Vapor pressure assisted void growth and cracking of polymeric films and interfaces

Pores and cavities form at filler particle-polymer matrix interfaces, at polymer film-silicon substrate interfaces as well as in molding compounds of IC packages. Moisture diffuses to these voids. During reflow soldering, surface mount plastic encapsulated devices are exposed to temperatures between 210 to 260 degreesC. At these temperatures, the condensed moisture vaporizes. The rapidly expanding water vapor can create internal pressures within the voids that reach 3 - 6 MPa. These levels are comparable to the yield strengths of epoxy molding compounds and epoxy adhesives, whose glass transition temperatures T-g range between 150 to 300 degreesC. Under the combined action of thermal stress and high vapor pressure ( relative to the yield strength at T-g), both pre-existing and newly nucleated voids grow rapidly and coalesce. In extreme situations, vapor pressure alone could drive voids to grow and coalesce unstably causing film rupture, film-substrate interface delamination and cracking of the plastic package. Vapor pressure effects on void growth have been incorporated into Gurson's porous material model and a cohesive law. Crack growth resistance-curve calculations using these models show that high vapor pressure combined with high porosity bring about severe reduction in the fracture toughness. In some cases, high vapor pressure accelerates void growth and coalescence resulting in brittle-like interface delamination. Vapor pressure also contributes a strong tensile mode component to an otherwise shear dominated interface loading. An example of vapor pressure related IC package failure, known as "popcorn" cracking, is discussed.