Second Source Component Strategy Guide

A second source is an alternate part that can be substituted for the primary one without changing the board. It protects a product from a supply interruption, from an allocation and from a quality problem at one vendor. The work is done at the design stage, because a substitute that needs a different footprint is not a second source at all but a redesign.

Why a Second Source Is Needed

A single sourced component turns a supply problem into a production stop. The causes range from a factory fire to an allocation, an obsolescence notice and a simple quality escape.

The risk is highest for a part with a long lead time, a single wafer fab or a package that only one supplier makes. Those are the parts that deserve the effort.

The cost of a shortage is measured in delayed revenue rather than in the price of the part, which is why the exercise is usually justified by the first problem rather than by a calculation.

The strategy belongs in the design release rather than in a purchasing review, because the alternative has to fit the board that already exists.

Levels of Compatibility

The strongest form of second source is a part that is identical in function, footprint and pin out, so that no change to the board or the firmware is needed.

A weaker form is a part that needs a different pad geometry or a different decoupling arrangement. It is still useful, but it requires a board revision to use.

The weakest form is a part that needs a firmware change or a different peripheral configuration. It is a fallback for an emergency rather than a second source.

The level should be recorded for every critical part, so that the team knows what a substitution would cost before the shortage happens.

What to Qualify

Component qualification is the work of proving that the alternate behaves like the primary, and it is the step that most projects skip when a shortage arrives.

The alternate part has to be checked electrically at the limits as well as at the nominal point, because two parts with the same typical figures can behave differently at the extremes.

The thermal behaviour has to be checked, since a package with a different thermal path can run hotter in the same position on the board.

The timing and the tolerance have to be checked where the part is in a loop or in a clock path, because a small difference can change the stability of the circuit.

The tolerance and reliability work covers the variation between parts, and the alternate has to be included in that analysis rather than added later.

Footprint and Layout

A footprint compatible alternate is the goal, and it should be verified against the package drawing rather than against a catalogue note that says compatible.

Where the two parts differ, a common footprint can sometimes be designed to accept both. A pad that is slightly longer or a courtyard that is slightly larger costs a little area and buys a supply option.

The pad standards should be followed for both parts, and the larger of the two requirements should be used where they differ.

The placement program and the inspection program also have to accept both parts, which is usually a matter of a library entry rather than a new program.

Documentation and Approval

The alternate part should be listed in the bill of materials with its status, so that a buyer can substitute without asking an engineer at midnight.

The approval record should state what was tested and what was not, because an untested substitution is a risk that is easy to take and hard to justify later.

The supplier of the alternate should be qualified as well, since a part from an unknown vendor is a different kind of risk from a part from the primary source.

The documentation should be reviewed when either part changes, because a revision at the primary source can break the compatibility that was the whole point.

Cost and Practicality

A second source costs engineering time and a small amount of board area, and it saves a production stop. The trade is usually obvious for a critical part and wasteful for a commodity.

The parts that deserve the effort are the ones with a long lead time, a single supplier, a custom package or a firmware dependency.

The rest can be handled by a generic approval rule, where any part with the same footprint and rating is acceptable without a separate qualification.

The cost review should include the supply risk, because the cheapest part is not the cheapest product if it stops the line.

Building the Substitution Plan

Write down the substitution before it is needed. A note in the bill of materials that names the alternate, the level of compatibility and the person who approved it turns a crisis into a purchasing decision that can be made in an hour.

Keep one board built with the alternate parts. A substitute that has only ever been analysed has not been proven, and the first build with the alternate is the worst moment to discover a firmware dependency or a thermal surprise.

Tell the contract manufacturer about the plan. If the alternate is not in the approved vendor list on the line, the substitution cannot be made at the moment it is needed, however well documented it is.

Review the plan once a year at least. Parts go obsolete, suppliers merge and packages change, and a list that was written three years ago describes a supply chain that no longer exists.

The list should also name the parts that have no alternate at all, because those are the single points of failure in the supply chain. Knowing which three parts could stop the product is more useful than a long list of comfortable substitutions.

Finally, treat the exercise as an insurance policy with a known premium. The engineering hours are small compared with the cost of a line stop, and the parts that need the cover are the same ones that everyone already worries about.

Keeping the Strategy Alive

A second source that is identified but never purchased is a paper exercise. The alternate should be bought and built at least once so that the substitution is proven.

The list should be reviewed at each design revision and at each obsolescence notice, because the supply situation changes faster than the design.

The review should include the purchasing team, since they see the lead times and the allocations before the design team does.

The record should be kept with the release package, so that the knowledge survives the departure of the engineer who did the work.

FAQ

What makes a good second source? A part with the same function, footprint and pin out, qualified at the electrical and thermal limits, and available from a supplier that has been approved.

How often should the second source list be reviewed? At every design revision and whenever an obsolescence notice arrives, since the supply situation changes more often than the design does.

Does a second source need a board revision? Only if the footprint differs. A compatible part can be substituted with no layout change, which is why footprint compatibility is the goal.