Initial interfacial reaction layers formed in Sn–3.5Ag solder/electroless Ni–P plated Cu substrate system

Analytical electron microscopy (AEM) was used to examine the initial interfacial reaction layers between a eutectic Sn–3.5Ag solder and an electroless nickel-immersion gold-plated (ENIG) Cu substrate during reflow at 255 °C for 1 s. AEM confirmed that a thick upper (Au,Ni)Sn₂ layer and a thin Ni₃Sn₄ layer had formed through the reaction between the solder and ENIG. The amorphous electroless Ni(P) plated layer transformed into two P-rich Ni layers. One is a crystallized P-rich Ni layer, and the other is an intermediate state P-rich Ni layer before the crystallization. The crystallized P-rich layer consisted of Ni₂P and Ni₁₂P₅. A thin Ni₂P layer had formed underneath the Ni₃Sn₄ layer and is believed to be a predecessor of the Ni₂SnP ternary phase. A Ni₁₂P₅ phase was observed beneath the Ni₂P thin layer. In addition, nanocrystalline Ni was found to coexist with the amorphous Ni(P) phase in the intermediate state P-rich Ni layer.