Modeling vapor pressure effects on void rupture and crack growth resistance

The phenomenon of vapor pressure assisted void growth and rupture is studied. Plastic electronic packages absorb moisture which condenses within numerous micropores in the substrate, solder mask and die attach materials as well as near their interfaces. During reflow soldering, the condensed moisture vaporizes with the result that these micropores as well as interfaces are subjected to high vapor pressure. Under extreme conditions, our study suggests that vapor pressures can attain high enough levels to drive the voids to grow to rupture, thereby causing package failure. Under other conditions, residual/thermal stresses assisted by vapor pressure can cause crack growth within the polymeric materials as well as along interfaces. Vapor pressure effects on void growth have been incorporated into the Gurson model for porous ductile material. Using this model, a finite element study shows that the combination of high vapor pressure and high porosity is very detrimental to fracture toughness.