Thermal fatigue life analysis and forecast of PBGA solder joints on the flexible PCB based on finite element analysis

Thermal fatigue life of PBGA (plastic ball grid array) solder joint on the FPC (flexible printed circuit) was analysed based on finite element analysis. According to symmetry theory, a quarter finite element model of 144-PIN PBGA was established. Both the stress and strain of lead and lead-free PBGA solder joints on the flexible PCB with the basic material of polyimide were studied by non-linear finite element analysis (FEA) respectively under -55~125degC thermal cycling. Based on the plastic strain calculated by FEA and Coffin-Manson formula, the thermal fatigue life of key lead and lead-free solder joints on the flexible PCB was calculated respectively. The results show that: (1) In the process of thermal cycling loading, the outside solder joint which is the farthest away from the inter-connect center of PBGA undergoes the largest alternating stress and strain; (2) Temperature has great impact on stress and strain in solder joint, and in the process of temperature remaining, the stress and strain has little change, however, it arise rapidly in the process of temperature decreasing while drops fast in heating-up period. (3) Plastic strain of dangerous point is accumulated in the process of temperature cycling, which will ultimately cause solder joint failure. (4) Compared with the lead-free solder joints, the lead ones of PBGA obviously have the longer fatigue life.