MICROSTRUCTURES RESULTING FROM THE USE OF SOLDER BALLS OF ONE COMPOSITION WITH PASTE OF A DIFFERENT COMPOSITION ON A COPPER SUBSTRATE
Authors: L. Snugovsky, et al. Company: University of Toronto and Celestica Inc. Date Published: 5/16/2008
ICSR (Soldering and Reliability)
Abstract: With the development and use of a variety of Pb-free solders, it is probable that some solder joints in electronic assemblies may be made with solders of two different compositions, one of which may be eutectic Pb-Sn. To investigate some possible resulting microstructures, small balls of four different SAC Pb-free solders were each melted with eutectic Pb-Sn solder paste, as well as with various SAC pastes, on a copper substrate. Two paste levels were used, 20 and 29 weight percent of the total sample weight. In all cases, the sample was held for 90 seconds at a temperature above the melting point of the solder paste. For samples made with PbSn paste, two different values of maximum temperature were used, 210oC and 220oC, while for samples made with SAC paste, the maximum temperatures used were 235oC and 245oC. The resulting microstructures were evaluated metallographically using both optical and scanning electron microscopes. It was observed that the use of Pb-Sn solder paste introduced some Pb-Sn eutectic microstructure and changed the ternary eutectic present from Ag3Sn-Cu6Sn5-Sn, normally observed in SAC alloys, to Ag3Sn-Pb-Sn. The high Sn content of the SAC alloys used led to rapid dissolution of the copper substrate, often resulting, on cooling, in formation of primary Cu6Sn5 crystals and Cu6Sn5-Sn binary eutectic regions. Similar experiments were also carried out using balls of the Sn-Zn-Al solder alloy.