Abstract
Zn addition to Cu under bump metallurgy (UBM) in the solder joint was the subject of this study. An alternative design was carried out to fabricate pure Sn as the solder and Cu-xZn (x=15 and 30 wt.%) as the UBM to form the reaction couple. As Zn increased from 15 to 30 wt.% in Sn/Cu-Zn system, the growth of both Cu3Sn and Cu6Sn5 could be suppressed. Besides, no Kirkendall void was observed at the interface in both Sn/Cu-Zn couples during heat treatment. After 40-day aging, different multilayered phases of [Cu6Sn5/Cu3Sn/Cu(Zn)] and [Cu6Sn5/Cu(Zn,Sn)/CuZn] formed at the interface of [Sn/Cu-15Zn] and [Sn/Cu-30Zn], respectively. Intermetallic compounds (IMCs) growth mechanism during aging was discussed and proposed on the basis of the composition variation in the joint assembly with the aid of electron microscopic characterization and Sn-Cu-Zn ternary phase diagram. According to these analyses of interfacial morphology and IMCs formation in Sn/Cu-Zn system, Cu-Zn may be a potential UBM for retarding the Cu pad consumption in solder joints. In the solid reaction of pure Sn solder and Cu-Zn alloy, Cu(Zn,Sn) and CuZn phases were formed adjacent to the Cu-Sn compounds in sequence. In this study, Cu(Zn,Sn) phase was precipitated inside Cu6Sn5 as the accumulation of Zn atoms in Cu6Sn5 reached the saturated concentration. Finally, Cu(Zn,Sn) phase would transform into the CuZn phase during annealing. The growth mechanism of CuZn phase was related to the suppression of the Kirkendall voids formation at the interface. In the liquid reaction of pure Sn on the different Cu-xZn (x= 0, 15 and 30 wt.%) substrates, Cu6Sn5 was the dominating product at each interface. After reflow at 250 oC, the Cu6Sn5 morphology was faceted pillar in the Sn/Cu-30Zn system and it grew along the specific direction with the extended wetting time. On the other hand, round and scallop-like Cu6Sn5 were formed in the Sn/Cu-15Zn and Sn/Cu systems, respectively. The phenomenon can be explained by Jackson’s parameter, which was used to correlate the morphology variation with the formation enthalpy. It was noted that the higher Zn concentration in the Cu6Sn5 led to the larger energy change in Cu6(Sn,Zn)5 formation. Moreover, the average thickness of total IMCs grew slowly as the Zn content in Cu-Zn substrate increased.