By Francy Akkara
Reliability of the new solder alloys has been a serious concern, especially in the harsh environment. Aging at elevated temperature alters the structural and mechanical properties of the solder joint and leads to the coarsening of the precipitates in the bulk solder and growth of the intermetallic compound layer, which in turn deteriorates the solder joint. Various new elements such as bismuth, antimony, and nickel are micro-alloyed into the SnAgCu based alloys to slow down this deterioration process and improve the reliability. There haven’t been many studies about the effect of solder sphere alloys with bismuth pastes on the harsh thermal reliability. In this paper, the effect of mixing solder sphere alloys (SAC105, SAC305, matched solder sphere – paste) with Bi-based solder pastes on the component reliability in accelerated thermal cycling is studied. Three surface finishes including OSP, ImAg, and ENIG are considered. The test vehicle consists of 15mm and 6mm Ball Grid Array (BGA) components. The assembled test boards were isothermally aged at 125oC for 12 months, which are then thermally cycled between -40oC to +125oC for 5800 cycles. It was observed that ENIG surface finish improves the component reliability comparing to OSP and ImAg. It is also observed that matching solder sphere with solder paste alloys improves the reliability in several cases, but a significant improvement was not observed when compared with the SAC305 solder spheres.