High Sn solder reaction with Cu metallization

When Sn based solders or high Sn solders are reflowed on to thin film Cu metallization, the fast reaction between Cu and Sn can convert the Cu layer completely into an intermetallic layer. After the conversion, prolonged exposure of the intermetallic to solder melt can cause dissolution of Cu from the intermetallic to the bulk solder. Excessive dissolution of the intermetallic from the interface impairs the interface strength, since it results in solder dewetting at the exposed Cr layer (solder does not wet Cr surface). Utilization of Cu-Cr phased layer is not sufficient to prevent dewetting from occurring at high Sn solders.