Aging and Long Term Storage of Assemblies

An assembly that has been in a warehouse for two years is not the same assembly that was packed. The solder joints have aged, the finishes have oxidised, the plastic parts have absorbed moisture and the electrolytic capacitors have lost part of their life.

What Changes in the Solder Joint

The intermetallic layer at the copper interface continues to grow at room temperature, slowly but without limit. The layer is harder and more brittle than the solder, and a thick layer is a place where a crack can start.

The growth rate roughly doubles for every ten degrees, so a warehouse at thirty degrees ages a joint far faster than one at twenty. The effect is small over a year and measurable over five, particularly for small joints where the intermetallic occupies a larger fraction of the total volume.

The tin in a lead free joint also undergoes a transformation at low temperature, which changes its mechanical properties. The transformation is slow at room temperature and accelerated by cold, so a joint that has been stored cold behaves differently after storage. Our quality notes describe the joint conditions that are checked after storage.

What Changes on the Surface

The exposed metal on the board tarnishes or oxidises according to the finish. A silver finish forms a sulphide layer that inhibits wetting, and a copper finish forms an oxide.

The soldermask and the legend may yellow slightly, which is cosmetic, but a mask that has absorbed moisture has lower adhesion and lower insulation resistance.

Contamination on the surface that was harmless when the board was dry becomes active if the storage environment allows condensation. Our solderability test notes describe how the surface condition is measured.

What Changes in Components

Electrolytic capacitors have a limited life that begins at manufacture, and their electrolyte slowly dries. A capacitor stored for years has consumed part of that life without having been used.

Batteries, whether primary or rechargeable, self discharge and degrade. A product stored with a battery installed may arrive with a dead battery and, in the worst case, with a leaked one.

Plastic parts absorb moisture and change dimension slightly, and some adhesives continue to cure. A connector that was within tolerance when built may be outside it after storage. Our component reliability notes describe the parameters that change.

Storage Conditions That Matter

Humidity is the dominant factor for the surface and for moisture sensitive parts. A controlled store keeps the relative humidity low and stable, and a barrier bag protects the assembly between the store and the user.

Temperature affects the ageing rate of the joints and the life of the electrolytic capacitors. A cool store is better, provided it does not cause condensation when the product is brought out.

Temperature cycling is worse than a constant temperature, because it drives moisture in and out and because it stresses the joints. A store that is hot during the day and cold at night is therefore worse than one at a moderate constant temperature.

Requalification After Long Storage

The requalification should test the properties that change: solderability of any surface that must be soldered, insulation resistance, and the function of the product at its temperature extremes.

Where the storage has been long, a sample should be opened and inspected rather than relying on the outside appearance of the packaging. A sealed bag with a humidity indicator card shows whether the protection held.

Where the assembly is complete, a functional test at temperature, plus a check of the capacitors and any battery, covers most of the risk.

Design Measures That Reduce the Risk

Choosing a finish that ages slowly, such as a gold over nickel, extends the storage life at a cost. Choosing components with a long shelf life, including solid capacitors instead of electrolytic ones where the design allows, removes the ageing mechanism.

Avoiding a battery in the stored product is the simplest measure of all. A product that must ship with a battery should have the battery isolated, or the battery should be a separate item. Our coating notes describe how a coated assembly behaves during storage.

Documenting the storage limit for each assembly and marking it on the packaging makes the limit enforceable. A limit that exists only in a specification is a limit that will be exceeded.

Process Control and Verification

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.

Verification and Records

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.

Points to Confirm at First Article

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.

Documented Process Control

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.

Inspection and Traceability Notes

Sampling is a compromise between cost and confidence, and the sample size should follow from the failure rate that has to be detected. The first article confirms that the setup matches the intent, and it is the cheapest point at which a wrong setup can still be corrected.

FAQ

Does a sealed bag stop the ageing? It stops the moisture and the surface oxidation, and it does not stop the intermetallic growth or the electrolytic ageing.

Is a cold store better? For the ageing rate, yes. It must not cause condensation, which requires the product to be brought to ambient before the bag is opened.

What does gopcb provide for stored assemblies? We provide the finish and material shelf life for each build, controlled storage with humidity and temperature records, humidity indicator cards in every sealed bag, and requalification testing for material that has been stored beyond its period.