Incoming Inspection of Bare Boards: What to Measure and Record

A batch of bare boards arrives, the box is opened, one board is held up to the light and the lot is accepted. That single glance is the whole of the receiving inspection process in many programs, and it is the reason a fabrication defect is usually discovered after assembly, when the cost of the discovery has multiplied several times over.

Why Incoming Inspection Is Not an Audit of the Supplier

Incoming inspection is often described as checking up on the fabricator, which makes it feel adversarial and therefore optional. The useful framing is different. The inspection verifies that the shipment matches the drawing and the purchase order, and every error it catches is one that would otherwise be caught by an assembly line or by a customer.

It also protects the fabricator, because a defect that is documented on arrival is clearly attributable to the lot. A defect discovered three weeks later, after cutting, assembly and reflow, has a disputed origin and a shared cost.

The process therefore belongs to the receiving organization and should be sized to the value of the boards, not to the size of the supplier relationship.

Documentation Before the Board

The first check is not physical. The lot should arrive with a certificate of conformance, the fabrication drawing revision it was built to, a stack-up record if one was specified, and coupon or test data where the purchase order required it.

Comparing the revision on the certificate against the revision in the drawing system takes a minute and catches the most expensive class of error, which is a lot built to a superseded drawing. Our fabrication notes checklist lists the notes that should be visible on the document being compared.

Where the requirement included impedance control, the coupon measurement should be recorded against the lot. A certificate that states compliance without data is a statement of intent rather than a measurement.

What to Look At First

Begin with the board outline. Measure the overall dimensions and check that the outline matches the drawing within its tolerance, because everything downstream is positioned relative to the edge. Our board outline rules describe how the tolerance is normally expressed.

Then check the tooling features: the position of the registration holes, their diameter and their relationship to the pattern. A board whose outline is correct but whose pattern is shifted will still fit the assembly fixture and will still place components in the wrong place, so the two checks are separate.

Finally check the thickness at several points across the panel and at several panels in the lot. Thickness variation across a panel indicates a pressing problem, and variation between panels indicates a stack-up that is not being held.

Copper Features and Annular Ring

The annular ring is the copper that surrounds a drilled hole after the drill has removed material. It is the feature most often reduced without any change to the artwork, because drill wear, registration error and etch variation all remove copper from the same place.

Measuring the annular ring on the smallest vias, and specifically on the vias in the corners of the panel where registration error is largest, gives a useful indication of the lot. A ring that is present but thin in the corner is a warning that the rest of the panel has less margin than the drawing assumed.

Alongside the ring, check conductor width and spacing against the drawing on a sample of traces, and look for the defects that survive etching: nicks, pinholes, residual copper shorts and excessive undercut. Our judging PCB quality notes describe how each appears under magnification.

Surface Finish and Solderability

Surface finish determines whether the board can be assembled, and it degrades with time and with handling. A hot air solder leveled finish should be examined for uniformity of coverage and for any area where the solder has not wetted the copper, since those areas will not wet during reflow either.

Immersion finishes such as silver, tin and gold are thinner and their appearance says less about their condition, so a solderability test on a sample is worth more than a visual check. Where the boards will be stored before assembly, the shelf life stated by the fabricator should be recorded and the packaging should be intact.

Soldermask and legend should be checked for alignment to the copper, for coverage over the copper that should be covered, and for any encroachment onto the pads that will be soldered. A mask shift of a fraction of a millimetre is invisible on a coarse board and marginal on a fine one.

Coupons and Test Data

Where a coupon was built on the panel, it carries the same copper, the same dielectric and the same plating as the production boards, which makes it the most direct evidence available about the lot. Our test coupon notes describe the structures normally included.

The coupon should be kept. Severing it and discarding it removes the only sample that can later be measured without destroying a production board, and it makes a subsequent failure investigation depend on new material rather than on the lot in question.

Where impedance control was specified, compare the coupon measurement against the design target and record the difference. A lot that is within tolerance but consistently at one edge of it is information about the process, and it deserves to be passed back rather than filed.

Sampling, AQL and the Statistics of Small Lots

Acceptable quality level sampling plans were designed for large lots of interchangeable parts, and their assumptions do not transfer well to a prototype order of twenty boards. The plans still provide a useful vocabulary and a defensible sample size, but the practical decision is often to inspect every prototype board and to sample only in production.

For a production lot, a common approach is to inspect the critical features on a sample and to treat the coupon as an additional sample. The critical features are those whose failure would be expensive: inner layer registration, impedance, plating in the holes and finish condition.

The statistics of small lots mean that a lot of thirty boards cannot be characterized by inspecting three of them with any confidence. Either inspect all of them or accept that the lot is unverified, and record which of the two decisions was taken.

Holes in the Finished Board

Hole size is measured on the finished board, after plating, and it is the finished size that the component lead must fit. A hole that is at the top of the tolerance and a lead that is at the top of its tolerance produce an interference fit that slows assembly or cracks a plated barrel.

Plating thickness in the holes is usually verified through the coupon rather than through the production board, since measuring it in a production hole requires sectioning. Where the coupon data is missing, the alternative is a microsection of a sacrificial board.

Also check for hole wall defects that can be seen optically: voids in the plating, resin smear and the barrel cracks that appear after thermal cycling. These are the defects that produce intermittent opens months later, and they are the reason plating quality is worth the cost of a section.

Recording the Result

An inspection that is not recorded cannot be used. The record should identify the lot, the drawing revision, the quantity, the sample size, the measurements taken and the disposition of the lot, and it should be short enough that it is actually completed for every delivery.

Photographs of any defect, with a scale in the frame, turn a description into evidence. A photograph of a marginal annular ring beside a coupon measurement is a complete case, while a note that the ring looked thin is an opinion.

The record also feeds the supplier discussion. A trend of registrations drifting toward one edge of the panel over several lots is visible only in the records, and it is the kind of finding that prevents a failure rather than explaining one.

When Something Is Wrong

Separate the lot before deciding what to do with it, and keep the packaging, the coupon and the certificate together so that the material is in the same condition it arrived in. Moving a suspect lot into general stock is the single most common way that a containment fails.

Raise the issue with the measurements attached rather than with a general complaint, and state which characteristic failed and by how much. A fabricator can act on a measured deviation, and cannot act on a description of disappointment.

Where the deviation does not affect function, document the acceptance and the reason, so that the decision can be reviewed later. A deviation accepted with a recorded rationale is engineering; the same deviation accepted silently is a defect waiting to be found.

FAQ

How long should incoming inspection take? For a prototype lot, fifteen minutes of documented measurement per board is realistic. For production, a sampled check plus the coupon data is enough to release a lot the same day.

Is it worth inspecting if the fabricator is qualified? Yes. Qualification describes the process, not the lot, and registration, plating and finish all vary within a qualified process.

What does gopcb provide with a shipment? We provide the certificate of conformance, the coupon with its measurements, the stack-up record, and lot traceability tying the boards to the panels and the process date. Where a customer requires additional data, we agree the measurements at order rather than after delivery.