A cooling solution for power amplifier modules in cellular phone applications

Abstract

Portable electronic products like cellular phones offer increasing features for enhanced communication. A number of digital and analog devices are used in such products. Multichip module (MCM) technologies allow unique opportunity for mixed signal computing and higher integration suitable for these products. Small form factor MCMs are currently being used in radio frequency (RF) MCM products for wireless applications. Depending on its size and functionality, each RF MCM product may have one to three CMOS, bipolar, and/or GaAs die, and 10 to 100 discretes interconnected on a laminate or ceramic substrate. We have studied the cooling path of a sample RF MCM structure that contains GaAs power amplifier (PA) die. The molded MCM package houses one GaAs, one CMOS, and >20 discretes on a 100 sq. mm four-layer laminate substrate. The MCM substrate has thermal vias and voltage planes for improved thermal performance. An improved cooling path is devised by implementing an extended heat removal path to transfer heat from the bottom surface of an MCM-to-board interface to the external antenna using a micro heat pipe. Modeling and experimental studies were conducted to demonstrate the effectiveness of using a micro-heat pipe structure in similar products. The analysis includes a highspeed transient thermal modeling, device sub modeling, and a system level modeling of the MCM in operation. The experimentally confirmed results show an almost 26% improvement in MCM package thermal performance by using a micro-heat pipe in the proposed implementation.