Effect of Gold Content on the Reliability of SnAgCu Solder Joints

摘要

Electroplated 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 AuSat IMC. Our comprehensive long-term reliability study did not confirm the finding by Ho et al. (2002) that the weak interface between (Au, Ni)Sn_4 and Ni_3Sn_4 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.