Tips for improving reliability?

[T3sl4co1l]

The partial pressure of the gas makes no difference -- if there is diffusion, the partial pressures inside and outside the enclosure will eventually mix.

Tim

[CatalinaWOW]

Actually using a hermetically sealed enclosure is not a good idea at all (unless you fill it with a gas like Nitrogen). There will always be moisture going in & out.

No, and this is contradictory!

If there's always moisture in and out, then how would fill gas change anything?  Dry air, N2, Ar, SF6 -- they'll all equilibriate with ambient gasses, including H2O, O2, even what small amounts of He and H2 are around.

The point is to have a seal that is impermeable even to those.  Vacuum apparatus does this every day.  Metal-metal and metal-glass seals are typical, but even gasketed joints can be used to a certain extent.

This is the difference between a truly hermetic seal that does not allow gaseous diffusion, and something that is merely sealed against relatively rapid pressure changes like a plastic IP67 enclosure.

A sealed, but non-hermetic, enclosure might equalize in days, years or centuries, but if it's permeable to gas, gas will diffuse in and out, and condensation and oxidation will be a problem at some point.  (Obviously, if it's in the centuries, that's good enough for commercial purposes, but still not truly hermetic.)

Tim

As soon as you allow gasket joints it is all an argument about rate.  If you can keep the moisture level below a set level for the service life of your product it is effectively "hermetic" for reliability purposes.   That set level is usually either the dew point for the lowest projected temperature exposure or freezing.

A fill with dry nitrogen can delay the arrival at the limiting moisture content by reducing the initial moisture load.   You can also use a desiccant cartridge, fill or structure to sop up some moisture allowing a higher permeation rate through your seals.   

How much of this you have to do depends on how much reliability you need.  Venting with conformal coating is good enough for many applications.

[patrick_2]

What I've seen is older ECUs were vented with conformal coating, newer stuff is sealed shut and no conformal coating. I guess the sealed is more reliable, but that would increase the cost a lot and kill the serviceability. It's common for sealed and vented ECUs from 20-30 years ago to still be working fine, so I think I'm going that route.

[CatalinaWOW]

ECUs share an advantage with all automotive electronics in reliability over several other applications.  Unless it is installed in a museum piece, the engine will be started and the automobile heated regularly.  This tends to dry out the electronics and reduces moisture related problems.  That is why you see unsealed (but splash protected) ECUs lasting decades.