Desiccant Storage And Moisture Control For PCBs
A printed circuit board that is stored in a sealed bag with a desiccant and a humidity indicator card is a board whose moisture content is known. The bag keeps the atmosphere around the board dry, the desiccant absorbs what enters, and the card shows whether the bag has held. The combination is simple and it is the cheapest way to avoid the whole family of problems that starts with a board that has absorbed water.
This article explains what the packaging does, how the desiccant quantity is chosen, how the indicator card is read, and how the storage is controlled and recorded.
What The Bag Does
The bag is a laminate of polymer films with a metallic or oxide layer that makes it a barrier to water vapour. The barrier is not perfect, so a small amount of moisture passes through over time, and that is what the desiccant is there to absorb. The combination of a good barrier and enough desiccant keeps the humidity inside the bag below the level at which the board gains moisture, for the shelf life that the packaging is rated for.
The bag also protects against contamination and handling damage, which is a secondary benefit. A board that is unpacked immediately before assembly is a board that has been touched once rather than several times, and the packaging is often the only barrier between the fabrication shop and the assembly line. Where the board will be coated after assembly, the moisture state matters twice, and the coating process is described under conformal coating and board protection.
<img src=”https://www.gopcba.com/wp-content/uploads/2026/09/225-1.jpg” alt=”Sealed moisture barrier bag with desiccant sachet” />
Desiccant And Its Quantity
The desiccant is usually a sachet of silica gel or of a clay based material, and its capacity is expressed in grams of water that it can absorb. The quantity needed depends on the surface area of the boards, the volume of air in the bag, the barrier of the film and the shelf life required, and the supplier’s table gives the combination. Using fewer sachets than the table requires is a common way to save a small amount of money and lose the protection.
The desiccant has to be active. A sachet that has been left open in a humid room has already absorbed what it can, and it will not protect anything. The sachets are stored in a sealed container and used promptly, and a sachet that is soft or visibly damp is discarded. The cost is small enough that there is no reason to reuse one.
Reading The Indicator Card
The humidity indicator card has a set of spots that change colour at defined humidities, usually 5, 10, 60 percent and sometimes 30 percent relative humidity. Each spot is a different chemical that changes from blue to pink, or from yellow to a darker colour, at its own threshold, and the card is read by noting the highest spot that has changed. A card whose spots up to 10 percent have changed indicates that the bag has held; one whose higher spots have changed indicates that moisture has entered.
The card is read immediately after the bag is opened, in the same conditions, because the card itself responds to the room. A card that is read after ten minutes in a humid room will show the room rather than the bag. The reading and the date are recorded, and the board is either used within its floor life or the bag is resealed with fresh desiccant.
Storage Conditions And Shelf Life
The sealed bag has a shelf life that the manufacturer states, typically twelve months for a board with an organic finish and longer for a board with a metallic finish. The shelf life assumes storage in a controlled environment rather than in an unheated warehouse, since the film barrier is less effective at high temperature and the desiccant is consumed faster.
The board inside has a floor life once the bag is opened, and the two figures are different. The shelf life describes how long the packaging protects the board, while the floor life describes how long the board can be exposed before it needs a bake or a re-bag. Both are recorded, and the floor life is the one that matters when a bag is opened and then closed again several times. The dimensional behaviour that moisture affects is described under PCB dimensional stability and expansion.
Opening, Resealing And Records
A bag is opened with a straight cut that leaves enough material to reseal it. The boards are removed, the indicator card is read, and the bag is resealed with the original desiccant or with a fresh one, depending on how much of the shelf life has been consumed. A bag that is left open for a shift has lost its protection and the boards inside have started to gain moisture.
The record that matters is the one that links a board to its moisture history. A label on the bag with the date of sealing, the desiccant used and the shelf life gives the starting point; a record of each opening completes it. Where the same bag is opened several times, the cumulative exposure is what determines whether a bake is needed, and it can only be known if each opening was recorded. The assembly sequence that follows is described under PCBA development process.
Verification
Verification is by the indicator card and by weight. The card is the routine check, and it is read against a reference photograph of a card at each threshold so that the reading does not depend on the ambient light in the room. Where the moisture content matters, a sample of boards is weighed before and after a bake, and the difference is the water that was present.
The packaging itself can be tested by sealing a bag with a known quantity of desiccant and a recording hygrometer inside, and measuring the humidity after a period at the storage temperature. The test is used to qualify a supplier or a new bag material rather than for routine control, and its result is the evidence that the shelf life claimed for the packaging is achievable in the actual store.
Process Control and Verification
On a design of this kind, storage is the item that decides how the rest of the board is arranged. 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.
FAQ
Can a desiccant sachet be dried and reused? Silica gel can be regenerated in an oven, and the practice is common in a workshop. It is only worth doing where the drying is controlled and the sachet is handled dry, and the cost of a new sachet is small compared with the risk.
What if the indicator card shows moisture? The boards are baked according to the material and the finish before assembly, or they are returned to the supplier. The bake removes the water, and the card is replaced when the bag is resealed.
Does the bag protect against contamination? It limits handling and it keeps dust off the surface, and it does not remove contamination that was already present. Its main function is the control of moisture.
