Interracial adhesion in plastic packages is one of the key requirements to achieving popcorn-resistant packages. As part of an Air Force-funded study to develop an Expert Design System for plastic packages (EPACK®), an extensive evaluation of various interfaces in plastic packages is being conducted. Three-point bending testing was conducted on molded composite beams embedded with either Si (with and without polyimide coating), or copper alloy. Two pre-crack lengths were tried. Testing was conducted at different temperatures and humidities. The effect of residual stresses (resulting in curved test specimens) was modeled and accounted for in the evaluation of the critical interracial fracture toughness. Eventually such material properties, together with package configurations and leadframe designs, will be entered as part of the EPACK database to provide design rules for achieving Level-1 (no dry bag) performance.