Reference · Quality
Four-Stage Quality Control
Where defects are caught, what is checked at each stage, and why the final stage inspects every unit rather than a sample.
Which inspection gate can actually reject a batch?
Each of the four gates has a different power: incoming material control (IQC) rejects components, in-process control (IPQC) stops a run, final QC (FQC) tests finished units, and pre-shipment inspection under AQL 2.5 sampling decides whether the lot ships. Ask who may stop the line, where the record is kept, and what the contracted defect remedy is — replacement, credit or rework — before the deposit, not after a failed container. The distinction between a sampling limit and a zero-defect claim is set out in the AQL guide.
Sorting is not control
Quality control that starts at the end of the line is sorting, not control. Each stage below exists to catch a class of defect that the next stage either cannot see or can only see expensively: a material substitution is invisible once the unit is assembled, and a water-resistance failure is invisible once the unit is boxed.
What is checked at each stage
| Stage | Sample rate | Checked |
|---|---|---|
| IQC — incoming | Per material batch | Material certificate, hardness and tensile on silicone, surface finish on ABS, battery capacity verification |
| IPQC — in process | Per production milestone | Curing parameters, assembly torque, solder joints, potting integrity |
| FQC — finished unit | 100% of units | Full function test, charging cycle, water resistance by model rating, surface finish, packaging |
| OQC — pre-shipment | AQL 2.5 sample | Random draws from packed cartons, documentation check, carton drop integrity |
How the sample size is set
| Lot size (units) | Code letter | Sample size | Accept | Reject |
|---|---|---|---|---|
| 281 – 500 | H | 50 | 3 | 4 |
| 501 – 1 200 | J | 80 | 5 | 6 |
| 1 201 – 3 200 | K | 125 | 7 | 8 |
| 3 201 – 10 000 | L | 200 | 10 | 11 |
Where the weight sits
Notice where the weight sits: AQL sampling runs on top of a 100% final function inspection, not instead of it. Sampling is a statistical control that tells you the process is behaving; it does not tell you that the unit in your customer's hand works. Only testing every unit does that, which is why the outgoing defect rate is held below 0.1% by inspection rather than by luck. The OEM process applies the same four stages to branded and custom runs.
The outgoing sample
The outgoing sample follows ISO 2859-1, where the acceptance number belongs to the plan rather than to the lot: the same defect count can pass one plan and fail another [International Organization for Standardization, 1999]. Reading a result without the plan it came from is how a compliant lot gets rejected on paper.
References
- International Organization for Standardization (1999). Sampling procedures for inspection by attributes — Part 1: Sampling schemes indexed by acceptance quality limit (AQL) for lot-by-lot inspection, ISO 2859-1. Geneva: ISO.
- Montgomery, D. C. (2020). Introduction to Statistical Quality Control, 8th ed. Hoboken, NJ: Wiley.
- Juran, J. M. & Godfrey, A. B. (1999). Juran's Quality Handbook, 5th ed. New York: McGraw-Hill.
Questions on this reference
Where does AQL inspection sit in the four stages?
At the end. AQL inspection is the outgoing sample: a lot is accepted or rejected on a statistical reading of a sample drawn to ISO 2859-1, after the first three stages have already passed every unit. It is a check on the process, not a substitute for one.
Can I send my own inspector?
Yes. Any third-party inspector you choose may book pre-shipment inspection directly at the factory address. There is no coordination fee and no restriction on who you use.
What is the difference between FQC and OQC?
FQC is a full functional check on finished units; OQC is a statistical sample of the packed cartons before sealing. One is about the product, the other is about the lot — and neither replaces the other.
Is AQL 2.5 applied to every order?
It is the standard outgoing level, but the AQL and the inspection level are contract terms. If your purchase order does not name them, you have accepted whatever the supplier uses. See the AQL guide.
What happens to a lot that fails?
It is rectified and resubmitted, and the resubmission is inspected more tightly. Who pays for the rework and the second inspection is a commercial term — settle it in the contract, not after the failure.
Next: what AQL 2.5 actually accepts — or how the batch record travels with an order.
Four stages only mean something if each one can reject something. AQL 2.5 in practice is the outgoing sample read as a buyer reads it, the audit checklist is the same process seen from the floor, and who runs this line is the part of the answer a document cannot give you.
Putting this reference to work
This page is the data. The four sections below are where it changes a decision: the terms it is quoted alongside, the guide that explains the reasoning, and the models the figures apply to.