Thermal cycling induced interconnect stability degradation mechanism in low melting temperature solder joints

With the increase of interest in low melting temperature solder alloys, in recent studies on Sn-Bi based system solder show relatively good thermal cycling performances comparable to conventional Sn-Ag-Cu based solder interconnects at a given thermal cycling profile. Sn-Bi eutectic system microstructures are similar to Sn-Pb eutectic microstructure but have different damage accumulation mechanism due to Bi crystal lattice with Rhombohedral A7 unit cell structure, which is less ductile compared to Sn-Pb, where Pb has face centered cubic crystal lattice. The nature of less ductility in Sn-Bi alloy system reveals a different damage accumulation process during thermal cycling compared to Sn-Ag-Cu solder material, although the thermal cycling performance is comparable with micro-elementalloying. To identify the degradation mechanism in Sn-Bi solder interconnects, the study presented here is a series of microstructure analysis on segmented thermal cycling completed components, which reveal gradual and localized microstructure evolution. 12x12 mm2 chip array BGA (CABGA) components were thermal cycled with a -40 to 100°C cycle profile and a 10min dwell time. The microstructure developments per component were analyzed with 200-250 cycles interval cross-sections until both Sn- Ag-Cu and Sn-Bi solder joints reached to full failure. The correlation between crack initiation, crack propagation and localized recrystallization were compared in a series of cross section analyses using polarized imaging and Electron- backscattered diffraction (EBSD) based strain and residual stress analysis. The analysis revealed the potential damage accumulation process in Sn-Bi solder joint under thermal cycling, which is discussed in this paper.