Bare Board Shelf Life and Storage Control

A bare board is not inert while it waits to be assembled. The copper oxidises, the finish changes, the laminate absorbs moisture and the mask cures further. The shelf life of a board is the time over which those changes stay inside the process window.

What Changes With Time

The surface finish is the most sensitive element. An organic finish degrades quickly and loses solderability; a metal finish oxidises slowly and mostly holds. The storage requirement follows the finish rather than the board.

The laminate absorbs moisture from the air, and the amount it absorbs depends on the humidity and on the material. A board that has absorbed moisture will release it during reflow and the release causes delamination and blow holes. Our laminate notes describe the mechanism.

Storage Conditions

The standard measure is a controlled environment with a defined temperature and humidity, and the board sealed in a moisture barrier bag with a desiccant and a humidity indicator.

The indicator is what makes the package useful. A bag that has been opened and left without the indicator gives no information when the board is used. Our fabrication notes notes list the attributes that should be stated.

Shelf Life by Finish

An organic finish has the shortest life and it should be assembled first. A metal finish stores longer, and the practical limit is set by the oxide that forms and by the handling the board receives.

The life should be stated as a condition rather than a number: sealed in the bag at the stated environment, the period applies; opened, it does not. Our surface finish notes describe how the choice of finish changes the storage requirement.

Before Assembly

A board that has been stored for a long period should be assessed before it is assembled rather than after. The assessment is a solderability test on a sample and a bake where the laminate allows it.

The bake drives out moisture but it also oxidises the finish, so the two effects have to be balanced. The bake schedule should come from the laminate data rather than from a general rule.

Handling and Contamination

Every time the bag is opened, the board is exposed to the room. The exposure is cumulative, and the contamination that matters is the ionic residue transferred from hands, packaging and the surface of the bench.

Bare boards should be handled by the edges with gloves, and boards that have been handled should be cleaned before assembly if the process allows it. Our contamination notes describe the risk that the residue creates.

Records and Rotation

The date of manufacture, the finish and the storage history should travel with the boards. Without the date, the oldest boards are used last, which is exactly the wrong order.

Stock should be rotated so that the oldest boards are consumed first, and the rotation should be visible in the records rather than in the operator’s memory. Our quality notes describe how the verification is recorded.

What Happens If the Life Is Exceeded

A board past its stated life is not necessarily scrap. It is a board that requires assessment, and the assessment may conclude that it is fully usable.

The decision should be recorded with the evidence that supported it, because the same question will return with the next batch and the same analysis should not have to be repeated.

Process Control and Verification

Where a value sits close to a process limit, the drawing should say so, since the shop can then open the process window rather than working to a nominal figure that carries no tolerance. A stack-up that is drawn rather than described removes most of the ambiguity from a quotation, and it lets the fabricator price the board against the dielectric and copper weights that will actually be used.

Where the requirement is not written down, the shop supplies its own default, and the default is chosen for the process rather than for the design. The measurements that matter are the repeatable ones: conductor width and spacing, annular ring, finished hole size, plating thickness and surface finish are all verifiable on a coupon that travels with the panel.

Running a first article through the same checks as the production panel confirms that the two agree, and that comparison is the cheapest form of process control available at prototype stage. Keeping a sample from the panel turns a dispute into a measurement, because the same coupon can be re-examined by both parties without rebuilding the batch.

Checks Before Release

The checks that matter are the ones performed on the product rather than on a sample kept for the purpose, because a coupon that travels with the panel is the only evidence about that panel. Where a requirement can be measured, it should be measured at the point of manufacture and recorded against the board or the lot it applies to.

A parameter that is set once and never re verified drifts, and the drift is usually discovered by a defect rather than by the record. The tooling, the material and the profile form one system, and a change to any of them should be assessed against the other two before it is released.

Where the process window is narrow, the measurement resolution has to be better than the window, or the data cannot distinguish a good part from a marginal one. A record that identifies the operator, the date and the settings is worth more than a record that identifies only the result.

Verification and Records

The acceptance criteria should be written before the work starts, so that the decision is made by the specification rather than by the person inspecting. Where an operation cannot be verified afterwards, it has to be controlled during the operation, and that control has to be visible in the record.

Sampling is a compromise between cost and confidence, and the sample size should follow from the failure rate that has to be detected.

FAQ

Does a moisture barrier bag need a desiccant? Yes, together with a humidity indicator. A sealed bag without either protects against dust and not against moisture.

Can a board be baked repeatedly? Each bake carries a thermal exposure and an oxidation cost, so the number should be limited and recorded rather than repeated until the solderability is lost.

What does gopcb provide for bare board storage? We provide finish specific shelf life definition with the storage condition stated, moisture barrier packaging with desiccant and indicator, handling and glove rules, pre assembly assessment by solderability test and controlled bake, stock rotation records, and documented disposition for boards past their stated life.