How to Achieve Reliable Tactile Pressure Distribution In Press-Pack High Power Semiconductor Stacks

Keywords: pressure distribution, pressure mapping, clamping force distribution, high power semiconductors, fuji prescale film Summary: High power semiconductor stacks rely on uniform tactile pressure with heatsinks to avoid temperature overload. Using simple clamps is not enough and Fuji Prescale films can help discover tactile pressure problems and suggest solutions too.

Written by Igor Mateski

Pressure Uniformity In Semiconductor Stacks

High power semiconductor stacks generate a lot of heat and require fine-tuned heat dissipation in order to function properly. Another issue that arises in stacked semiconductors is the issue of parallelism deviations, where the bottom and top side of a semiconductor element are not perfectly parallel. So when more elements are stacked into a unit, these parallelism deviations may significantly affect the cooling properties of heatsinks. With big tightening forces semiconductor elements can even fail because of unparallel sides and load concentration. Designing for pressure uniformity is not always easy and the complexity should not be underestimated.

Pressure Distribution In Simple Clamps

Simple solutions, such as clamping the device between two rectangular plates by bolting down the corners will exhibit poor reliability. Because of the mentioned parallelism problem, it is recommended to use load distribution elements, semi-spherical spacers that focus the load force into a single point. Spherical load distributors help in balancing out some of the parallelism issues in the stack.

However, even in proper stacks, to verify that the pressure distribution is uniform, ABB recommends the use of Fuji Prescale film medium pressure grade, 1 - 5 kN/cm2.

Tactile Pressure Mapping In Properly Clamped Stacks

Using spherical cups to accommodate for parallelism issues and convey the loading force into a single point is the first step in achieving proper clamping of semiconductor stacks. The other best practice is to use four-point bolting if the stack is larger than 2 elements. Using different heat sinks for different stacks also helps in even distribution of the bolting force. The diagram below shows how a well designed semiconductor stack achieves uniform tactile pressure.

When building semiconductor stacks, technicians and engineers should always verify the tactile pressure distribution between stack elements. To do this, semiconductor manufacturer ABB suggests using Fuji Prescale films between the stack elements.

As the illustrations shows, the Fuji Prescale film first needs to be cut to size and placed between semiconductor elements and heatsinks. Then the nominal clamping force is applied by bolting down the assembly. This bolting load causes the color micro-capsules to burst and thus develop the Fuji Prescale film. These micro-capsules are pressure sensitive, so the greater the pressure, the stronger pigmentation appears.

In the image above, the first developed Fuji Prescale film shows pressure peaks on the peripheral areas of the semiconductor, with pressure valleys around the center. This will inevitably result in poor heat dissipation and may even result in failure. In a properly designed stacks, the developed Fuji Prescale film shows uniform tactile pressure and engineers can be certain that heatsinks will properly dissipate heat from the semiconductor elements.

With proper verification methods like the Fuji Prescale pressure mapping, multiple bolting points, good load distribution and separate heat sinks result in a uniform tactile pressure between semiconductor elements.

Thanks to known industry best practices, and verifying results with a Fuji Prescale film, engineers can build durable and reliable high power semiconductor stacks easily and cheaply.