My question is how deep the vacuum has to be to be able to do a valid test on (space) electronics working under space vacuum conditions ?
It depends. We finally got a space vacuum chamber that can pump down to .00001 Torr. Beginning at atmospheric 760 Torr. So it can pump to nearly one-hundred-millionth of atmospheric pressure.
The roughing pump (typical lab pump) can probably get it down to .01 which I always thought would be plenty good for space testing.... A high-cost turbine pump runnning at 27,000 RPM takes it the rest of the way down. I had always figured that .01 Torr (one one-hundred thousandth of atmospheric woiuld surely be good enough for most putposes.. Until this year, when we did a thermal lab and forgot to turn on the turbine pump.
We were demonstrating the properties of radiative heat transfer (the only way to get rid of heat from components under a vacuum after you remove all the convection cooling by air. Turns out, not until we got that final factor of 1000 by turning on the turbo pump, did we actually get the resluts we expected.
Remember, I posted that here back in late November or so. That is, the inability of aluminum to radiate away any heat (it is only 3% as effective as Black). At that time, the point of my posting, was that of-course, the aluminum has to be CLEAN! Just the surface oil from handling completely masked the effect. But it was during this test that we also found that convection cooling still existed even at .01 Torr. Though we did not document it... We simply turned on the turbinge pump.
Notice, we were looking for a 3% value compared to a 90% value of heat transfer. When even that 90% is completely masked by convection cooling which is orders of magnitude still higher. So that is why the closer to pure vaccum the better.
But almost all ectronics that are going to get hot are going to be colored BLACK or other high emissive coating, and so the radiative cooling will probably be effective at higher pressures.
So. The answer is, what are you testing for? Any component that has entraped air or volatiles inside shouldn't be flown anyway (electrolytic caps). Anything else that might have entrapped air would change its characteristics under a vacuum... But with most modern electronics, I woiuld guess that it is the cooling effects you are most interested in... But then anything that should get hot, you are going to use a conductive heat sink on it, and so again, vacuum wont make much difference.. Etc..
I don't know the right answer, but those are our experiences...
Good luck. Bob, Wb4aPR