This page has important hints on short cuts to make your potting easier and more efficient. Contents of this page (Click to go to the area of interest):
I. Properties of that effect performance
Printed circuit boards that have components that generate high amounts of heat present problems when they are potted. The design engineer wants the potting compound to dissipate the heat so they specify thermally conductive potting compounds. This would seem like a logical idea, but thermally conductive potting compounds have problems and much higher cost for very little increase in heat removal. The discussion that follows gives some insight in to the situation.
Thermally conductive potting compounds generally have Thermal Conductivity of 10 Btu-in/oF- ft. Normal potting materials are 2 Btu-in/oF- ft when filled with calcium carbonate and 1 Btu-in/oF- ft when unfilled. Comparing that to air at 0.01 Btu-in/oF- ft, it seems pretty good but when compared to aluminum at 1200 Btu-in/oF- ft, potting materials are very low. This is why we suggest heat sinks as the primary means to remove the heat even though the difference between thermally conductive and normal potting compounds is 5 times the difference between two small numbers is small when compared to the very high number of aluminum’s heat transmission.
In a real life situation, a customer cast a hockey puck of one of a normal potting compound that was filled with calcium carbonate and a thermally conductive product. They placed them on a hot plate at 150o C with a hockey puck of Aluminum of the same dimensions, let them come to equilibrium and measured the temperature on the top of the three pucks. As expected, the top of Aluminum puck was 148°C since the transfer of heat is high. The normal potting compound was only 110°C but the potting compound that was five times more thermally conductive was only 113°C. Thermally speaking, the potting material acts like a heat insulator. It is better than air so it does help, but then use the less expensive normal potting compounds. The cost difference between the two systems was 20%. The other factor not discussed was that the filler used in this high thermally conductive potting compound is very abrasive and will wear out MMD equipment very fast.
The designer still needs to remove the heat so we recommend the placement of the heat generating units close to a metal heat sink, closer the better. Use a flame-retardant potting compound that uses a filler called Hydrated Aluminum Oxide. This is a soft filler that will not wear the MMD equipment, but gives an intermediate thermal conductivity of 4-6 Btu-in/oF- ft. and it’s cost is not that more expensive than the normal potting material.
There is one situation that we would recommend the high thermally conductive potting compound. This is when the component exceeds its temperature limit when potting. Even though the thermally conductive potting materials do not conduct much heat, they do lower hot spot temperatures – the temperature next to the component. This is because they have a higher specific heat i.e.; can absorb more Calories or BTU’s. The filler in them requires more BTU’s or Calories to raise the temperature. Your decision to use a thermally conductive potting compound is basically determined by how sensitive the component is to the thermal rise. If it is really critical, then use a thermally conductive potting compound.
The CTE of all potting compounds are higher than the parts being potted. Therefore, many designers are concerned about the coefficient of thermal expansion of the potting compound in order to reduce stress on the device being embedded and to stop cracking on thermal cycling. Sometimes they make their decision of what material to use solely on the CTE property and not considering other factors that also effect the stress. These other factors are shrinkage, the temperature at gel, operating temperature extremes of the part, resiliency or flexibility of the potting compound and the Tg of the potting compound. These all can all help reduce the stress induced on the part because of the difference of the resin system CTE and the part CTE.
1. Shrinkage - All potting compounds shrink when they change from the liquid to the solid state with a chemical reaction. Shrinkage put added stress on to the part to be potted. A slow, orderly hardening of the potting compound can minimize the amount of shrinkage and the stress it brings. With a fast hardening resin system the shrinkage is higher due to the exothermic heat given off during hardening and the fact that the reaction speeds up when the temperature is raised further increases the speed of hardening and more shrinkage. Therefore, to reduce shrinkage, lower the hardening temperature to minimize exotherm and slow the reaction to allow orderly flow in to shrinking areas. By reducing the mass of the potting compound less exotherm will develop which will help reduce the shrinkage. Also, using a potting compound that has a long gel time and has filler in it helps reduce the fast hardening. Since the polymer is the only item that shrinks, using a filled systems give less shrinkage since it does not shrink. Filled resin systems also have a lower CTE.
2. Temperature at Gel - The temperature at which the potting compound is gelled is the zero stress point. So if it gels at 40°C and then cools to room temperature at 25°C, the potting compound will shrink base on its CTE. This puts stress on the part. The temperature at gel can be changed as discussed in shrinkage, by reducing the mass, lowering the temperature at which it is hardened and the choice of a low exotherm potting compound. A potting compound gels at the highest exothermic temperature so the zero stress is at a high temperature even though the curing temperature is room temperature. Reduction of the stress during thermal cycling is further discussed in operating temperature extremes of the part.
3. Operating Temperatures If the temperature extremes are only 10° to 30°C then the stress on the part will be small, but if the extremes are –40°C to 150°C the difference in temperature is 190°C which is high. Of course, this is when the CTE comes in to play – lower the better. We have found that the low temperature is normally the greatest problem with stress since as the temperature goes down the potting compound gets harder, see Tg. A method to reduce the stress is to gel the potting material at the lowest temperature so the zero stress temperature is lower and therefore the difference between the low temperature and zero stress temperature is reduced. This reduces the stress when the part is cooled. Using our example of –40°C to 150°C temperature range, if we gel the potting at 25°C rather than 60°C we get only 65degree vs. 100 degree difference. So the shrinkage will be less: CTE x 65 versus CTE x 100.
