Solder/Substrate Interactions between Lead-Free Solders and CuNi Alloys

ABSTRACT Dissolution rate into lead-free solders and intermetallic growth rate during soldering of Cu, Ni, and CuNi-alloys were studied. Immersion tests were made in lead-free and eutectic PbSn solder baths, and it was seen that the dissolution rate into the solder was significantly faster for the lead-free solders than for PbSn. In the immersion tests, the fraction of the substrate converted into intermetallic was small compared to the amount consumed by dissolution. The dissolution and intermetallic growth depend on the relative amounts of solder and substrate material. When there was a large amount of solder available, thick intermetallic layers did not have a chance to grow. When there was a small amount of solder available compared to the size of the substrate, thick intermetallic layers (up to 15 microns after 1minute reflow at 250°C) were observed. This was attributed to fast saturation of the solder, after which dissolution into solder decreases and fast intermetallic growth can occur.