Josh Koffman gmail.com> writes: > I had an idea for a project that will be subjected to some pretty low > operating temperatures (say down to -20C). I know there are some > tricks when using through hole components (ie leaving a bit of extra > lead to act as a flex point when things contract at different rates), > but I don't know much about using SMT in low temperatures. I would > guess that using as fat tracks as possible would be good to try to > avoid them getting broken. Would a metal clad PCB be an asset or > liability? I can think of arguments both ways. Any other tips? SOIC > would probably work size-wise (and have the largest leads), but 0603 > resistors have zero flex. Extended temperature range modules use Alumina boards for that reason. The thermal expansion coefficient is the same as for all alumina and ceramic ba= sed parts (SMD resistors and most caps). Your most important concern is likely complete dry packaging including probably void elimination (potting in sili= con with ultrasonication might work well). Any void will collect condensate and= that will expand and contract with thermal cycling eventually causing whatever i= t is under to separate or crack. So one uses conformal coating to create a surfa= ce that is mostly convex on the outside. You also probably want to limit the s= ize of each module as much as possible. Take a good look at a ready made hybrid circuit, it comes close to what you need. I am not an expert in the field b= ut I recently read the relevant information for a project. The easyest way to de= al with cold it so avoid it. A PTC and a few resistors will provide enough hea= t to keep the board above the dew point and make most special measures above avoidable. The simplest and oldest way to deal with this seems to be to pro= vide a small incandescent lightbulb in the case, undervolted with a resistor. Li= fe expectancy is >10,000 hours (over 12 years) for an undervolted bulb, but it= will likely still be bright enough to serve as 'Power' indicator. -- Peter --=20 http://www.piclist.com PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist .