Thermal stability of electroless-nickel/solder interface: Part A. Interfacial chemistry and microstructure

The thermal stability of the interface between the Sn-Pb eutectic alloy and an electroless Ni-P coating was examined by cross-sectional transmission electron microscopy (TEM). The interface was formed by reflowing the eutectic Sn-Pb solder alloy between electroless Ni-P deposits. The microchemistry and microstructure of the interface were analyzed in the as-reflowed, mild-aged, and overaged conditions, by energy-dispersive spectroscopy (EDS), selected-area electron diffraction (SAD) convergent-beam electron diffraction (CBED), and bright- and dark-field imaging. In the as-reflowed condition, the interfacial microstructure consisted of a thin Ni₃Sn₄ intermetallic layer and a thin P-rich layer. The P-rich layer was composed of two phases, face-centered cubic (fcc) Ni and Ni₃P, and the excess P was primarily due to the ejection of P from the electroless Ni-P when it reacted with Sn in the reflow process. Following the mild aging, a trilayer interfacial microstructure was found, including a coarsened Ni₃Sn₄ layer, a P-rich layer with increased P concentration, and a P-deficient layer. With overaging, a multilayer interfacial microstructure was developed, which consisted of two Ni-Sn intermetallic layers, Ni₃Sn₄ and Ni₃Sn₂, and three distinct P-rich layers, Ni₁₂P₅, Ni₁₂P₅+Ni₃P, and Ni₃P+Ni.