Obsolescence: Preparation, Placement and Process Control

Every component on a board has a life that is decided by its maker rather than by the product it sits in. A part that is perfectly good in service can be discontinued because the wafer fab moved on, the package fell out of favour or the volume no longer justifies the tooling. Obsolescence management is the discipline of seeing that coming early enough to do something cheap about it.

Why Parts Disappear

The usual cause is silicon rather than electronics. A process node is retired when the foundry needs the capacity, and the components built on it go with it, however long the automotive or industrial market would like them to last.

Packaging and materials follow. A lead frame alloy, a plating chemistry or a mould compound can be discontinued on environmental grounds, and the part number changes even though the die is identical. Each of these routes produces a different amount of warning, and the short ones are the dangerous ones.

Reading the Warning Signs

Lifecycle status is published by the larger suppliers, and it moves through stages that have names like active, not recommended for new designs and obsolete. The intermediate stage is the one that matters, because it is the last point at which a replacement can still be qualified without a redesign.

Less formal signals matter too. A distributor changing from stock to non stock, a minimum order quantity jumping, or a lead time stretching from weeks to months all precede the formal announcement. Watching the supply channel is often faster than watching the status field.

The Last Time Buy Decision

A last time buy converts a future shortage into an inventory problem, and the calculation is about quantity rather than about whether to act. The quantity has to cover the remaining production, the expected attrition, the storage life of the part and the risk that the forecast is wrong.

Storage is the part that is usually underestimated. Moisture sensitive devices need dry storage and a bake before use, and the cost of maintaining a controlled store for years is part of the decision. Our article on component selection beyond the datasheet describes the other properties that should be checked before a part is committed to.

Finding and Qualifying an Alternate

An alternate part is only useful if it is form, fit and function compatible, and that phrase covers more than the footprint. Electrical limits, derating, thermal behaviour, tolerances and the behaviour of the part at the corners of its range all have to be compared.

Where a pin compatible second source exists, qualification can be as short as a functional test on a sample build. Where the replacement is electrically different, the work becomes a design change, and the cost rises accordingly. The evidence that supports the change is the same evidence described in our article on first pass yield analysis.

Records That Make the Work Possible

The approved vendor list, the bill of materials and the assembly drawing have to agree with each other, and the alternate has to be recorded against the correct level. A change note that names the replacement without naming the assemblies it applies to is a future source of confusion.

Traceability matters here for a different reason than in production: when a part is replaced, the record has to show which units used which version. Our article on PCBA traceability labelling covers the label side of that requirement.

Design Choices That Reduce Exposure

Using parts from more than one maker, avoiding the newest and smallest packages, and keeping a generous margin on electrical parameters all reduce the chance that a single discontinuation becomes a crisis.

Standard packages are also easier to replace than proprietary ones, and a part that is available from several distributors is more likely to survive a supply shock. None of this costs much at design time and all of it costs a great deal later.

Running the Process

The work should be a scheduled review against the bill of materials rather than a reaction to a shortage. A quarterly check that reports status changes, lead time movement and any new alternates is enough for most product lines.

The output of that review should be an action list with owners and dates, because an item that has been noticed is not the same as an item that has been addressed.

Checks Before Release

On a design of this kind, obsolescence is the item that decides how the rest of the board is arranged. Where the supplier and the user both measure the same property, they should agree on the method before the first delivery. 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 first article confirms that the setup matches the intent, and it is the cheapest point at which a wrong setup can still be corrected. The cost of verification is small compared with the cost of a field failure, and it is paid at a point where the product can still be corrected.

Documentation exists so that a person who was not present can reproduce the work and reach the same conclusion. 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.

The environment around the process, including temperature, humidity and cleanliness, sets limits on what the process can hold. A result that cannot be reproduced is not a result, and reproducibility should be demonstrated rather than assumed.

Verification and Records

On a design of this kind, obsolescence is the item that decides how the rest of the board is arranged. 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. Consumables have a life measured in cycles, and the replacement point should come from the measurement rather than from a failure.

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. Where two operations share a tolerance, the allocation between them should be explicit rather than left to whichever is measured first.

Points to Confirm at First Article

On a design of this kind, obsolescence is the item that decides how the rest of the board is arranged. Where a decision is made by judgement, a boundary sample makes the judgement repeatable between operators and between shifts. A record that identifies the operator, the date and the settings is worth more than a record that identifies only the result.

FAQ

Is a last time buy always the right answer? Only when the remaining volume justifies the inventory and the part will survive the storage period. For a low volume product it is often cheaper to redesign once than to hold stock for years.

Can a datasheet identical part be used as a drop in replacement? Only after the electrical and thermal limits, the tolerances and the derating have been compared. Identical headline figures do not mean identical behaviour at the corners.

How early should alternates be identified? Before the product enters volume production, ideally, so that the qualification runs alongside the main design rather than against a shortage deadline.