Effect of alloying W in Ni(5P) metallization of solder joints

As lead free solders are being implemented in commercial electronic goods, reliability of electroless Ni(P)/Pb-free solder joint has become an important issue because formation of Ni
₃
P and Ni
₃
SnP at solder/UBM interface increased the susceptibility to brittle cracking through these layers. In this report, composition of electroless Ni(P) under bump metallization(UBM) was modified by co-depositing W, and interfacial reactions between electroless Ni-5P-xW and Sn-3.5Ag solder were investigated using SEM and XRD. Results indicate that addition of W to the electroless Ni(5P) layer suppressed the formation of Ni
₃
Sn
₄
, increased the temperate of Ni
₃
P formation, and increased the crystallinity of Ni-5P-xW film.