Reliability of Aging in Microstructures for Sn-Ag-Cu Solder Joints with Different Surface Finishes during Thermal Cycling

Abstract

A direct and deleterious effect on packaging reliability has been observed during elevated temperature isothermal aging for fine-pitch ball grid array (BGA) packages with Sn-1.0Ag-0.5Cu (SAC105) and Sn-3.0Ag-0.5Cu (SAC305) solder ball interconnects. Package sizes ranging from 19 mm with 0.8 mm pitch BGAs to 5 mm with 0.4 mm pitch BGAs with three different board finishes (ImAg, ENIG, ENEPIG) were evaluated.

The aging temperatures were 125oC, applied for a period of 6 months and 12 months. Subsequently, the specimens were thermally cycled from -40oC to 125oC with 15 min dwell times at the high temperature. The Weibull characteristic lifetime is dramatically reduced during isothermal aging at 125oC for SAC105/305 on a variety of finishes. In particular, SAC105 undergoes a considerable lifetime reduction during aging and illustrates the risk in using SAC105 solder balls in applications where thermal fatigue failure is a concern. IMC analysis showed the IMC is thicker at corner side solder balls compared to center balls in most cases. For the 0.8mm pitch 15mm package size BGA, the investigating of degradation charts shows that the characteristic lifetimes for both SAC105 and SAC305 decrease as: ENIG > ENEPIG > ImAg. However, the order is ENIG ≈ ENEPIG> ImAg for 0.4mm pitch 10mm package size BGA. In all cases, ENIG performs better than Immersion Ag for applications involving long-term isothermal aging. SAC305, with a higher relative fraction of Ag3Sn IMC within the solder, performs better than SAC105. SEM and polarized light microscope analysis show most cracks happened at package side, propagated from corner to center or even to solder bulk, which eventually cause fatigue failures. Three factors are discussed: IMC, Grain Structure and Ag3Sn particle. The continuous growth of Cu-Sn intermetallic compounds (IMC) and grains increase the risk of failure, while Ag3Sn particle seems helpful to block the crack propagation.