Effect of volume in interfacial reaction between eutectic Sn–Pb solder and Cu metallization in microelectronic packaging

Eutectic Sn–Pb is historically the most important solder alloy, thus its reactions will serve as relevant references for Pb-free solder substitutes. Here, an investigation has been carried out to compare the dissolution kinetics of the Cu pad of the ball grid array (BGA) substrate with the molten conventional eutectic Sn–Pb solder having different volumes. BGA solder ball of 760 and 500 μm of diameter were used on the Cu pad having a circular area with a diameter of 650 μm. A scanning electron microscope was used to measure the consumed thickness of Cu and also the thickness of the intermetallic compound. The soldering reaction was carried out at 190, 200 and 210 °C for 1, 5, 10 and 20 min. The Cu consumption was much higher for the Sn–Pb solder with higher volume. On the other hand, the mean thickness of the intermetallics for solder with smaller volume was thicker than that of the bigger solder balls. No significant difference was observed of solder volume on the ripening/coarsening flux of soldering reaction. Cu3Sn compound was also observed at the interface in between Cu6Sn5 IMCs and Cu substrate for longer reflow for the smaller solder balls. Larger Cu6Sn5 IMCs are observed in the bulk of the solder with bigger volume.