Effect of intermetallic compounds on vibration fatigue of μBGA solder joint

This paper studies the vibration fatigue failure of μBGA solder-joints reflowed with different temperature profiles, and aging at 120 °C for 1, 4, 9, 16, 25, 36 days. The effect 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_η), 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_η is near 500s°C. Moreover, the lifetime of the solder joint decreases almost linearly with the increasing fourth root of the aging time. The SEM/EDX inspection shows that Ni₃Sn₄ IMC and Cu₆Sn₅/Cu₃Sn IMCs are formed at the interface of the solder/nickel-plated PCB pad, and the no-aging solder/component-metallization, respectively. And during long term aging, Ni₃Sn₂ and NiSn were found at the Ni/Solder interface with X-ray diffraction, except Ni₃Sn₄. For nonaged solder joint, the fatigue crack generally initiates at the interface between the Ni₃Sn₄ IMC and the bulk solder. Then it propagates mostly near the Ni/solder, and occasionally in the LMC layer or along the Ni/solder interface. After aging, the fatigue track mostly initiates and propagates in the Cu₆Sn ₅ -phase/bulk-solder interface or the Cu₃Sn/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 toughness and strength of the solder joint, which results in mechanical failure