Potting Compounds – A Guide to the Essentials
2020/07/01
The majority of RECOM’s products are encapsulated in a potting compound. Why is this?
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.
Encapsulation considerations
All of the advantages listed above only apply if the potting creates a gas-tight and water-tight seal. If the potting material contains air inclusions or ‘voids’, then it may not be such an effective method of protection. Voids can reduce the insulation effectiveness by allowing internal arc-over or surface tracking and, of course, will reduce the thermal conductivity. Also, an inclusion could create mechanical stresses such as cracking due to very high or low air pressures or expansion/contraction of air in the void with temperature.
Fig. 1: Example of air voids in epoxy potting compound (cross-sectional photo).
There are several techniques that can be used to eliminate significant air inclusions in the potting compound, the most effective being vacuum mixing and dispensing. The potting compound is prepared and mixed under a vacuum which causes any air bubbles to rise to the surface. It is then pumped into dispensing syringes via a pressurized system that does not allow air to re-enter into the compound. Some products, such as our RAC-K series AC/DC power supplies, use the potting as part of their approved safety isolation barrier and safety agencies require that the penetration of potting compound and void-free performance must be controlled in production. This is guaranteed by the vacuum impregnation process along with periodic monitoring by ‘sectioning’ or X-ray.
However, the majority of our products are not potted under a vacuum as this speeds up the production capacity greatly, important when you are manufacturing more than one million products each month. RECOM can still avoid air bubbles in the potting compound by using other techniques, which in combination, can eliminate almost all included voids. The first of these is the correct potting process; firstly, the case is partly filled with the potting compound, then the pre-tested and assembled PCB is inserted into the case and the case filled almost to the top. The partly-completed converter is then placed in a resting oven which is warm, but held below the curing temperature of the compound. The potting compound becomes very liquid at the warm temperature and any trapped air bubbles rise to the surface and, if necessary, the case can be placed on a shaker table to encourage the bubbles to rise. Finally, the converter case is filled completely and transferred to a curing oven at a higher temperature.
Two-component epoxy potting material is hydroscopic – it will absorb moisture from the air leading to a mottled surface appearance called “amine bloom”, so it is important to use fresh chemical compounds that have been stored in a low-humidity, temperature-controlled environment. Also, the PCB itself should be clean and free from residue. As RECOM has its own SMD production lines, we can control the cleanliness, humidity and temperature in our production area closely. A clean, bare-board converter will allow the potting compound to adhere well to the components and PCB to reduce air inclusions.
Finally, we can use X-ray inspection to check the converters for voids. For new designs, some air bubbles can be unexpectedly trapped under components or under the PCB and be difficult to remove. In this case, we can use the results of the X-ray inspection to either reposition the components or to add strategic holes in the PCB to allow trapped air to escape during the potting process.
Factors affecting the choice of potting compound
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:
Fig.3: Shore hardness scale (source: Smooth-on, Inc.)
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.
Epoxy/polyurethanes/silicones
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)
Epoxy resin: this is the main type of encapsulant used. Two-component epoxy consists of a polymer resin and a hardener, which, when mixed together causes a chemical reaction which cross-links the chemical bonds in the polymer chains to create a tough, rigid and strong compound.
The advantages of epoxy potting materials are:
The disadvantages are:
Polyurethanes: Polyurethane (PU) is a type of thermoset plastic. Like epoxy, it is typically a two-component compound consisting of a base resin with an isocyanate curing agent. Unlike most epoxies, it sets to a softer, more rubbery state.
The advantages of PU potting materials are:
The disadvantages are:
Silicones: Silicone rubber is a synthetic polysiloxane polymer that uses an additive platinum catalyser to transition from the liquid to the solid state. The result is a rapid-cure compound even at room temperatures.
The advantages of silicone potting materials are:
The disadvantages are:
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.
Fig 4: Evaluation prototypes with different potting materials (epoxy, silicone and PU). In this particular case, the PU compound had the best overall performance and adhesion qualities
We are here to help
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.
RECOM: We Power your Products
--------------
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.
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.
Encapsulation considerations
All of the advantages listed above only apply if the potting creates a gas-tight and water-tight seal. If the potting material contains air inclusions or ‘voids’, then it may not be such an effective method of protection. Voids can reduce the insulation effectiveness by allowing internal arc-over or surface tracking and, of course, will reduce the thermal conductivity. Also, an inclusion could create mechanical stresses such as cracking due to very high or low air pressures or expansion/contraction of air in the void with temperature.
Fig. 1: Example of air voids in epoxy potting compound (cross-sectional photo).
There are several techniques that can be used to eliminate significant air inclusions in the potting compound, the most effective being vacuum mixing and dispensing. The potting compound is prepared and mixed under a vacuum which causes any air bubbles to rise to the surface. It is then pumped into dispensing syringes via a pressurized system that does not allow air to re-enter into the compound. Some products, such as our RAC-K series AC/DC power supplies, use the potting as part of their approved safety isolation barrier and safety agencies require that the penetration of potting compound and void-free performance must be controlled in production. This is guaranteed by the vacuum impregnation process along with periodic monitoring by ‘sectioning’ or X-ray.
