Thermal and vapor pressure effects on cavitation and void growth

Abstract

Vapor pressure and thermal stress induced growth and coalsecence of microvoids near interfaces of die/die-attach and die-pad/moulding compound is recognised as the precursor to interface delamination and popcorn cracking of plastic electronic packages. When exposed to humid ambient conditions prior to reflow soldering, plastic electronic packages absorb moisture, which condenses within the numerous micropores in the substrate, die attach, moulding compounds and in the vicinity of interfaces joining the different material layers. During reflow soldering, moisture in these micropores is vaporized. A representative material cell containing a single microvoid is used to investigate void growth under combined vapor pressure and thermal stress. A critical surface traction, the sum of the internal vapor pressure and the externally applied stress, defines the onset of unstable void growth. Existence of “large” microvoids lowers significantly the critical stress levels for unstable void growth. These stress levels are consistent with generally accepted estimates of stress levels for popcorn cracking given in the literature. Vapor pressure effects can be incorporated into a continuum description of stresses and strains and the Gurson-Tvergaard void growth model providing a new capability for full-field analysis of popcorn cracking in electronic packages.