PCB Incoming Inspection: Verifying Materials

Incoming inspection is the last point at which a defect can be stopped before it enters the production flow, and the first point at which supplier quality becomes visible as a number. The temptation is to inspect everything, which is impossible, or to inspect nothing, which is expensive in a different way. The workable middle is material verification against a short list of characteristics that matter, checked against acceptance criteria agreed with the supplier before the material arrived, backed by a sampling plan that reflects the risk of the item.

What Incoming Inspection Is For

Incoming inspection has two purposes and they are often confused. The first is to prevent defective material from entering production. The second is to collect data about the supplier so that the buying decision can be informed. A check that catches a bad lot but records nothing has done half the job, and in the long run the recorded data is worth more because it prevents the next bad lot from being ordered.

It is also not a substitute for supplier control. A shop cannot inspect its way to good material, and an inspection programme that is catching a high proportion of defects is a signal that the supplier should be changed or supported rather than a signal that the inspection is working well. The measure to watch is the trend of rejects, not the number of checks performed.

Characteristics Worth Checking

For laminate the characteristics that matter are the grade and the supplier marking, the thickness, the copper weight, the dimensions, and the appearance including scratches and resin richness. For prepreg they are the grade, the resin content, the gel time and the roll condition. These are simple checks that a trained goods inwards operator can complete in minutes, and they catch most of the mistakes that actually occur, which are usually wrong grade or wrong thickness rather than a subtle material failure.

For components the priority shifts to identity, packaging and traceability. The part number, the date code, the manufacturer marking and the packaging condition are the checks that catch counterfeit and mislabelled material, and they matter more than an electrical measurement that cannot be performed on a reel in any case. A visual check against a reference sample catches most mislabelling without any measurement at all. Where a part is safety related or single sourced, the identity check should be mandatory rather than sampled.

Acceptance Criteria

Acceptance criteria should be written before the first shipment arrives and should be the same criteria used in the purchase specification. A check that is performed against an unwritten standard produces a disagreement every time it fails, because the supplier will have a different expectation. The criteria should state the characteristic, the method, the limit and what happens when the limit is not met.

The criteria should also distinguish between a defect that rejects a lot and a defect that is recorded and accepted with a concession. A scratch on a panel that will be trimmed away is not the same as a scratch across a circuit, and treating both as a rejection will produce a stream of trivial disputes that erodes the working relationship and hides the real problems.

Sampling Plans

A sampling plan sets how many units are inspected from each lot and what number of defectives causes a rejection. Sampling is used because inspection is a cost and because most material is good, and the plan should be written so that the risk of accepting a bad lot is understood. A plan that inspects three panels from a lot of five hundred is a plan that detects gross errors and nothing else, which is often exactly what is needed.

The plan should vary with the item and the history. A new supplier or a new grade should be inspected more heavily until a record is established, and the sampling can then be reduced. An item with a history of problems should not be sampled at all but inspected in full, because the cost of the inspection is smaller than the cost of the disruption that a bad lot would cause. The plan should be reviewed whenever the supplier or the grade changes, since a plan written for a stable source is not appropriate for a new one.

Records and Traceability

Every incoming inspection should produce a record that links the material to the shipment it arrived in, the batch it was released into and the boards that were built from it. That link is what allows a shop to react to a late material problem without scrapping an entire month of production, and it is the same record that a customer audit will ask to see.

The record belongs with the fabrication notes for the job, so that a trace from a finished board back to a material lot takes minutes rather than days. Where the trace cannot be made, the practical consequence is that any material problem becomes a full recall of everything built since the last known good shipment, which is a very expensive way to learn that the lot number was not written down.

Working With Suppliers

Incoming inspection data should be shared with the supplier rather than kept in the shop. A defect report with the lot number, the characteristic, the measurement and a photograph can usually be resolved in a single exchange, while a complaint without evidence usually produces a replacement and no change. Suppliers can only improve what they can see.

The data should also drive the purchasing decision. A supplier whose rejects are consistently low can be given a reduced inspection plan, which saves money and is a genuine incentive. A supplier with recurring problems should be placed on a controlled plan with full inspection until the record improves, and the escalation should be a written rule rather than a decision made in the middle of a shortage. Recording that rule, and the date it was applied, keeps the decision consistent between shifts and gives the supplier a clear signal of what has to improve.

Practical Rules

Write the acceptance criteria before the first delivery, check the characteristics that catch real mistakes, and vary the sampling plan with the risk and the history. Record every result against the lot, and keep the trace back from the finished board to the material batch. A trace that takes minutes is worth far more than one that takes days, because it is only used when time matters.

Share the data with the supplier, and use it to justify reduced inspection for good performers and full inspection for poor ones. Incoming inspection works best when it is small, evidence based and linked to the buying decision, because a quality system that inspects everything equally ends up trusting nothing.

FAQ

Should all incoming material be inspected? No. Inspect the characteristics that catch real errors and vary the effort with the risk. Full inspection is reserved for items with a poor history.

What matters most on laminate? Grade, thickness, copper weight and supplier marking. Most actual errors are wrong grade or wrong thickness rather than a subtle material fault.

How long should records be kept? Long enough to trace any board back to its material lot for the life of the product or the period the customer requires.