical conductive adhesives via introducing silver–graphene nanocomposites, J. Mater. Sci.: Mater. Electron. 25 (3) (2014) 1149–1155, https://doi.org/10.1007/s 10854-013-1671-7. [7] L. Wang, C. Wan, Y. Fu, H. Chen, X. Liu, M. Li, Study on the effects of adipic acid on properties of dicyandiamide-cured electrically conductive adhesive and the interaction mechanism, J. Electron. Mater. 43 (1) (2014) 132–136, https://doi.org/10. 1007/s11664-013-2765-y. [8] G.S. Xiao, E.Q. Liu, T. Jiu, X.F. Shu, Z.H. Wang, G.Z. Yuan, X.X. Yang, Mechanical properties of cured isotropic conductive adhesive (ICA) under hygrothermal aging investigated by micro-indentation, Int. J. Solids Struct. 122–123 (2017) 81–90, https://doi.org/10.1016/j.ijsolstr.2017.06.003. [9] R. Durairaj, L.W. Man, Effect of epoxy and filler concentrations on curing behaviour of isotropic conductive adhesives, J. Therm. Anal. Calorim. 105 (1) (2011)
4. Conclusions In summary, the weldability was focused on for the practical applications of conductive adhesion to surface conductive treatment of piezoelectric composite material. The conductive mechanism, welding spot bonding mechanism, bonding strength, and shear strength of welding spot with co-doping of metal material Al and oxide material ZnO were studied. When Al and ZnO doped separately, Al-doping would increase the volume resistivity and adhesion strength for welding spot, and a proper amount of ZnO-doping would decrease the volume resistivity and improve adhesion strength for welding spot. Co-doping with Al and ZnO could decrease volume resistivity and enhance adhesion strength of welding spot concurrently, and the typical properties were 1.119 × 10−4 Ω cm for volume resistivity, 34.84 MPa for shear 245
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Weldability modification of conductive silver adhesion for piezoelectric composite material by co-doping of metal material and oxide material
T
Xingli Zhou, Likun Wang, Qingwei Liao , Lei Qin ⁎
R
esearch Center of Sensor Technology, Beijing Information Science & Technology University, Beijing 100192, China
ARTICLE INFO
ABSTRACT
Keywords: Adhesion Weldability Piezoelectric composites
The weldability is essential for the practical applications of conductive adhesion. In this paper we focused on the weldability through microstructure, bonding strength, and shear strength of welding spot with co-doping of metal material Al and oxide material ZnO. When Al and ZnO doped separately, it can be seen from the results that Al-doping would increase the volume resistivity and adhesion strength for welding spot, and a proper amount of ZnO-doping would decrease the volume resistivity and improve adhesion strength for welding spot. Co-doping with Al and ZnO could decrease volume resistivity and enhance adhesion strength of welding spot concurrently, and the typical properties were 1.119 × 10−4 Ω cm for volume resistivity, 34.84 MPa for shear strength, and 17.00 MPa for bonding strength with addition of 4 wt% Al and 7 wt% ZnO.
1. Introduction Piezoelectric composite material which composed of organic polymer and piezoelectric ceramics is the core functional materials of underwater acoustic transducers [1,2]. It was insulating material, and only could be used for electrical testing and practical application after the conductive treatment of their surface. The conductive silver adhesion held both characteristic of electrical conductivity and adhesiveness and had a simple coating process and excellent electrical conductivity, making it an ideal choice for the surface metallization of piezoelectric composites. However, at present, most researches on conductive silver adhesion focused on following aspects: conductive properties, mechanical properties, and curing mechanisms [3–11]. There was hardly to see researches on the strength of conductive silver welding spot adhesion, and piezoelectric composite materials can only be practically applied with welding leads. In addition, due to the deformation and cracking of the piezoelectric composite material at high temperatures, the wire bonding treatment must be performed under the temperature lower than 100 °C. Our group focused on ultra-low temperature curable adhesion in recent years and many interest and useful results were obtained [12]. But in the later practical applications, the weldability became a one kind of scientific point should be studied due to lack of weldability for common conductive silver adhesion with most our results or commerce products. ZnO nanomaterials have quantum size effects, surface effects, and tunneling properties, and were widely used in electronics, chemicals, and ceramics [13–25]. Nano powders have
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Corresponding author. E-mail address: [email protected] (Q. Liao).
https://doi.org/10.1016/j.chemphys.2018.10.021 Received 15 August 2018; Accepted 29 October 2018 Available online 30 October 2018 0301-0104/ © 2018 Published by Elsevier B.V.
