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I am trying to find the correct potting compound for a device that will sit on top of lead acid batteries. It will have SMTs, and we would like it to tolerate temperatures as low as -40. Because of this we need to make sure whatever compound we select won't break the solder joints when the device and potting compound get very cold.

What is the more important value to be concerned about when potting SMTs; Tg or CTE. I was recommended a very low CTE epoxy for our solution but am interested in hearing other opinions.

Options that I have considered are: Hotmelt (fear it won't last) Silicon (acid resistance??) Epoxy (will it break SMTs off?) Urethane (Acid resistance, will the higher CTE effect SMTs)

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    \$\begingroup\$ To recommend a technology to use, we need to know what purpose the potting is for, e.g. why you couldn't just use conformal coating. \$\endgroup\$ – PlasmaHH Oct 28 '16 at 15:14
  • \$\begingroup\$ Protection from physical shock ie: throwing a wrench at it, Protection from environmental factors; water, battery acid the board is approx 5"x 2" \$\endgroup\$ – Danniard Oct 28 '16 at 15:18
  • \$\begingroup\$ so anything hard is out of the question then I guess. Not sure if anything will be able to handle that much abuse, you should think about additionally putting it into a proper casing. \$\endgroup\$ – PlasmaHH Oct 28 '16 at 15:20
  • \$\begingroup\$ I was pretty sure that most of the options I listed would fulfill the physical shock req. (willing to be wrong) a couple mm thick epoxy was my initial selection; would this break off SMTs at low temps? \$\endgroup\$ – Danniard Oct 28 '16 at 15:25
  • \$\begingroup\$ I don't know the difference in thermal expansion coefficients between common epoxies and PCB materials, but the harder a material for potting is, the less it will obviously dampen the wrench you throw at it, thus delivering more energy directly to the parts, which (depending on what they actually are) they will not likely like. \$\endgroup\$ – PlasmaHH Oct 28 '16 at 15:26

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