Derating: Component Selection Beyond the Datasheet

Selecting a component is a design decision that continues past the datasheet, because the part has to be placed, soldered, inspected and purchased. Each of those adds a constraint that the electrical specification does not capture.

Package and Footprint

The package determines the footprint, and the footprint determines the pitch, the stencil aperture and the inspection method. A package that is electrically ideal but whose pitch is at the limit of the process will cost yield on every board.

The footprint also determines the thermal path and the mechanical strength. A package with a large thermal pad requires a via array, a stencil pattern that avoids voiding and a reflow profile that can drive the heat through the pad. Our thermal management notes describe the provisions.

Voltage and Thermal Derating

A component operated at its rated voltage and temperature will not last as long as one operated at a fraction of both. Derating is the practice of choosing a part whose rating exceeds the applied stress by a defined margin.

The margin depends on the application. A consumer product may accept a smaller margin than an industrial one, and the difference should be a stated policy rather than a designer’s preference. Our component reliability notes describe the parameters.

Tolerance and Its Effect on Yield

A tight tolerance costs more and may reduce availability. The question to ask is what the circuit requires rather than what is available, because a one per cent resistor in a position that only needs ten per cent adds cost without adding performance.

Where a tolerance is critical, it should be marked as such on the bill of materials so that a substitute is not accepted on value alone.

Availability and Lifecycle

The lifecycle status of a part determines whether the product can be built in three years, and the risk is highest for specialised parts. A part that is active today may be on allocation tomorrow and obsolete the year after.

The design response is either a footprint that accepts a replacement or a second source that has been evaluated. Both are cheaper than a redesign triggered by a purchase failure. Our bill of materials notes describe the fields that record the risk.

Moisture Sensitivity and Handling

Plastic packages absorb moisture, and the absorbed moisture turns to steam during reflow. The moisture sensitivity level states how long a part may be exposed to the floor environment after the packaging is opened before it must be baked.

The level affects the assembly plan rather than the electrical design, but it belongs in the design record because it determines how much of the board can be built at once.

Second Sources and Substitution

An approved second source is worth having for parts on the critical path, and the approval has to cover more than the electrical value. The package, the pinout, the tolerance, the temperature range and the height all have to match.

Where a substitute differs in a parameter that does not matter to the circuit, that should be recorded, so that a later reviewer does not treat the difference as a defect.

Cost in the Context of the Assembly

The price of a part is a small part of what it costs to fit it. A component that requires a special stencil aperture, a different profile or a manual operation costs more to assemble than its price suggests.

The comparison should therefore be made at the level of the assembled board rather than at the level of the component list. A slightly more expensive part in a more convenient package is often the cheaper choice.

Documenting the Choice

The bill of materials should record the parameters that are critical for each part, together with the reason the part was chosen where that reason is not obvious. The record is what allows a substitute to be evaluated later without repeating the analysis.

Where a part was chosen for a reason that has since changed, the record makes that visible. Without it, the choice survives as a tradition rather than as a decision. Our design release checklist lists the fields.

Process Control and Verification

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. A stack-up that is drawn rather than described removes most of the ambiguity from a quotation, and it lets the fabricator price the board against the dielectric and copper weights that will actually be used.

Where the requirement is not written down, the shop supplies its own default, and the default is chosen for the process rather than for the design. The measurements that matter are the repeatable ones: conductor width and spacing, annular ring, finished hole size, plating thickness and surface finish are all verifiable on a coupon that travels with the panel.

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.

Verification and Records

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.

Points to Confirm at First Article

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.

Documented Process Control

Where an operation cannot be verified afterwards, it has to be controlled during the operation, and that control has to be visible in the record.

Inspection and Traceability Notes

Sampling is a compromise between cost and confidence, and the sample size should follow from the failure rate that has to be detected. 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.

FAQ

How much derating is appropriate? Enough that the applied stress is a defined fraction of the rating, with the fraction set by the application’s life requirement. The figure should be written down, because a policy that exists only in experience is applied inconsistently.

Is the cheapest part always the wrong choice? It is the right choice when it meets every requirement, and the way to establish that is to check the requirements rather than the price. The failures come from parts that were accepted on price without a check.

What does gopcb check on a component list? We check the package against the footprint, the pitch against our assembly capability, the moisture sensitivity level against the assembly plan, the lifecycle status where the data is available and the tolerance where the design marks it as critical.