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Dehydrating moisture sensitive chips at home
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clearchris:
So I bought some stm32 chips to repair a 3d printer board.  They were on the shelf in the bag with dessicant for 6 months, but the moisture detector strip shows under 60% moisture, but over 10% moisture.  I need to bake them.


--- Quote ---STM32 AN2639 Application Note:
The stm32 After opening a dry pack, soldering should be done within 24 hours. SMD products stored over the specified storage period need to be baked at 125 °C for 24 hours (under nitrogen atmosphere).
--- End quote ---

I can bake the chip at 125C for 24 hours, but "under ntirogen atmosphere" is a bit of a problem.  I have read that the nitrogen is required because even at 125C the air could have as much as 20% humidity.  That seems a bit high to me, but I don't have a hygrometer, so that's hard to verify.

What do you recommend for drying these chips?  I was going to reflow them, but I think that's out at this point, I just need them OK for hand soldering or soldering with hot air (recommendations?).  There's no pad underneath the chip, it's not bga, just smt. 

Here's are some items that could be relevant in making a solution
I have a reflow oven ( https://hackaday.io/project/171619-not-just-a-reflow-oven )
dessicant
vacuum pump
3d printers with heated beds that could be used as a hot plate up to 120C or so
40cf nitrogen

My oven isn't airtight enough to run a nitrogen purge, but I could do something like find a small jar, put the dessicant in with a the chip, drill a small hole in the lid and bake in the oven.  Or I could build a small vacuum chamber and run the vacuum pump.  Anyone have thoughts or ideas?

Thanks!



David Hess:
For hand soldering nothing needs to be done.

I would place them under a vacuum for hours to days to remove the moisture.  Under a vacuum and heated would be even better.
KE5FX:
This is the kind of thing you need to worry about in commercial production, but unless you plan on assembling thousands of units at home you will almost certainly never have a problem.  Don't sweat it.
jogri:

--- Quote from: David Hess on April 22, 2021, 07:05:16 pm ---For hand soldering nothing needs to be done.

I would place them under a vacuum for hours to days to remove the moisture.  Under a vacuum and heated would be even better.

--- End quote ---

You need a rather good vacuum to remove water... Should be well below 10 mbar. I've used that method to dry stuff, but it takes quite some time (or a vacuum pump that can reach 0.01-0.1 mbar). You add a bunch of fresh desiccant to the chamber, evacuate it and let it sit for a few days.
clearchris:

--- Quote from: David Hess on April 22, 2021, 07:05:16 pm ---For hand soldering nothing needs to be done.
--- End quote ---


--- Quote from: KE5FX on April 22, 2021, 08:12:52 pm ---This is the kind of thing you need to worry about in commercial production, but unless you plan on assembling thousands of units at home you will almost certainly never have a problem.  Don't sweat it.

--- End quote ---

Hmm.  In the past, I had attempted to tranplant an stm32 chip from a blue pill to the printer board, and failed each time.  IIRC I tried 2 or 3 times, and it was by hand with hot air.  Maybe I destroyed the chips removing them with the hot air.  I have always hated using chipquik on chips that I'm going to re-use, it's near impossible to get that stuff off.


--- Quote from: jogri on April 22, 2021, 08:40:33 pm ---You need a rather good vacuum to remove water... Should be well below 10 mbar. I've used that method to dry stuff, but it takes quite some time (or a vacuum pump that can reach 0.01-0.1 mbar). You add a bunch of fresh desiccant to the chamber, evacuate it and let it sit for a few days.

--- End quote ---
I have used my pump to reach 25 micron, though usually I wouldn't take it that low.  Large diameter evacuation hoses really speed the process.  ( https://appiontools.com/mh120006eak/ ) Would be easier to raise the temperature of the chamber to boil off the water faster, though I might be concerned about popcorning at that point.

Ok, thanks everyone, I think I'll bake it for 24 hours, maybe with dessicant, but no nitrogen, and hand solder with an iron.  I want to be extra sure of success this time.  Wish me luck, fourth time is the charm?






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