Board FIT Calculator — Failure Rate and MTBF from a BOM

Enter what is on the board and the temperature it runs at. The calculator returns a board-level failure rate and MTBF, scales it to your ambient, and ranks which component classes are actually spending it.

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What a board FIT number is for

A failure rate on its own settles very little. Its value is that it is the input to two other questions. The first is commercial: what warranty return rate does this design imply, and can the business live with it. The second is safety: a hardware safety analysis begins with a failure rate per element, and if you cannot produce one, the analysis cannot start.

That second use is why this calculator sits next to the safety tools rather than on its own. A board FIT total is the raw λ that the FMEDA rollup consumes. Split across elements, with a safe-fault fraction and a diagnostic coverage argued for each, it becomes SPFM, LFM and PMHF — and only then does it say anything about an ASIL.

Temperature is usually the biggest lever

Failure rates are quoted at a reference temperature, and they do not stay there. The standard correction is the Arrhenius relation:

With an activation energy of 0.7 eV, moving from 40 °C to 85 °C multiplies the rate by roughly twenty-six — the example board above goes from about 180 years of continuous operation to about 7. That single number swamps almost any component-selection argument, which is why a mission profile that has not been agreed is the most common reason two analyses of the same board disagree. Change the ambient field above and watch the total move; then consider that the ambient inside an enclosure is not the ambient on the datasheet.

Activation energy is not universal — 0.7 eV is a common default for silicon, but electrolytics, solder joints and connectors age by different mechanisms with different energies, and a rigorous analysis applies them per class rather than globally. This tool applies one value to everything, which is a real simplification.

Where the rate actually comes from

Look at the share column rather than the total. On most boards a handful of classes supply the majority of the failure rate, and they are rarely the parts anyone worried about during design. Electrolytic capacitors and connector pins routinely outweigh the microcontroller, because reliability follows count and stress, not complexity or cost. If you want a lower number, the ranking tells you the only three lines worth touching.

What this leaves out

• Stress ratios. Real models derate per part by applied voltage, power and temperature rise above ambient. A capacitor at 80% of rated voltage is not the same part as one at 30%, and this tool cannot tell them apart.
• Wear-out. This is a constant-rate model — the flat middle of the bathtub curve. Electrolytic capacitor dry-out, solder-joint fatigue under thermal cycling and connector fretting are wear-out mechanisms with their own lifetime models, and they are frequently what actually ends a product’s service life.
• Environment. Vibration, humidity, condensation and thermal cycling are absent. A ground-benign figure applied to automotive underhood is not a conservative estimate, it is a wrong one.
• Assembly and process. Workmanship, cleanliness and test escape rates do not appear in a parts-count model at all, and on a new production line they can dominate everything above.

Where this fits

PCB design and FMEDA/FMEA review are both primary service lines here, and this calculator sits exactly where they meet. If you need a board failure rate you can defend — sourced, derated, with a stated mission profile and a written rationale — that is an FMEDA gap review or a from-scratch build, one to two weeks and three to eight weeks respectively. Both end in a report you keep.

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