Development of a solder material process to relieve the plastic constraint associated with thin joints

Abstract

High aspect ratio (large diameter/thickness) solder joints which are plastically constrained develop large hydrostatic stresses (Friction Hill) greatly in excess of their yield strength. Because the local high triaxial stresses arising from the Friction Hill prevent homogeneous yielding and, in a strain controlled system, will localize plastic deformation within the regions near free surfaces, abrupt brittle fracture through an intermetallic or along an interface can occur. In such situations, the service life of the joint during fatigue situations such as thermal cycling will be greatly reduced. The prevention of triaxial stress build up within such a strain controlled environment which can occur in, for example, leadless chip carrier solder joints requires a distribution of internal free surfaces within the joint. The solder system developed in this study is a thin porous metal film with a regular distribution of pores. The solder material is formed from the usual components, tin and lead. Small lead or tin particles are coated with a thin film of the other component, mixed with flux paste, and the temperature is raised to just above the eutectic temperature. Solid state diffusion occurs across the lead-tin interface until its composition reaches the melting point. The particles then are interconnected by a thin near eutectic liquid film. Additional metal from the solid particle dissolves into the liquid increasing its position and, thus its melting point. Diffusion into the liquid continues until it solidifies isothermally. This forms an interconnecting network of solder “mini-elements” with a dense pore structure.