In high power amplifiers, the understanding of heat transfer phenomena is critical to ensure components operate within acceptable temperature limits. System operational reliability has been shown to reduce considerably when individual components are allowed to exceed maximum-rated junction temperatures. Of particular interest in this study are high power amplifiers used in the telecommunications industry. In such applications, heat sinks in forced airflow have traditionally been used to provide cooling due to their low cost and ongoing maintenance requirements. Here, the causes of failure of an amplifier circuit board used for high-frequency signal transmission have been closely studied. Analysis of failure data gathered over a four-year period was used to identify the main component level causes of system failure, and preliminary experiments were conducted to estimate junction temperatures of these components under field conditions. Subsequent experiments were then carried out to measure the thermal performance of the existing extruded aluminium heat sink over a range of Reynolds numbers from 10,000 to 100,000. A correlation for thermal resistance was developed and compared to experimental results obtained by other researchers for aluminium heat sinks of varying fin geometries. Finally, a commercially available computational fluid dynamics package was used to model the current heat sink geometry. Experimental results were used to validate the numerical model, which can be used for future optimization of key features of the amplifier cooling system.