SMT Component Overage and Attrition Buy Quantity Calculator
A BOM line says 12 pieces per board, but a build of 500 boards rarely consumes exactly 6,000. Parts are lost in pickup, setup and first-article runs. This calculator turns attrition and setup loss into a realistic buy quantity and then rounds it to the packs your distributor sells.
Your numbers
Pickup misses, tombstones and drops. Small passives often run higher than ICs.
Parts used for feeder setup, nozzle checks and the first boards.
Reel, tray or tube quantity the distributor sells.
Your results
Planning aid. Use your line's measured attrition by package type for the best result.
Order quantity vs placement attrition
Sweeps Placement attrition from 0 to 10, holding your other inputs fixed.
Overage rules inside your ERP
Optional: a short review of how attrition and pack rounding could be applied in your planning.
Optional. The tool and every export work without this. No spam.
How to use it
Enter the number of boards and the quantity per board, then your measured placement attrition and the setup loss for the line. Add the standard pack size and any minimum order quantity. The result shows the requirement with attrition, the actual order quantity after pack rounding, and the leftover stock and its value. Run it per BOM line or for groups with similar package types, since a 0402 capacitor behaves very differently from a BGA.
The formula
Required quantity equals boards times quantity per board, times one plus the attrition percentage, plus a fixed setup loss. The order quantity is the larger of the MOQ and the requirement rounded up to a whole number of packs. Setup loss is fixed per line, so it matters most on small builds. Attrition scales with volume, so it matters most on large builds.
Reading the result
If the effective overage is large, the pack size is driving the buy, not attrition. On low-volume builds of cheap passives that is fine, since leftover stock is cheap. On expensive ICs, consider cut tape or a partial reel from the distributor, even at a higher unit price. The chart shows how the order quantity steps up as attrition rises, so you can see when a small change pushes you into another full reel.
Common mistakes
Applying one overage percentage to the whole BOM is the classic error: it overbuys ICs and underbuys tiny passives. Another is forgetting that scrap from rework, reflow and test can consume parts after placement. Finally, using the supplier MOQ as the only constraint ignores pack multiples, which many distributors enforce.
Frequently Asked Questions
What attrition rate is typical?
It varies by line, feeder condition and package. Many shops see well under one percent on mid-size passives and higher on very small or odd-form parts. Measure your own from line reports rather than relying on a generic figure.
Should setup loss be per reel or per job?
Per job and per line item is simplest. If you change over feeders mid-run or split a build across lines, add the setup loss for each setup separately.
How does this relate to ERP scrap factors?
Most ERPs have a component scrap percentage on the BOM line. This tool helps you pick a defensible value and a fixed setup quantity to feed into those fields.
What about kitting for contract manufacturers?
Agree overage in the kit contract. Many contract manufacturers request specific overage by part type, and any shortage or surplus is then clearly allocated.
Netray can help encode your measured attrition by package type into your ERP planning parameters, so the purchase proposal already reflects it.
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