Si Power Device|Evaluation

Heat Dissipation Performance of PMDE Package (Simulations)


Points of this article

・In the PCB mounted state, as the copper foil area increases, heat is spread more readily, so that heat dissipation improves.

・If the copper foil area is too small, the PMDE Rth(j-a) value becomes larger than that for PMDU, and adequate heat dissipation is not exhibited.

・As the copper foil area increases, the PMDE heat dissipation performance improves, exceeding that of the PMDU package.

・The thicker the copper foil, the smaller the copper foil area at which the PMDE heat dissipation performance exceeds that of the PMDU package.

・If the copper foil area is made larger than necessary, the heat dissipation performance reaches saturation, and an effect equivalent to the area is no longer obtained. Hence an appropriate size should be chosen for the copper foil area.

Simulations were used to compare the heat dissipation performances of the PMDE and PMDU packages.

Thermal Simulation Method

As in the diagram below, simulations were conducted of the element temperature Tj and the manner of propagation of heat when the size of the copper foil on which a device was mounted on a 50x50x0.8 mm PCB (printed circuit board) was increased in steps. As the copper foil area was increased, heat was diffused more broadly, and we see that heat dissipation improved. We also see that heat transmission to the glass epoxy part of the PCB is extremely limited. From these results, it can be said that the magnitude of the copper foil area for mounting is extremely important for adequate heat dissipation.

Visualization of heat dissipation on a PCB (simulated)

Thermal Resistance Rth(j-a) of the PMDE Package and Copper Foil Area

Based on the above thermal simulation results, the relationship between the junction-to-ambient thermal resistance Rth(j-a) and the copper foil area for the PMDE and PMDU packages is shown below. The relative error in Rth(j-a) for the PMDE package with reference to the PMDU package is also shown.

Rth(j-a) vs. copper foil area for PMDE and PMDU packages

Relative error in Rth(j-a) for PMDE package with PMDU package as reference

If the copper foil area is small, the effect of PMDE rear-surface heat dissipation is not sufficiently exhibited, and the PMDE Rth(j-a) is a larger value than that for the PMDU package. As the copper foil area increases, the heat dissipation effect of the PMDE package improves, the difference with the PMDU package diminishes, and for a copper foil thickness of t=35 μm and area of about 90 mm2, or for t=70 μm and an area of about 60 mm2, the thermal resistance is equal to that of the PMDU package. When the copper foil area is larger still, the PMDE package heat dissipation is further improved, surpassing that of the PMDU package. And, in the vicinity of 2000 mm2, saturation begins.

From these results, we see that in order to obtain satisfactory heat dissipation characteristics using the PMDE package, an appropriate copper foil area must be secured. If the copper foil area is too small, the heat dissipation performance is inferior to that of the PMDU package; if on the other hand it is too large, an improvement in heat performance comparable to the foil area cannot be obtained, and board area is wasted. Also, thicker copper foil means a more satisfactory heat dissipation characteristic than for the PMDU package can be obtained at a smaller foil area.

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