Component Derating for Temperature and Voltage Margin

Derating is the deliberate use of a component below its rated limit so that it has margin. It costs money and board area, and it is the cheapest reliability measure available when it is applied to the parts that actually limit the life of the product.

Why the Rating Is Not the Working Point

A rating is a limit at a defined condition, usually a case temperature, a load life and an end of life criterion. Working at the rating means accepting the shortest life the part is allowed to have.

The rating also assumes an ideal thermal path. A resistor in free air on a test board and the same resistor surrounded by components on a real assembly are not at the same temperature. Our thermal notes describe how the actual temperature is established.

Temperature Derating

The temperature derating is applied to the junction or the hotspot, not to the ambient. The difference between the two is the thermal resistance of the path, and it is where most derating calculations fail.

The margin should be enough to cover the spread of the thermal path between units as well as the ambient variation. A part at 90 percent of its limit at nominal conditions has no margin at all.

Voltage and Current Derating

Capacitors and semiconductors are derated for voltage because the failure rate rises steeply as the applied voltage approaches the rating. The mechanism is different in each family, so the derating factor should come from the manufacturer rather than from a general table.

Current is derated for the temperature rise it produces, which depends on the copper and the airflow around the part. Two resistors of the same value in the same circuit can need different derating because of where they sit. Our current capacity notes describe how the rise is calculated.

Power and Thermal Cycling

The power rating is the least useful of the ratings without a case temperature, because the two are linked. A part run at half its power rating into a hot environment can be further from its limit than one run at full rating in a cool one.

Thermal cycling is a separate limit. A part that survives a steady temperature may fail from the fatigue caused by repeated expansion, so the derating should consider the number of cycles as well as the peak.

Where to Apply It

Derating should be applied where the consequence is severe and where the part is already close to its limit. Applying a large margin everywhere increases cost and size without changing the reliability of the product.

The parts that matter are usually the ones with the highest failure rate, the ones that are hardest to replace, and the ones whose failure takes the rest of the assembly with them. Our component selection notes describe how the ranking is made.

Recording the Margin

The applied stress and the resulting margin should be recorded for each critical part, with the temperature it is based on. A derating figure without a temperature is not a calculation.

Where the margin is small, the reason should be recorded as well, so that a later change to the load or the environment can be evaluated against the original assumption. Our design release notes cover where the record belongs.

Verification

The margin is verified by measuring the temperature of the critical parts in the finished assembly under the worst case load, and by confirming that the measurement agrees with the calculation.

Where the measurement is higher than predicted, the thermal path is the usual cause, and it should be corrected before the derating is reduced to match the measurement. Our quality notes describe how the result is recorded.

Process Control and Verification

On a design of this kind, derating is the item that decides how the rest of the board is arranged. 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. 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.

Process Control and Verification

On a design of this kind, derating is the item that decides how the rest of the board is arranged. 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. 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.

Process Control and Verification

On a design of this kind, derating is the item that decides how the rest of the board is arranged. 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.

A first article check confirms that the process and the drawing agree on the points listed above, and that the coupon data supports the values used in the design.

<img src="https://www.gopcba.com/wp-content/uploads/2025/08/16-1-1.png" alt="Junction temperature estimated from a case measurement” />

Where a measurement falls outside the expected window, the sample is retained so that the cause can be established before the balance of the batch is released.

FAQ

Is a larger derating always better? No. Beyond the point where the margin is adequate, the extra margin buys cost, board area and mass rather than reliability.

Should every component be derated? Every component with a stress limit should be evaluated, but only the ones near the limit or with a severe consequence need a recorded margin.

What does gopcb provide for derating? We provide critical part identification, temperature measurement on the real assembly, junction temperature calculation from the measured case, voltage and current derating to supplier data, thermal cycling review, and a record of the applied stress and margin with the temperature it is based on.