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
On a design of this kind, storage is the item that decides how the rest of the board is arranged. 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. A short note on the drawing about handling, storage or packaging is often worth more than an extra decimal place on a tolerance.
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.
Process Control and Verification
On a design of this kind, storage is the item that decides how the rest of the board is arranged. 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. A short note on the drawing about handling, storage or packaging is often worth more than an extra decimal place on a tolerance.
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.
Process Control and Verification
On a design of this kind, storage is the item that decides how the rest of the board is arranged. 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. A short note on the drawing about handling, storage or packaging is often worth more than an extra decimal place on a tolerance.
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.
Process Control and Verification
On a design of this kind, storage is the item that decides how the rest of the board is arranged. 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. A short note on the drawing about handling, storage or packaging is often worth more than an extra decimal place on a tolerance.
A first article check confirms that the process and the drawing agree on the points listed above, and that the coupon data supports the values used in the design.
Where a measurement falls outside the expected window, the sample is retained so that the cause can be established before the balance of the batch is released.
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.
