MODEL AND ANALYSES FOR SOLDER REFLOW CRACKING PHENOMENON IN SMT PLASTIC PACKAGES

A quantitative model has been developed to predict solder reflow cracking resistance versus encapsulant properties, package geometry, and storage conditions for SMT plastic packages. Sensitivity analyses have shown that the encapsulant's moisture diffusivity, moisture solubility, modulus of elasticity, and high temperature flexural strength are important to crack resistance. Their cumulative effect is quantified by the ''cumulative property performance index'' (CPPI) with a larger CPPI indicating better package crack initiation resistance. A master sensitivity analysis plot allows the crack resistance of any mold compound to be predicted from its basic measured properties. Modeling results demonstrated the difficulty of achieving good solder reflow cracking performance by simply improving the mechanical and moisture properties of mold compounds, if the intrinsic mold compound-die pad adhesion is poor. This is especially, true for the new thin plastic packages. For excellent adhering interfaces, the overall cracking resistance improves even though the relative effect of other mold compound properties decreases. Encapsulants with low moisture diffusivities continue to perform somewhat better with respect to crack initiation.