high surface activity and are easily adhered to silver grains, inhibit silver grain growth, and change the microstructure of conductive silver adhesion [26–30]. Doping them in the conductive silver adhesion would change the colloidal conductivity by changing the carrier concentration and the carrier mobility and the colloidal welding joint strength would be enhanced by changing the degree of refinement of the colloidal structure [25–29]. Therefore, in this paper, the weldability of conductive silver adhesion for piezoelectric composite material promoted via co-doping of nano metal material Al and nano oxide material ZnO was studied. The weldability through microstructure, bonding strength, and shear strength of welding spot was discussed. 2. Experimental section The Al/ZnO co-doped conductive silver adhesion was prepared by two-step method. The first step was to prepare resin matrix solution: Heating the E51 epoxy in 120 °C for 30 min to remove water, after it cooled down, added curing agent (Ethylenediamine), accelerator (Triethanolamine), plasticizer (Dibutyl Phthalate, C16H22O4, Analytical Pure), flexibilizer (Carboxylated- terminated liquid acrylonitrile rubber, Analytical Pure), coupling agent (Silane Coupling Agent, Analytical Pure) in proportion of 100:50:10:10:10:10, Stiring it thoroughly with stirrer, then placed it in vacuum oven (DZF-6090, Bluepard instruments) for vacuum deaeration, finally obtained the matrix resin. The second step was to prepare conductive silver adhesion: Thoroughly mixed the matrix resin, nano silver powder, nano-aluminum powders
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(100 nm), nano-zinc oxide (100 nm), acetone (Analytical Pure) in proportion of 30: (40–75): (0–9): (0–9):10, stir well until obtain the final conductive silver adhesion. The volume resistivity was test with the precision LCR digital bridge (TH2821B, measuring range is 0.0001 Ω–9.999 MΩ, Tonghui, Changzhou). The welding spot bonding strength and the welding spot shear strength was test with the microcomputer control electron universal testing machines (20 kN specification, 0.5 precision grade, Reger, Shenzhen). The surface morphological was observed using a Scanning Electron Microscope (SEM: EVO18, ZEISS, Germany). The thermal analysis was carried out by thermogravimetric analysis (SDT Q600, TA, USA) and differential scanning calorimetry (DSC: SDT Q600, TA, USA).
excess silver particles cannot be fully bonded. The number of conductive channels reached the largest value and would no longer increase significantly, so that the volume resistivity would not show a significant downward trend. Considered the cost and colloidal concentration after doping, the silver content was determined to be 65 wt% in subsequent experiments. Fig. 2 shows the SEM photographs of conductive silver paste with different silver content and the corresponding pictures processed by color selection. It can be seen that the average size of the silver flake after curing was between 1 and 2 μm, and a small part of the silver flake reached 3 μm. In addition, the degree of densification of the conductive adhesion system increased with the increasing of silver content, and the silver flake become closely connected by partial contact. When the content of silver powder less than 50 wt%, the gap between the silver flake was became larger, and most of them are filled with resin and cannot contact each other, leaded to the resin play a barrier role in the formation of the conductive channel, and the large gap between the silver sheets made the current unable to pass. Therefore, the conductive adhesion has a large insulation resistance and poor electrical conductivity. When the content of silver powder increased to 50 wt%, the density of the silver flake increased significantly, the gap between the silver flake shrunk, the tunnel resistance decreased sharply, and the conductive adhesion initially formed a better conductive network. When the content of silver powder was 60 wt%, the conductive adhesion silver flakes were pressed against each other, and directly contacted, hence, the conductive network gradually completed. When the content of silver powder reached 65 wt%, the density of the silver flakes in the resin matrix increased, and the continuity of the conductive network became better. Fig. 2(e) shows an SEM image of a conductive silver adhesion processed by color selection which could simply type–token distinct between silver flakes and insulated gap. The color range selection processing is performed on the SEM photograph of the conductive silver adhesion. The sampling color was a silver color, and the color tolerance was 50 wt%. After processing, the silver area is a black portion, and the white line is a silver film connecting edge that was, an insulated gap portion between the silver flakes. In this way, the degree of compactness of the silver flake connection could be more intuitively seen. It could be that when the silver powder content increased, the white line became smaller, the silver filling area became larger, and the more conductive paths formed in the unit volume. These results in agreement with the volume resistivity data of conductive silver paste with different silver content. Fig. 3 shows the conductive mechanism of the conductive silver paste. The point of conductive mechanism of the conductive silver paste is close related to the degree of contact between the conductive filler silver powder. In the curing process, when the amount of silver powder is added to a certain extent, the internal stress generated by the contraction of the polymer matrix causes decreasing of the distance between the silver particles, hence, the insulating gap became smaller, and part of the silver particles grew together, as a result, the direct contacted forming the conductive path. The electrical current also could generate between the non-direct contacts which had the gap smaller than 10 nm. On the contrary, when the gap between silver particles larger than 10 nm, the conduct current was not going to happen, and the adhesion were in its non-conducting state. Therefore, the total resistance of the conductive silver adhesion was mainly composed of silver sheet insulation resistance (R1), intrinsic resistance (R1′), tunnel resistance (R2) and contact resistance (R2). The resistivity of silver (1.62 × 10−6 Ω cm) was much smaller than the insulation resistance, tunnel resistance and contact resistance. When the silver powder is nanoscale, the silver particles cannot be directly contacted. When the amount of silver powder was small and the spacing between the silver flakes was large (> 10 nm), there was no current conduction between the particles, and the conductivity was determined by the insulation resistance, and the whole adhesion was almost non-conductive. When the distance between the silver plates was small enough
