Laser soldering of flip-chips

Laser soldering of flip-chips

ARTICLE IN PRESS Optics and Lasers in Engineering 44 (2006) 112–121 Laser soldering of flip-chips K. Korda´s, A.E. Pap, G. To´th, M. Pudas, J. Ja¨a¨...

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ARTICLE IN PRESS

Optics and Lasers in Engineering 44 (2006) 112–121

Laser soldering of flip-chips K. Korda´s, A.E. Pap, G. To´th, M. Pudas, J. Ja¨a¨skela¨inen, A. Uusima¨ki, J. Va¨ha¨kangas Microelectronics and Materials Physics Laboratories, University of Oulu, P.O. Box 4500, Oulu FIN-90570, Finland Received 14 December 2004; accepted 22 March 2005 Available online 24 May 2005

Abstract A novel process for laser soldering of flip-chips on transparent printed circuit board assemblies is presented. The experiments were carried out on silver test patterns printed on glass wafers using a roller-type gravure offset printing method. The contact pads, where the bumps of the flip-chips are positioned, were covered with a thin layer of additional solder paste. The aligned samples (solder pad—solder paste—chip bump) were illuminated through the glass substrate using an Ar+ laser beam (l ¼ 488 nm, P ¼ 0:623:0 W, d ¼ 100 mm at 1/e) to heat the printed pad and melt the solder paste, thus forming a joint between the printed pad and the chip bump. The heat-affected zone was modeled using computer-assisted finite element method. The solder joint cross-sections were analyzed using optical and electron microscopy as well as energy dispersive X-ray element analyses. The laser-soldered joints were of good mechanical and electrical quality and the process proved to be suitable for manufacturing customized circuit prototypes. r 2005 Elsevier Ltd. All rights reserved. Keywords: Laser; Flip-chip; Soldering; Finite element modeling; Gravure offset printing

1. Introduction Soldering using various lasers (CO2, Nd:YAG, etc.) is a well-established technology applied in printed circuit board (PCB) assembly for over 20 years. In a Corresponding author. Tel.: +358 8 553 2741; fax: +358 8 553 2728.

E-mail address: [email protected].fi (G. To´th). 0143-8166/$ - see front matter r 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.optlaseng.2005.03.002

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conventional laser soldering process, the component leads together with the solder and the PCB are heated until the solder melts and flows around the lead and on the printed circuitry. When the illumination is stopped, the temperature decreases and the solder solidifies forming a joint between the lead and the PCB. Depending on the laser power and the thermodynamic parameters of the joint (heat capacity, thermal conductivity, specific heat of melting, etc.), the process takes from a few milliseconds up to a few tenths of a second. In the case of laser soldering—compared to reflow soldering techniques—the applied heat is localized, thus the active regions of electronic components are not exposed to high temperatures. Therefore, assembly of heat-sensitive components can be accomplished. In addition, laser soldering enables mounting of individual components and customized modifications of PCB assemblies [1–8]. Difficulties with the conventional laser soldering process arise when flip-chips or other tile-type components are mounted, since the joints to be formed are between the component and the PCB. In such cases, the laser beam has to be applied from the side (parallel to the PCB) or either through the component or through the PCB (both perpendicular to the PCB). Due to simple optical reasons, the first option (illumination from the side) enables soldering along the perimeter of the component. Considering a numerical aperture of 0.1 for the focusing optics (which is already a rather small value) and a typical flipchip and PCB distance of 100 mm, maximum soldering distance from the side of the chip is 1 mm. Therefore, in the case of large chips/arrays, the inner joints cannot be realized. The second alternative for laser soldering—with a beam through the component— can be dismissed due to scattering of the beam on the metal patterns of the chip and due to the high risk of a possible component failure caused by the high-intensity laser beam. Since the only feasibility criterion for illumination through the PCB is high transparency of PCB dielectrics at the wavelength of the laser beam, the third option appears to be the obvious one. Though the most common PCB dielectrics, FR4 and PI, are fairly transparent near IR, no such applications were found in the literature. In the first related publication [9], laser soldering of flip-chips was performed on copper/polycarbonate tapes using a Nd:YAG laser. Successful soldering of commercial solder balls to metallized glass (Cr/Ti adhesion and Cu/Ni top layer) using a near UV continuous wave laser beam (355 nm) was also reported [10]. In this study, laser soldering of flip-chips on silver-on-glass substrates using a visible beam is presented. The novelty of the process is the relatively easy visualization/monitoring, by which customized circuitry fabrication and repair for high-density packages becomes available.

