Substantiation of numerical analysis methodology for CPU package with non-uniform heat dissipation and heat sink with simplified fin modeling

Abstract

Thermal design in electronic packaging is driven by the maximum allowable junction temperature of a CPU. An inadequate thermal design that underestimates the junction temperature may adversely impact the electrical performance of the CPU, making predicting the junction temperature a crucial step in package and system thermal design. A numerical model of a heat sink and thermal test package with a uniform and non-uniform power dissipation was created and used to predict their temperatures. The uniform power dissipation case was used to calibrate the numerical model's TIM2 thermal impedance. In the non-uniform power cases, the maximum heat flux was over four times higher than the average heat flux. The numerical analysis results in the non-uniform power cases yielded junction temperatures within 2 degrees of the measured values. The heat sink used in the tests as well as numerically modeled contained a vapor chamber base and a plate heat sink. Three different heat sink modeling approaches were used, including: detailed modeling of the heat sink, effective convection coefficient heff, and effective thermal conductivity keff. Test data was used to establish the effective heat transfer coefficient and effective thermal conductivity. A simplified heat sink numerical model allows the computational grid density to be significantly reduced, resulting in fast convergence. Alternate heat sink designs were also considered.