Moisture Barrier Bag Calculation

For calculating the life of moisture barrier bags used for long-term storage, I need to give the initial volume of the "vacuum-sealed" moisture barrier bag as input to one of my calculations.

After vacuum sealing the MBB, it occupies the shape of whatever object was sealed inside in such a way that the thickness becomes near zero. So, how do I calculate the volume of vacuum-sealed bag?

S.R.

Expert Panel Responses

We solved a very similar issue by immersing the irregular shaped sample in a measured volume water. The volume of water displaced is the volume of the object.

Mitch Holtzer
Director of Reclaim Business
Alpha Assembly Solutions

I've been in the soldering materials/applications industry for 25 years. Since joining Alpha, Ive been the global product manager for preforms, wave soldering flux, solder paste and more recently the Director of the soldering materials reclaim business.

Please use J-STD-033 for calculations. The calculations is not based on the collapsed bag, but the bag volume regardless of sealed or unsealed.

David Cormier
Engineering Manager
Circuit Technology Center, Inc.

Manufacturing Engineer of 20+ years. Involved in Industries relating to all sectors of defense, Commercial product Industries, RF - Microwave and Semiconductor industries. Vast knowledge and experience relating Mil-STD’s, IPC-STD’s, EAI-STD’s, GEIA-STD’s, J-STD’s and MIL-PRF-STD’s.

The most common way to find the volume of an irregular shape is to submerse it in water and note its displacement. This seems a bit counterintuitive since the goal is to keep the parts dry, but if the MBB is sealed, it should not matter to give it a quick dip.

However, I'm sure a good estimate of length x width x height (even if only a few mm) would be good enough for your calculations.

Paul Austen
Senior Project Engineer
Electronic Controls Design Inc

Paul been with Electronic Controls Design Inc. (ECD) in Milwaukie, Oregon for over 39 years as a Senior Project Engineer. He has seen and worked with the electronic manufacturing industry from many points of view, including: technician, engineer, manufacture, and customer. His focus has been the design and application of measurement tools used to improve manufacturing thermal processes and well as moisture sensitive component storage solutions.

Take the largest assembly that fits inside the bag and then vacuum-seal it.

Second, change everything to mils, for example a 6 X 8 inch bag now is 6000 X 8000.

Third, measure the bag thickness including component. Let's say is 25 mils.

Fourth, multiply 6000 X 8000 X 25 = 1200000000 cubic mils.

Fifth, convert to cubic inch = 1.2 cubic inches. You can then convert the result to other volume measurements.

Edithel Marietti
Senior Manufacturing Engineer
Northrop Grumman

Edithel is a chemical engineer with 20 year experience in manufacturing & process development for electronic contract manufacturers in US as well as some major OEM's. Involved in SMT, Reflow, Wave and other assembly operations entailing conformal coating and robotics.

I'm not sure of what he is referring to when he states "volume of vacuum-sealed bag?" It's nebulous. Without knowing what he is seeking I can't answer the question.

Jerry Karp
President
JSK Associates

Based in. Northern California since 1971. Founded JSK Associates in 1979. Actively involved in soldering, cleaning, chemistries. 30 years experience in EOS/ESD control.

We solved a very similar issue by immersing the irregular shaped sample in a measured volume water. The volume of water displaced is the volume of the object.

Mitch Holtzer
Director of Reclaim Business
Alpha Assembly Solutions

Reader Comment

Vacuum sealing is to keep moisture in products. (think freezer burn in food) Do NOT vacuum seal electronics because you want to do the opposite, you want to remove moisture. If you vacuum seal then the moisture has no path to get to the desiccant. Also the vacuum can draw moisture through the moisture barrier bag and poke holes at sharp corners. Lightly evacuate the air, and then seal. If the bag conforms to the contents then you have taken out too much air.

Stephen Olan, Stim Canada

With regard to Mr. Olan's comment regarding the purpose of vacuum sealing food, one is in fact sealing to keep the moisture in. For electronics packaging, however, the parts are first baked dry and then immediately vacuum sealed to keep them FREE of moisture ingression (dry).

The premise is that you are sealing dry parts to keep them dry. Therefore, you want to remove as much air from the bag so as not to leave behind any moisture-laden air pockets. Since some small amount of air is always left behind, a desiccant pouch is typically vacuum-sealed inside the bag with the parts to absorb any remaining moisture, along with a humidity indicator card (HIC) so one can readily see the moisture level upon opening.

If a good dry desiccant pouch of sufficient capacity is used, you are (somewhat) correct, an absolute vacuum that is punctured or could damage the parts is not required, but a good vacuum seal IS required otherwise any large pockets of air sealed into the bag with the parts can hold so much moisture that not even the desiccant bag or pouch is enough to keep the moisture out of the parts.

I have dry-packed or worked with electronic components, assemblies, and even chemicals that were drypacked for as long as 36 years for rework and repair of Legacy programs such as train controls, large earth-moving equipment used in open-pit mines, C-scans used in hospitals and other medical equipment, missile control systems, satellite systems and other equipment expected to have a functioning life (and replacement parts available) of perhaps 50 years or more.

I have never opened a properly prepared drypak to find the HIC card indicating the parts are not dry. Not once in thousands of rework/repair jobs. You can't use sandwich baggies for this; but standard moisture barrier bags will do nicely. They are quite tough and not easily penetrated. A "good"vacuum seal also helps to identify any that are punctured.

Richard D. Stadem
Advanced Engineer/Scientist
General Dynamics

Richard D. Stadem is an advanced engineer/scientist for General Dynamics and is also a consulting engineer for other companies. He has 38 years of engineering experience having worked for Honeywell, ADC, Pemstar (now Benchmark), Analog Technologies, and General Dynamics.

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