PCB Storage Humidity Control Guide

A finished printed circuit board is not inert. The laminate absorbs moisture from the air, the surface finish reacts with the atmosphere, and a board that has been stored badly will fail during the reflow soldering that was supposed to be the easy part. PCB storage and humidity control are therefore part of the assembly process, and the rules are simple enough that they are often ignored.

Why Moisture Matters

The laminate is a composite of resin and glass, and the resin absorbs water from the air. The absorbed moisture does not change the board at room temperature, but at soldering temperature it turns to steam.

The steam has nowhere to go inside a multi layer stack, so it builds pressure until the resin separates from the glass or from the copper. The result is a blister or a delamination that appears during reflow.

The same moisture also affects the solderability of the surface. An oxide layer grows faster on a damp surface, and a finish that has been exposed to a humid atmosphere for months will not wet as readily as a fresh one.

The effect on the mechanical properties of the material is described in the laminate properties guide, and the dimensional consequences appear in the stability guide.

How Boards Absorb Moisture

Absorption depends on the resin system, the temperature and the relative humidity of the air. An epoxy absorbs a fraction of a per cent by weight when it is exposed to a humid room for a long period.

The absorption is also a diffusion process, so it takes time. A board that is unpacked for a day in a humid room takes up far less moisture than one that has been on a shelf for six months.

Bare laminate, a finished board and an assembled board all behave differently, because the mask and the coating are barriers that slow the diffusion at the surface while leaving the edges exposed.

The edges of the board and the walls of the holes are the fastest paths, since the laminate is exposed there and the surface area is large. A board with many holes absorbs faster than one with few.

Shelf Life and Packaging

A board with an organic solderability preservative has the shortest shelf life, because the preservative itself degrades and the copper underneath oxidises. A few months is typical in a sealed bag.

An immersion gold finish has a long shelf life, because gold does not oxidise. A board with a gold finish can be stored for a year or more with little change in solderability.

The packaging is part of the specification. A moisture barrier bag with a desiccant and a humidity indicator card is the standard, and the bag has to be sealed rather than folded.

The indicator card is the useful part, because it shows at a glance whether the bag has been breached. A card that has changed colour means that the board has been exposed and the shelf life clock has restarted.

Baking Before Assembly

Baking drives the moisture out, and it is used when a board has been stored for too long or when the packaging has been compromised. The temperature and the time depend on the thickness and on the finish.

A thin board can be baked at a moderate temperature for a few hours, while a thick board needs a longer time because the moisture has further to travel. The supplier of the laminate publishes a schedule for each material.

The temperature has to be limited for a board with an organic finish, and for an assembled board the limit is set by the most sensitive component. A plastic package that has absorbed moisture splits when it is heated, which is the same problem in a different place.

Baking is also a stress relief in its own right, and a board that is baked and then cooled too quickly can warp. A slow cool in the oven is part of the procedure.

Popcorning and Delamination

Popcorning is the name given to the blister that forms when the trapped steam expands and lifts the resin at the package or board surface. The shape of the blister resembles the food, and the failure is permanent.

On a board the blister appears as a bump in the mask or as a visible separation between the layers. It is often found after reflow, and it is usually attributed to the laminate rather than to the storage.

A cross section of a blistered area shows the separation clearly, and it is the only way to confirm the cause. The section also shows whether the failure is at the resin and glass interface, which is typical of moisture, or at the copper interface, which is typical of an adhesion problem.

A board with a blister cannot be repaired. The affected area has to be scrapped, and if the blister is inside the board the whole assembly is a loss.

Storage Room Practice

The room should be held at a controlled temperature and a relative humidity below about forty per cent, and the value should be recorded. A room without a record is a room with no control at all.

Boards should be kept in their sealed bags until they are needed, and a bag that is opened should be resealed with fresh desiccant or used within the working life. A bag that is left open on a bench is the most common cause of a moisture failure.

The stock should be rotated so that the older boards are used first, and the date of manufacture should be on the label rather than in a file. A board that has been in stock for two years should be baked before use, not simply used.

The coating applied after assembly is a long term barrier, while the storage practice is a short term one. Both are needed, and neither replaces the other.

Practical Rules

Record the humidity and the temperature of the storage area, and keep the log with the material records. The record is what allows a moisture failure to be traced.

Keep the boards in a sealed barrier bag with a desiccant and an indicator card, and treat an opened bag as a change of state rather than a minor inconvenience.

Bake a board that has exceeded its shelf life or that has been stored outside the bag, and use the schedule from the laminate supplier rather than a rule of thumb.

Check the finished board after reflow for a blister or a delamination, and record the result as part of the quality record so that a batch problem can be identified early.

Additional Considerations for This Build

Practical attention to moisture absorption pays for itself here, because it decides whether the finished board behaves as the drawing intended. Where the requirement is not stated on the fabrication drawing or in the assembly notes, the shop has to assume a default, and that default is rarely the value the design was simulated with. Stating moisture absorption explicitly, together with the tolerance that applies, removes the assumption and keeps the result predictable from batch to batch.

FAQ

How long can a bare board be stored? It depends on the finish. An organic preservative is measured in months in a sealed bag, while an immersion gold finish lasts a year or more.

When should a board be baked? When the packaging has been opened for longer than the working life, when the indicator card has changed colour, or when the shelf life has been exceeded.

Does baking damage the board? It can if the temperature is too high or the cooling is too fast. A controlled schedule from the laminate supplier avoids both problems.