Effects of phosphorus content on the reaction of electroless Ni-P with Sn and crystallization of Ni-P

The reaction between electroless Ni-P and Sn and the crystallization behavior of Ni-P were investigated to better understand the effect of P content on the Ni-P layer. Electroless Ni-P specimens with three different P contents, 4.6 wt.%, 9 wt.%, and 13 wt.%, were used to study the effect of the P content and the microstructure of Ni-P on the subsequent crystallization and intermetallic compound (IMC) formation during the reaction between Ni-P and electroplated Sn. Ni₃Sn₄ was the major phase formed in all samples heated up to 300°C, which totally transformed into Ni₃Sn₂ when samples were heated up to 450°C and the Sn layer was 0.5-μm thick. The IMC formed on the nanocrystalline Ni-P showed stronger texture compared to that formed on the amorphous Ni-P. Both the IMC thickness and density decreased with P content in the Ni-P layer, and Ni₃Sn₄ morphologies varied with P content. Dissolution of Ni into Sn increased with P content, which made IMC size in the bulk Sn increase with P content.