APPLICATION NOTE: AN031 PCB Design Guidelines to Maximize Cooling of eGaN FETs Designing a PCB for maximum heat dissipation The various PCB design characteristics that have thermal characteristics are board area, copper area, copper thickness, vias, and device proximity. Each independent characteristic has a thermal performance and an impact on the electrical system and will be discussed next. PCB Area for Improved Cooling One of the main characteristics that influences heat transfer to the ambient is the area of the PCB. A larger board area allows for more heat transfer between the PCB and the environment as there is more area for convection to occur.
This relationship is shown in Figure 2, where two EPC2218 [2] devices dissipating 1 W each are simulated for varying PCB surface areas. Maximum board temperature sharply rises as the PCB gets smaller, but there are diminishing returns in cooling as board size increases past a certain threshold. The area of the GaN device itself also has an impact on heat transfer. Larger devices have more solder bumps connecting to the PCB, resulting in more heat flow from the device into the copper on the board. Figure 3 shows how temperature rise changes for different device sizes on the same board area of 2.56 cm 2 and the same power dissipation of 1 W total.
There is also a nonlinear trend in the relation between these two factors as the decrease in temperature rise as the devices continue to increase in size plateaus very quickly. 100 60 90 Temperature Rise (°C) 80 Temperature Rise (°C) 50 70 60 40 50 30 40 30 20 20 10 10 0 0 5 10 15 20 25 30 35 40 PCB Area (cm 2 ) 0 0 2 4 6 8 10 12 14 16 Device Area (mm 2 ) Figure 2. Effect of board size on temperature rise Figure 3. Effect of device size on temperature rise for a fixed board area A 4 in 2 , 4-copper layer board will serve the baseline on which the PCB-only cooling design techniques are examined.
Impact of Copper Trace Width and Plane Area on Thermal Performance Copper is the dominant path for heat to spread from the FET and to transfer to the ambient. This thermal heat spreading effect is primarily affected by the trace geometry, area, and thickness of the copper layers [3]. The most impactful layer for heat-spreading is that to which the device is soldered to. The width of the copper traces that the part is connected to as well as the total area of the copper must be considered for keeping thermal resistance as low as possible. Example layout designs of the top layer for an EPC PQFN device are shown in Figure 4.
Figure 4(a) shows a design that would result in a higher thermal resistance throughout the top layer of the board due to the narrow traces and lack of large thermal dissipating area, raising the thermal resistance from junction to ambient. The thin traces that the solder bars of the device rest on in combination with the disconnected copper planes of small area would serve to trap the heat dissipated from the device in the pad area, resulting in a penalty to thermal performance.
Figure 4(b) shows the design of a layer that maximizes the area of the copper planes around the device with wider traces connected to large areas of copper, which in turn keeps the thermal resistance from junction to ambient low. Impact of Copper thickness Using thicker copper traces for low electrical resistance also benefits thermal resistance and provides a high thermal-conductance medium at each layer in the PCB. The effectiveness of increasing the thickness of copper layers is shown in Table 1 for two EPC2218 devices, each dissipating 1 W of power. Increasing the copper thickness from 1 oz to 2 oz results in a decrease in R θJA of 21%.
Further increases in thickness beyond 2 oz copper though results in diminishing returns in R θJA reduction. Copper thickness greater than 2 oz also presents challenges in etching fine EPC – POWER CONVERSION TECHNOLOGY LEADER | EPC-CO.COM | ©2024 | For more information: info@epc-co.com | 2 16.52 cm 2 25 .81 cm2 2.56 in2 4 .00 in2 9.29 c m2 1.44 in2 60 50 40 Rise [°C] 30 Temperature 20 See F igure 3 10 00 2 4 10 00 2 4 Impa ct of De vice Are a D6evice Ar8ea [mm102] 12 14 16 16 .52 cm2 2.56 in2