My company is working on developing a product that will go into commercial freezers, so my boss asked me to provide operating temperature specifications for the product. I can find "operating range" temperatures listed for everything but the PCB itself, which is just plain old FR-4.
Wikipedia helpfully lists "Temperature Index" (whatever that means) as 140 C, but there are no indications of a minimum temperature.
I'm not really worried, as I'm sure that the other components on the board will be the limiting factors, but for the sake of completeness, I would like to have it listed.
Does anyone known the minimum operating temperature of FR-4? (And what would the failure mode be?)
Best Answer
FR4 PCB is glass-reinforced epoxy laminate. Several research studies have been published of the effect of low temperatures on such material.
A specific quote from the paper "Dynamic failure behavior of glass/epoxy composites under low temperature using Charpy impact test method" (Shokrieh et al):
This study applied a temperature of -30oC, not low enough for spontaneous crystalline brittleness.
Another study, "The Influence of High and Low Temperatures on the impact properties of glass-epoxy composites" (Putic et al) takes the temperature down to -50oC, and finds the appearance of brittle cracks in the material at such temperatures.
These are two key assumptions regarding the device touched upon in the question:
If either of these assumptions is invalid, the material in question needs to be reconsidered. There do exist special-purpose industrial ceramics / alumina PCB substrates designed specifically for extremely low temperatures, typically used for device deployment in space or in cryogenic equipment. Those materials might be more suitable in this case.
The point to note in such environments is the possible fracture of packages, casings and solder junctions for the electronic components on the board, not just the PCB itself.
A commonly recommended procedure for deploying circuit boards in extreme temperature conditions, is to bring the device to the desired temperature in slow stages, thus avoiding any rapid shrinkage or thermal shock to board or parts.