PCB Packaging, Storage and Shelf Life

A board that is perfect when it leaves the fabricator can be unusable when it arrives at the assembly line. The change happens in storage and transport, and almost all of it is preventable with packaging and handling rules that cost very little.

What Happens to a Board in Storage

The surface finish is the first thing to change. A thin immersion finish oxidises, a silver finish tarnishes, and a solder leveled finish slowly forms an intermetallic layer at the copper interface. All of these reduce the ease with which solder wets the pad.

The laminate also absorbs moisture from the air. Absorbed moisture is released during reflow and can produce blisters, measling and delamination, particularly on thick boards and on boards that have been stored in a humid environment without protection.

Neither change is visible in a way that predicts its effect. A board that looks normal can have a finish that no longer wets, which is why the storage conditions and the shelf life are recorded rather than judged by appearance.

Moisture Barrier Packaging

The purpose of a moisture barrier bag is to isolate the board from the humidity of the surrounding air. The bag is a laminate with a low water vapour transmission rate, and it works only if it is sealed and contains a desiccant.

A humidity indicator card inside the bag shows whether the protection has held. Reading the card at the moment of opening is the check, and its colour is the evidence that the boards were stored in the condition that was intended.

Vacuum packaging is sometimes used for large panels and for boards with fine features, partly to restrict movement in transit. The vacuum is not itself a moisture barrier; the bag material is what provides the protection.

Cleaning Before Packaging

Boards should be clean before they are packed, because residue trapped under the packaging is in contact with the surface for the whole of the storage period. Handling with bare hands deposits salts and oils that accelerate the change in the finish.

Gloves are required for any board that will be stored, and the correct glove material is one that does not leave residue, which rules out some powdered types. The requirement applies to inspection as much as to packing.

A final rinse and dry before packaging is common for boards with tight features, because water trapped in a hole or under a component site can cause corrosion during storage. Our judging PCB quality notes describe the surface condition checks that precede packing.

Shelf Life and Its Basis

Shelf life is quoted for a finish and a packaging method, and it is the period for which the fabricator will stand behind solderability. It is not a prediction that the board becomes unusable the following day, and it is not a guarantee that an older board will fail.

The quoted period assumes the bag remains sealed and the storage conditions are within the stated range. A bag that has been opened and resealed with fresh desiccant starts a new period, and the resealing should be recorded.

Where the shelf life is exceeded, the options are to test the solderability on a sample, to bake the boards to remove moisture, or to use them for applications where solderability is less critical. Disposing of them is the last resort and is often unnecessary.

Transport and Mechanical Protection

Boards are damaged in transit by impact and by flexing rather than by vibration alone. A stack of panels that can move within its carton will rub at the edges, and the damage appears as nicks and as finish worn away at the corners.

Edge protection, interleaving and a carton that fits the stack are the practical measures. Where the board carries components with fine leads, the packaging must also prevent the leads from bearing the load.

Panels should be supported so that the stack cannot bow. A stack supported only at its middle or only at its edges will deform under its own weight and under the loads of transport, and the deformation leaves a set that affects assembly.

Storage Conditions on Site

Humidity and temperature control in the store matters more than the specific values. A store that is dry but that swings in temperature will condense moisture on cold boards moved into a warm room, and the condensation is more damaging than steady humidity.

Where the boards are moved between environments, they should be allowed to reach the ambient temperature before the bag is opened. Opening a cold bag in a warm room is the situation that produces condensation directly on the board.

Stock rotation by date code ensures that the oldest material is consumed first, which is what makes the shelf life meaningful. Without rotation, some material will always be at the back of the shelf beyond its period.

What the Assembly House Needs

The assembly house needs the finish type, the packaging date, the shelf life and the storage history. Without them, the incoming inspection has no baseline and the process cannot be adjusted for a finish that has aged.

Where a board has been stored for a long period, the assembly house can often compensate by adjusting the flux or the reflow profile, but only if it knows the condition. Discovering the age after a batch has failed to wet is the expensive way to find out.

Our solderability test notes describe the test that settles the question when the history is uncertain.

Records and Traceability

Recording the finish, the packaging date and the lot for each delivery makes the shelf life enforceable. It also allows a later problem to be traced to a shipment rather than to a general suspicion about the supplier.

A simple label on the bag carrying the lot, the finish and the date is enough. The label does not need to be elaborate, and it needs to be applied consistently, which is a matter of process rather than of equipment.

Our fabrication notes checklist and our conformal coating notes cover the related requirements for boards that will receive additional processing after arrival.

Process Control and Verification

On a design of this kind, shelf life is the item that decides how the rest of the board is arranged. 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.

FAQ

How long can a board be stored? The figure is quoted by the fabricator for the finish and packaging used, and it typically runs from several months for a bare copper or silver finish to a year for a packaged board with a robust finish.

Is baking a board before assembly always safe? It removes moisture and it also accelerates the growth of the intermetallic layer on the finish. Where the finish is already marginal, baking does not restore solderability.

What does gopcb provide for storage and shelf life? We provide the finish type and its shelf life on the certificate, moisture barrier packaging with a desiccant and an indicator card, sealed date marking on every bag, and solderability testing where the storage period has been exceeded. Where the assembly house requires a specific packaging method, we agree it at order.