Intermetallic compound growth on Ni, Au/Ni, and Pd/Ni substrates with Sn/Pb, Sn/Ag, and Sn solders [PWBs]

The growth mechanism of the Ni₃Sn₄ intermetallic compound (IMC) during aging was studied with three different solders (100Sn, Sn-3.5Ag, and Sn-37Pb) on three different substrates (Ni, Ni/Au, and Ni/Pd), at the temperatures of 75, 100, 125, and 160°C from 1 to 36 days. The growth rates of Ni₃Sn ₄ with Sn on Ni and Ni/Au substrates were similar, growing to about 6 μm after 36 days at 160°C, but only to about 1-2 μm after 36 days at a temperature below 100°C. The growth rate of Ni ₃Sn₄ with Sn-37Pb on Ni/Au substrate was close to that with Sn for the same substrates. However, the Sn-3.5Ag solder showed a slower growth rate of Ni₃Sn₄ on both Ni and Ni/Au substrates, resulting in only about half the thicknesses when compared to Sn on the same substrates. In addition to the Ni₃Sn₄ compound, a PdSn₄ compound was observed on the NiPd substrates. The growth rate of Ni₃Sn₄ on the Ni/Pd substrate is much slower than that on either the Ni or the Ni/Au substrate, possibly due to the existence of the PdSn₄ layer between Ni and the solder. At temperatures lower than 100°C, there is hardly any Ni₃Sn ₄ detected for Sn-3.5Ag and Sn-37Pb solders for up to 36 days. The apparent activation energies, Q, are in the range of 3 to 12.8 Kcal/mole, and Q for Ni₃Sn₄ with Sn is the highest for the three solders on both the Ni and Ni/Pd substrates, and those for Sn-3.5Ag the lowest. However, Q for Ni₃Sn₄ growth with Sn-3.5Ag is the highest on the Ni/Au substrate. A thick Ni₃ Sn₄ layer may pose potential reliability issues as evidenced by the fractured morphology of the intermetallics due to a 10.7% volume shrinkage during the transformation from solid phase Sn and Ni to the Ni₃Sn₄ compound