Topic about Power Module Gluing

What is the advantage of potting of bare board?

It could?protect?it?against moisture in the air, corrosive chemicals, and gases (especially sulphur, which can eat away at the copper of devices and PCB traces), protects against mechanical shock, vibration, and supports and cushions delicate or fragile components such as ferrite used in transformer cores. It also supports the PCB pins and relieves most of the stress, allowing the strength of the connection between the pins and the board to be more than just a solder joint.

?

Potting replaces the air around the PCB with a highly insulating dielectric, avoiding arc discharges caused by overvoltage stresses within the power supply, especially at high altitudes. It also avoids contaminants such as moisture, dust and dirt from degrading the insulation between input and output or creating electrical traces on surfaces. Thermally conductive potting materials reduce temperature differential stresses on power devices by conducting heat to the enclosure and homogenising the heat gradient to reduce hot spots within the power module. Fire protection is also provided, as the potting compound will not catch fire or continue to burn once cured. Comparatively, potting provides longer storage time and service life.

?

Precautions for power module potting

If the potting compound contains air or air bubbles, the thermal conductivity will be reduced, and air bubbles will reduce the insulation effect due to internal arc discharge or surface traces. Inclusions can also cause the potting compound to rupture due to mechanical stresses caused by extremely high or low air pressure or air in the void expanding or contracting with temperature.

?

The most effective way to eliminate air is vacuum mixing and dispensing. Preparing and mixing the potting compound under vacuum causes the air bubbles to rise to the surface and then dispense it into the syringe through a pressurised system, preventing the air from flowing back into the material. This can be ensured by a general vacuum injection process and by regular monitoring of cross-section or X-ray results.

?

Manufacturing processes can also be used to avoid air bubbles in the potting compound. For example, the casing is first partially filled with potting compound, then a tested PCBA is inserted and the casing is filled to almost full capacity. Then put the power module semi-finished product into the oven, the oven temperature is kept lower than the curing temperature of the potting compound, the bubble will rise to the surface in the liquid state. Placing on a vibration table will also cause the bubbles to rise smoothly, and finally the case is completely filled and the power module is placed in a warmer oven.

?

All potting compounds shrink as they change from liquid to solid. Most potting compounds shrink very little, but any degree of shrinkage can put mechanical stress on the device causing micro-cracks or gaps that allow liquid or gas to enter. The solution is to use a soft potting compound that retains some softness when fully cured, so the elasticity of the potting compound eliminates the mechanical stresses caused by shrinkage and provides a tight bond to the case, pins and device.