3. Results and discussions Fig. 1 shows the relationship between volume resistivity and the silver content. The volume resistivity testing of conductive silver adhesion referred to the national military standard GJB548a-1996. It can be seen from the Fig. 1 that the volume resistivity decreased as the silver content increasing. When the content of silver powder was below 40 wt%, the resistance tended to be infinite, the system was in an insulated state. When the amount of silver powder was increased from 40 wt% to 45 wt%, the volume resistivity decreased from 87.2 × 10−4 Ω cm to 39.7 × 10−4 Ω cm, and the electrical conductivity was poor. With the silver filling amount increased to 50 wt%, the volume resistivity of the conductive adhesion dropped sharply to 3.586 × 10−4 Ω cm. It indicated that the amount of silver powder has exceeded the percolation threshold and the resistance has dropped sharply to meet the basic electrical properties. When the silver filling amount is 60 wt%, the volume resistivity was reduced to 1.557 × 10−4 Ω cm, which was 56.6% lower than the 50 wt% filling amount, and the conductivity is obviously improved. However, when the silver filling amount continued to increase to 70%, the volume resistivity decreased to 1.248 × 10−4 Ω cm, which was only 19.8% lower than the 60 wt% filling amount, and the decreasing speed became slower. When the content of silver powder reaches 75 wt%, the volume resistivity was 1.155 × 10−4 Ω cm, and the value seemed stable. When the amount of silver powder was lower than the percolation threshold, the spacing between the silver sheets was large, making the charge transfer is difficult, and a complete conductive network cannot be formed. When the silver filling amount reaches 60 wt%, the formation of the colloidal conductive network tends to be completed, and the volume resistivity was significantly reduced. However, when the silver powder reached to 70 wt%, the density of the silver powder in the polymer matrix has reached a relatively large value, and the silver flakes are in close contact. Even if the silver content kept increasing, the
Fig. 1. The relationship between volume resistivity and the silver content. 238
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Fig. 2. The SEM image of conductive silver paste with different silver content and the corresponding pictures processed by color selection.
(< 10 nm), the electrical conductivity was mainly determined by the tunnel resistance, and the tunnel resistance was determined by the distance between the silver plates. The smaller the distance between the silver sheets, the easier the charge transfer and the smaller the tunnel resistance. However, when the nano-silver powder was cured and sintered and connected in micrometers, some of the silver flakes were in direct contact. At this time, the conductivity of the conductive silver paste was mainly determined by the contact resistance. The contact resistance was determined by the contact area of the contact point. The smaller the contact area, the greater the contact resistance [31–35]. ZnO was easily adsorbed by silver grains due to the high surface activity of it when an appropriate amount of ZnO was doped. As the number of ZnO adsorbed on the surface of silver increases, the surface energy of grains decreases, and the grain growth is inhibited. At the same time, the adsorption of ZnO will hinder the diffusion of Ag, reducing the diffusion flux of Ag particles, making the spacing between grains smaller, hence, the tunnel resistance and contact resistance were reduced, and the conductivity of the conductive adhesion was increased. However, when ZnO content increasing, ZnO tended to agglomerate at the interface between Ag and ZnO, forming a ZnO-rich region, making the conductive adhesion too viscous, impeding the spread and
dispersion of Ag particles, and deteriorating the conductive and mechanical properties of the conductive silver paste. When ZnO and Al are co-doped, since the ionic radius of Al3+ (0.0535 nm) is smaller than that of Zn2+ (0.074 nm), the difference in ionic radius is -20.5%, and Al3+ easily replaces Zn2+, reducing the grain size of ZnO and simultaneously lowering ZnO. The possibility of particle agglomeration increases the wettability between Ag and ZnO, reduces the defects and holes between Ag and ZnO, and increases the carrier concentration, the density and conductivity of metal particles of the conductive silver adhesion. Inversely, when the doping amount was not enough, excess Al3+ ions cannot effectively replace Zn2+, the carrier concentration would not increase any more, and Al3+ would enriched on the ZnO crystal grain surface or grain boundary as Al2O3 amorphous phase or cluster [36], becoming the scattering center at the time of carrier migration, reducing the carrier mobility and increasing the resistivity of the conductive paste. Fig. 4(a) shows the welding spot bonding strength and welding spot shear strength with different Ag content. It can be seen that the solder joints of conductive silver paste without added dopants had very low shear strength and bond strength, which made the solder joints were easily dropped and also difficult to solder during soldering. The 239
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Fig. 3. The conductive mechanism of the conductive silver paste. (a) Conductive path diagram; (b) the total resistance; (c) doping mechanism diagram; (d) the atomic number, ion radius and electronegativity of doping element.
