Soft solders are used instead of hard solders in applications were there are large coefficient of thermal expansion mismatches between semiconductor die and leadframes. A hard solder can cause die cracking due to stresses built up in the assembly. Examples of this are 65Sn/25AG/10Sb and 80Au/20Sn. Cracking is generally prominent with larger die.
The other place stress can build up is in the solder itself. Brittle alloys, such as bismuth solders can not handle the stress of some applications. Thicker solder interfaces can help alleviate some of the stress – but choosing a good solder like 92.5Pb/5Sn/2.5Ag is the best way to fix the problem.