Evaluation of adhesion strength of semiconductor package with the Weibull stress

The idea of the Weibull stress, which is based on the “local approach”, has been applied to delamination failure analyses of semiconductor packages. Interface fracture strengths between package mold resin and 42 alloy lead frame are obtained by a conventional shear test with three resin sizes and two shear heights. Six different facture strengths are converted to a same adhesion strength value which is expressed in terms of the critical Weibull stress. The Weibull stress can be used as non-relative indices of adhesion strengths unaffected by testing conditions. Delamination failures in a TSOP (Thin Small Outline Package) which occur during reflow soldering have been analyzed with the Weibull stress. The Weibull stresses of molded resin in the package at the temperature of reflow soldering are estimated on several spots where either delaminated area or non-delaminated area is distinguished by SAT (Scanning Acoustic Tomograph). Evaluated Weibull stress parameters at delaminated spots exceed the critical Weibull stress which is obtained by the shear test at 240°C. The result is opposite at points where delaminations are not observed. They are smaller than the critical value. This approach can be expected to predict delamination failures in semiconductor packages.