maximum shear strength was only 2.935 MPa, and the maximum bond strength was 1.249 MPa. When there was an external force, the solder joint and silver layer can easily separate leaving a layer of gray and black organic impurities, which was shown in Fig. 4(b). The measurement shows that the drop layer resistance of the solder joint reached MΩ level, which was almost non-conductive. In addition, it can be seen from the Fig. 4(a) that the bond strength of solder joints was less than the shear strength. Fig. 5 shows the microscopic mechanism of weld spot for piezoelectric composite. Fig. 5(a) is a schematic diagram of bonding process and shear stretching direction. During the welding process, the welding iron head cannot directly contact the surface of the conductive silver adhesion so as not to damage the conductive surface due to the conductive silver adhesion layer is thin and the organic content cannot bear the high temperature. Therefore, solder needed to melt on the welding
iron first and then cool on the conductive silver adhesion surface after touching the solidified solder and solder the wire to the silver layer gently. The solder joint shear direction is parallel to the silver layer and evenly distributed at the joint of the entire solder joint. When wires and solder joints were subjected to tensile force, the crystal grains shifted, and stress deformation occurred at the solder joints. The stress deformation continuously accumulates at the position of the solder joints. When the stress deforming reached a certain level, microscopic cracks would appear within the solder joints, and the cracks continued to expand, eventually leading to the dropping of welding spot. The solder joint was oriented perpendicular to the silver layer, and the stress mainly concentrated on the junction between the wire and the junction of solder joint and silver layer, thus, the stress was not uniform. To sum up, the stress from the shear direction of the solder joint was evener than that where from the bonding direction, therefore, the shear
Fig. 4. (a) The welding spot bonding strength and welding spot shear strength with different Ag content; (b) the photo of welding spot fallen area and it’s the conductive behavior. 240
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Fig. 5. The microscopic mechanism of weld spot for piezoelectric composite. (a) bonding process and shear stretching direction; (b) diffusion schematic of welding layer; (c) SEM photograph of interface layer between welding spot and conductive adhesion.
strength of the solder joint was much greater than that of the solder joint. Fig. 5(b) shows the schematic diagram of the diffusion layer of the solder. The principle of welding was the phenomenon of mutual diffusion between atoms. When the solder melted at a high temperature and solidified on the conductive silver layer at room temperature, tin atoms and silver atoms crossed each other and entered the lattice of each other. Finally, Ag3Sn, Ag6Sn5 and Sn63Pb37 compounds were formed in the intermediate layer [37]. Fig. 5(c) shows an SEM photograph of the interface between the solder joint and the silver layer. It can be seen that there was a clear compound layer at the interface between the solder joint and the silver layer. Fig. 6(a) shows the structure of the epoxy resin AG-80. AG-80 had low viscosity, strong adhesion, good mechanical properties, with the main functional group of epoxy. The two carbon atoms of the epoxy group and the one oxygen atom were in the same plane, making the epoxy group resonant. The epoxy CeC bond (0.147 nm) is longer than the CeO bond (0.145 nm) and the ∠COC (61°24′) is greater than ∠OCC (59°18′). The epoxy group had a large tilt with chemically active. Since the electronegativity of oxygen was larger than that of carbon, resulting in electrostatic polarization, the density of electron clouds near oxygen atoms increased, and the density of electron clouds near carbon atoms decreased. Therefore, positive and negative charged active centers were
