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Moisture Barrier Bag Issues



Moisture Barrier Bag Issues
Moisture sensitive chip components have been exposed to the atmosphere beyond specification. Can they be packaged and sealed with desiccant? Jim Hall and Phil Zarrow, The Assembly Brothers, answer this question.
Board Talk
Board Talk is presented by Phil Zarrow and Jim Hall of ITM Consulting.
Process Troubleshooting, Failure Analysis, Process Audits, Process Set-up
CEM Selection/Qualification, SMT Training/Seminars, Legal Disputes
Phil Zarrow
Phil Zarrow
With over 35 years experience in PCB assembly, Phil is one of the leading experts in SMT process failure analysis. He has vast experience in SMT equipment, materials and processes.
Jim Hall
Jim Hall
A Lean Six-Sigma Master Blackbelt, Jim has a wealth of knowledge in soldering, thermal technology, equipment and process basics. He is a pioneer in the science of reflow.

Transcript


Phil

And welcome to Board Talk with Phil Zarrow and Jim Hall, the Assembly Brothers, who by day go as ITM Consultants. Coming to you from high atop Mount Rialto. We're in the component handling room today.

Jim, we actually have a question from E.R. E.R. proposes, Let's say we have moisture sensitive chip components that have been exposed to the atmosphere slightly beyond the specification allowed. They are then re-packaged and sealed with desiccant and a humidity indicator card.

After about a week's time would the card indicate humidity? Could the desiccant alone remove the moisture in the chips or is baking necessary? Would vacuum sealing the bag make any difference?

Well, that is interesting. If I understanding this question correctly, what E.R. is asking is whether he can get away with not baking just by putting these things in a moisture barrier bag.

Jim

And that is why I put in a reference to J Standard 033, which is our Bible for handling components. It specifically defines short term exposure and drying within the moisture barrier bag.

For a level 3 part if your exposure out of the bag is less than twelve hours you can put the parts back in the moisture barrier bag and reseal them. I want to point out to make the moisture barrier bag work you always have to vacuum seal it. That is a given any time.

If you don't vacuum seal your bag you cannot count on them maintaining a less than 10% relative humidity inside the bag, regardless of how much desiccant you put in there. You always need to vacuum seal the bag.

But if you do that and a level 3 part has not been exposed to more than 12 hours then the part will dry out and will go back to zero. Reset the clock to zero, in five times the exposure.

If you were right at twelve hours, you put it back in the bag in 60 hours, or about three days, staying in that sealed bag it would be reset to zero. Anything greater than twelve hours for level 3, and I think it is 8 hours for level 4 and level 5, check 033, which you should be very familiar with if you are dealing with moisture sensitive pars.

But beyond that, you've got to the end of your out of the bag life. If you are beyond that, which is what you are implying. This typical type 3 part, you have 7 days, 168 hours.

Let's say it is out there for 170 hours, you know you have to bake the part. Putting it back in the bag with desiccant is not going to reset it to zero. You are going to have to do a higher temperature bake for two days or more, depending on the temperature that you bake it, which is all covered in 033.

After about a week's time would the card indicate humidity? This is the idea of putting parts that have moisture in them back in the bag, seal them up with a desiccant and temperature humidity card.

Would you ever have a situation where the humidity card would indicate that you are above 10%? I honestly don't know. I suppose if you had a lot of parts that were over-exposed, or let's say they were near their exposure.

Theoretically I got a stack of ten matrix trays that for some reason got left out for 160 hours. So they are still under the limit for a type 3 part. I quickly put them back in the bag, seal them up with some desiccant. Over time some of that moisture will come out of those parts and get absorbed by the desiccant.

If the desiccant relative to the number of parts, and how much moisture they had and how long, I could envision a situation where you could saturate the desiccant and raise the temperature above 10% and trigger the indicator card.

No idea what it would take to do that, how little desiccant and so forth. I guess that could happen. Obviously if you have any bag of components where the moisture indicator says over 10% all bets are off and you should bake those cards just to be safe.

Phil

Very good. Well, we hoped we answered E.R.'s question.

Just remember when you are dealing with parts that are under-exposed or over-exposed, or whether you yourself are under-exposed or over-exposed, whatever you do please don't solder like my brother.

Jim

And even though it's not mentioned in J Standard 033, don't solder like my brother.

Comments

We are a manufacturer of flexible and rigid flex circuits which are hydroscopic and need to be desiccated fairly often. Baking is our most common method, but when we need to desiccate quickly we use one of our vacuum chambers and pull vacuum until it reached 31 millitorrs. It is very quick and handy when baking might be detrimental to a part.
Bob Burns, Printed Circuits LLC
We often hear of the metal parts adsorbing more moisture than the approved desiccant bags. This creates a problem for the ability of the HIC to read correctly. One way to tell, is to weigh both the desiccant pack and the metal part individually, then weight them again after moisture has developed.
Dan Jenkins, Steel Camel
One should consider what happens to the moisture inside the bag. Does adsorb to the Silica and still remain wet? or does it absorb to another type of desiccant and become inert. Also consider that as you seal the bag, you can very well be trapping in moisture. Therefore, one might consider leaving plenty of air in bag for proper circulation and with a proper absorbing desiccation method.
Dan Jenkins, Steel Camel
I have found that most people's experience with vacuum sealing is with food and try to apply that to ICs. But they are sealed for exact opposite reasons. Food is vacuumed sealed for freezing to prevent moisture from being removed causing freezer burn. We seal ICs to keep moisture out. Food we want moisture so we vacuum seal, ICs we don't want moisture so we lightly evacuate air. Too strong a vacuum can pull moisture into the MBB. It can also cause tears at the corners of trays.
Stephen Olan, Stim Canada
The standard says that the HIC should not touch the desiccant inside the MBB. Do you have any insight into this and do you think that perhaps taping the HIC to the reel so that it stays in place would be an issue?
Dawn
In reference to the comment from Dave. It is correct what he says about 033. By having vacuum the essicant inside the MBB will not work well. The vacuum will contrast the moisture absorbtion by the dessicant so the results are more like as not having essicant inside MBB. And what HIC would show would be just the effect of not corret essicant behavior. Try and you'll see.
Gabriele Sala, GSC
Dave from EMAC read my mind. Let's not confuse "Vacuum Sealing" with "Sealing". While on the topic, MANY suppliers seem to be ignorant of, or just plain disregard, that very clause. I've seen parts where you can essentially read the marking on the top surface because the package is vacuumed so tight.
Rick Wyman, Benchmark Electronics
In reference to the comment from Jim about having to vacuum seal bags. 033C-1 3.3.5.3 states that in actual practice air evacuation is not required. Light air evacuation may be used to reduce the packaging bulk. Furthermore Full evacuation SHALL NOT be used as it will impede desiccant and HIC performance and possibly lead to MBB puncture. Can you please comment on this discrepancy?
Dave, EMAC Inc.

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