Early microstructural indicators of crack initiation in lead-free solder joints under thermal cycling

Abstract

The microstructural analysis of as-reflowed SAC305 solder joints showed a highly textured microstructure. During thermal cycling tests, the as-solidified microstructure gradually transformed into a less textured structure with a high population of misoriented grain boundaries through a recrystallization process. Ag3Sn IMCs coalescence is also another strong phenomenon of lead-free solder microstructure evolution since the bigger and the more spaced they are, the less dislocation pinning can prevent recrystallization from occurring. The main observation is that recrystallization accompanied by Ag3Sn IMCs coalescence are preliminary leading to intergranular propagation in the high strain regions. This work presents the Sn3.0Ag0.5Cu (SAC305) solder joints microstructural evolution at different thermal cycling levels. Electron Back Scattered Diffraction (EBSD) analysis was conducted to assess the SAC305 microstructure corresponding to a specific number of thermal cycles. Microstructural properties as $\beta$-Sn grain size and crystallographic orientation, grain boundary angles and the size of Ag3Sn intermetallic compounds (IMC) are investigated to characterize the different stages of microstructural changes under thermal cycling. Results show that crack initiation in chip resistors solder joints starts very early and precedes recrystallization process. However, solder joint lifetime is controlled by recrystallization and Ag3Sn IMCs coalescence which are the most important phenomena leading to the intergranular crack propagation.