Spalling behaviors of intel-metallic compounds during the etting reaction of Sn(3.5Ag) on electroless Ni-P metallization

Abstract

Intel-metallic 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 Sn 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.