Effects of Gold on the Properties of Tin-Antimony Solder in Flip-Chip-Pin-Grid-Array (FCGPA) Packages

See Bee Kee', Luay B Hussain', YL Khong ', Dennis Chandran Prem Kumarl ' School of Materials & Mineral Resources Engineering, Univ Science Malaysia

2Materials Science Dept, Asian Inst. for Medicine, Science & Technology, Malaysia 'Intel Technology (M) Sdn. Bhd, Malaysia

Abstract The physical characteristics of the tin-antimony solder

used in FCPGA semiconductor packages change with the diffusion of gold from the gold plated layers in the solder pads or pin. This must he considered in the setting of parameters for the subsequent re-flow processes and also the possible impact on the quality and reliability of the joints. This work describes the physical and micro-structural properties of the 95Sn5Sb parent material alloyed with 1.0, 2.0, 2.5, 3.0, 3.5,4.0 and 5.0 wt% ofgold. In particular, the melting behaviour, electrical and thermal conductivity and hardness of 95Sn5Sb solder doped with gold are discussed.

1. Introduction For applications in microelectronic packaging, dual

solder systems are used to allow for attachment of various parts at different stages of the packaging process. In the flip-chip-pin-grid-array (FCPGA) systems, the pins are attached first to the organic substrate. These pins must remain intact during the subsequent chip attach process. Hence the solder for the pins must withstand the temperatures of the chip attach solder during the re-flow process.

95Sn-5Sh has good electrical conductivity and mechanical strength [ I ] . The addition of antimony is known to have the desirable effect of retarding whisker growth and inhibiting allotropic transformation of tin [2] which ultimately weaken joint strengths. Together with its higher melting temperature, these properties make 95Sn-SSb a good candidate for packages requiring dual solder systems for attachment of parts at different processes. In the case of the FCPGA above, the 95%-5Sb solder (liquidus temperature 24OOC) for pin attachment and the 63Sn-37Ph eutectic system (liquidus temperature of 183'C) for chip attachment are appropriate.

Immersion of gold on electro-less nickel layers over copper are commonly used on electrical conductors in micro-electronic packages [3,4]. The thin layer of gold deposited protects the underlying metal layers from oxidation and increases solder wenability and therefore adhesion. During the soldering process, the gold layer is fully consumed through diffusion into the tin base solder system [ 5 ] . The solder forms a joint with the nickel under- layer. However, with the high dissolution rate of gold in tin containing solders, AuSn4 inter-metallic compounds are observed especially with solders with high proportion of tin re-flowing on gold plated layers [6]. The thickness of the plated gold layer contributes to the amount of AuSn, inter- metallic that affects the strength and reliability of the joints formed [7].

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The tin-antimony solder system is used in electronic packages to affix gold plated parts like the pins and pads as described. It is thus important to understand the effects that gold has on the properties of the tin-antimony solder system and relate this to the impact it has on the subsequent processing involving the FCPGA package.

In this work, the microstructure, melting, electrical and thermal conductivity and hardness behavior of the 95Sn5Sb doped with 1.0, 2.0, 2.5, 3.0, 3.5,4.0 and 5.0 wt% of gold are characterized. The possible impact on the processing of FCPGA packages shall be discussed.

2. Materials and Experimental Procedure The solder alloy materials of appropriate stoichiometry

were purchased from Alpha Metals Inc. A Perkin Elmer DSC7 was used for the thermal analysis.

Calibrations were performed using indium and tin and approximately 17 mg were used for each scan. The thermal profile comprise a hold time of 1 minute at 1 9 0 T followed by a ramp of l°C/min up to 250°C.

Cuboids with sample dimensions of approximately 3xZx43mm were prepared for electrical conductivity measurement using a Wheatstone bridge circuit. The conductivity of the samples are expressed in terms of the lntemational Annealed Copper Standard percentage (%IACS) [XI .