2. Experimental The electronic circuitry was made using roller-type gravure offset printing on ordinary soda-lime glass plates having thickness of 0.8 mm. This method involves the

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Fig. 1. Illustration of gravure offset printing. (a) First, the ink is doctored into the grooves of the gravure plate, then (b) picked up from there using a rolling offset cylinder, and finally (c) deposited on the substrate by pressing the rolled cylinder.

doctoring of silver ink into the grooves of a gravure plate, from which the ink pattern is picked up by rolling a silicone polymer blanket over it. In the next step, the blanket with the ink is rolled over a glass substrate, onto which the ink is transferred (Fig. 1). Finally, the as-printed circuitry is fired at a 600 1C peak temperature in a belt furnace. The process enables the fabrication of thick (2–10 mm) metallic patterns with a lateral resolution of 25 mm [11–13]. In our experiments, two kinds of soldering pad shapes were used. The first type of pad was thin (2 mm) and had three parallel slits each 30 mm wide with 30 mm spacing. These samples enabled good visual monitoring of the process. The second type was thicker (5 mm) and bulky (without any optical slits). Flip-chips (DS18B20X digital thermometer from Dallas Semiconductor, US) with four semi-spherical bumps (Sn5-Pb95, 350 mm in diameter) were soldered on the solder pads of circuitries printed on glass wafers using an additional layer of Sn62Pb36-Ag2 solder paste (Heraeus, F352). The solder paste was dispensed manually over the solder pads. Laser soldering was carried out using a defocused (the sample is placed 500 mm behind the focal plane) beam of an Ar+ laser. The laser spot diameters (1/e) at the planes of focus and printed pads were 3 and 100 mm, respectively (Fig. 2). The process was monitored using a microscope and a CCD set on the optical axis, thus direct observation during laser processing was possible. Micrographs of the solder joints were taken using optical and scanning electron microscopy (SEM, Jeol JSM-4600F). Chemical element analyses were performed on cross-sectioned samples using an EDX analyzer installed on the SEM. The heat-affected zone was modeled using computer-assisted (Ansys) finite element method (FEM).

3. Results and discussion The laser power was increased gradually in order to ensure even heating of the whole solder pad, paste and bump, as well. Since the temperature of the solder paste

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Microscope objective 10 × NA = 0.28

cw Ar + laser beam λ = 488 nm P = 0.6 – 3.0 W

focal plane

printed pads on glass

Alumina fixture

additional solder paste

flip-chip with solder bumps

XYZ-translation 20-500µm/s

Fig. 2. Schematic experimental setup of flip-chip laser soldering.

could not be measured due to the very small dimensions, the soldering profile of the solder paste (temperature versus time) was followed using visual observations, where the only fixed point was the melting temperature of the paste (179 1C). The laser power and heating time for the drying and heating period were adjusted so that the paste did not melt, but its temperature was just below the phase transition (o179 1C). After that, the laser power was increased again to form a molten phase of the paste and was kept for a short period (45 s), according to the temperature profile recommended by the solder paste supplier. Finally, the laser power was gradually decreased to ensure proper cooling profile. The optimal experimental parameters for manufacturing solder joints of high quality are listed in Table 1. During the process, first the resin melted and started to evaporate. It was followed by the collapse of the warmed solder paste balls and the flow of the molten phase between the chip bump and printed pad. A partial dissolution of the solder pads into the liquid-phase paste was also observed (Fig. 3). When the laser power was decreased, solidification of the molten phase and formation of a laser-soldered joint took place. In the case of thick homogeneous pads—where the peak laser powers were much higher compared to the ones applied to the thin (hollow) pads—partial melting of the solder bumps was also observed. Quantitative EDX analyses revealed an increase in silver concentration in the solder (2 wt% - 5 wt%) and higher tin concentration in the chip bump (5 wt% - 35 wt%) at the interface. The temperature distribution was analyzed using FEM. The structure of bump/chip interface was investigated by SEM and EDX measurements (Fig. 4). It

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Table 1 Laser powers, heating rates and illumination times used in soldering on two pad types 2 mm pads with slits, 5 mm bulky pads Laser power (W) Heating rate (mW/s) Duration (s)

Drying

Pre-heating

Melting

Cooling

0.6-1.5 0.6-2.4 15 30 60

1.5 2.4 — — 150

1.5-1.8 2.4-3.0 150 150 45

1.8-0.6 3.0-0.6 20 40 45

Fig. 3. SEM image (a) and EDX element map of a laser soldered flip-chip joint cross-section. Images (b)–(d) show the distribution of tin, lead and silver, respectively.

was found that the under bump metallization (UBM) structure is 1 mm Ti and 3 mm Cu on 4 mm Al pad. The passivation layer is SiOx with a thickness of 5 mm. In the model, 130 000 elements each with eight nodes and a single degree of freedom (temperature) were used. The elements were gradually densified towards the volume of interest. The natural convection in ambient air (20 W/m2 on all surface

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Fig. 4. SEM image of the chip cross-section. The inset shows a magnified (Sn5-Pb95)/UBM/chip region.

