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Advantages and precautions of gluing power modules

Article Source：Kinri Energy | Author：Kinri Energy | Issuing Time：2024.04.15
The advantages of gluing the power module

Bare board gluing can prevent moisture, corrosive chemicals, and gas erosion in the air (especially sulfur can corrode copper in devices and PCB circuits), and prevent mechanical impact, vibration, support, and buffering of precision or fragile components, such as ferrite used in transformer cores. Simultaneously supporting PCB pins and reducing most of the stress, so that the connection strength between pins and circuit boards does not solely rely on solder joints.



Gluing uses highly insulating media to replace the air around the PCB, avoiding arc discharge caused by overvoltage stress in the power supply, especially in high-altitude areas. At the same time, it also avoids pollutants such as moisture, dust, and dirt that reduce the insulation between input and output or create electric marks on the surface. Thermal conductive sealing materials can transfer heat to the casing and evenly distribute the heat gradient to reduce hotspots within the power module, thereby reducing the temperature difference stress on the power device. Fire protection can also be provided, as once the sealing adhesive solidifies, it will not catch fire or continue to burn. Relatively speaking, after gluing, it has a longer storage time and service life.

Precautions for gluing the power module

If the sealing adhesive contains air or bubbles, it will reduce the thermal conductivity, and the bubbles will reduce the insulation effect due to internal arc discharge or surface electric marks. Inclusions can also cause mechanical stress due to extremely high or low air pressure or the expansion or contraction of air in the gaps with temperature, leading to the rupture of the sealing adhesive.

The most effective way to eliminate air is through vacuum mixing and packaging. Preparing and mixing sealing adhesive under vacuum will cause bubbles to rise to the surface, and then distribute them into the syringe through a pressure system to prevent air from flowing back into the material. The general process of vacuum injection and regular monitoring of cross-sectional or X-ray results can ensure this.

Manufacturing processes can also be used to avoid bubbles in the sealing adhesive. For example, first fill a portion of the sealing adhesive in the shell, then insert a tested PCBA, and then fill the shell to almost full. Then place the semi-finished power module in the oven, keeping the oven temperature below the curing temperature of the sealing adhesive, and the liquid bubbles will float on the surface. Placing it on the vibration table will also allow the bubbles to rise smoothly, ultimately filling the shell completely and placing the power module in a higher temperature oven.

All sealing adhesives shrink when they transition from liquid to solid. The shrinkage of most sealing adhesives is small, but any degree of shrinkage will apply mechanical stress on the device, causing micro cracks or gaps, allowing liquid or gas to enter. The solution is to use soft potting adhesive, which retains a certain degree of softness after complete curing. Therefore, the elasticity of potting adhesive can eliminate mechanical stress caused by shrinkage and tightly bond with the shell, pins, and devices.

There is also a glass transition temperature at which the sealing adhesive will become brittle. Even at very low temperatures, the sealing adhesive must maintain flexibility, and high-temperature performance is also important. One method to check the integrity of the sealing adhesive is temperature cycling test. For example, using soft or hard sealing adhesive and then conducting temperature shock cycles from -40 ° C to+85 ° C to monitor the differences in electrical performance between the two types of sealing adhesive. In applications with low or slow temperature changes, the use of hard or soft potting adhesive has little practical difference, but in extreme or harsh environments, softer materials are more suitable.

When conducting fault analysis, elastic materials can usually be broken by hand after removing the casing, but hard materials need to be cut or ground off using sharp tools.
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