The effect of pretinning on the strength of a chip carrier solder joint

Abstract

Pretinning of gold-plated components is often done to avoid the formation of brittle AdSn intermetallic compounds, which may lower the mechanical strength of the joints. In this investigation the reliability of 32LCCs surface mounted on alumina boards is tested in internal and external temperature cycling, as well as in shear tests, and the effect of pretinning is studied. One thousand times of temperature cycling between -55 and +125"C and 2000 h of power cycling with an upper joint temperature of +75"C did not cause any electrical opens.Shear tests made after a 168 h heat treatment at +125"C and 1000 temperature cycles showed that forces of approximately 1000 N were required to break the chip camer joints. The microstructure of Sn62tPb36tAg2 solder joints containing gold was examined and the existing phases were identified using electron diffraction X-ray analysis. The joints fail at the AdSn intermetallics at the chip carrier metallization or at the CdSn compounds formed at the substrate thick filmtsolder interface. No deterioration of the overall joint strength due to AdSn intermetallics was seen, when the gold content in the joints was approximately 4 weight per cent.