Reliability of large conductive polymer flip chip assemblies for multichip modules (MCMs)

Work has already been reported on two thermoset adhesives for solder replacement flip chip attachment using both gold and copper flip chip test vehicles. The one drawback with any thermoset material is that rework is difficult should there be a failure after assembly so precise alignment is necessary. Thermoplastic material offers the advantage of reworking. This paper discusses work with thermoplastic epoxy. However, the new material being tested is so different from the previously tested conductive pastes that new fabrication and assembly procedures had to be developed. Initial work has begun in evaluating the performance of thermoplastic epoxy. Not only is this thermoplastic paste viscous enough to prevent smearing during the stencil printing process, but also rigid enough to create 2 mil height bumps by stencil printing twice onto the same substrate. Some disadvantages of this paste is that processes after the bumps are applied must be performed at low temperatures and the paste appears to be more compatible with palladium than gold