Potting Compounds – A Guide to the Essentials

There are several good reasons for potting electronics. Embedding the circuit board into a potting compound gives environmental protection against water ingress and corrosion due to moisture in the air or caustic chemicals and gasses (sulphur in particular attacks the copper used in the components and PCB tracks). Potting also gives protection against the effects of mechanical shock and vibration, supporting and cushioning delicate or fragile components such as the brittle ferrites used in transformer cores. It also supports and takes much of the strain away from the PCB pins, so that the strength of the pin connection to the circuit board is not reliant only on the solder joints.

Potting also replaces the air around a converter PCB with a highly insulating medium¹ , avoiding arc-over within the power supply with over-voltage stress, particularly at high altitudes, and the effects of pollution such as moisture, dust and dirt which could reduce the insulation between the input and output, or allow tracking to occur across surfaces. Thermally conductive potting materials also reduce hot-spots within the converter, conducting heat away to the case and levelling-out the thermal gradients, to reduce temperature-difference stress on the components. Finally, potting offers fire protection (UL94-V0) because once cured, the compound will not ignite or maintain a flame.

A potted DC/DC converter also has an extended storage and operating lifetime. The materials used in RECOM’s products have a ten-year shelf-life and we occasionally get requests for replacement converters for products that were manufactured in the 1990’s and have been operational for decades. This longevity is partly due to the hermetic seal of the potting compound which maintains a clean, stable environment for the electronic components used in the converter.

¹ The epoxy potting compound that RECOM uses for the majority of its DC/DC converters has an voltage withstand capability of 15kV/mm of solid material.

All potting compounds shrink when they transition from the liquid to the solid state. The amount of shrinkage is small for most of the common compounds used for potting electronics, but any shrinkage places mechanical stress on the components or can open up microscopic cracks or gaps that allow the ingress of liquids or gasses. The solution is to use potting compounds that are not too hard, but retain some softness once fully cured. The mechanical stress caused by the shrinkage is then dissipated by the elastomeric (“rubbery-ness”) property of the potting compound, and the seals to the case, pins and components remain tight. Further, the potting compound should also have a low CTE (Coefficient of Thermal Expansion) so that the effects of thermal cycling do not cause mechanical stress on the components or PCB, which may have a different CTE. A certain softness in the cured potting material also mitigates against such thermally-induced mechanical stresses.

As a rule of thumb, epoxy potting should show a slight indentation when pressed with a fingernail (medium hard) and polyurethanes and silicones (see later) should have a more rubbery feel (medium soft). The hardness of potting compounds is measured on the Shore scale:


Another important factor is the glass transition temperature, the temperature below which the potting material becomes brittle. Almost all RECOM products have an operating temperature down to -40°C, so it is important that the potting material remains flexible even at these very low temperatures. At the other end of the scale, high temperature performance is also important. While the majority of RECOM’s products are specified to work at full load up to an ambient temperature of +85°C, a relatively benign maximum temperature as far as the potting compound is concerned, the converters must also survive a high temperature wave-soldering process without the seal being damaged.

One way of checking the integrity of the potting material is to do temperature cycling tests. In the following example, a set of DC/DC converters was potted with either a soft potting material or a harder potting material and then subjected to -40°C to +85°C shock temperature cycling. After each sequence of 250 thermal shock cycles, the converters were tested to see how many had failed:


It can be seen that up to 750 thermal cycles that there is no detectable difference between hard or soft potting on the electrical performance, but after 1000 cycles, the hard potting starts to cause the converters to fail. The difference in the thermal expansion between the potting compound and the components gradually causes mechanical stress failures. In applications with low temperature variations or slow temperature variations, there would be little practical difference between hard or soft potting materials and the tougher, harder potting material may be beneficial. Only in extreme or harsh environments would a softer potting material be preferable.

A final consideration is the ability to remove the encapsulation in order to undertake analysis and fault-finding. While elastomeric compounds can usually be pulled apart by hand, hard epoxies need to be chipped away with a sharp tool or ground off. This can make fault-finding very difficult with epoxy-potted converters.

There are three main potting compounds used in the electronics industry. Each fulfils the main objectives of encapsulation, namely, environmental protection, protection against mechanical stresses, and good electrical insulation and thermal conductivity, but each has its own particular advantages and disadvantages.

RECOM uses all three types, depending on the type of converter and intended application (for example, the automotive industry does not allow silicone potting because the compound can still “outgas” after curing and form a film on adjacent surfaces which can affect the quality of the paintwork or the effectiveness of electrical contacts. The railway industry also does not accept silicone potting because although it will not burn in a fire it could release smoke, which could cause panic in an enclosed space like a train carriage. The requirements of both of these applications can be addressed by using alternatives such as epoxy or polyurethane potting compounds).

The image below shows a prototype of a custom AC/DC converter that was tested with all three types of potting compound to evaluate which encapsulant had the optimum performance with the best flow, temperature withstand and adhesion characteristics.

Recom prides itself on designing efficient, cost-effective and reliable AC/DC and DC/DC potted power supplies, whether they be off-board or board-mounted. We use epoxy, polyurethane or silicone encapsulation materials depending on the type of converter and intended application, to create reliable converters that are protected against the environment, protected against mechanical shock and vibration stresses, and have good electrical insulation and high thermal conductivity.

As with all mass-production processes, the performance of the encapsulation in its cured state is dependent not only on the choice of compound used, but its careful preparation and exact application by well-trained staff in a temperature- and humidity-controlled environment. Failure to follow detailed SOPs (Standard Operating Procedures) could result in voids or poor adhesion of the potting compound, potentially leading to early failures. Therefore, the potting procedure is one of the most carefully controlled and overseen stages in our entire production process.