Incoming Inspection of Bare Boards
Why Inspect on Receipt
An incoming inspection is the last opportunity to reject a board before value is added to it. Once components are placed and reflowed, a fault that came from the bare board is indistinguishable from an assembly fault until a considerable amount of diagnosis has been done, and the cost of scrapping the unit is many times the cost of the board. The purpose of the inspection is not to test every board but to verify that the shipment matches what was ordered and that the process that produced it was in control, which is what a sample-based check can establish.
Documents and the Certificate
The first check is documentary: the part number and revision on the packaging against the purchase order, the quantity, the date code, and the certificate of compliance with the specification. Where the board has a controlled impedance requirement, the certificate should include the coupon measurements, and where the board has a surface finish specified by thickness, the certificate should report the plating thicknesses. The certificate is a claim by the supplier, not evidence, but a certificate that is missing a required measurement is a signal that the requirement was not understood, and it is much easier to raise at receipt than after a fault.
Visual Checks
A visual check on a sample looks for defects that would cause a fault or a reliability problem: scratches that cross conductors, solder mask that has lifted or that contains slivers, residue or contamination on the pads, exposed copper, silkscreen printed onto pads, and damage to the board edge. Under magnification the check also covers the mask registration, the annular rings on the outer layers, and the quality of the finish on the pads. Where the design has plated edges or castellations, those should be examined specifically, because they are damaged by handling more easily than flat surfaces.
Dimensional and Hole Checks
The board outline and the position of the tooling holes are what the assembly fixtures rely on, so they are worth measuring on a sample. The hole diameters matter for any connector, press-fit or mechanical part, and a pin gauge check on a few holes of each size is quick and definitive. Where the board has a card edge or a castellation, the critical dimensions are on the connector interface and should be measured rather than assumed. A board that is dimensionally wrong will fail at assembly in a way that looks like a fixture problem, which is a difficult diagnosis to make from the symptoms.
Coupons and Test Reports
Where the fabricator provides a coupon, it can be checked for the properties that the product cannot be inspected for: the plating thickness in the barrel, the dielectric thickness, the impedance of a controlled line and, on a demanding product, the results of a thermal cycling or a CAF test. The coupon is the only part of the shipment that can be destroyed without losing a product, which is what makes it valuable. Where no coupon is supplied, the electrical test data should be requested, and where the product is critical, a sample cross section is worth the cost.
Handling, Packaging and Moisture
Boards arrive with a surface finish that is sensitive to handling, and the packaging is part of the protection. Vacuum-sealed bags with desiccant are the norm for a finish with a limited shelf life, and the indicator card inside the bag should be checked on opening: if the indicator has changed, the packaging has failed and the shelf life has been consumed. Bare boards should never be handled by the pads, and gloves are used where the finish or the cleanliness requirement demands it. The packaging should also be checked for physical damage, since a box that has been crushed is a shipment that may contain cracked boards.
Storage and Shelf Life
The shelf life of a bare board begins at fabrication, not at receipt, and it depends on the finish. An organic finish has a limited life, a nickel-gold finish lasts far longer, and a tin finish sits between the two. The useful discipline is to record the date code and the finish on receipt, to store the boards in their sealed packaging in a controlled environment, and to bake them before assembly where the storage period or the humidity warrants it. Where a board has been stored beyond its finish’s life, the surface should be checked for solderability before it is used, since the alternative is to discover the problem after reflow.
Rejection and Communication
A rejection is only useful if it is communicated with evidence. The report should identify the defect, its location on the board and on the panel, the quantity affected, and the acceptance criterion it violates, ideally with a photograph and the coupon or test data that supports the finding. Where the defect is systematic rather than random, the shipment should be quarantined rather than sorted, because sorting a systematic defect leaves the cause in place. Where the defect is cosmetic and within the agreed specification, it should be recorded rather than returned, since an inspection that rejects boards the specification allows is an inspection that will be ignored.
Coupons and Test Coupons on the Panel
A coupon is useful only if it matches the product. Where the coupon has a different copper density, a different trace width or a different dielectric thickness from the board next to it, it monitors the process in general rather than the board in particular, and its results have to be read with that limitation in mind. Where a design has several impedance classes or several hole types, the coupon should carry a representative of each, and the report should identify which line corresponds to which class. A coupon that is cut away and discarded before the boards are shipped removes the evidence, so the fabricator should retain the data and include it with the certificate. Where a critical product is involved, keeping one coupon from each panel in a sample bag is a cheap way to preserve a record for later failure analysis.
FAQ
How many boards should I inspect? A sample based on the shipment size, with a tighter sample for a new supplier or a new design, and a full check only where the risk justifies it.
What is the most useful check? The coupon, where one is supplied, because it reveals the internal properties that cannot be inspected on the product.
Should I check every hole? No. A pin gauge check on a few holes of each size confirms the process, and a full check belongs with the fabricator.
Does the shelf life start at receipt? No. It starts at fabrication, so the date code and the finish decide how much life remains.
When should boards be baked? Where the storage period or the humidity has been significant, or where the packaging has been opened, before assembly.
Conclusion
Incoming inspection is a sample-based verification that the shipment matches the order and that the process was in control, supported by the certificate, the coupon and a small number of dimensional checks. Record the date code and the finish, protect the boards in storage, bake them where the storage justifies it, and communicate any rejection with evidence. Fabrication capability and specifications are described under PCB capabilities, the processes behind the acceptance criteria in PCB manufacturing, and the verification discipline belongs to quality management. Boards are then released to PCB assembly in 2026.
