Thermal Investigation of Interconnect Technologies for GaAs RF Power Amplifier in Handheld Applications

Thermal simulation and infrared characterization are conducted to evaluate and compare the thermal performance of three interconnect options: wire bond, direct die attach, and flip chip interconnect. The test vehicle is a dual band RF PA module with low and high-band GaAs devices. As only one band is operated at a time, the focus is on the low-band GSM GaAs die. For direct die attach interconnect, the wire bond device is reused with all RF in/outs being connected from the backside of the GaAs device through individual vias in the device. The effects of through via design, solder attach material and thickness, and the thermal via layout in the substrate are evaluated. For flip chip interconnect, the original wire bond device was redesigned to accommodate metallic bumps for RF in/out, control and grounding. The ground bumps placed directly above the heat source area provide the main heat transfer path to the substrate. The size of the ground bumps, its material and the thermal vias layout in the substrate are investigated. Infrared thermal characterizations were conducted on both wire bond and direct die attach options to validate the simulations, with good agreement.