The investigation of dopants on SAC solder for board level reliability

The benefits of using the low Ag solder in CSP package has been known in terms of the performance due to its intrinsic characteristic. In this study, SAC alloy dopanted D1 and D2 are selected to compare with the general solder alloy, SAC105, in terms of thermal fatigue fracture performance. The 0.5mm-pitch TFBGA packages on Halogen-free FR4 PCB treated with 150 C thermal aging at different period of time and -40/125°C thermal cycling aging at various cycle are examined so as to observe the microstructure evolution of different solder joints. In this study, the polarized image and orientation image microscope are applied to observe the phenomena of Sn grain recrystallization and the statistic method are used to assist in getting a more credible analysis result. Orientation image microscope offers possibilities for characterizing cracks in solders and can unambiguously identify them as intergranular or transgranular after the thermal cycling test. Statistical analysis of grain boundaries can identify the types of grain boundaries and orientation variations that may be assistant or resistant to cracking. The package array was divided into four regions, outer-corner, outer-edge, inter-corner and inter-edge. Experimental result shows that the solder joints are transformed obviously from single-grain to multi-grain when the thermal cycles increased while thermal aging has a relatively small impact on the grain transition. The polarized image mapping shows the outer solder joint recrystallization degree is more evident relatively inter solder joint, and the corner solder joint recrystallization degree is more evident than edge solder joint, no matter the low Ag solder with Dl or D2 dopanted. Compared with the ratio of single to multi grain transformed on Dl and D2 dopanted, the polarized images show that the ratio is increased obviously in the D2 dopanted while the Dl dopanted doesn't have significant transformation. According to the cross-section analysis result, we can find the fatigue fracture on solder balls that mostly happen on solder bulks, and recrystallization area exhibit apparent intergranular fatigue crack via the polarized image can be observed.