formed around carbon atoms and oxygen atoms, respectively. After adding the curing agent ethylenediamine, the amino group attacked the carbon atoms and reacted rapidly, causing CeO bond cleavage, ringopening polymerization of the epoxy group, and finally generating a huge network structure molecule. In order to rapidly cure the conductive silver paste at a low temperature, 10 wt% of triethanolamine was added at the same time, and the main functional group of triethanolamine was a tertiary amine, which accelerated the curing reaction of the conductive silver adhesion as a catalyst. Fig. 6(b) shows the thermogravimetric analysis of a conductive adhesion with a silver content of 65 wt% and no dopant. The total weight loss was 13.6%. Fig. 6(c) shows the thermogravimetric analysis of a conductive adhesion doped with 3 wt% of Al with a silver content of 65 wt%. The total weight loss rate was 13.2%. Fig. 6(d) shows the thermogravimetric analysis of conductive adhesion doped with 7 wt% of Al with a silver content of 65 wt%, and the total weight loss rate was12.4%. It can be found that when adding the dopant, the weight loss of the conductive silver paste reduced, which indicated that the addition of the Al and the ZnO could enhance the colloidal stability to some extent. Fig. 7 shows the volume resistivity of conductive silver adhesion with different content of Al and ZnO for the silver powder content of 65 wt%. It can be seen that the volume resistivity became larger when Fig. 6. (a) The structure of the epoxy resin AG-80; (b) the thermogravimetric analysis of a conductive adhesion with a silver content of 65 wt% and no dopant; (c) the thermogravimetric analysis of a conductive adhesion doped with 3 wt% of Al with a silver content of 65 wt%; (d) the thermogravimetric analysis of conductive adhesion doped with 7 wt% of Al with a silver content of 65 wt%.
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material with a wide bandgap (3.37 eV) and high exciton binding energy (60 meV)[37]. Adding it to the conductive silver adhesion could reduce the contact resistance between the silver flakes and increase the colloidal carrier concentration. However, when the amount of ZnO is too large, the specific surface area and the specific surface energy of ZnO were increasing, and the surface activity was in the high lever. Therefore, it is easy to agglomerate together to increase the viscosity of the conductive adhesion, resulting in local zinc-rich area, which is not conducive to the diffusion filling of silver flakes, making silver flakes are easily stacked together and forming a large number of defects. As a result, the conductivity of conductive adhesion deteriorated. In a word, the best conductivity could be obtained when the content of silver powder was 65 wt%, and the ZnO content is 6 wt%. Fig. 8 shows the SEM of conductive silver adhesion doped with different Al contents (5 wt%, 9 wt%) and different ZnO contents (5 wt% and 9 wt%). It can be seen that the aluminum powder adhered to the silver flakes and the silver flakes was wrapped with the Al doping, which hindered the growth of the silver flakes. There were many holes and obvious gaps between the silver flakes. The matrix resin filled in and the tunnel resistance was very high. When the ZnO powder is added by 6 wt%, the silver flakes were surrounded by ZnO, and showed relatively uniform in particle size, dense and non-porous, and could form good conductive paths. But when ZnO powder is added up to 9 wt%, ZnO dispersed unevenly, agglomeration formed locally, the silver flakes formed a stack, the delamination was obvious, and the conductivity was poor. It was consistent with the data in Fig. 7. In order to enhance the bonding strength and shear strength of welding spot, ZnO and Al were adopted to dope into silver adhesion respectively. Firstly, the schematic diagram of ZnO pinning effect is shown in Fig. 9(a). After being doped with nano-ZnO powder, ZnO pinned at the grain boundary to hinder the grain boundary sliping and inhibit the diffusion of silver atoms and tin. Fig. 9(b) shows the comparison picture of doping and un-doping with ZnO welding spot. Fig. 9(c) and (d) show the shear strength and bonding strength of conductive silver adhesion with different ZnO content and different Al content, respectively. It can be seen that when ZnO or Al was doped, both the bonding strength and the shear strength of the conductive silver adhesion increased first and then decreased. The peak point of 24.31 MPa for shear strength and 12.44 MPa for bonding strength, respectively, was obtained as the content of ZnO was 7 wt%. It was
Fig. 7. The volume resistivity of conductive silver adhesion with different content of Al and ZnO.