4. Flexibility of the potting compound - If the compound has some flexibility it will reduce the stress on the device enough to allow it to be used. This is most successful if the device is not thermally shocked rapidly. The hardness of the compound along with the Tg, glass transition temperature, helps reduce the stress to the point were the CTE is no longer a factor. Potting compounds have hardness values from Shore 000 = 50 to Shore D = 95. This is a hardness of Jell-O to the hardness of a plastic counter top.5. Glass Transition Temperature (Tg) The glass transition temperature is the temperature at which the potting compound changes structure, similar to a crystalline structure change, and becomes harder and more brittle. Some polyurethane systems have glass transition temperature below –50 C. This means less stress since the potting compound does not become hard at those temperatures so it can deform rather than putting pressure on the part. Therefore, for SMT boards, polyurethane is recommended so solder bonds are not broken, see below.
II. Special Parts
Potting through-hole PC boards can use most any potting compounds but potting of PC boards with surface mount components on them present a special problem. This problem is the breaking of the solder connections of the surface mount components by the potting compound. This happens when the PC board with SMT is exposed to lower than room temperatures. Because the CTE of potting compounds is higher than the PC board the potting shrinks more than the board and pushes against the surface mount component. If the temperature is below the Tg of the potting compound the compound becomes harder and gives more pressure to the component until it breaks the solder connection.The solution to this problem is to use low Tg potting compounds so they remain flexible and deform rather than moving the component or use a very low CTE material and other modifications, see the discussion of CTE. The most practical solution is to use the low Tg potting compounds like urethane and silicone. Epoxy, acrylics and polyesters have high Tg and are so hard at the lower temperatures they break the solder connection. The lowest Tg epoxy that we have seen is –22°C. The type of urethane which is most effective is Poly BD which have a Tg in the –50°C range.
Components such as frequency generating crystals, glass incased reed switches, and powered iron parts are a few parts which will change their properties when pressure is applied. They require some special care in selection of the potting compound and or the method of potting so their properties are not changed
1. Low Stress Potting Compounds- Potting compounds that are soft and have a low Tg would be recommended. The hardness can be as soft as Jell-O. These potting compounds are called re-enterable potting compounds because it can be torn out of the part very easily so that a bad part can be replaced and then re-potted. These materials are not only soft but have low modulus and low strength but also high elongation so they do not but much stress on the part. If the Tg is low then the stress level will stay low even as the temperature goes to –40°C. See the discussion of CTE for more understanding.
2. Cocoon the Part – This is a process of protection of the sensitive part by the method of potting. The method is to pot the part twice, once with a very soft, low Tg material and then with a harder potting compound over the soft compound. This may take the form of only the sensitive section of the part having the soft material and all other sections would have the harder material. This is complicated. In cases were the hard outer shell is required it is only method we know that works. Processing of this method has some problems but they can be overcome.
If you are potting a RF part that has an antenna attached to it, special processing techniques for success operation after potting. The potting compound is not electrically conductive so it acts as a capacitor and changes the electronic circuit especially at high frequencies. This shifts the frequency of the output of the circuit. There are methods to allow the RF part to be potted but are complex.
A. Don't have Potting Shells?
If you need to pot your device but do not have a potting shell you can use a plastic ice cube tray to put the units in and then pour the resin system in to the tray. Because the ice cube tray is polyethylene or polypropylene the resin will not stick to it and you can remove the potted unit the same way you empty ice. For more information on potting shells see Potting Shells
B. Need to remove the potting material from the device?
There are two ways to remove potting compound either physically or chemically remove it. To physically remove the potting compound it is common to heat the part until the potting compound becomes soft and it can be torn off. If it is a high Tg potting compound it could be cooled and then hit to shatter the resin since it is brittle at low (-40C) temperatures. Chemical removal of the potting compound is done by soaking in solvents to soften the resin so it can be torn off. The solvents are toxic so care must be used. Commercial mixtures are available - see accessories supplier page
C. Underwriters Laboratory Recognition of Potted Part
If you require Underwriters Laboratory (UL) recognition of your part, use suppliers have received UL recognition of their potting compounds for UL flame retardant and or UL 1446 insulation systems. By using those materials and using other recognized pieces such as the plastic case or potting shell you will save time and money since UL will not require as extensive testing of your potted part. UL for the US and CSA for Canada have harmonized their requirements so most suppliers of potting compounds have both UL and CSA recognition. You can contact UL from their web site at www.ul.com and view the suppliers systems that are recognized. They will be under Customers, then click on the UL Online Certifications Directory in the first paragraph, and then search by company or UL File Number.
For some applications a shiny surface is a requirement. Many of the potting compounds have a mottled appearance which are unacceptable. This mottled appearance happens with certain epoxy hardeners when the humidity in the air is high.Polyurethane compounds normally do not have this problem. Here is a picture comparing a heavily mottled surface with a shiny surface. There are two methods to obtain a shiny surface. One is to use a potting compound which has a hardener which does not have this mottling called "amine blush". If this is not possible because of the other requirements then cure the potting compound in an oven at slightly elevated temperatures, 100°F. Although the surface will not be as shiny as the non-blushing compound it will not give the heavy mottle appearance shown in the picture.
Color of the potting compound normally is not a consideration and most people use black or natural tan color. The preference for black usually is because potting cases or shells are black to reduce costs. If the potting compound spills on the case it is not easily seen and no cleaning of the spill is required. If color is an important requirement then be prepared to pay more for the potting compound.
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Last Modified: July 20, 2012