Desiccant and Moisture Barrier Packaging for PCB Storage
Bare boards and moisture sensitive components absorb water from the air, and that water turns to steam when the part is heated. The result is delamination, blisters, cracked packages and, in the worst cases, a board that fails during reflow. Moisture barrier packaging exists to interrupt that process between manufacture and use, and its effectiveness depends on doing several small things correctly.
Why Packaging Matters
A board that leaves the fabricator dry can absorb enough moisture during a week in a humid warehouse to fail a reflow cycle. Laminate, solder mask and mould compounds all take up water, and the amount depends on the material, the temperature and the relative humidity of the storage environment.
The purpose of packaging is to hold the item below a defined moisture level for a defined period, so that the assembly process can start from a known condition. It is a controlled process with a specification, not simply a bag put around a box, and the controls are what make the difference between a protected shipment and one that arrives with an invisible problem.
How Moisture Enters
Water enters through the bulk of a polymer by absorption, and it enters a package through any opening in the barrier. Absorption is driven by the difference in humidity between the material and the air around it, so a dry part in humid air gains moisture steadily until it reaches equilibrium. The rate depends on the material and on temperature, and the material properties are compared in this guide to laminate material properties.
Diffusion through the packaging film is the other route, and it is slow but continuous. Even a good barrier has a measurable water vapour transmission rate, and a bag that is sealed with a pinhole or a poor closure effectively has a much higher one. That is why bag integrity matters as much as the film specification.
<img src="https://www.gopcba.com/wp-content/uploads/2024/09/Metal-Base-PCB.jpg" alt="PCB in a moisture barrier bag with a desiccant pack and humidity indicator card” />
Moisture Barrier Bag Construction
A moisture barrier bag is a laminate of several layers, typically including a metallised or foil layer that blocks water vapour, a structural layer for strength and a sealable inner layer. The barrier property comes from the whole construction, so a bag cannot be judged by its appearance alone, and the specified water vapour transmission rate is the number that matters.
Bags also provide electrostatic shielding in many constructions, which is convenient because the same items usually need both protections. The two properties are independent, however, and a bag that is adequate for ESD may not meet the moisture requirement. Where both apply, the specification has to state both.
Desiccant Selection and Quantity
Desiccant absorbs the moisture that enters the bag during storage and shipping, and the quantity required depends on the bag volume, the barrier’s transmission rate, the storage duration and the conditions. Suppliers publish tables and calculators that give a unit quantity for a given combination, and using them is more reliable than choosing by bag size.
Desiccant has a finite capacity, and it becomes less effective as it is consumed. Units that have been exposed to air during handling should be replaced rather than reused, because the amount of capacity left is unknown. This is one of the simplest controls to get right and one of the most commonly neglected.
<img src="https://www.gopcba.com/wp-content/uploads/2026/05/机械臂PCBA.png" alt="Impulse sealer closing a moisture barrier bag before vacuum sealing” />
Vacuum Sealing and Bag Closure
Sealing is where most packaging failures occur. The seal has to be continuous across the full width of the bag, free of wrinkles that create channels, and strong enough to survive handling. Impulse sealers with a controlled temperature and dwell give repeatable results, while hand sealing without temperature control does not.
Vacuum sealing removes most of the air and reduces the load on the desiccant, but the vacuum level matters. Too much vacuum can stress the bag and the contents, particularly on a thin board with tall components, while too little leaves air that has to be dried by the desiccant. The bag should also be sized so that the seal area is flat and accessible.
Humidity Indicator Cards and Labels
A humidity indicator card changes colour across a printed scale to show the humidity inside the bag, and it is read through the transparent side without opening the package. It gives an immediate indication of whether the packaging has held, and it is the first thing to check when a bag is opened.
The label carries the rest of the information: the moisture sensitivity level, the date of sealing, the shelf life, the maximum floor life after opening and the baking conditions. A bag without a legible label loses most of its value, because the person opening it cannot tell whether the contents are still usable.
Storage, Shelf Life and Floor Life
Shelf life is the period the sealed bag is expected to keep the contents below the specified moisture level, and it is calculated from the barrier’s transmission rate and the desiccant capacity. Floor life is a different quantity: the time a part may remain out of the bag in the assembly environment before it must be baked or used.
Both depend on the humidity of the environment, so a floor life figure quoted for thirty degrees and sixty percent relative humidity does not apply in a room at eighty percent. Where a facility cannot control humidity, the safe approach is to shorten the floor life rather than to assume the published figure applies.
Opening, Baking and Resealing
When a bag is opened, the card should be read and recorded, and the parts should be used within the floor life or baked before use. Baking removes the absorbed moisture, but it also affects solderability, and the correct schedule depends on the material and the package thickness rather than on a single generic temperature and time. Baking also changes the surface, which is why a solderability check is often wise after a high temperature bake.
Partially used bags can be resealed if the desiccant is replaced and the seal is made properly, and the label should be updated with the new date. What must not happen is a bag being closed with tape or a clip and treated as if it were sealed, because the moisture level then rises without any indication on the card.
Auditing the Packaging Process
Auditing means checking the film type, the desiccant quantity, the seal quality and the label content, along with the conditions in which the packing is done. Packing in a humid room with the bag open for a long time can undo the benefit of a good desiccant, so the environment and the handling discipline are part of the specification and of the wider quality checks applied to a finished board.
Seal strength and integrity are checked by a peel or dye penetration test on a sample, and the results are recorded. The requirement should be written into the fabrication notes alongside the material and finish specifications, so that it is reviewed with the rest of the job rather than added at the end.
FAQ
How long can boards stay sealed in a moisture barrier bag? That depends on the barrier, the desiccant quantity and the storage conditions, and it is calculated rather than assumed. A shelf life of twelve months is typical for well specified packaging, but it applies only to bags that are intact, correctly sealed and stored within the stated temperature range.
Can desiccant be reused? Some types can be regenerated by drying, but the capacity remaining in a unit that has been exposed to air cannot easily be verified. Replacing desiccant when a bag is resealed is cheap insurance compared with the cost of a delaminated board or a cracked package.
What should I do when the humidity indicator shows high? The parts should be baked according to the schedule for that package before use, and the reason for the high reading should be investigated. It usually means the seal failed, the bag was open too long or the desiccant was exhausted.
