Abstract: The solder joint reliability of a 0.5mm lead pitch, 240-pin quad flat package(QFP) was studied by nonlinear finite element analysis(FEA). The stress/strain distributions within the solder joints and the maximum plastic strain range of the solder joints were determined. Based on the calculated maximum plastic strain range the thermal fatigue life of the solder joints was calculated using Coffin-Manson equation. The influences of shape parameters including volume of solder joint, pad size and stand-off on the thermal fatigue life of the solder joints were also studied. The results show that the stress and strain distribution in the solder joint are not uniform; the interface between the lead and the solder joint is the high stress and strain region; the maximum stress and stain occur at the topmost point where the solder joint intersects with the inner side of the lead. The solder joint cracks should occur firstly at this point and propagate along the interface between the solder and the lead. The solder joint with the pad size of 1.25mm×0.35mm, the stand-off of 0.02mm and the solder volume of 0.026mm³ has longer fatigue life than that of any others. These optimal parameters have been applied in practice to assemble the 240-pin, 0.5mm pitch QFP.

Key words: fine pitch; solder joint shape; nonlinear finite element analysis; thermal fatigue life