When considering adopting high-power semiconductors in your designs, understanding proper mounting and handling procedures is vital not only to achieve mechanical reliability, but also to ensure the right thermal and electrical performance.

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Written by: Littelfuse

High-power semiconductors provide dependable performance in a broad range of applications, from rail, marine and mining transportation, to renewable energy systems, to power supplies for industrial welding, induction melting and electrochemical systems.

At first glance, these might appear to be highly durable, robust parts—yet despite their rugged appearance, they require careful handling and mounting. If you are considering adopting high-power semiconductors in your designs, you will need to understand proper mounting and handling procedures—not only to achieve mechanical reliability, but to ensure the right thermal and electrical performance going forward.

Keeping Temperature Under Control

Often, disc devices containing diodes, thyristors, IGBTs and/or GTOs are stacked to create the desired topology and to support dedicated demands. Doing so requires careful handling and precise alignment of the power semiconductors; the critical factors in these stacked assemblies are the treatment of all the contact surfaces, thermal management, and the proper distribution of mounting forces.

Differences between types of press-pack technologies.

Let’s discuss temperature first. In disc devices, the contact area not only carries electrical current, it also helps dissipate thermal energy for proper cooling. This means the contact area’s partner is essentially both a bus bar and a heat sink, making it critical for those interfaces to maintain stable contact throughout the equipment’s lifetime.

You will need to consider both surface geometry and surface finish in your design to minimize thermal resistance. For instance, a non-plated contact surface (such as an extruded aluminum heat sink) should be lightly abraded to remove oxide films before mounting, with just enough electrically-conductive contact grease used to fill any microscopic gaps without hindering current flow.

Avoiding Excessive Force

Assembling the device requires using the necessary amount of assembly force while homogenously distributing that force at the same time. This is not a trivial consideration, and requires the right kind of mechanical fixtures. Make sure you are using the right hardware for the task. For example, Littelfuse supplies clamp systems designed specifically for use in assembling components in high-power semiconductor applications.

In any case, no matter the design, you will need to spread the assembly force properly. In theory, an even distribution can be achieved by applying force to a single point aligned with the center line of the semiconductor, using mechanical means such as a steel cone and contact block to spread that force across the contact area. In this scenario, to properly spread the applied force, the cone’s diameter must match the semiconductor’s diameter and the point angle must not exceed 90 degrees.

Ideal scenario for spreading mounting force for uniform distribution.

In many high-power applications, semiconductor devices are stacked to achieve higher blocking voltages or to form a desired topology. When those stacks include more than two devices, proper assembly calls for special attention to achieve the right amount of pressure and to avoid overstressing any device within the stack.

Again, component suppliers often provide parts, kits and/or complete assemblies to help you avoid these challenges and let you develop stacks more quickly.

Verifying Your Design

Since you cannot measure or monitor pressure distribution in every component stack, you need to define the mounting procedure and the stack’s clamp system to ensure a suitable result.

You can do so by monitoring the pressure in a fully-assembled system. One recommended method is to use a pressure-sensitive, color-changing film inserted between the semiconductor device and the corresponding contact block. After applying pressure, you can disassemble the stack and inspect the film to check for uniform pressure distribution.

Verifying pressure distribution with pressure-sensitive film.

For more information on mounting and handling these semiconductor devices, consult the Littelfuse application note Mounting & Handling Press-Pack Semiconductor Devices.