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.

Figure 1 Material rejected at the first stage never reaches a later one, so the cost of a failure rises with every stage it passes. Batch records — including the measurements taken on that batch — ship with the goods.

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, and how often
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

Typical single sampling plan, general inspection level II, AQL 2.5
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

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.

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.

GuideAQL 2.5 in practiceWhat the numbers mean when you receive goods.GuideFactory audit checklistWhat to look at when you visit.EvidenceQuality control detailThe four stages described.CompanyThe manufacturerWho runs this line and since when.

Questions on this reference

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.

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