Thermal Assessment of a Power Amplifier Module in Wireless Handsets

Increased functionality of microelectronic packages for commercial applications leads to the necessity of identifying packaging solutions with high standards for thermal performance. A detailed numerical analysis examines the thermal characteristics of a power amplifier module for time division multiple access (TDMA), using commercially available software. The increasing trend in power levels and densities leads to the need of design thermal optimization, either at the module level or at the system (module board stack-up) level. Several designs are investigated for thermal performance and the best thermal design is identified. Initial study focuses on assessing the thermal performance of a baseline design. The peak temperature reaches 144°C, about 60°C temperature increase over the reference temperature. The peak temperature value is slightly below the limit of 150°C, and is calculated based on the optimal (temperature constant) heat sink scenario attached to the bottom face of the module. Several alternatives are investigated, by modifying the thermal via array structure and Cu plating thickness. The increase in copper plating from 0.025 mm to 0.05 mm (1 to 2 mils) has the largest impact on module’s thermal performance. The addition of solder material and radio board increases by almost 50% the overall thermal resistance, hence the estimated peak temperatures reached by the heat stages would exceed the limit. A detailed sensitivity study was completed to assess the importance of each element in the module-board stack-up. Finally, a comprehensive experimental study was completed to validate the numerical simulation. The results indicate that the error between measurements and simulation range between 5–8%.