added Al compared to non-doping sample, and the electrical conductivity deteriorates. On the one hand, the chemical properties of aluminum are very active, aluminum powder particles are small, and the specific surface area is very large, when it was exposed to air, Al3+ and O2− in the air are mutually dispersed, forming a dense Al2O3 film which is insulated on the surface of Al powder. This layer of Al2O3 film structure defected, making it had a very high surface energy, resulting in the aluminum powder surface could adsorb a lot of gas and oxidation temperature was reduced, thus, it could be oxidized at room temperature. On the other hand, since the doped aluminum powder is a nanomaterial which shows insulated because of the quantum size effect. Therefore, after the addition of aluminum powder, the conductivity of the conductive silver paste decreased [38]. When only ZnO was added, the volume resistivity first decreased and then increased with the increase of ZnO content. The volume resistivity reached a minimum value of 0.845 × 10−4 Ω cm with doping 6 wt% ZnO. Compared with the undoped one, the volume resistivity is reduced by 45.7%. When the ZnO content higher than 6 wt%, the volume resistivity of the conductive paste increased. This is because ZnO is a semiconductor
Fig. 8. The SEM photograph of Al-doping (5 wt%, 9 wt%) and ZnO-doping conductive silver adhesion. 242
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Fig. 9. (a) The schematic diagram of ZnO pinning effect; (b) the comparison picture of doping and un-doping with ZnO welding spot; (c) the shear strength of welding spot with different ZnO content and different Al content; (d) bonding strength of welding spot with different ZnO content and different Al content.
because that the stability and elastic modulus of ZnO were very high. When adding it to the conductive silver colloid, ZnO particles were dispersed in the matrix and pinned at the grain boundary to act as a diffusion barrier to inhibit the diffusion of silver atoms and tin atoms, which could prevent the grain boundary slippage, increase the dislocation density, inhibit dislocation movement, and at the same time reduce the surface energy and surface tension of the conductive silver adhesion, and also improve the wettability of the conductive adhesion, thereby improving the shear strength and bond strength of the solder joints [39,40]. However, excessive ZnO content would increase colloidal viscosity, hinder colloidal spreading, weaken wettability, and decrease bonding strength and shear strength [41]. The peak point of Al-doping sample was 19.8 MPa for shear strength and 12.44 MPa for bonding strength when the Al content was 8 wt%. It was because that when Al was added in an appropriate amount, on the one hand, the density of Al was low and it would accumulate in the interface region of the solder joint. The distribution of the nano-Al powder at the interface played a certain role in strengthening the stability of the solder joint; on the other hand, Al was an inert reinforcing phase that did not react with the solder or silver layer, thus, it could maintain its own stable morphology, and have a good reinforcing effect on the strength of solder joints. In addition, the interface compounds between Ag and Sn were
mostly brittle phases which would have an adverse effect on the stability of solder joints. The addition of Al powder could reduce the formation of compounds, inhibit the coarsening of compounds in the structure, and improve the stability of solder joints. However, when the amount of Al powder was excessive, Al tended to form Al-rich phase, and there were many internal structural defects in aluminum powder. A large amount of Al powder aggregated to adsorb a large amount of gas on the surface and increased the degree of surface oxidation, hence, made solder joints difficult to solder. Fig. 10 shows the thermogravimetric analysis of conductive adhesion co-doped with 3 wt% Al and 7 wt% ZnO, and the total weight loss was 13.4%. At 81 °C, the maximum weight loss rate was reached, and the curing reaction proceeded fastest. After 108 °C, the weight loss tended to be stable, and the post-curing reaction was mainly carried out at this time, and the reaction tended to the end and the performance was stable. Thus, it can be found out that when the Al powder and the ZnO powder are co-doped, the weight loss rate was not very different from that in Fig. 6, which indicated that the dopant properties were relatively stable and the weight loss in the curing reaction was small. In addition, it can also be proved that the addition of dopants had little effect on the curing reaction of the conductive silver adhesion and did not change the conductive silver adhesion curing system. Fig. 11 shows the SEM image of co-doped conductive silver adhesion with different contents of Al and 7 wt% ZnO. It can be seen that when 5 wt% of Al powder was added, the silver sheet showed the best spreadability, the crystal grains were fine, and the structure was dense. The ZnO nanoparticles and the Al powder were uniformly filled in the gaps of the silver sheets and were connected to form a good conductive network. For non-Al-doping sample, ZnO agglomerated, unevenly distributed between the silver flakes, and the silver particle size was relatively large. With the increase of Al content, the growth of silver flakes was significantly suppressed, the particle size decreased, and the spacing between silver flakes decreased. When the Al powder filling amount reached to 9 wt%, the dopant content saturated and overflowed, and the dopant adhered to the silver plate. Excessive Al3+ and O2− combined to form Al2O3 molecules, which hindered the conductive path and degraded the conductivity. The undoped ZnO was N-type semiconductor, and its conductivity was not high. Al dopant as a group III element, would provide donor levels, the number of carriers would increase, and the conductivity would increase. Therefore, different content of Al powder was added to the conductive silver adhesion with 65 wt% silver content and 7 wt%
Fig. 10. The thermogravimetric analysis of conductive adhesion co-doped with 3 wt% Al and 7 wt% ZnO. 243
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Fig. 11. The SEM photograph of Al and ZnO co-doping conductive silver adhesion.
