Alternate Part Qualification and Change Control

An alternate part is a second source for a component that is already qualified, and qualifying it is a process activity rather than a purchasing one. The two parts have to be equivalent in the ways that matter to the assembly and to the product.

What Form Fit and Function Covers

Form is the package outline and the terminal geometry, fit is what it has to mate with, and function is the electrical behaviour. A change in any of the three affects something downstream.

The comparison should be made against the drawing rather than the datasheet alone. Our component selection notes describe the differences that the datasheet omits.

Differences That Matter to the Process

Terminal finish, moisture sensitivity level, body height and tape packaging all affect the line. A part with a different finish needs a different profile, and a part with a different body height changes the stencil or the nozzle clearance.

Each of those should be checked before the part is released. Our defect notes describe what a finish change does.

Revalidation and Its Scope

The scope of revalidation follows the differences. A different terminal finish requires a solderability and a profile check, and a different thermal mass requires a profile check on a board that carries the part.

A blanket rule that requires everything is as unhelpful as one that requires nothing. Our solderability notes describe the test.

Marking, Polarity and Traceability

A different manufacturer may mark the part differently, which changes the assembly drawing and the silkscreen. The marking should be recorded so that the two sources can be distinguished later.

Our silkscreen notes describe the marks that matter.

Mixed Supply and Its Risks

Once both parts are approved they may appear on the same board, and the process has to tolerate both. Where they differ in a process-relevant way, the process window has to cover the wider of the two.

The window should be established with both parts present. Our yield analysis notes describe how the comparison is recorded.

Records and Withdrawal

The qualification record should state what was tested, with which part and on which board, so that a later withdrawal can be traced. A part that fails in the field has to be traceable to the boards that carry it.

Our design release notes describe the change control that covers it.

Verification

The verification is a form fit and function comparison against the drawing, an assessment of every process relevant difference, a revalidation scoped to those differences, and a record that identifies the part used on each lot.

Our quality notes describe how the records are kept.

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.

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.

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.

FAQ

Is a datasheet comparison enough? It is a starting point. The process relevant differences are often not on the datasheet, which is why a sample check is part of qualification.

Can both sources be used interchangeably? Only where the process window covers both. Where they differ, the board has to be built to the wider requirement.

What does gopcb provide for alternate parts? We provide a form fit and function comparison against the drawing, an assessment of finish, moisture level, body height and packaging, a revalidation scoped to the actual differences, and a record that identifies which source was used on each lot.