Why Do Electronic Potting Compounds Still Have Bubbles After Vacuum?

Introduction

Electronic potting compound is commonly used to encapsulate power modules, controllers, sensors and other electronic components. During potting, the compound must fill the gaps around the components and form a continuous covering layer after curing.

Bubbles, however, are common in production. Some compounds look fairly smooth on the surface after vacuum degassing but still show internal voids after curing. In other products nothing unusual is seen at potting, and bubbles only appear after some time or once curing is complete.

This shows that although vacuum degassing helps remove part of the gas in the compound, it cannot deal with every source of bubbles. To find the cause, distinguish whether bubbles form at the mixing stage, are introduced during pouring, or form during curing.

Why bubbles remain after vacuum

• Mixing keeps drawing in air

  When potting compound is stirred, dispersed or loaded with powder filler, air is easily carried into the system. In high-filler formulations especially, a stirring speed that is too fast, a poorly placed stirring blade, or an obvious vortex in the mixing vessel may entrain a lot of bubbles. Vacuum makes bubbles expand and escape, but if high-speed stirring, transfer or improper pouring follows mixing, air can get back into the compound. So degassing depends not only on the vacuum treatment but also on the earlier mixing method and the handling after degassing.

• High viscosity keeps bubbles in

  Potting compound usually contains resin, hardener and different types of filler. A high filler ratio, a resin system with poor flow, or a strongly structured system hinders bubble movement inside the compound. Under vacuum bubbles may expand but not rise and burst in time. If the compound starts to thicken soon after degassing, bubbles are even more likely to stay inside. Simply extending vacuum time may then not solve the problem; judge by compound viscosity, temperature, pot life and equipment. When adjusting temperature, consider the reaction speed and use requirements of the material, and do not ignore curing characteristics just to lower viscosity.

• Air hidden in gaps and under parts

  Not all bubbles come from the compound itself. Under electronic components, connectors, wire harnesses and complex structures there may be narrow gaps or areas from which air cannot easily escape. When the compound flows in from outside and the air has no suitable escape path, it can be trapped and form local voids. Even a degassed compound cannot guarantee that such bubbles will not appear after pouring. Check the pouring direction, the flow path of the compound, venting space and pouring speed. For complex products, cut-open specimens or other suitable inspection methods can confirm whether voids concentrate in particular positions.

• Bubbles can also form during curing

  If the compound is in normal condition when poured but new bubbles appear after curing, the curing stage needs further checking. Moisture in raw materials, compatibility problems between materials, or volatiles released by the curing reaction and temperature changes can all cause bubbles at a later stage. The specific cause differs by resin system; epoxy, silicone and polyurethane potting compounds each have to be judged by their own reaction characteristics. So when more bubbles are found after curing, do not conclude directly that vacuum degassing was insufficient. Check raw material storage, component ratio, moisture in the substrate and curing conditions.

What can a defoamer solve?

A defoamer can help bubbles break or reduce foam stability, but its effect depends on the specific formulation and material system.

• Air drawn in by mixing

  A well-matched defoamer may help improve degassing behaviour.

• Air trapped under parts, or gas formed in curing

  Adjusting the defoamer alone usually cannot fix the root cause.

• Matching the system

  A defoamer must be compatible with the resin, filler and other additives. After adding it, watch bubble changes, compound appearance, storage stability, curing performance and, where necessary, electrical properties, to avoid new compatibility or surface defects.

• High-filler potting compound

  A sound mixing process, vacuum degassing conditions, pouring path and defoamer choice should work together rather than relying on a single measure.

Troubleshooting by the stage where bubbles appear

• Bubbles right after mixing

  Check the stirring method, how filler is added and compound viscosity first.

• Bubbles still there after degassing

  Check the vacuum level, treatment time, compound temperature and whether air is drawn in again after degassing.

• Local voids after potting

  Check component structure, pouring direction, flow path and venting conditions.

• Bubbles only after curing

  Check moisture in raw materials, component ratio, material compatibility and curing conditions.

Separating when and where bubbles appear, then running small comparison tests, usually helps find the problem better than directly adding more defoamer or extending vacuum time.