Inhibiting the formation of (Au_1−xNi_x)Sn₄ and reducing the consumption of Ni metallization in solder joints

Abstract

The effects of adding a small amount of Cu into eutectic PbSn solder on the interfacial reaction between the solder and the Au/Ni/Cu metallization were studied. Solder balls of two different compositions, 37Pb-63Sn (wt.%) and 36.8Pb-62.7Sn-0.5Cu, were used. The Au layer (1 ± 0.2 µm) and Ni layer (7 ± 1 µm) in the Au/Ni/Cu metallization were deposited by electroplating. After reflow, the solder joints were aged at 160°C for times ranging from 0 h to 2,000 h. For solder joints without Cu added (37Pb-63Sn), a thick layer of (Au_1−xNi_x)Sn₄ was deposited over the Ni₃Sn₄ layer after the aging. This thick layer of (Au_1−xNi_x)Sn₄ can severely weaken the solder joints. However, the addition of 0.5wt.%Cu (36.8Pb-62.7Sn-0.5Cu) completely inhibited the deposition of the (Au_1−xNi_x)Sn₄ layer. Only a layer of (Cu_1-p-qAu_pNi_q)₆Sn₅ formed at the interface of the Cu-doped solder joints. Moreover, it was discovered that the formation of (Cu_1-p-qAu_pNi_q)₆Sn₅ significantly reduced the consumption rate of the Ni layer. This reduction in Ni consumption suggests that a thinner Ni layer can be used in Cu-doped solder joints. Rationalizations for these effects are presented in this paper.