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I recently came across a reference to the use of stencils to consistently apply thermal paste/grease between device and heatsink. I'm having trouble finding much better explanation of that concept. Are such stencils commonly used? If so, how does one use, construct, and/or obtain them?

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    \$\begingroup\$ Link to the reference, if it's public? \$\endgroup\$ Apr 1, 2014 at 21:23
  • \$\begingroup\$ It was a comment at a conference, sorry. \$\endgroup\$ Apr 1, 2014 at 21:24
  • \$\begingroup\$ Thermaltake got you covered my friend tomshardware.com/news/… \$\endgroup\$
    – Josh
    Aug 12, 2020 at 1:00

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A stencil could be used as Scott mentions, or a "silk" screen with adequately large mesh for the viscosity of the thermal grease. I put "silk" in scare quotes because most screens these days are polyester mesh.

silk screen image

I have not used these for this particular purpose, but I have done more than enough silk screen printing to know it would be no problem to print arbitrary patterns of thermal grease on the flat surface of a heat sink (for example, 12 TO-220 package outlines could be printed at once).

The problem comes in if the surface has any protrusions (such as fins) which could interfere with the screen or the screen frame.

In the case of stencils, the viscosity is similar to solder paste, so printing should be similar. You could create a gerber file for the stencil maker as you would for a PCB layer. In the case of a screen, they normally work from graphic arts files such as Adobe Illustrator or PDF.

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    \$\begingroup\$ This Dow Corning PDF shows screen printing of thermal grease \$\endgroup\$
    – endolith
    Oct 16, 2014 at 19:08
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    \$\begingroup\$ @endolith A couple nice things about screen printing the thermal grease are that you can get a fairly controlled even thickness (crucial for controlling thermal resistance) and it doesn't dry out so the screening operation will be much easier than with fast-drying inks. \$\endgroup\$ Oct 17, 2014 at 1:43
  • \$\begingroup\$ Great point about using traditional tools to prepare gerbers for manufacturing. I wonder, though, how flexible are they with the thickness of the stencils? I bet they are carefully adjusted for solder paste application, which most likely is significantly different from thermal paste thickness. \$\endgroup\$
    – Maple
    Feb 29, 2020 at 20:21
  • \$\begingroup\$ @Maple A 0.1mm stencil is appropriate with appropriate dimensions of honeycomb layout of thermal paste. That's standard from my stencil supplier, though a bit thinner than the standard 0.12mm. \$\endgroup\$ Feb 29, 2020 at 21:17
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I haven't heard of them, so I'll answer with respect to what I'm sure of, and then move off into SWAG territory. You'd make them the same way you make a stencil for solder paste. Pick a layer, and use the gerb processor to put that layer in a gerber file, and send that file to a stencil maker. Use a tool like gerbv to make sure you have it right before you waste money.

Now the part I don't know -- It would be very difficult to use such a stencil when there's been any populating of the board at all. It can't be done before the board is populated, or you'll screw it up during solder stencil printing. It can't be done after, because there are components in the way. Maybe its doable as part of a wave solder assembly, or with a small hand-held stencil. I can't think of any particular case where thermal paste is a real precision job, though. I can picture some situations where you might want a metered grease delivery, but there are better tools for that than a stencil.

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    \$\begingroup\$ If heatsink mounting is a part of automated manufacturing then an easy way to do it is to apply paste to heatsinks while they are sitting nice and even in the supply tray. \$\endgroup\$
    – Maple
    Feb 29, 2020 at 20:31
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Yes they are. it is a method which not only ensure a reasonable repeatability on the application but also is the fastest and less "messy" method during manufacture.

Ideally a pad is used (phase change, graphite etc...) as this is the cleanest method for manufacturing to use (it's typically a good idea to not use fluids in manufacture (glue, paste etc)

BUT if paste is to be used a stencil is idea.

https://www.infineon.com/dgdl/Infineon-AN2006_02_Application_of_screen_print_templates-AN-v1.0-en.pdf?fileId=db3a304412b407950112b40ed3f71297

enter image description here

https://www.semikron.com/dl/service-support/downloads/download/semikron-application-note-thermal-paste-application-en-2018-01-19-rev-00/

enter image description here

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  • \$\begingroup\$ very interesting links... I suspected that there is more to it than just "swap with old credit card and slap in place" method, but never seen any research on the issue other than numerous "testing thermal paste" videos \$\endgroup\$
    – Maple
    Feb 29, 2020 at 20:25

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