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Vacuum Bags

Views: 3459


aj

#50236

Vacuum Bags | 16 May, 2007

Hi all,

Are there special type bags which you need to use for Vacuum sealing Boards/tray parts etc. or are they just the standard anti-static bags ?

aj...

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#50238

Vacuum Bags | 16 May, 2007

You want Moisture Barrier Bags (MBB). It is possible you are calling these bags, standard anti-static bags. If you can see anything through the bag, they are not what you want to use for vacuum sealing. In any case make sure you get MBBs.

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#50242

Vacuum Bags | 16 May, 2007

Hi Stephen,

What if you used an anti-stat bag with a desiccant inside? Would this be the same affect? Sorry to ask a question over another question - just curious.

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#50243

Vacuum Bags | 16 May, 2007

Moisture barrier bags are polymer coated foil so are nearly gas impermeable. There's always some coating left at the weld (or it wouldn't stick) so they're not completely sealed but that's as good as it gets.

Usually people use the vacuum to remove air and then replace with nitrogen to eliminate oxidation and a medium for bacterial growth. Why anyone would need to vacuum pack a pcb is beyond me, though.

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#50244

Vacuum Bags | 16 May, 2007

3.3.2.1 Moisture Barrier Bag (MBB) The moisture barrier bag shall meet MIL-PRF-81705, TYPE I requirements for flexibility, ESD protection, mechanical strength, and puncture resistance. The bags shall be heat sealable. The Water Vapor Transmission Rate (WVTR) shall be �0.002 gm/100 in2 in 24 hrs at 40�C after flex testing per condition ��E��ASTM F 392.

The above is from IPC/JEDEC J-STD-033A.

I read a study a while back by IBM that showed too strong a vacuum would cause too much moisture to enter the bags.

I don't know how long desicant can absorb the moisture let in by an ESD bag. But basically if it doesn't meet MIL-PRF-81705, then it doesn't meet IPC/JEDEC J-STD-033A.

And if you are running MSD components on one side of the board then not reflowing the other side for a while you would be better off with a good sized dry cabinent.

On a side note, ever notice how if someone doesn't know an answer they say something along the lines of "standard such and such says..."?

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#50245

Vacuum Bags | 16 May, 2007

"I read a study a while back by IBM that showed too strong a vacuum would cause too much moisture to enter the bags."

That's what the nitrogen is for. If you pull a vacuum w/o backfilling with something dry, like nitrogen, pretty soon the vacuum overcomes the bag permeability (and pump, vacuum chamber, etc.) and outside air ends up getting in. Granted, if you're pulling that kind of vacuum you're probably perforating your bag on components anyway so it's kind of pointless.

Playing with vacuum is fun. You'd be amazed the trouble you can get into wth a system capable of pulling down to <.1 Torr and a Tupperware container with 1/2" of water in it. :)

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#50247

Vacuum Bags | 16 May, 2007

Oh sorry. Let me try again.... If I seal a desiccant in an ESD bag and such says, would it be the same standard such affect?

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#50248

Vacuum Bags | 16 May, 2007

For a day or a week probably. Possibly a month. But I"m sure not for a year or such. There would be too many variables to know exactly how long it would be before the desicant became saturated. And it wouldn't meet the Jedec standard.

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aj

#50251

Vacuum Bags | 16 May, 2007

"Usually people use the vacuum to remove air and then replace with nitrogen to eliminate oxidation and a medium for bacterial growth. Why anyone would need to vacuum pack a pcb is beyond me, though. "

We might get a batch of 400 pcbs in but the kit might only clear for 200 - these boards are either left on the shelf or they need to be baked and then vacuum packed for storage ( am I putting extra work on us for doing this ?)

We have undoubtedly seen an increase in issues with PCBs since the transition to Leadfree and on one particular batch which we seen delamination - baking these boards got rid of the problem. So what I am trying to do now is get stores to bake and vauum pack any boards that are causing us problems.

Is Vacuum packing PCBs not done elsewhere ???????

aj...

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#50252

Vacuum Bags | 16 May, 2007

Can't you schedule the bake to coincide with the production run so it just becomes part of the process flow?

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