ZnO content, and the volume resistivity was tested. Fig. 12 shows the volume resistivity of conductive silver adhesion of 7 wt% ZnO-doping with different Al-doping. It can be seen that the volume resistivity of the conductive silver adhesion first decreased with increasing Al content. When the Al doping up to 5 wt%, the volume resistivity of the conductive adhesion reached the lowest value, which was 0.947 × 10−4 Ω cm. This was because that when a small amount of Al powder was added, the lattice position of Al3+ instead of Zn2+ achieved donor doping and could be expressed as:
ZnO + xAl3 +
ZnO
formation of silver-tin compounds and increase bonding strength. Fig. 12(b) and (d) show the SEM photographs of solder joint fallen area in the shear direction and the bonding direction, respectively. It can be seen that when the solder joint was subjected to the force from the shear direction, the silver sheet slipped and partially stretched. Al and ZnO were pinned between the silver sheets in the conductive silver adhesion, preventing the grain boundary from shifting and increasing the friction. When the tensile force exceeded the maximum capacity of the silver sheet, ductile fracture between solder joint and substrate occurred. When the solder joint was subjected to the force along the bonding direction, the ZnO and Al were almost completely dropped because the bonding direction was perpendicular to the substrate, and the transverse friction force was not hindered during the stretching. The direction of the silver strip falling was the z-axis direction and the silver flakes fell. The area was much larger than the shear drop area, which was also the reason why the bonding strength of the solder joint was far less than the shear strength of the solder joint. Fig. 13(a) and (c) show the shear strength and bonding strength of welding spot for Al and ZnO co-doping conductive silver adhesion, respectively. It can be seen that as the Al content increasing, the shear
Zn12 +x (Al3 +·e )x O + xZn2+
Zn12 +x (Zn2 +·2e) x O12
x
+
x O2 2
Al3+ had one more positive charge than Zn2+, so each Al3+ provided one electron for the substitution of Zn2+, and each oxygen hole provided two electrons. To maintain electrical neutrality, Al3+ weakly binds a valence electron. This binding force was much smaller than that of the normal lattice for valence electrons. Therefore, valence electrons easily escaped from the bound center, and the oxygen ion vacancy concentration and carrier concentration increased. On the other hand, the ionic radius of Al3+ is smaller than that of Zn2+ and Ag+, thus, Al3+ easily entered lattice, filled gaps or defects between crystal grains, improved Hall mobility of carriers, thereby, conductivity had been improved. However, when Al was added overmuch, the Al atoms would escape from the position of the replacement Zn, occupying the gap position, suppressing the growth of crystal grains, increasing the interfacial energy at the grain boundary, and forming Al2O3 at the grain boundary to hinder the carrier migration. As a result, the resistivity increased and the conductivity deteriorated [42]. Fig. 12(a) and (c) show the shear strength and bonding strength of welding spot for Al and ZnO co-doping conductive silver adhesion, respectively. It can be seen that as the Al content increasing, the shear strength and bonding strength first increased and then decreased. The peak shear strength with value of 34.84 MPa and peak bond strength with value of 17.00 MPa both for the 4 wt% Al sample. It was because that Ag atoms and Al atoms had large difference in electronegativity and strong affinity. Al atoms can be filled in holes and defects between ZnO and Ag, and the density of metal atoms in the conductive silver adhesion increased, the wet force increased, and wetting time reduced. At the same time, addition of trace amounts of Al could reduce the
Fig. 12. The volume resistivity of conductive silver adhesion of 7 wt% ZnOdoping with different Al-doping. 244
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Fig. 13. (a) The shear strength of welding spot for Al and ZnO co-doping conductive silver adhesion; (b) The SEM photograph of solder joint fallen area in the shear direction; (c) The bonding strength of welding spot for Al and ZnO co-doping conductive silver adhesion; (d) The SEM photograph of solder joint fallen area in the bonding direction.
strength and bonding strength first increased and then decreased. The peak shear strength with value of 34.84 MPa and peak bond strength with value of 17.00 MPa both for the 4 wt% Al sample. It was because that Ag atoms and Al atoms had large difference in electronegativity and strong affinity. Al atoms can be filled in holes and defects between ZnO and Ag, and the density of metal atoms in the conductive silver adhesion increased, the wet force increased, and wetting time reduced. At the same time, addition of trace amounts of Al could reduce the formation of silver-tin compounds and increase bonding strength. Fig. 13(b) and (d) show the SEM photographs of solder joint fallen area in the shear direction and the bonding direction, respectively. It can be seen that when the solder joint was subjected to the force from the shear direction, the silver sheet slipped and partially stretched. Al and ZnO were pinned between the silver sheets in the conductive silver adhesion, preventing the grain boundary from shifting and increasing the friction. When the tensile force exceeded the maximum capacity of the silver sheet, ductile fracture between solder joint and substrate occurred. When the solder joint was subjected to the force along the bonding direction, the ZnO and Al were almost completely dropped because the bonding direction was perpendicular to the substrate, and the transverse friction force was not hindered during the stretching. The direction of the silver strip falling was the z-axis direction and the silver flakes fell. The area was much larger than the shear drop area, which was also the reason why the bonding strength of the solder joint was far less than the shear strength of the solder joint.
