Thermal Evaluation of GaAs-Based RF Power Amplifier Packages for Handheld Portable Applications

Abstract

Abstract This study characterizes the thermal performance of the Thermally Enhanced Plastic Very Thin Fine Pitch Quad Flat No Lead Package (HP-VFQFP-N) family for GaAs-based RF power amplifier applications. This package uses perimeter lands on the bottom of the package to provide electrical contact to the printed wiring board (PWB). The package also provides thermal enhancement by having the die attach paddle exposed on the bottom of the package surface to provide an efficient conduction path when soldered directly to the printed wiring board. Thermal model assumptions and development for the 0.5 mm pitch HP-VFQFP-N package are provided. Due to the relatively low thermal conductivity of GaAs, power dissipation in the individual stage areas on the die were modeled instead of uniform surface heat flux or uniform heat generation rate approximations. A conjugate heat transfer problem, in which radiative losses from the exposed surfaces of the package and PWB to the external environment were accounted for, was solved for horizontal natural convection cooling. The model is benchmarked with infrared thermal imaging measurements obtained for a 20 I/O, 0.5 mm pitch, 4 mm HP-VFQFP-N package with a 2-Stage GaAs Power Amplifier (PA) integrated circuit (IC). Predictions for junction-to-ambient thermal resistance were within 10% of measured values. Parametric studies were performed for HP-VFQFP-N sizes of 4, 5, and 6 mm with GaAs die sizes from 1.465 × 1.369 mm to 2.7 × 2.354. IC layout considerations and associated power distribution on the die were also examined. 2-Stage PA, 3-Stage PA, and 3-Stage Dual-Band PA IC configurations were examined. Some improvement in thermal performance was noted with increasing package size (for the same IC configuration/die size). GaAs die size had minimal impact on thermal performance of the QFN package but the IC layout/power distribution significantly impacted product/package thermal performance. Additional studies were performed to evaluate mold compound and die attach thermal conductivity effects on package thermal performance.