Abstract: The crack growth in solder joint of SMT during thermal fatigue process is investigated by experimental method. Two kinds of land shapes and two kinds of solders, including lead-tin eutectic solder and lead-free solder (SAC305), are selected. In order to investigate the crack growth in solder joint, the solder joint is sliced and the crack on the cross section of solder joint is observed. Some conclusions are drawn: During thermal fatigue process the solder joint experiences two stages of crack growth, including crack initiation and crack propagation, in which the period of crack initiation is shorter. In the late period of thermal fatigue, crack goes through the solder joint, to fail. And there are two kinds of crack growth modes observed in the experiment. The crack growth mode of lead-free solder joint is obviously different from that of lead-tin solder joint. Experimental results also show that the solder joint with small land shape has a relatively poor thermal fatigue resistance and the lead-free solder joint of SAC305 has a thermal fatigue resistance better than that of lead-tin solder joint.

Key words: Crack, Solder joint, Surface mounted structure, Thermal fatigue