Second Source: Second-Source Qualification: Proving a Replacement Is Equivalent

A second source is introduced to reduce risk, and introducing one carelessly creates a different risk in its place. The work is in demonstrating that the replacement behaves like the original in the ways that matter, which is usually a smaller set than the data sheets suggest.

Why Second Sources Are Introduced

The reasons are supply continuity, cost and lead time. All three are legitimate and all three create the same obligation: the design must be shown to work with the alternative.

The obligation is not symmetric. A second source for a passive component is a small exercise, while a second source for a microcontroller with different silicon requires a full validation of the firmware and the peripherals.

The effort should therefore be proportional to the difference between the parts, and the first step is to characterise that difference rather than to start testing.

Establishing Equivalence

Two parts are equivalent for a purpose when they meet the same requirements. The requirements are the ones the design places on the part: the values, the tolerances, the temperature range, the physical size and the electrical behaviour at the frequencies used.

A comparison table built from the two data sheets identifies the differences, and the differences are ranked by their effect on the design. A difference in tolerance that the design already accommodates is not a difference that needs testing.

Our component selection notes describe the parameters that are frequently omitted from a data sheet but are important in practice.

The Validation Plan

The plan should specify the tests that distinguish the two parts, not the tests that both will pass. A test that measures a parameter where the parts are identical establishes nothing.

Where the difference is electrical, the test is the measurement of that parameter in the assembly. Where the difference is mechanical, the test is the assembly and inspection of the joint or the fit.

Where the difference is in a secondary characteristic such as the moisture sensitivity level, the test may be a process qualification rather than a product test. Our design release checklist notes where the resulting requirements are recorded.

Sample Size and Confidence

A single sample of the second source proves only that one unit works. The sample size should be enough to detect the difference the validation is looking for, and it should include material from more than one lot where the difference is likely to be lot dependent.

For a passive component the sample can be large because the test is cheap. For an expensive integrated circuit the sample is small, and the validation then relies more on the analysis of the difference than on the test.

The result of a small sample should be reported with its limit, since claiming equivalence from three units overstates what has been shown.

Process Considerations

A substitute part sometimes differs in its packaging, its tape and reel dimensions or its solderability. Each of these affects the assembly line rather than the product, and each requires a check.

A different reel pitch requires a different feeder setup, and a different body height may affect a fixture. These are inexpensive checks that are often omitted because the electrical comparison was done.

The moisture sensitivity level of the substitute may differ from the original, which changes the floor life and the baking requirement. Our fabrication notes describe how such differences are recorded.

Change Control After Approval

Once the second source is approved, both parts must remain approved. A change by either supplier invalidates the comparison that was performed, and the change must be assessed against the same requirements.

The assessment should ask whether the change affects a parameter that the validation relied on. A change in the mould compound affects the moisture sensitivity; a change in the die affects the electrical behaviour.

Where the supplier does not notify changes, the incoming inspection is the only protection. Our quality notes describe the lot level checks that detect a change.

Documenting the Decision

The record should state which parts were compared, which requirements were considered, which tests were performed, what the results were and what was concluded. It should also state what was not tested.

The statement of what was not tested is the part that is usually missing and the part that is most useful later, because it defines the boundary of the approval.

A second source approved for a benign environment is not approved for a harsh one unless the validation covered it, and the record is what makes that distinction visible.

Process Control and Verification

On a design of this kind, second source 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.

Process Control and Verification

On a design of this kind, second source 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.

Process Control and Verification

On a design of this kind, second source 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.

FAQ

Can a second source be approved on the data sheet alone? Where the parts are identical in every parameter that the design uses, yes. That case is rarer than it appears.

Does a second source need its own part number? It needs to be distinguishable in the records, either by a separate part number or by a supplier code, so that a failure can be attributed.

What does gopcb provide for second source qualification? We provide a comparison of the two parts against the design requirements, a validation plan that tests the differences, assembly trials that cover the packaging and process differences, and a record of the tests performed and the limits of the approval. Where a difference cannot be resolved by testing, we say so before the part is released to production.