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The phenomenon of package cracking during solder reflow continues to trouble surface mount plastic packages for integrated circuits (ICs). The mechanism has been explained as moisture absorption, vaporization, delamination (loss of adhesion), deformation, and final package cracking. The effect of diminished mold compound material properties in saturated moisture and elevated temperature environments is a crucial factor. Current specifications of flexural strength at room temperature and elevated temperature flexural strengths for mold compounds do not correlate well with known failures during reliability/qualification preconditioning tests. Characterization of mold compound using uniaxial tensile tests have furthered the understanding of the mechanical nature of the material. This paper explains the correlation between mold compound material characterization under tensile load tests and reliability test results. The types of fractures and delaminations that occur during solder reflow appear to be mainly Type I (tensile) in nature for both thick and thin packages (see Figure 1). The contribution of the ductility of the copper leadframe and the hydrostatic pressure within delamination gaps is evident in this tensile gap.