The ratio of units that meet quality specifications without any rework or repair on their first pass through a defined process step or sequence, computed as (good units leaving the step on first pass) ÷ (units entering the step).

Principle

FPY measures a process’s ability to produce conforming units without corrective effort; when applied across serial steps, individual FPYs multiply to produce rolled throughput yield, so modest losses at multiple steps compound into substantial total loss.

Demonstration

Illustrative scenario → A PCB assembly receives 1,000 boards at process start. After the final inspection, 930 boards require no rework. FPY = 930/1000 = 0.93 (93 %). Recognition: a high FPY indicates low rework burden and lower rework cost; a low FPY directs root‑cause analysis to the steps contributing defects.

Misapplication

Including units that were repaired or reworked as 'first pass' or computing FPY using inconsistent inspection criteria or across product variants without normalization. The semantic error is substituting post‑rework net yield for true first‑pass performance, which hides the actual rework burden and lead‑time impact.

Consequence

Correct use: targets defect reduction at source, reduces rework cost, shortens lead time and lowers process variability. Misuse: underestimates rework capacity needs, misallocates cost and capacity by planning on net output rather than first‑pass throughput, leading to missed delivery targets or hidden costs.

Reversal

FPY is less meaningful where rework is an intended part of the process (authorized repair stations), where inspection is destructive, or where 'first pass' cannot be unambiguously defined (iterative inspection with feedback loops). In bespoke or highly variable processes FPY may not indicate actionable improvement potential.

Boundary

Clearly within: discrete manufacturing lines with defined, inspectable process steps and identifiable units. Boundary case: processes with staged inspection and rework loops where defining 'first pass' requires explicit protocol. Clearly outside: continuous chemical processes or services where discrete units and discrete rework actions are not definable.

Semantic Tension

FPY (quality‑at‑source emphasis) can conflict with throughput‑focused metrics that accept downstream rework to meet delivery schedules; the tension is between minimizing immediate defects and maximizing on‑time delivered quantity.

Synthesis

FPY is a targeted measure of first‑time conformity that is most useful when paired with step‑level diagnostics and rolled throughput calculations; it must be distinguished from net output after rework and normalized before cross‑product or cross‑line comparisons.