Moisture Barrier: Design Rules and Process Limits
A moisture barrier bag is the last control between a dry part and the humidity of the room, and it only works if the bag, the desiccant, the humidity indicator and the seal are all correct. A bag that is correctly specified and badly sealed is a bag that does not work.
What the Bag Has to Do
The bag has to limit the rate at which water vapour passes through it, so that the desiccant inside can absorb what enters and hold the internal humidity low for the shelf life. The transmission rate is a property of the film and of the seams.
A bag with a good film and a poor seal has the transmission rate of the seal rather than of the film, which is why the seal is verified rather than assumed. Our board quality notes describe how the packaging is inspected.
Desiccant Sizing
The desiccant is sized from the bag volume, the shelf life and the transmission rate of the film. A unit that is adequate for a small bag is not adequate for a large one, and the calculation is a supplier figure rather than a guess.
The desiccant also has a capacity, so it will eventually saturate. That is the basis of the shelf life figure, and it is the reason a bag that has been open and resealed has a shorter remaining life.
The Humidity Indicator Card
The indicator card shows the humidity inside the bag at the moment it is read, and it is the only simple check that the packaging worked. The reading has to be taken before the bag is opened, because the card responds to the room as soon as it is exposed.
The card has a threshold and the threshold has to be compared with the requirement for the parts inside. A card that shows a mid-level reading does not by itself mean the parts need baking; it means the level has to be compared with the specification. Our joint criteria notes describe what the parts are being protected for.
Sealing and Its Verification
The seal is made by heat, and its quality depends on the temperature, the pressure and the dwell. A seal that is too cold is weak and one that is too hot is brittle, and both fail in handling rather than in storage.
Verification is by a visual check of the seal width and by a peel or a burst test on a sample. Doing the test on a sample from each shift is more useful than testing one bag per lot, because the sealer is what drifts.
How Parts Are Loaded
The bag contains parts, desiccant and the indicator card, and the loading order matters. A card that is placed against a reel can be masked, and the reading that follows is meaningless.
The loading also affects the vacuum. A vacuum that is too deep damages the desiccant pouch and can crush the card, so the vacuum is set at a level the contents tolerate.
Process Control and Verification
On a design of this kind, desiccant is the item that decides how the rest of the board is arranged. The measurements that matter are the repeatable ones: conductor width and spacing, annular ring, finished hole size, plating thickness and surface finish are all verifiable on a coupon that travels with the panel. Running a first article through the same checks as the production panel confirms that the two agree, and that comparison is the cheapest form of process control available at prototype stage.
Keeping a sample from the panel turns a dispute into a measurement, because the same coupon can be re-examined by both parties without rebuilding the batch. Reviewing the design before the data is released is cheaper than correcting it after the panel is in the tank, because every step downstream inherits the decision made at the front end.
Documenting the assumption is part of the design work, and a short note on the drawing prevents a question that would otherwise arrive a day later and cost a day of schedule. The process window is set by the narrowest step in the flow, so an improvement anywhere else shows up as margin rather than as yield until that step is addressed.
Process Control and Verification
On a design of this kind, desiccant is the item that decides how the rest of the board is arranged. The measurements that matter are the repeatable ones: conductor width and spacing, annular ring, finished hole size, plating thickness and surface finish are all verifiable on a coupon that travels with the panel. Running a first article through the same checks as the production panel confirms that the two agree, and that comparison is the cheapest form of process control available at prototype stage.
Keeping a sample from the panel turns a dispute into a measurement, because the same coupon can be re-examined by both parties without rebuilding the batch. Reviewing the design before the data is released is cheaper than correcting it after the panel is in the tank, because every step downstream inherits the decision made at the front end.
Process Control and Verification
On a design of this kind, desiccant is the item that decides how the rest of the board is arranged. The measurements that matter are the repeatable ones: conductor width and spacing, annular ring, finished hole size, plating thickness and surface finish are all verifiable on a coupon that travels with the panel. Running a first article through the same checks as the production panel confirms that the two agree, and that comparison is the cheapest form of process control available at prototype stage.
Opening and the Remaining Life
Once the bag is opened, the clock starts. The parts have a floor life at the ambient condition of the assembly area, and that life is different from the shelf life in the bag.
The two are often confused, which is why the label carries both. Our reflow defects notes describe what happens when the floor life is exceeded and the part is heated anyway.
Handling and Storage
Bags are stored in a controlled area, and they are not stacked in a way that creases the film. A crease in the film is a weak point and a path for moisture, and it is often created by the way the bag was put on a shelf.
The storage temperature matters as well, because a bag that is moved from a cold store to a warm room condenses water on the inside if it is opened too soon. The conditioning period belongs in the instruction.
FAQ
Can a damaged bag be resealed and used? It can if the desiccant and the indicator are replaced, since the exposure has already consumed their capacity.
Does a humidity indicator card replace baking? No. It reports the condition; the decision to bake is made from the card, the floor life and the sensitivity level together.
What does gopcb provide for moisture barrier packaging? We provide bag and desiccant sizing from the volume and shelf life, indicator cards read before opening, seal parameters verified by peel or burst test on a sample, controlled loading and vacuum, and storage and conditioning instructions that protect the parts.