areas) was applied as boundary condition. The applied load was the heat flux generated by the Ar+ laser source. The effective laser intensity has been evaluated by the fact; the solder paste is still in solid phase below a threshold value of 2.4 W laser power. Further increase in the power melts some parts of the paste (the melting temperature is 455 K), and by applying 3 W the paste completely melts. To determine the optical losses, test simulations were made by changing the relevant parameters in the heat flow equation. In addition to the given 19% reflection loss on the optical components, an 80% scattering/reflection loss for the silver-glass interface was obtained. Therefore the term of the heat source q in the heat-flow equation is q ¼ ð1  0:19Þ  ð1  0:80Þ  I 0  exp ðr=wÞ2 , where I 0 ¼ Pð2pw2 Þ1 , r is the distance from the center of the spot and w is the radius of the beam (50 mm) at 500 mm distance from the focal plane. Based on the finite element model, it was concluded that the heat-affected zone (HAZ) is very well confined into the solder pad/solder paste/chip bump region (Fig. 5). The peak temperature in the UBM region of the chip is about 380 K, which proves that the heat sensitive parts are prevented from damages caused by overheating. The main reason is the SiOx layer located between the bump and chip provides good thermal insulation; hence the chip is protected from the heat generated during the soldering process. The role of the alumina fixture turned out to be important as well in the soldering process. Because of the good thermal conductivity (237 W/m K at 300 K) and large surface area of the alumina sample holder, it acts as a heat sink, by which the temperature of the chip is significantly reduced. Partial melting of the chip bump resulted in higher mechanical strength, measured by pulling tests (Cahn DCA-312). The minimum force needed to pull off the chips from the substrate was as high as 0.7 N (thin pads with slits) and 1.4 N (thick bulky pads). Usually, the joints broke and detached at the glass–silver interface, indicating

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Fig. 5. Results of FEM simulation: Temperature as a function of position along the geometrical axes of a joint (a). Cross-section contour plot (temperature field) of the volume of interest for laser powers of (b) 2.4 W and (c) 3.0 W.

excellent strength of the laser-soldered joint (Fig. 6). Considering a total joint crosssectional area of 1.9  103 cm2 (the surface area of three smaller 200 mm pads and a big 350 mm solder pad), the adhesion of the thin pads and the bulky pads was 3.7 and 7.4 MPa, respectively. Since two bumps (ground) are directly connected inside the chips, the electrical resistances of such joint pairs could be measured simply on a test PCB chip mount using the four-point method. The typical resistance values were between 0.1–0.2 O for each joint. The resistance values did not change even after 4 weeks of loaded testing (100 mA DC; 672 cycles: 15 min at 40 1C, 15 min heating, 15 min at 125 1C and 15 min cooling) in a thermal cycling chamber (Vo¨tsch Industrietechnik, VTS 7027-15).

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Fig. 6. Micrographs of a detached solder joint: (a) bottom view of a flip-chip, (b) side view of a flip-chip and (c) a corresponding residual silver pattern on glass. The solder and also some portion of the solder pad are attached to the solder-ball of the flip-chip.

Fig. 7. Photograph of laser-soldered flip-chip arrays (in the background, the array is upside down).

4. Summary and conclusions Laser soldering of flip-chips and flip-chip arrays on silver-on-glass printed circuitry was accomplished using a diverging Ar+ laser beam (Fig. 7). Laser irradiation and thus heating of the solder pads was carried out through the substrate. Prior to soldering, the solder pads were covered with a thin layer of additional solder paste to ensure good solder joint formation. During laser soldering,

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the silver pads dissolved partially in the solder. When higher laser powers were applied, the surface of the chip bumps melted in addition to the solder paste and a 5 mm thick transition zone with a composition of Sn35-Pb60-Ag5 formed. Finite element analysis was carried out to verify the possible damages caused by overheating. The results show that the HAZ is very well confined outside of the chip. The maximum temperature in the chip is 380 K, therefore the chip does not deteriorate during the process. Preliminary thermal, electrical and mechanical tests revealed that the joints formed by laser soldering are of good quality (0.1–0.2 O/joint resistance and up to 7.4 MPa adhesion). For comparison, similar adhesion forces were measured (0.4–2.2 N) with conventional laser-soldered joints [6]. In order to provide uniform heating along the solder pads and bumps, the diameter of the laser beam has to be compatible with the size of the objects to be soldered. Therefore, in the case of future components having microscopic leads or bumps, a downscaled process will provide novel alternatives for manufacturing micro-solder joints. In addition, a smaller soldering volume requires lower laser powers and enables a faster parallel laser soldering process by applying proper intensity patterns generated by illuminated phase masks [5].

Acknowledgments Financial support from the EU (MARVEL, project code: IST-1999-29043) is acknowledged. Ge´za To´th and Andrea Edit Pap would like to thank the EMPART Research Group of Infotech, Oulu for the financial support. Andrea Edit Pap acknowledges the grants given by Oulu Yliopiston Tukisa¨a¨tio¨ and Naisten Tiedesa¨a¨tio¨. Krisztia´n Korda´s is grateful for the Academy Research Fellow post received from the Academy of Finland and also for the Nokia Visiting Scholarship given by the Nokia Foundation.

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