Vibration fatigue of μBGA solder joint

This paper studies the vibration fatigue failure of μBGA solder-joints reflowed with different temperature profiles, and ageing at 120°C for 1, 4, 9, 16, 25, 36 days. The effect of the thickness of the Ni₃Sn₄ and Cu-Sn intermetallic compound (IMC) on the fatigue lifetime is also reported. During the vibration fatigue test, in order to identify the failure of μBGA solder joint, electrical interruption was monitored continuously through the daisy-chain network. Our results show that the fatigue lifetime of the solder joint firstly increases and then decreases with increasing heating factor (Q_n), which is defined as the integral of the measured temperature over the dwell time above liquidus (183°C) in the reflow profile. The greatest lifetime occurs when Q_n is near 500 s°C. Moreover, the lifetime of the solder joint decreases linearly with the increasing fourth root of the ageing time. The SEM/EDX inspection shows that only Ni₃Sn₄ IMC and Cu₆ Sn₅/Cu₃Sn IMCs are formed between the solder and the nickel-plated PCB pad, and the solder/component-metallization interface respectively. For non-aged samples reflowed with different profiles, the fatigue crack generally initiates at valleys in the rough surface of the interface of the Ni₃Sn₄ IMC with the bulk solder. Then it propagates mostly near the Ni/solder, and occasionally in the IMC layer or along the Ni/solder interface. For aged samples, the fatigue crack mostly initiates and propagates in the Cu₆Sn₅-phase/bulksolder interface or the Cu₃ Sn/Cu₆Sn₅ interface on component-metallization. Evidently, the intermetallic compounds contribute mainly to the fatigue failure of μBGA solder joints. The thicker the IMC layer, the shorter the fatigue lifetime of solder joint. The initial formation of the IMCs at the interface during soldering ensures a good metallurgical bond between the solder and the substrate. However, a thick IMC layer influences the solder joint strength, which results in mechanical failure due to volume shrinkage during the transformation from solid phase to the intermetallic compound