Research on temperature gradient effect to solder joint reliability

In order to evaluate the solder joint reliability under different thermal treatment condition, the thermal cycling test was usually used as a benchmark method to evaluate the quality of different electronic products. However, in real case, the product would suffer power cycling more than the thermal cycling, particularly personal portable devices, like laptop computer, mobile phone, and tablet PC etc., which would often be on and off. For the size limitation, a cooling system will be used to dissipate the heat from chip by cooling the PCB in these kinds of products. It means a large temperature gradient will be set at two sides of solder joint. The temperature of die side would be hot, while the substrate side would be cool. The temperature gradient condition would affect the metal diffusion inside solder, change the intermetallic compound grain growth, and finally influence the solder joint reliability. A new test method was introduced in this paper. A temperature gradient would be applied on the two sides of BGA chip - heat at die side and cool at substrate side. Three solder materials - eutectic solder (EU), lead-free solder (LF) and high-lead solder (HL) were introduced and compared. And three thermal profiles are applied in this research. FEM simulation was also applied to compare the temperature gradient difference effect and the stress distribution of whole package. The cross-section figures of solder bump before and after the experiments are compared and observed by SEM and EDX. It found that the metal migration phenomenon in different solder materials and the cycling thermal loading would be the two key facts to make the crack occur easily.