PBGA warpage and stress prediction for efficient creation of the thermomechanical design space for package-level reliability

In the pursuit of reliable thermomechanical designs for thin, chip-scale (CSP), plastic ball grid array (PBGA) packages, research conducted by the ESPRIT BATEL 24366 program has extended the research on plastic package warpage to specifically include that for PBGAs. Because PBGAs are asymmetric devices about their cross section and are composed of two plan-view areas, consisting of a die area and a mold area, different rates of thermal contraction due to coefficient of thermal expansion (CTE) mismatch between layered materials causes two distinct curvatures in the package upon cooling from post-mold cure to ambient temperature. The overall warpage of the PBGA is described by two curvatures, a die-area curvature and a mold-area curvature. Physical samples of PBGAs measured in this research have verified the existence of this dual curvature. The analytical development for dual-curvature beam bending is presented and compared with previous predictions for multi-layered beam bending and 30 PBGA numerical simulations.