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)