Cylindrical specimens nominally 13.5 mm in radius and 31 mm in length were prepared to measure their thermal conductivity using a heat conduction apparatus by Armfield Limited. The heat flow throughout was maintained at 5 W.

For Brinell hardness testing, a tester by G. Melchrs & Co., Germany was used. The load applied during the test was 62.5 kgf and the diameter of the ball used was 5 mm.

Micro-structural examinations were performed using an optical microscope and a scanning electron microscope (SEW. 3. Results and Discussion

Liquidus and Solidus Temperatures Table 1 and figure la and Ih show the solidus and

liquidus temperatures obtained by differential scanning calorimetry (DSC) for the different wt% of Au in 95Sn5Sb. The solidus and liquidus temperatures of the 9SSn-5Sb are 235.3 and 241.3"C respectively which are in agreement with previous work [9]. With the addition of 1 wt% of gold, the solidus temperature falls rapidly to 214°C. For subsequent increases in the gold content, the solidus temperature did not appreciate considerably. The liquidus temperature however decreases in an approximately linear fashion from 243 to 236°C with the gradual incorporation of gold.

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Table 1: The values obtained of the (a) solidus and (b) liquidus temperatures, (c) electrical conductivity, (d) thermal conductivity and (e) Brinell hardness with wt% of gold in 95Sn-5Sb.

Comparing the phase diagram of the Sn-Sh, (fig.2a), and Sn-Au binary systems (fig.2b) [IO], it is expected that there will he some lowering of the liquidus and solidus temperatures of the 95Sn-Sb system with the addition of gold. However, the liquidus lines should follow the liquidus surface bounded by liquidus line in fig.2a and liquidus line in fig.2b. Similarly the solidus lines should follow the solidus surface bounded by soildus line in fig 2 and solidus line in fig 2b. Therefore, it is not surprising to observe a sudden drop in the solidus temperature with a IW% addition of gold, which is consistent with the solidus line that lies at a lower temperatures at low Au percentage as shown in the Sn-Au phase diagram (figure 2 b) in the temperature region of concem. From about 1 to 10% Au in Sn, the solidus temperature is essentially unchanged (Eutectic reaction) at approximately 217°C. Hence, as observed for the Au in Sn-Sb system studied, little change was observed in the solidus temperatures with addition of gold up to 5 wt % (figure 1 a) indicating that the gradient of solidus surface with this range of Au addition is not significant in comparison with liquidus surface after lwt%Au addition.

(a)

-I --I -I -. -I cl

Wtgb of G o l d in 955n-6Sb

Figure 1: The variation of (a) solidus and (b) liquidus temperatures, (c) electrical conductivity, (d) thermal

conductivity and (e) Brinell hardness with wt% of gold in 95Sn-5Sb.

(4 (bl

0 10 0 10 Figure 2: (a) Portion of Sn-Sh binary system showing peritectic reaction up to 10wt% Sb. (b) Portion of Sn-Au binary system showing Eutectic reaction up to lOwt% Au.

The rapid decrease in the solidus temperature observed with the incorporation of small amount of Au suggests that this would be detrimental to an application where 95Sn-5Sb is used in conjunction with gold plated layers. In the case of FCPGA packages, if gold plated pins are attached to the substrate using the 95%-5Sb system, the subsequent chip attach process with nominal temperatures in excess of 215°C would cause the pins to move about their origin positions and in worst case, drop off. In several instances, this occurred in the as received packages during the re-flow process to attach the chip.

Electrical Conductivity Variations of the conductivity of the samples expressed

in IACS% is shown in Table 1 and Figure IC. The incorporation of gold clearly results in an increase of electrical conductivity. A faster rate of conductivity increase was observed with the first 1 wt% of gold incorporated. This increased conductivity is desirable from the electrical connectivity perspective especially for high speed electronic packaging.

