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 e lectron microscopy (TEM). The interface was formed by reflowing the eutecti c 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), sel ected-area electron diffraction (SAD) convergent-beam electron diffraction (CBED), and bright- and dark-field imaging. In the as-reflowed condition, t he interfacial microstructure consisted of a thin Ni3Sn4 intermetallic laye r and a thin P-rich layer. The P-rich layer was composed of two phases, fac e-centered cubic (fcc) Ni and Ni3P, and the excess P was primarily due to t he ejection of P from the electroless Ni-P when it reacted with Sn in the r eflow process. Following the mild aging, a trilayer interfacial microstruct ure was found, including a coarsened Ni3Sn4 layer, a P-rich layer with incr eased P concentration, and a P-deficient layer. With overaging, a multilaye r interfacial microstructure was developed, which consisted of two Ni-Sn in termetallic layers, Ni3Sn4 and Ni3Sn2, and three distinct P-rich layers, Ni 12P5, Ni12P5 + Ni3P, and Ni3P + Ni.