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Effect of Gold on Reliability of SnAgCu Joints
Analysis Lab |
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Authored By:Jianbiao Pan, Patrick Hyland California Polytechnic State University, San Luis Obispo, CA USA Julie Silk, Mike Powers Agilent Technologies, Santa Rosa, CA USA TranscriptElectroplated nickel/gold over copper is a popular metallization for PCB finish as well as for component leads, especially wire-bondable high frequency packages, where the gold thickness requirement for wire bonding is high. The general understanding is that less than 3% by weight of gold is acceptable in tin-lead solder joints. However, little is known about the effect of gold content on the reliability of tin-silver-copper solder joints. The purpose of this study is to determine the acceptable level of gold in sack 305 solder joints. Three different package platforms with different gold thicknesses were assembled on boards with two different gold thicknesses using a standard surface mount assembly line in a realistic production environment. The assembled boards were divided into three groups. All boards were then subjected to accelerated mechanical reliability tests including random vibration and drop testing. The results show that solder joints with over 10% by weight of gold were unacceptable. If copper is available to dissolve in the solder joint, then a gold content under 5 % by weight will not significantly degrade the reliability of the solder joint. When nickel layers are present on both the board and component sides of the interface, this limits the ability of copper to dissolve into the solder joint and hence a gold content under 3% by weight is acceptable. SummaryElectroplated Ni/Au over Cu is a popular metallization for PCB finish as well as for component leads, especially wire-bondable high frequency packages, where the gold thickness requirement for wire bonding is high. The general understanding is that less than 3 wt% of Au is acceptable in SnPb solder joints. However, little is known about the effect of Au content on the reliability of SnAgCu solder joints. The purpose of this study is to determine the acceptable level of Au in SAC305 solder joints. Three different package platforms with different Au thicknesses were assembled on boards with two different Au thicknesses using a standard surface mount assembly line in a realistic production environment. The assembled boards were divided into three groups: as-built, isothermally aged at 125°C for 30 days, and isothermally aged at 125°C for 56 days. All boards were then subjected to accelerated mechanical reliability tests including random vibration and drop testing. The results show that solder joints with over 10 wt% Au are unacceptable. If Cu is available to dissolve in the solder joint, then an Au content under 5 wt% will not significantly degrade the reliability of the solder joint. When Ni layers are present on both the board and component sides of the interface, this limits the ability of Cu to dissolve into the solder joint and hence an Au content under 3 wt% is acceptable. The failure mechanism for solder joints with high Au content is fractures through the AuSn4 IMC. Our comprehensive long-term reliability study did not confirm the finding by Ho et al. (2002) that the weak interface between (Au, Ni)Sn4 and Ni3Sn4 results in brittle interfacial failure. Additional findings confirmed the danger of placing parts near high stress areas and that a high level of voiding reduced reliability. ConclusionsA comprehensive study has been conducted investigating the effect of Au content on lead-free solder joint reliability. The results show that SAC305 solder joints with over 10 wt% Au are unacceptable. If Cu is available to dissolve into the solder joint, then an Au content under 5 wt% will not significantly degrade the reliability of the solder joint. When Ni layers are present on both the board and component sides of the interface, limiting the ability of Cu to dissolve into the solder joint, an Au content under 3 wt% is acceptable. Additional findings confirmed the danger of placing parts near high stress areas and that a high level of voiding reduced reliability. When the Au content in a solder joint is less than 3 wt%, AuSn4 IMCs are small and won't play a significant role. The failure mechanism of such a solder joint is fracture within the Sn matrix when the joint is subjected to a very high stress level. If the Au content is high or large AuSn4 IMCs are present in a solder joint, the failure mechanism is fractures through the AuSn4 IMCs. Fractures through the AuSn4 IMCs were found in the bulk solder and/or near the solder/metallization interface. Ho et al. (2002) claimed that the weak interface between (Au, Ni)Sn4 and Ni3Sn4 results in brittle interfacial failure. However, our comprehensive long-term reliability study did not confirm this finding. Initially Published in the IPC Proceedings |
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