strength, and 17.00 MPa for bonding strength with addition of 4 wt% Al and 7 wt% ZnO. Acknowledgements This work was supported by the National Natural Science Foundation of China (No. 11604363, 61471047, 61671068), and the Beijing College Innovation Capability Promotion Plan of Beijing Municipal Institutions (No. TJSHG201510772015). The project is also supported by The Importation and Development of High-Caliber Talents Project of Beijing Municipal Institutions (No. CIT& TCD201504053) and Beijing Municipal Education Commission Science and Technology Plan Project (No. KM201811232021). References [1] V.K. Varadan, Y. Ma, V.V. Varadan, Scattering of acoustic waves by piezoelectric composites, J. Acoust. Soc. Am. 80 (Suppl. 1) (1986) S74, https://doi.org/10.1007/ 1-4020-4131-4_19. [2] L. Qin, L.K. Wang, Y. Lu, Decoupling in multi-elements composite for transducer array application, Curr. Appl Phys. 11 (3) (2011) S368–S370, https://doi.org/10. 1016/j.cap.2011.03.048. [3] D. Klosterman, L. Li, J.E. Morris, Materials characterization, conduction development, and curing-effects on reliability of isotropically conductive adhesives, IEEE Trans. Compon. Packag. Manuf. Technol. Part A 21 (1) (1996) 23–31, https://doi. org/10.1109/95.679028. [4] J.M. Lin, W.N. Chen, C.Y. Lin, C.F. Lin, J.C. Chang, A novel highly electrically conductive silver paste, Int. Conf. Electron. Packag. IEEE (2016) 615–618, https:// doi.org/10.1109/ICEP.2016.7486902. [5] H. Ma, X. Tian, S.C. Yan, Z. Li, X.H. Guo, Y.Q. Ma, L. Ma, Silver flakes and silver dendrites for hybrid electrically conductive adhesives with enhanced conductivity, J. Electron. Mater. 47 (5) (2018) 2929–2939, https://doi.org/10.1007/s11664-0186145-5. [6] X. Peng, F. Tan, W. Wang, X. Qiu, F. Sun, X.L. Qiao, J.G. Chen, Conductivity improvement of silver flakes filled electrical conductive adhesives via introducing silver–graphene nanocomposites, J. Mater. Sci.: Mater. Electron. 25 (3) (2014) 1149–1155, https://doi.org/10.1007/s 10854-013-1671-7. [7] L. Wang, C. Wan, Y. Fu, H. Chen, X. Liu, M. Li, Study on the effects of adipic acid on properties of dicyandiamide-cured electrically conductive adhesive and the interaction mechanism, J. Electron. Mater. 43 (1) (2014) 132–136, https://doi.org/10. 1007/s11664-013-2765-y. [8] G.S. Xiao, E.Q. Liu, T. Jiu, X.F. Shu, Z.H. Wang, G.Z. Yuan, X.X. Yang, Mechanical properties of cured isotropic conductive adhesive (ICA) under hygrothermal aging investigated by micro-indentation, Int. J. Solids Struct. 122–123 (2017) 81–90, https://doi.org/10.1016/j.ijsolstr.2017.06.003. [9] R. Durairaj, L.W. Man, Effect of epoxy and filler concentrations on curing behaviour of isotropic conductive adhesives, J. Therm. Anal. Calorim. 105 (1) (2011)
4. Conclusions In summary, the weldability was focused on for the practical applications of conductive adhesion to surface conductive treatment of piezoelectric composite material. The conductive mechanism, welding spot bonding mechanism, bonding strength, and shear strength of welding spot with co-doping of metal material Al and oxide material ZnO were studied. When Al and ZnO doped separately, Al-doping would increase the volume resistivity and adhesion strength for welding spot, and a proper amount of ZnO-doping would decrease the volume resistivity and improve adhesion strength for welding spot. Co-doping with Al and ZnO could decrease volume resistivity and enhance adhesion strength of welding spot concurrently, and the typical properties were 1.119 × 10−4 Ω cm for volume resistivity, 34.84 MPa for shear 245
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