|Ask the Experts|
January 12, 2021
Conformal Coating Over Heat Sinks
We have PCB assemblies that will be conformally coated using a had spray acrylic coating. Should we apply masking to the heat sinks to prevent them from being coated? If the conformal coating is applied over the heat sinks, will it impact their performance?
|Expert Panel Responses|
In my opinion, yes, you should mask the heat sinks. Each conformal coating will have some impact on heat flow. Ask the coating vendor for the emissivity figure of the coating. A perfect emitter of heat will have a value of 1. The closer you are to 1, the better a heat radiator the coating is.
In general, the ability of a heat sink to move heat, at least those mounted to the tops of components, is based on an assumption of radiating to air. If you reduce that amount, then you might not be getting the full cooling effect that the design engineer assumed. For most of our products, we do not allow conformal coating of heat sinks.
Masking does not need to be a complex issue. Companies like Shercon or Dempsey can make stock plastic boots that fit over components and come off easily after coating.
Principal Materials and Process Engineer
Yes, definitely mask them; the coating will impede performance. The bulk thermal conductivity of a copper heat sink is close to 380W/(m*K). The bulk thermal conductivity of most polymers is at or below 0.2W/(m*K), or nearly 2000 times lower.
The presence of the coating will impede heat transfer because the heat must be transported through the coating to the surface before being transported to the environment by convection or radiation.
Applying conformal coating in general to a heat sink or an associated accumulator sink will affect its ability to dissipate the heat. There may be some coatings that have a relatively high thermal conductivity but in general coatings will retard the dissipation rate.
Although this adds another process step, I'd recommend masking off the sink with an easily removed peel-able mask. Feel free to contact me for a sample of some of the masks that we manufacture.
Conformal Coatings are usually 2-6 mls. This would have very little effect on the heat dissipation. However in a perfect world you would mask. There are some easy ways to do this quickly.
Question is Why conformal coating need to put on heat sinks where there is no evidence heat sink will have environmental adverse effects.
You may need to mask heat sink provided keep lead join of heat sink open for coating.
Supplier Quality Leader
It will not stop the heat sinks from working but may reduce their efficiency, but leaving them uncovered also increases the risk of the conformal coating not working 100% (depending on how well it is applied).
Using automated machines you can selectively coat areas of a PCB but hand spraying can give thicker or thinner coatings and if this occurs you may see areas where the results do not offer good protection.
All things being considered I would try to mask the heat sinks and hope the results are OK.
Global Product Champion
Regarding coating of heat sinks with conformal coating.Here is a calculation based on the thermal conductivity of acrylic resin being 0.2 w/mk.
Epoxyis about 0.35 w/mk.
0.2watts/mK = 0.002 watts/cmK =y
Coating thickness 50 μm = 50 x 10E - 4 cms = x
x/y = 2.5degrees C/watt for a 1 cm2 area.
Put in a simple comparison context, a highly filled thermally conductive polymer would be about 4 watts /mK giving 0.125 degrees C/watt cm2 for the same 50um layer. The lower the number the better, i.e. less resistance toheat passing through the coating!
But it's not that simple because then you have to factor in the thermal impedance of the area of a board dissipating to free air which depends on its topography, (actual surface area not just the flat square), and thermal emissivity etc etc!
The upshot is, don't coat heat sinks or surfaces of components that may be used in thermal interfaces.There is no significant effect on the normal heat dissipation of standard components.
Masking should be done with a non silicone based solvent resistant adhesive tape and if the board is of high sensitivity to static, use a silicone free anti static masking tape.Both of the above are available on request.
Selective coating will prevent over spray and allow you flexibility and consistent results. If you are using an acrylic, it's important to ensure coverage is void free and curing is done in a controlled environment for consistent results PCB to PCB.
If Selective coating is not available, you can develop a "boot" to cover the Heat Sink and then manually apply coating around the component that the HS is associated with. Generally speaking, if this is a Power component, you'll need to coat the solder pedestal and/or Solder interface to the board.
Defining the coating and it's application can be difficult task, so extra time should be taken to document this on your Assembly Drawings and/or those provided by customers.
Capital Equipment Operations Manager
Specialty Coating Systems
To be safe the answer is yes and yes. Covering a heat sink even with a thin layer of conformal coating will impact its performance.
Having said this the impact should be relatively small and may not be sufficient drop the overall thermal performance of the assembly below the system requirements. If this is the case the it may be acceptable to have the heat sink coated.
Senior Applications Chemist
Although a conformal coating will have a significantly lower thermal conductivity than the heat sink, it is applied in such a thin layer that I do not think it will significantly reduce performance, some differences may be seen, however.
My concern would be in relation to the coating quality achieved over a heat sink. Spray coating a heat sink using a hand gun may involve coating gathering in certain areas due to the complex geometry.
If areas of coating do gather there are a number of problems that may occur,such as solvent entrapment, breaks or gaps in the coating and reduced heat dissipation in areas of thick coating.
In addition, coating over the heat sink will make it more difficult to remove and could affect the integrity of the coating, leading to moisture ingress. I think the safer option would be to mask the connector using an appropriate material or guard.
European Technical Support Specialist
Of course coating reduce thermical conductivity. In practice it's often negligible, because of the low thickness of the coating, and because heat don't take only this patch. We have experience of products which started to burn because of high moisture in some countries, and no coating between PCB and heatsink.Jean-Jacques FABRE, ACTIA
Moisture used the isolator sheet (between PCB and heatsink) and generate short circuit between different vias of the PCB... We remove the masking operation of coating, we saved money, resolved the problem, and didn't measure significant difference of component temperature. So if you have the same configuration, I would recommend to test the difference and let the coating.
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