Repair Formulas

How to Calculate Core Depot Repair Metrics: Yield, Lead Time, NFF and Cost Per Unit

Step-by-step math for the five metrics that run an electronics repair depot, from first-pass yield to cost per repaired unit, with worked numbers and unit sources.

Start with first-pass repair yield because everything downstream depends on it. First-pass yield equals units repaired correctly on the first attempt divided by total units entering repair. If a depot receives 1,240 boards in a week and 1,041 pass final functional test without a second pass, yield is 1,041 / 1,240 = 0.839, or 83.9 percent. Pull the numerator from your final test pass log and the denominator from RMA intake, not from shipped units, since scrapped and no-fault-found boards still consumed labor. The First-Pass Repair Yield calculator handles the ratio, but the discipline is defining 'first attempt' consistently across technicians.

No-fault-found rate is deceptively simple: NFF units divided by total units screened. If 96 of those 1,240 boards passed incoming diagnostics with no reproducible defect, NFF rate is 96 / 1,240 = 7.7 percent. The input trap is time-boxing. A board flagged NFF after a 4-minute smoke test is not the same as one that survived a 45-minute soak. Standardize the screen duration before you trust the rate. The No-Fault-Found Rate calculator gives you the percentage, but you supply the test protocol that makes the denominator honest.

RMA queue lead time is a flow calculation, not a single duration. Total lead time equals queue wait plus diagnostic time plus rework time plus test and burn-in plus outbound staging. If a unit waits 3.5 days in queue, takes 0.4 hours to diagnose, 1.2 hours to rework, 8 hours in burn-in, and 0.5 days to stage, convert everything to consistent units. In days at an 8-hour shift: 3.5 + 0.05 + 0.15 + 1.0 + 0.5 = 5.2 days. The RMA Queue Lead Time calculator wants each stage separately so you can see that queue wait, at 67 percent of total, is your real constraint.

Board rework time drives labor capacity. Estimate it as setup plus component removal plus placement plus inspection. For a single BGA reball: 6 minutes setup, 4 minutes controlled removal, 5 minutes paste and placement, 3 minutes AOI and touch-up equals 18 minutes, or 0.30 hours. Multiply by defects per board. A board averaging 1.4 rework sites needs 0.30 times 1.4 = 0.42 hours of bench time. The Board Rework Time calculator lets you build these element times once and reuse them, which beats estimating per job and drifting 20 to 30 percent between quotes.

Burn-in test capacity is a throughput calculation gated by chamber slots and dwell time. Capacity per day equals (chamber slots times available hours) divided by dwell hours per unit. With 240 slots, 20 available hours after loading windows, and a 12-hour dwell, capacity is (240 times 20) / 12 = 400 unit-slots per day. If each unit occupies one slot for the full 12 hours, that caps you at 400 units regardless of bench headcount. The Burn-In Test Capacity calculator exposes this ceiling early so you size chambers to yield, not to hope.

Cost per repaired unit ties the labor math together. It equals total repair cost divided by units successfully repaired, where the denominator excludes scrap and NFF. If weekly repair cost is 42,600 dollars and you shipped 1,041 good units, cost per repaired unit is 42,600 / 1,041 = 40.92 dollars. Note the denominator: dividing by all 1,240 intake units understates true cost to 34.35 dollars and hides the burden of failed repairs. The Cost Per Repaired Unit calculator and the Diagnostic Labor Cost calculator feed each other, since diagnostic minutes are often 30 to 40 percent of that per-unit figure.

Parts salvage value is a per-board harvest calculation you run before scrapping. Salvage value equals the sum of reusable component market values minus harvest labor. A failed motherboard might yield a 14-dollar DC jack, 3 dollars of DDR, and a 9-dollar controller, totaling 26 dollars, against 0.25 hours of desolder labor at 38 dollars per hour, or 9.50 dollars. Net salvage is 26 minus 9.50 equals 16.50 dollars. The Parts Salvage Value calculator makes the harvest-versus-scrap call quantitative rather than a technician's gut feel on a busy Friday.

Chain the calculations to sanity-check each other. First-pass yield of 83.9 percent means 16.1 percent of intake needs a second pass, which inflates average rework hours and therefore cost per repaired unit. A 5-point yield gain from 83.9 to 88.9 percent on 1,240 units removes roughly 62 second passes, and at 0.42 hours each that frees 26 labor hours weekly. Run the metrics as a connected set, matching numerators and denominators, so a yield improvement shows up correctly in lead time and cost rather than getting lost in inconsistent unit definitions.

Published 2026-07-01.