Step Soldering Factors Affecting the Reliability of Ag-Pd Thick Film Conductor Pads

Abstract

One of the essential thick film materials on the hybrid circuit is the conductor which performs as an interconnection between discrete components. However, metallurgical reaction between the conductor and solder during the soldering or thermal aging/cycling may cause electrical failure and/or loss of adhesive strength due to the reaction of Ag/Pd with Sn and subsequent intermetallics formed. A step soldering process, which applies a high-Pb solder layer over the conductor layer before the eutectic or near-eutectic Sn-Pb solder is applied, has been shown to prevent contact between the conductor and Sn in the solder, as long as the high-Pb layer does not dissolve into the low-Pb layer. Experiments were carried out to quantify the isothermal interdiffusion reaction rate and the interface displacement between the 88Pb-10Sn-2Ag and the eutectic 63Sn-37Pb solder layers between 195 and 235°C. The results were analyzed by the methodology developed by Lommel and Chalmers (1959) which was based on Nernst-Brunner theory of dissolution (Moelwyn-Hughes, 1947). The displacement of the interface follows an Arrhenius relationship with an activation energy of 98 KJ/mole, which is close to the activation energy of solid Sn diffusion into Pb. A process window was constructed to show that temperature is a more sensitive parameter influencing the reaction rate than time. A mathematical model was applied to predict the interfacial displacement during the reflow process and was found to correlate well with experimental results.