Investigation of thin small outline package (TSOP) solder joint crack after accelerated thermal cycling testing

Solder joint crack is a common failure mode of printed circuit board assembly (PCBA) for electronic products. In order to investigate the crack behavior of fine-pitch SMT solder joints, accelerated thermal cycling (ATC) up to 1500 cycles was performed on advanced PCBAs with low-profile thin small outline package (TSOP). The functional examination result shows that the failure rate of TSOP solder joint open after ATC is 2.5%, 7.5% and 27.5% after 1000, 1250 and 1500 cycles, respectively. Optical inspection by microscopy demonstrates obvious solder joint cracks at many locations. Cross sectional study has confirmed serious crack in the bulk solder, resulting in electrical failures. Also, microstructural information has been obtained by metallurgical analysis with the aid of Scanning electron microscopy (SEM). In addition, finite element analysis (FEA) was performed to simulate the thermal-mechanical stress in solder joints under ATC. Based on the modelling, the fatigue life of solder layer was calculated. Both experimental study and modelling confirmed that solder fatigue crack upon concentrated stress during ATC is a major contributor to the open failure of TSOP solder joints.