Spalling behaviors of intermetallic compounds during the wetting reaction of Sn(3.5Ag) on electroless Ni-P metallization

Intermetallic compound (IMC) spalling from electroless Ni-P film was investigated with lead-free solders (Sn-3.5 wt.% Ag and pure Sn) in terms of solder deposition method (electroplating, solder paste and solder preform) and P content in Ni-P layer (4.6, 9, and 13 wt.% P). The reaction of Ni-P with Sn3.5Ag paste easily led to IMC spalling after 2min reflow at 250 °C while IMCs adhered to the Ni-P layer after 10min reflow with electroplated Sri or Sn3.5Ag. The IMC spalling in Sn3.5Ag preform was moderate. The spalling increased with P content in the Ni-P layer. Ni₃Sn₄ intermetallics formed as a needle-shaped morphology in an early stage and changed into a chunky shape. Needle-shaped compounds exhibited a higher propensity for spalling from the Ni-P layer than the chunky shaped because a molten solder can easily penetrate into the interface between the 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 is attributed to the IMC spalling. Dewetting of solder from the NiSnP layer, however, didn´t occur even after spalling of most IMCs.