However, the majority of our products are not potted under a vacuum as this speeds up the production capacity greatly, important when you are manufacturing more than one million products each month. RECOM can still avoid air bubbles in the potting compound by using other techniques, which in combination, can eliminate almost all included voids. The first of these is the correct potting process; firstly, the case is partly filled with the potting compound, then the pre-tested and assembled PCB is inserted into the case and the case filled almost to the top. The partly-completed converter is then placed in a resting oven which is warm, but held below the curing temperature of the compound. The potting compound becomes very liquid at the warm temperature and any trapped air bubbles rise to the surface and, if necessary, the case can be placed on a shaker table to encourage the bubbles to rise. Finally, the converter case is filled completely and transferred to a curing oven at a higher temperature.
Two-component epoxy potting material is hydroscopic – it will absorb moisture from the air leading to a mottled surface appearance called “amine bloom”, so it is important to use fresh chemical compounds that have been stored in a low-humidity, temperature-controlled environment. Also, the PCB itself should be clean and free from residue. As RECOM has its own SMD production lines, we can control the cleanliness, humidity and temperature in our production area closely. A clean, bare-board converter will allow the potting compound to adhere well to the components and PCB to reduce air inclusions.
Finally, we can use X-ray inspection to check the converters for voids. For new designs, some air bubbles can be unexpectedly trapped under components or under the PCB and be difficult to remove. In this case, we can use the results of the X-ray inspection to either reposition the components or to add strategic holes in the PCB to allow trapped air to escape during the potting process.
Factors affecting the choice of potting compound
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:
Fig.3: Shore hardness scale (source: Smooth-on, Inc.)
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:
| Number of thermal cycles | Failures with soft potting | Failures with hard potting |
|---|---|---|
| Initial test | 0 (out of 10 samples) | 0 (out of 10 samples) |
| After 250 cycles | 0 (out of 10 samples) | 0 (out of 10 samples) |
| After 500 cycles | 0 (out of 10 samples) | 0 (out of 10 samples) |
| After 750 cycles | 0 (out of 10 samples) | 0 (out of 10 samples) |
| After 1000 cycles | 0 (out of 10 samples) | 2 (out of 10 samples) |
| After 1250 cycles | 0 (out of 10 samples) | 5 (out of 8 samples) |
| After 1500 cycles | 0 (out of 10 samples) | 2 (out of 2 samples) |
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.
Epoxy/polyurethanes/silicones
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)
Epoxy resin: this is the main type of encapsulant used. Two-component epoxy consists of a polymer resin and a hardener, which, when mixed together causes a chemical reaction which cross-links the chemical bonds in the polymer chains to create a tough, rigid and strong compound.
The advantages of epoxy potting materials are:
- High rigidity & tensile strength (tough and durable in the cured state)
- High temperature withstand capability (-50°C to +150°C or more)
- Low shrinkage
- Good adhesion
- Good chemical resistance
- Good moisture resistance
- High electrical insulation (17kV/mm)
The disadvantages are:
- The curing process is exothermic (generates heat), so epoxy potting must be cured slowly over several hours to allow the internal heat generated to dissipate
- The slow curing time means that the epoxy potting process can be a bottleneck in production
Polyurethanes: Polyurethane (PU) is a type of thermoset plastic. Like epoxy, it is typically a two-component compound consisting of a base resin with an isocyanate curing agent. Unlike most epoxies, it sets to a softer, more rubbery state.
The advantages of PU potting materials are:
- Elastomeric (flexible and rubbery in the cured state)
- High resistance to abrasion
- Good for delicate components such as ferrites
- Good for dissimilar materials or larger components with high thermal expansion rates
- Customizable flow and curing properties (including fast cure)
- Low glassing temperature (typically -70°C)
The disadvantages are:
- Can be susceptible to moisture ingress over time (unless polybutadiene type is used)
- Maximum temperature is limited to around 130°C
Silicones: Silicone rubber is a synthetic polysiloxane polymer that uses an additive platinum catalyser to transition from the liquid to the solid state. The result is a rapid-cure compound even at room temperatures.
The advantages of silicone potting materials are:
- Elastomeric (flexible and rubbery in the cured state)
- Stays soft at extremes of temperature (-50°C up to +200°C)
- Good thermal conductivity
- Good for thermal shock, thermal cycling
- Good adhesion
- Safest for the environment (no by-products from the curing process)
The disadvantages are:
- More expensive compared with PU or epoxy
- Some industries ban silicones because the material can continue to out-gas after curing, depositing microscopic layers of silicone rubber on adjacent components. This can affect solderability, paint work and interfere with relay electrical contacts
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.
Fig 4: Evaluation prototypes with different potting materials (epoxy, silicone and PU). In this particular case, the PU compound had the best overall performance and adhesion qualities
We are here to help
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.
RECOM: We Power your Products
--------------
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.
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