Vibration fatigue of /spl mu/BGA solder joint

This paper studies the vibration fatigue failure of /spl mu/BGA solder-joints reflowed with different temperature profiles, and ageing at 120/spl deg/C for 1, 4, 9, 16, 25, 36 days. The effect of the thickness of the Ni/sub 3/Sn/sub 4/ 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 /spl mu/BGA 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/sub n/), which is defined as the integral of the measured temperature over the dwell time above liquidus (183/spl deg/C) in the reflow profile. The greatest lifetime occurs when Q/sub n/ is near 500 s/spl deg/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 Ni/sub 3/Sn/sub 4/ IMC and Cu/sub 6/Sn/sub 5//Cu/sub 3/Sn 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 Ni/sub 3/Sn/sub 4/ 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 Cu/sub 6/Sn/sub 5/-phase/bulksolder interface or the Cu/sub 3/Sn/Cu/sub 6/Sn/sub 5/ interface on component-metallization. Evidently, the intermetallic compounds contribute mainly to the fatigue failure of /spl mu/BGA 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.