Abstract
This paper studies the vibration fatigue failure of /spl mu/BGA solder-joints reflowed with different temperature profiles, and aging at 120 /spl deg/C for 1, 4, 9, 16, 25, 36 days. The effect 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 /spl eta//), 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 /spl eta// is near 500s/spl deg/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 Ni/sub 3/Sn/sub 4/ IMC and Cu/sub 6/Sn/sub 5//Cu/sub 3/Sn 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, Ni/sub 3/Sn/sub 2/ and NiSn were found at the Ni/Solder interface with X-ray diffraction, except Ni/sub 3/Sn/sub 4/. For nonaged solder joint, the fatigue crack generally initiates at the interface between the Ni/sub 3/Sn/sub 4/ 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 Cu/sub 6/Sn/sub 5/ -phase/bulk-solder 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 toughness and strength of the solder joint, which results in mechanical failure.
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