Thermal Conductivity The thermal conductivity increases in an approximate

linear fashion from 63.9 to 77.3 Wlmm-K as the percentage of gold increases from 0 to 5 %. This means that up to 20% extra transfer of heat from the chip through the substrate to the pins via the gold doped Sn-Sb solder is possible. Any problems associated with this increase in beat transfer are not known. From the reliability perspective, increased heating of the pins hastens the degradation on the pin-socket connection. However, further work is required to investigate this since pin-socket reliability is also dependent on other factors like moisture and contamination.

If we compare figure I C and d, it is apparent the Wiedmann-Franz law [ 111 which requires the thermal and electrical conductivity to be linearly related apparently does

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not strictly apply for the ternary system studied here. The electrical conductivity increases more rapidly compared to the thermal conductivity at low percentage of Au. We speculate that the electrical conductivity of this solder system is more sensitive to the micro-structural environment compared to the thermal conductivity.

Hardness With increasing Au content from 0 to 5% in 95Sn-5Sb

solder, the hardness, as measured on the Brinell hardness scale increases from 15 to 32.7 (Table 1 and figure l e ). The rate of increase is faster with the first 1% of Au incorporation.

It is expected that the increasing hardness would make the pin solder joints more susceptible to brittle fracture. Hence, pull or shear tests are recommended for joints formed with parts containing gold plated surface used with 95Sn-5Sb solders, especially, if the gold plating thicknesses are not well controlled or well determined.

Microstructure A SEM micrograph displaying the inter-metallics

observed is shown in figure 3. The elemental compositions of the various stmctures were confirmed by EDX.

Figure 3: SEM micrograph showing the dendritic structures of AuSn4 in 3% gold doped 95Sn-5Sh.

The AuSn4 inter-metallics are characterized by their needle-like structures. The tin-antimony inter-metallics appear as black agglomerates in the optical micrograph. In the hener resolved SEM micrographs, they appear globular- like.

The optical micrographs show increases in the density of the AuSn4 inter-metallics with increasing gold doping. At higher magnifications of the SEM micrographs, apart from slight thickening of the AuSn, needles at 5 wt% doping, little change was observed in this inter-metallic structure from 1 to 4 wt% gold doping.

The tin-antimony globules spreaded almost uniformly throughout the parent material studied. No fixed pattems were apparent in the distribution of the globules across the samples. There are no systematic variations in the sizes of these globules with gold doping either.

As in the previous work, the presence of AuSm inter- metallic is of concern [12]. This was shown to significantly weaken the solder joints and also cause reliability issues. It

is thus important to control the amount of gold from the parts to he joined to suppress the formation of this inter- metallic in the tin-antimony solder system.

It was mentioned previously that the solidus temperature, electrical conductivity and hardness showed a rapid change between the undoped and 1% Au doped sample. The changes in these parameters become more gradual with higher percentage of gold doping. Overall, this is an indication that these parameters are more sensitive to micro- structural changes, namely the. formation of the SnAu4 inter- metallics. Within the limits of experimental uncertainty, the thermal conductivity however was found to show a more linear relationship indicating this parameter tends to he affected by the overall hulk structure.

4. Conclusions The addition of gold lowers both the solidus and liquidus

temperatures of the parent 95Sn-5Sb solder. Consequently, during the subsequent re-flow processes carried out at temperatures in excess of 215°C to melt the eutectic Sn-Ph solder, the gold plated pins attached by 95Sn-5Sh on occasions would move out of their originally soldered position and in some cases they even drop off. The electrical and thermal conductivity and hardness of the 95Sn-5Sb solder increases with the increase of gold diffused into the solder material. Similar to the other gold incorporated tin based solder systems, the needle like AuSn4 inter-metallics were found to be present in gold doped 95Sn- 5Sb.

Acknowledgments The authors (B.K. See and L.B. Hussain) are grateful for

a research grant provided by Intel Technology Malaysia to cany out this work. Special acknowledgement is also directed to Prof. K. Nair for his useful comments and review of script.

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