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 surface mount technology (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 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 demonstrate 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. For interfaces with excellent adhesion the overall cracking resistance is always improved. Encapsulants with low moisture diffusivities perform somewhat better with respect to crack initiation