Reliability studies of mu BGA solder joints - Effect of Ni-Sn intermetallic compound

This paper studies the bending and vibration effects on the fatigue litetim e of (ball grid array (BGA) solder joints. The correlation between the Fati gue lifetime of the assembly and the heating factor (Q(n)), defined as the integral of the measured temperature over the dwell time above liquidus (18 3 degreesC) in the reflow profile is discussed. Our result shows that the f atigue lifetime of mu BGA solder-joints firstly increases and then decrease s with increasing heating factor. The optimal heating factor Q(n) is found to be 300-680 s degreesC. In this range, the assembly possesses the greates t fatigue lifetime under various mechanical periodic stress, vibration and bending tests. The cyclic bending cracks always initiate at the point of th e acute angle where the solder joint joins the PCB pad, and then propagate in the site between the Ni-Sn intermetallic compound (IMC) layer and the bu lk solder. Under the vibration cycling, it is found that the fatigue crack initiates at valleys in the rough surface of the interface of the Ni-Sn IMC with the bulk solder. Then it propagates mostly near the Ni-Sn IMC layer, and occasionally in the IMC layer or along the IMC/nickel interface. Eviden tly, the Ni-Sn IMC contributes mainly to the fatigue failure of the mu BGA solder joints, The SEM and EDX inspection show that only Ni3Sn4 IMC forms b etween the tin-based solder and the nickel substrate. Moreover, no brittle AuSn4 is formed since all the Au coated on the pad surface is dissolved int o the solder joint during reflowing. The formation of the Ni3Sn4 IMC during soldering ensures a good metallurgical bond between the solder and the sub strate, However, a thick Ni3Sn4 IMC influences the joint strength, which re sults in mechanical failure. Based on the observed relationship of the fati gue lifetime with Ni-Sn IMC thickness and Q(n), the reflow profile should b e controlled with caution in order to optimize the soldering performance.