Real-time scanning electron microscopy and auger spectroscopy of wetting in Sn-3.3Ag-4.8Bi solder paste wetted to Ni-Au

Abstract

Surface and thin film techniques reveal an immediate and intense surface reaction during wetting of Sn-3.3Ag-4.8Bi solder paste to Ni-Au board finishes. Rutherford backscattering spectroscopy (RBS) shows that a typical, vendorsupplied Ni-Au board finish consists of ϳ750 Å of Au above the Ni. Stylus profilometry indicates that the finish surface is rough with average vertical height variations of ϳ1-2 µm. The board finish seen by the solder alloy during wetting contains a large ϳ45% atom fraction of adventitious surface C observed by Auger spectroscopy (AES). The AES and energy-dispersive x-ray spectroscopy (EDX) indicates that Ni has fully intermixed throughout the alloy within seconds of wetting. Dissolution of the Au cap layer into the alloy and Bi and Ag surface segregation is observed. Auger point analysis and maps at the leading edge of the wetting front show no evidence of flux advance ahead of the alloy. Bismuth does not move out with the spreading wetting front. A unique feature of this work is the acquisition of real-time, in-situ electron microscopic observations of the spreading Sn-3.3Ag-4.8Bi alloy front across the Ni-Au substrate during wetting. The observations have been recorded on videotape and converted into computer files accessible on the World Wide Web with standard media players (ftp: //131.204.44.20).