Investigation of recrystallization in BGA package to evaluate the low Ag solder with different dopants after board level test

The benefits shown in the reliability performance when using the low Ag solder in BGA package has been known due to its intrinsic characteristic. In this study, SAC alloy with the dopant D1 and D2 respectively are selected to compare with the widely-used solder alloy, SAC105, from the perspective of the thermal fatigue fracture. The 0.5mm-pitch TFBGA packages on Halogen-free FR4 PCB pre-treated at 150°C for two different durations are examined so as to observe the microstructure evolution of different solder joints. After then, the board level tests, TCT and the drop test, are applied to the samples based on the specifications of IPC-9701 and JESD22-B111 respectively. The polarized image and orientation image microscope are utilized to observe the phenomena of Sn grain recrystallization, and the statistic method is used to assist in getting a more credible analysis result. The TCT result shows that the solder with D1 dopant has the worst performance, while the drop test does not reveal significant difference between three types of solders. The effect of thermal aging on the drop life is more apparent than on the TCT life. The reason of life reduction brought forth by aging is atom diffusion on interface between the IMC layer and solder bulk, which the driving forces are activation energy from the aging and the porosity existence from the reflow process. The experimental evidence shows that the solder joints are transformed obviously from single-grain to multi-grain when the thermal cycles are increased, while thermal aging has a relatively small impact on the grain transition. Although the aging and thermal cycling both provides the activation energy to transform the solders from single-grain to multi-grain, the TCT also induces the PCB warpage, the external stress which is another contribution to the grain transformation. The polarized images show the outer solder joint recrystallization degree is more evident than inner solder joint in all three experiment- - al solders. When the ratio of transformation from single to multi grain in D1-doped and D2-doped solders is investigated, the polarized image mapping shows that the ratio is increased obviously in the D2-doped solder while the D1-doped solder doesn´t have significant transformation. That is because the D2-doped solder has lower reduction in thermal supercooling than D1, which is verified via the DSC analysis result. It is known that the heterogeneous IMCs which are produced from freezing in alloy will make the formation and slip of dislocation easily when a minor stress is applied. According to the fractography and metallography observed, the evidence shows that the recrystallization area occurs at the locations of strong lattice curvature and new crystals appear at the locations of high lattice strain energy. Besides, at high temperatures and low strain rates, the grain boundary loses their strength more rapidly than do the crystals. So we can find the fatigue crack happened on solder bulks where is close to IMC layer and exhibit apparent intergranular fracture.