Reliability studies μBGA solder joints-effect of Ni-Sn intermetallic compound

This paper studies the bending and vibration effects on the fatigue lifetime of ball grid array (BGA) solder joints. The correlation between the fatigue lifetime of the assembly and the heating factor (Q _n), defined as the integral of the measured temperature over the dwell time above liquidus (183°C) in the reflow profile is discussed. Our result shows that the fatigue lifetime of μBGA solder-joints firstly increases and then decreases with increasing heating factor. The optimal heating factor Q_n is found to be 300-680°C. In this range, the assembly possesses the greatest fatigue lifetime under various mechanical periodic stress, vibration and bending tests. The cyclic bending cracks always initiate at the point of the 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 bulk 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. Evidently, the Ni-Sn IMC contributes mainly to the fatigue failure of the μBGA solder joints. The SEM and EDX inspection show that only Ni₃Sn₄ IMC forms between 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 into the solder joint during reflowing. The formation of the Ni₃Sn₄ IMC during soldering ensures a good metallurgical bond between the solder and the substrate. However, a thick Ni-Sn IMC influences the joint strength, which results in mechanical failure. Based on the observed relationship of the fatigue lifetime with Ni-Sn IMC thickness and Q_n, the reflow profile should be controlled with caution in order to optimize the soldering performance