For this level of power dissipation, a heat pipe / radiator fin design was the most effective choice. Today’s high end computer graphics cards utilize a similar system to dump their heat; however, most of these are actively cooled with a small on board fan. In order to keep the system passive, Boyd decided to look at a dual slot design where a larger fin stack would occupy the second slot.
Boyd created a detailed CFD base model which included FPGA, PHY,DDR, DDS, Omap, Bridge and LTM packages. The PCB thermal properties were defined by the number of ground and power layers specified by the customer. Other components in the system, such as the RF and HeatSink boards, were modeled as heat dissipating PCB boards. The system was then analyzed at an ambient temperature of 55°C with an altitude pressure of 750 hPa and a flow rate of 300LFM.
From the base CFD simulation, Boyd was able to determine flow and temperature maps over the board, component junction temperatures and case thermals at the above conditions. This base model was then altered by the following to finalize the thermal solution:
• For components at or above their thermal limit, calculations were done to find the minimum required heat sink thermal resistance to bring them into safe operating ranges.
• Designed the thermal solution for all the components exceeding the minimum junction temperature requirements.
• Optimized the heat sink for DFM, cost and performance.