Model and analyses for solder reflow cracking phenomenon in SMTplastic packages

摘要

A quantitative model has been developed to predict solder reflowcracking resistance vs encapsulant properties, package geometry andstorage conditions for SMT plastic packages. Sensitivity analyses haveshown that the encapsulant's moisture diffusivity, moisture solubility,modulus of elasticity, and high temperature flexural strength areimportant to crack resistance. Their cumulative effect is quantified bythe `cumulative property performance index' (CPPI) with a larger CPPIindicating better package crack initiation resistance. A mastersensitivity analysis plot allows the crack resistance of any moldcompound to be predicted from its basic measured properties. Modelingresults demonstrated the difficulty of achieving good solder reflowcracking performance by simply improving the mechanical and moistureproperties of mold compounds, if the intrinsic mold compound-die padadhesion is poor. This is especially true for the new thin plasticpackages. For excellent adhering interfaces, the overall crackingresistance improves even though the relative effect of other moldcompound properties decreases. Encapsulants with low moisturediffusivities continue to perform somewhat better with respect to crackinitiation