Thermal assessment of RF integrated LTCC front end module (FEM)

The thermal performance of Front End Module (FEM) incorporating Low Temperature Co-fired Ceramic (LTCC) substrate is investigated. An Infrared Microscope System was used to measure device surface temperature with both RF and DC power at various duty cycles (25 to 100%). The maximum junction temperature (∼112°C) occurs at the second stage. By powering the module with DC only, he comparison between numerical and experimental data indicates good agreement, with less than 10% difference in the peak temperature values. When replacing the common 2-layer organic substrate with a 14-layer LTCC substrate and silver paste metallization, the peak junction temperature reaches 130.1°C, ∼51% higher than before. However, by increasing the silver paste thermal conductivity from 90 to 350 W/mK, a significant drop in peak temperatures occurs, indicating the impact on module´s overall thermal performance. The top metal layer thickness (10 vs. 30 microns) only contributed to 5-8% changes in peak junction temperature. An improved FEM design incorporates a higher thermal conductivity silver paste material (300 W/mK) with new thermal via array structure (25 vias, 150 microns in diameter each) in the LTCC substrate. The module junction temperature reaches 96°C (based on 25°C reference temperature, 100% duty cycle), corresponding to a junction-to-substrate (R_js) thermal resistance of 14°C/W. Further study reveals that 20% voiding placed at the die center has no impact on FEM thermal performance, while the voiding placed at the die comer (under the heat dissipating stages) increases the stage peak temperature significantly by more than 40°C. Last part of the study focuses on design optimization: two particular designs provide the optimal thermal performance when reducing the thermal via number/costs by almost 40%.