Assembly Material Shelf Life Control
Every consumable in an assembly line has a shelf life, and the ones that matter are not always the ones that are tracked. Solder paste is carefully dated, while adhesives, flux, wick, cleaning chemistry and even the desiccant in a bag are often stored until someone notices a problem.
Material control is a simple discipline: a defined storage condition, a visible expiry, a first in first out rule and a record. Where any of the four is missing, the material will eventually be used outside its specification.
Shelf Life and Storage Life
Shelf life is the period in the original sealed package, while storage life describes the material after it has been opened. Many materials have a long shelf life and a short storage life, which is where confusion arises.
The storage condition is part of the specification: temperature, humidity and, for some materials, protection from light. A material stored outside its condition has a shelf life that is shorter than the label suggests.
Materials With Short Lives
Solder paste, two part adhesives once mixed, activated flux and some cleaning chemistries have working lives measured in hours. These are the materials where a visible clock is essential.
Underfill, potting compounds and conformal coatings fall in the same group, since a mixed batch begins to cure immediately. The mixed quantity should be matched to the work available.
Materials With Long Lives
Laminate, mask, connectors and passive components have long shelf lives that are nevertheless finite. Solderability degrades with age and storage condition, particularly on a component with a tin finish.
The long lived materials are the ones that are least likely to be checked, so a periodic review of the stock is worth more than a label alone.
First In First Out
First in first out is the rule that makes shelf life control work, because it ensures that the oldest material is consumed first. It requires that the storage location is arranged so that the new material goes behind the old.
The rule fails when a delivery is put at the front because it is convenient. The discipline should be verified by the audit rather than assumed.
Labelling and Identification
Every container should carry the material, the lot, the receipt date and the expiry. Where a material is transferred to a smaller container, the label should travel with it.
An unlabelled container at a workstation is a finding, because it cannot be traced and its age is unknown. The same traceability principle that applies to boards applies to the materials that build them.
Storage Conditions and Monitoring
A refrigerator that holds paste should have a recorded temperature, and the recording should be checked rather than merely collected. A refrigerator that has failed overnight may have frozen the paste without anyone noticing.
Humidity controlled storage is required for moisture sensitive components, and the indicator cards in the bags provide a check on the packaging as well as on the storage.
Expiry and Disposition
Material past its expiry should be quarantined rather than left in the same location as usable stock, because the two will eventually be mixed. The disposition should be recorded as consumption, return or disposal.
Where a material is used past its date with a concession, the decision should be documented with the reason and the affected boards identified. A concession that is granted informally is a risk that is carried silently.
Working Life on the Line
The clock that matters most is the one that runs after the material is opened. A jar of paste has a defined jar life, a mixed adhesive has a pot life and a cleaning bath has a usable concentration range.
These should be recorded at the station, since the information is not available from the label alone and the material may be entirely within its shelf life. This is the practice described for paste storage and thaw.
Records and Audit
The records should link the material lot to the boards that were built with it, which is what allows a containment when a material problem is found. A batch record without the material lot is incomplete.
The audit should check the storage conditions, the labels, the quarantine area and a sample of the records. It should also observe the practice, because a rule that is understood and not followed looks identical in the paperwork.
Additional Considerations for This Build
Practical attention to expiry labelling 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 expiry labelling explicitly, together with the tolerance that applies, removes the assumption and keeps the result predictable from batch to batch.
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.
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.
Related reading: our fabrication notes, board quality and design release notes cover the same ground.
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.
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.
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 material expire if it is unopened? Most do, and the shelf life is set by the chemistry rather than by exposure.
Can an expired material be used with a concession? Only with a documented decision and a defined period, since the effect of age on the process is not always visible.
Why is first in first out so important? Because it is the only practical way to ensure that material is consumed before its expiry without checking every container by hand.
What is the most commonly overlooked item? The working life after opening, which is not on the container and is the one that affects the process most directly.
