Moisture Barrier Bag Sealing: Verifying the Seal Before It Ships
A moisture barrier bag is the last process step that protects a dry assembly, and it works only while the seal holds. The bag itself is rarely the problem, because a laminated film is a poor transmitter of water vapour; the seam where it is closed is where most protection is lost. The metal layer is what makes the bag a barrier, and the plastic layers protect the metal from abrasion.
Verification therefore concentrates on the seal rather than on the film, and on the desiccant and the indicator card that report what has happened inside the bag since it was closed.
What the Bag Has to Do
The bag has to keep water vapour out, keep electrostatic charge away from the contents and survive transport without puncturing. Those three requirements are met by a laminate of aluminium foil and polyester, which combines a very low transmission rate with a conductive or dissipative layer.
The bag also has to match the moisture sensitivity of what is inside. A level 3 assembly and a level 5 assembly need different amounts of desiccant and different floor life after opening, which is the framework set out in the notes on moisture sensitivity levels.
Seal Width and Seal Geometry
Seal width is the distance between the inner edge of the seam and the outside of the bag, and it is normally specified between 6 and 12 mm, with 8 to 10 mm typical. A narrow seal has little margin for a fold or a wrinkle, and a wrinkled seal leaks.
Geometry matters as much as width. The seal should be parallel to the bag edge, clear of the contents, and free of creases where the film doubles over. Contents that press against the seal line during evacuation cause a channel that no width can compensate for. The bag should be sized so that the contents do not touch the seal line once it is evacuated.
Seal Temperature, Dwell and Pressure
The sealing jaws have to bring the inner layer of the laminate above its melting range without burning through the outer layer. Depending on the film, that means a jaw temperature of roughly 120 to 180 °C with a dwell of one to three seconds and a jaw pressure set so that the two layers are pressed into one.
Each of the three settings is verified with a seal sample rather than trusted, and the sample is examined for a continuous weld across the full width. A seal that looks closed but pulls apart at the edges has been run too cold or too briefly. The sample is kept with the record, because a seal that passed at the start of a shift says nothing about one made six hours later.
Vacuum Level and Air Removal
Evacuation removes most of the air, which carries water vapour and reduces the volume the desiccant has to protect. The vacuum is usually taken to a few hundred millibar absolute, which is enough to flatten the bag without crushing the contents.
Too much vacuum is a real failure mode. A bag evacuated hard against the corner of a connector or a shield can develop a pinhole during transport, and the damage is invisible at packing. Contents should be cushioned, and the bag should not be drawn down over sharp edges. A corner guard or a card insert is enough to prevent that damage in most packages.
Leak Testing the Seal
Seal integrity is checked with a dye penetration test on a sample, in which a coloured solution is applied to the seal and the assembly is held under pressure so that the dye is forced into any channel. A visible line of dye on the inside face of the seal is a leak.
The test is destructive and slow, so it is applied to a sample from each shift or each sealing setup rather than to every bag. A visual check of every bag, looking for wrinkles, folds and incomplete welds, catches the gross failures, and the dye test catches the ones that look acceptable. The two checks are complementary and neither replaces the other.
Desiccant Quantity and the Indicator Card
Desiccant is sized from the bag volume, the surface area of the contents and the floor life that has to be protected. The calculation is done by the package designer for the bag size, and the shop verifies that the quantity actually placed matches the specification. Too little desiccant produces a card that changes colour before the shelf life is reached, and too much is a cost nobody notices.
The humidity indicator card is the evidence that the bag worked. A card with spots at 10, 20 and 30 percent relative humidity should be readable through the bag without opening it, and a card that has changed colour on receipt means the bag leaked or the desiccant was exhausted, in which case the parts are treated as though the floor life had been exceeded.
Static Shielding Requirements
A moisture barrier bag used for assemblies with static sensitive devices also has to be a shielding bag, which means a surface resistivity in the dissipative or conductive range and a metal layer that limits the energy of a discharge reaching the contents.
The requirement applies to the whole package, including the labels and the tape used to close it, because an insulating label on a shielding bag creates an isolated charged area. The bag type should be specified once and not substituted at packing. Where a substitution is unavoidable, it is approved against the shielding requirement rather than accepted because the bag looks similar.
Storage, Transport and Receiving Checks
Sealed bags are stored in a controlled area, typically between 5 and 30 °C with humidity below 70 percent, away from direct sunlight and sharp objects. Shelf life is stated by the packaging supplier, and the desiccant has to be reclaimed or replaced if the bag is stored beyond that figure. Bags should also be stacked so that the seals are not creased under load.
On receipt, the checks are the seal appearance, the indicator card and the bag condition. Where a card has changed colour, the assembly is dried rather than risked, and the bake requirements for the package decide the schedule that follows.
Records and Deviations
The records that matter are modest: the bag and desiccant part numbers, the sealing settings, the operator, the date and the result of the seal sample. A photograph of the seal sample attached to the record is often more useful than a description of it.
Where a bag is resealed after a partial use, the original protection is not fully restored, and the deviation should be recorded with the desiccant that was added and the new expiry. The resealed bag is labelled with the new date so that the next user is not misled by the original one. The notes on desiccant and barrier packaging describe how that allowance is calculated.
FAQ
How wide should a moisture barrier bag seal be? Between 6 and 12 mm, with 8 to 10 mm typical. The seal also has to be free of wrinkles, because a crease forms a channel that a wider seal will not close.
How is the seal actually tested? By a dye penetration test on a sample from each sealing setup, combined with a visual check of every bag. The dye test finds channels that look perfectly sealed.
What if the indicator card has changed colour? Treat the contents as though the floor life had been exceeded, verify with a bake, and record the deviation rather than repacking the bag as it stands.
