MICROSTRUCTURES RESULTING FROM THE USE OF SOLDER BALLS OF ONE COMPOSITION WITH PASTE OF A DIFFERENT COMPOSITION ON A COPPER SUBSTRATE

摘要

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, 210°C and 220°C, while for samples made with SAC paste, the maximum temperatures used were 235°C and 245°C. 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 Ag_3Sn-Cu_6Sn_5-Sn, normally observed in SAC alloys, to Ag_3Sn-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 Cu_6Sn_5 crystals and Cu_6Sn_5-Sn binary eutectic regions. Similar experiments were also carried out using balls of the Sn-Zn-Al solder alloy.