Reducing The Convective Cooling Requirements By Choosing The Right MCPCB

In today’s post, I will analyze the effect of MCPCB’s (metal core printed circuit board) through-plane thermal conductivity on the chip’s maximum temperature by using a simplified analytical thermal model, i.e. a composite slab model of three layers that consist of chip, PCB, and heat sink. The heat sink is assumed to be cooled by the air through convection.

The goal of this exercise is to see if the convective cooling requirements can be lowered by strategically lowering the conduction resistance of the PCB layer. It is assumed that the semiconductor chip of size 30mm by 30mm and power dissipation of 4 Watts can only operate below 75 degC. In this case, what type of MCPCB should be used?

The junction temperature is calculated from the model by varying both the PCB’s thermal conductivity ‘K2’ and the cooling air heat transfer coefficient ‘h4’. The results are plotted in the attached 2D graphs (Note: another way to present the results is with a 3-D surface graph by plotting the junction temperature (z-axis) vs K2(x-axis) and h4 (y-axis)).

For the assumed thermal target of 50 degC, the relationship between K2 and h4 was found by taking a horizontal slice of the 3-D surface at 50 degC. The first thing to notice is that the convective heat transfer coefficient ‘h4’ will decrease by increasing the thermal conductivity of MCPCB ‘K2’, which means less cooling power is required for the heat sink if a higher thermal conductivity MCPCB board is selected in the design. Furthermore, the most “effective” combination of K2 and h4 was found to be (K2=5 W/m-K and h4=200 W/m^2-K) for the thermal target of 50 degC, since increasing the thermal conductivity of MCPCB beyond 5 W/m-K will not reduce the convective cooling requirement significantly as the value of h4 flattens out beyond the point.Similarly, for the thermal target of 75 degC, the most “effective” combination was found to be K2=2 W/m-K and h4=100 W/m^2-K.

In conclusion, by choosing the right MCPCB, the convective cooling requirement can be significantly reduced, and every thermal target has an optimized combination of K2 and h4.