Ask the Experts
November 8, 2018
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 someimpact on heat flow. Ask the coating vendor for the emissivity figure ofthe coating. A perfect emitter of heat will have a value of 1. Thecloser you are to 1, the better a heat radiator the coating is. Ingeneral, the ability of a heat sink to move heat, at least those mounted to thetops of components, is based on an assumption of radiating to air. If youreduce that amount, then you might not be getting the full cooling effect thatthe design engineer assumed. For most of our products, we do not allowconformal coating of heat sinks. Masking does not need to be a complexissue. Companies like Shercon or Dempsey can make stock plastic bootsthat fit over components and come off easily after coating.
Principal Materials and Process Engineer
Doug Pauls has a bachelors in Chemistry & Physics, Carthage College, BSEE, Univ of Wisc Madison. He has 9 years working experience for US Navy - Materials Lab, Naval Avionics Center Indianapolis. 8 years Technical Director, Contamination Studies Laboratories. 11 years Rockwell Collins Advanced Operations Engineering.
Yes, definitely mask them; the coating will impede performance.The bulk thermal conductivity of a copper heatsink is close to 380W/(m*K). Thebulk thermal conductivity of most polymers is at or below 0.2W/(m*K), or nearly2000 times lower. The presence of the coating will impede heat transfer becausethe heat must be transported through the coating to the surface before beingtransported to the environment by convection or radiation.
Fritz's career in electronics manufacturing has included diverse engineering roles including PWB fabrication, thick film print & fire, SMT and wave/selective solder process engineering, and electronics materials development and marketing. Fritz's educational background is in mechanical engineering with an emphasis on materials science. Design of Experiments (DoE) techniques have been an area of independent study. Fritz has published over a dozen papers at various industry conferences.
Applying conformal coating in general to a heatsink or an associated accumulator sink will affect its ability to dissipate theheat. There may be some coatings that have a relatively high thermalconductivity but in general coatings will retard the dissipation rate. Althoughthis adds another process step, I'd recommend masking off the sink with aneasily removed peel-able mask. Feel free to contact me for a sample of some ofthe masks that we manufacture.
Pierce Pillon is the Laboratory Manager and lead formulations chemist at Techspray, a division of Illinois Tool Works (ITW) and a leading manufacturer of chemical products for the electronics industry.
ConformalCoatings are usually 2-6 mls. This would have very little effect on theheat dissipation. However in a perfect world you would mask. Thereare some easy ways to do this quickly.
Wayne Wagner has over 25 years in the conformal coating industry and is the president of Krayden Inc., a leading distributor of engineered materials.
Questionis Why conformal coating need to put on heat sinks where there is no evidenceheat sink will have environmental adverse effects. You may need to mask heatsink provided keep lead join of heat sink open for coating.
Supplier Quality Leader
Subrat has 10 year of extensive experience in PCB assembly process optimizing for quality, process includes screen printing, wave, reflow. He has a copyright in stencil design published in Apex Expo2010 at Las Vegas US.
Itwill not stop the heat sinks from working but may reduce their efficiency, butleaving them uncovered also increases the risk of the conformal coating notworking 100% (depending on how well it is applied) Using automated machines youcan selectively coat areas of a PCB but hand spraying can give thicker orthinner coatings and if this occurs you may see areas where the results do notoffer good protection. All things being considered I would try to mask the heatsinks and hope the results are OK.
Global Product Champion
Richard Boyle is a Global Product Champion at Henkel Electronics. He has over 25 years experience in the electronics assembly industry and is responsible for the global technical service of all of Henkel's solder materials.
Regarding coating of heat sinks withconformal coating.Here is a calculation based on thethermal 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 asimple comparison context, a highly filled thermally conductive polymer wouldbe 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 thethermal impedance of the area of a board dissipating to free air which dependson its topography, (actual surface area not just the flat square), and thermalemissivity etc etc !
The upshot is, don't coat heat sinks or surfaces of componentsthat may be used in thermal interfaces.There is no significant effect on the normal heat dissipation ofstandard components.
Masking should be done with a non silicone based solvent resistantadhesive tape and if the board is of high sensitivity to static, use a siliconefree antistatic masking tape.Both of the above are available on request.
Chris Palin is currently managing European sales and support for HumiSeal Conformal Coatings. His expertise is in test & reliability, solder technology, power die attach and conformal coating.
Selective coatingwill prevent over spray and allow you flexibility and consistent results. If you are using an acrylic, it's important to ensure coverage is voidfree and curing is done in a controlled environment for consistent results PCBto PCB.
If Selective coatingis not available, you can develop a "boot" to cover the Heat Sink andthen manually apply coating around the component that the HS is associatedwith. Generally speaking, if this is a Power component, you'll need tocoat the solder pedestal and/or Solder interface to the board.
Defining the coatingand it's application can be difficult task, so extra time should be taken todocument this on your Assembly Drawings and/or those provided by customers.
Capital Equipment Operations Manager
Specialty Coating Systems
Rodney is currently Operations manager at SCS coatings, Global Leader in Parylene and Liquid Coating equipment. Rodney applies his BS in Computer Integrated Manufacturing from Purdue University, along with 20+ years of Electronic manufacturing and Equipment Assembly, to direct the Equipment business at SCS Coatings. "We provide unique, value added coating equipment solutions for our customers". Including conformal, spin and Parylene coating expertise.
To be safe the answeris yes and yes. Covering a heat sink even with a thin layer of conformalcoating will impact its performance. Having said this the impact shouldbe relatively small and may not be sufficient drop the overall thermalperformance of the assembly below the system requirements. If this is the casethe it may be acceptable to have the heat sink coated.
Senior Applications Chemist
Dr. Poole is a Senior Applications Chemist in Henkel Technologies, electronics assembly materials application engineering group. He is responsible for all of Henkel's assembly products including soldering products, underfills, PCB protection materials, and thermally conductive adhesives.
Although a conformalcoating will have a significantly lower thermal conductivity than the heatsink, it is applied in such a thin layer that I do not think it willsignificantly reduce performance, some differences may be seen, however. Myconcern would be in relation to the coating quality achieved over aheat sink. Spray coating a heat sink using a hand gun mayinvolve coating gathering in certain areas due to the complex geometry.
Ifareas 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 heatdissipation in areas of thick coating. In addition, coating over the heat sinkwill make it more difficult to remove and could affect the integrity of thecoating, leading to moisture ingress. I think the safer option would be tomask the connector using an appropriate material or guard.
European Technical Support Specialist
Jade Bridges is the European Technical Support Specialist for Electrolube. She is responsible for technical support within Europe, offering assistance to customers with product selection, implementation and after sales support across the range of Electrolube products. Her expertise is carried over from her position as R&D Manager for Electrolube, where she was responsible for the new product development and technical support across an array of chemical products for the electronics industry, including conformal coatings, encapsulation resins, thermal management products, contact lubrication and electronic cleaning solutions.