Approved Component Substitution Process

A component shortage forces a decision: wait, or use a part that is not the one on the drawing. The second option is legitimate and it has to be managed, because an unmanaged substitution is a design change made by whoever was at the desk when the order was placed.

Form, Fit and Function

The three tests are the starting point and they are not sufficient. Form is the package and the pinout, fit is the footprint and the placement, and function is the electrical behaviour in the circuit.

A part that passes all three can still differ in derating, in thermal behaviour and in reliability. Those are the items that a substitution review adds to the three. Our component selection notes describe the rest of the incoming information.

Differences That Pass a Datasheet Comparison

Two parts with the same headline specification can differ in the tolerance on that specification, in the temperature range over which it holds, and in the behaviour at the extremes. A datasheet comparison at the nominal condition does not show those.

The second class of difference is in the process: a different lead finish, a different moisture sensitivity level, a different tape and reel pitch. Those affect the assembly rather than the circuit. Our board quality notes describe how a change in the assembly process is detected.

Qualification should be proportionate to the risk. A second source of an identical part needs a certificate and an incoming check; a functionally equivalent part from a different manufacturer needs a qualification plan.

The plan covers the electrical limits, the temperature range, the reliability screen that the part will see and the effect on the assembly process. Where the part is safety related, the plan is not negotiable.

The substitution is approved by the function that owns the design, not by purchasing and not by the line. The approval is recorded against the part number and the date, so that the drawing and the bill of materials agree afterwards.

An approval that is not written into the documentation produces a product that differs from its own specification. That is a finding in an audit and a problem in a recall.

A different package height changes the stencil and the placement, a different finish changes the solderability and the shelf life, and a different moisture level changes the bake requirement. Each is a process change and each has to reach the shop.

The route for that is the same as for any engineering change, which is why a substitution is treated as one. Our thermal cycling notes describe the verification that a change of this kind normally needs.

The record has to connect the approved substitution to the units it was used on. A substitution that is approved and not traced produces a population of boards whose composition is unknown.

That connection is a line in the assembly record rather than a separate file, and it is worth the effort the first time a field question has to be answered.

Process Control and Verification

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.

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.

Can a distributor propose a substitution? A proposal is useful and the approval rests with the design owner.

Is a second source always acceptable? Where the part is identical and the source is qualified it usually is, and the incoming check still applies.

What does gopcb provide for substitution control? We provide a review that goes beyond form, fit and function to derating, reliability and process effects, qualification scaled to the risk, approval recorded against the part number and the drawing, and traceability from the approved substitution to the units that used it.

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.