Study of Spalling Behavior of Intermetallic Compounds during the Reaction between Electroless Ni-P Metallization and Lead-free Solders

Electroless Ni-P has been widely used for under bump metallization (UBM) of flip chip and surface finish layer in microelectronics packaging because of its excellent solderability, corrosion resistance, uniformity, selective deposition without photolithography, and also good diffusion barrier. However, the brittle fracture of solder joints and the spalling of intermetallic compound (IMC) associated with electroless Ni-P are critical issues for its successful applications. The mechanisms of the brittle fracture have been extensively investigated, but not fully understood yet. On the contrary, the spalling phenomenon has not been studied in detail, though IMC spalling could weaken the interfacial adhesion and also cause dewetting of flip chip solder bumps.

In this study, IMC spalling from an electroless Ni-P film was investigated with lead-free solders in terms of solder deposition methods (electroplating vs solder paste), P content in Ni-P layer (4.6, 9, and 13 wt.% P), and solder thickness (120 vs. 200µm). The reaction of Ni-P with Sn3.5Ag paste easily led to IMC spalling after 2 min reflow at 250°C while IMCs adhered to the Ni-P layer after 10 min reflow for the reaction with electroplated Sn or Sn3.5Ag. It has shown that both solder composition and deposition method are important for IMC spalling from a Ni-P layer. The spalling increased with P content in a Ni-P layer as well as with solder volume. Ni3Sn4 intermetallics formed as a needle-shaped morphology in the early stage and changed into a chunky shape. Needleshaped compounds exhibited a higher propensity for spalling from aNi-P layer than chunky shaped compounds because molten solder can easily penetrate into the interface between needle-shaped IMCs and the P-rich layer. A reaction between the penetrated Sn and the P-rich layer formed a NiSnP layer. The poor adhesion between the Ni3Sn4 compound and the NiSnP layer caused the IMC spalling. Dewetting of solder from the NiSnP layer, however, didn’t occur even after spalling of most IMCs. The wetting reactions of Pb-free solders on electroless Ni-P layers are also discussed.