PCB Visual Inspection Under IPC Class 2 Criteria

A board arrives with a visible mark near a via, and the line stops. Someone must decide whether that mark is cosmetic or a real defect. Under IPC Class 2, the answer depends on measurement, location, and the agreed acceptance criteria. This guide walks through the checks an inspector should make and the questions a buyer should ask before the first lot is built. The goal is a repeatable decision, not a personal opinion.

IPC Class 2 covers general electronic products where continued function matters more than cosmetic perfection. Most industrial, consumer, and computing boards fall into this class unless the customer specifies otherwise. That baseline gives inspectors a shared language for judging appearance and workmanship. It does not remove the need for customer drawings, because those documents often tighten limits beyond the default. Always confirm which class and which customer notes apply before inspection starts.

Dimensional Checks First

Start with the outline, because a board that does not fit cannot be saved by good cosmetics. Check length, width, thickness, routed features, and V-CUT dimensions against the fabrication drawing and its stated tolerances. A common shop control for length and width is plus or minus 0.15 mm, but the drawing governs when it is tighter. Record the measured values rather than a pass or fail tick, so the data can be reviewed later. First-article approval should include these numbers.

V-CUT scoring deserves its own check because it affects depanelization. The remaining web thickness should be consistent across the panel, so the board separates cleanly without cracking pads or laminate. Requirements vary with board thickness, material, and the depanelization method used downstream. Ask the fabricator how the web is controlled and how it is measured. A loose web shows up later as edge damage during assembly, and that damage is often blamed on the wrong process.

Bow and Twist Limits

Flatness affects paste printing, component placement, and soldering. Measure bow and twist across the full board and express the result as a percentage of the relevant length or diagonal. For boards carrying surface-mount parts, the commonly referenced maximum is 0.75 percent. For other board types, the usual reference is 1.5 percent. The applicable IPC specification and customer requirements decide the final acceptance, so treat these figures as defaults rather than absolutes.

Measure after the board has settled, not straight from a hot press. Thermal history changes flatness, and a board that looks warped on arrival may relax once it reaches room conditions. Use a flat surface and a consistent measuring method, and write the method into the inspection plan. When a board fails, note the direction of the curve, because that tells the process engineer which side of the stackup is unbalanced. That note is more useful than the number alone.

Solder Mask and Laminate

Solder mask coverage should be checked around pads, vias, holes, and exposed copper. Where the specification requires it, cured mask above a copper pad should stay within the applicable IPC limit. A commonly referenced value is roughly 50 micrometers above the copper surface. Inspectors should look for mask on pads, mask skips, and thin coverage over traces. Each condition has a different risk, so describe what you see before assigning a verdict.

Haloing appears as whitening or local separation around holes and mechanical features. It comes from stress in the laminate during drilling or routing. Minor haloing does not automatically mean rejection. The decision rests on size, location, severity, and whether electrical or mechanical integrity is affected. Check whether the halo reaches a conductive feature or grows toward the board edge. If it stays cosmetic and stable, it can often be accepted with a note.

Exposed glass fabric means resin did not fully cover the reinforcement. A small patch is not automatically a defect. Evaluate location, extent, and effect on performance, and give special attention to areas near conductors, holes, and board edges. Fabric near a pad or between traces can trap moisture and affect insulation over time. Judge against the applicable IPC requirement, not against a photograph from another job. Document the finding with a measured size and a location sketch.

Voids and Separation

Surface voids are cavities within the surface or laminate. Not every void is functional damage. Consider size, quantity, depth, and location, and ask whether the void sits in a path that carries current or supports a joint. Cosmetic voids in a noncritical area are treated differently from voids under a land. A proper inspection separates appearance from risk, and that separation protects both the yield and the field reliability of the product.

Delamination is separation between layers, while blistering shows as a raised area on the surface. Both are treated more seriously than cosmetic marks because they can affect structure and long-term reliability. Look for them after thermal exposure, since assembly reflow may reveal a weakness that shipping hid. Boards with significant separation should be evaluated against the applicable IPC criteria and customer requirements before acceptance. Keep the samples, because they help trace the cause back to material or process.

Plating, Marking, and Traces

Plating voids are areas where copper is incomplete, especially inside plated holes. Plated holes carry connections between layers, so integrity matters. Inspect for voids, thin plating, and barrel cracks, and use cross sections when the lot is critical. A defect that breaks connectivity is not cosmetic and should not be waived. Where the project moves into PCB assembly, these same holes must survive reflow without opening. Plating quality decided at fabrication sets the ceiling for assembly yield.

Silkscreen should be legible, correctly placed, and consistent with the approved data. Check that marking does not sit on pads, contacts, or areas that will be soldered. Poor marking causes placement errors and slows repair. Many marking problems start in the design files, so a review before fabrication prevents them cheaply. Our team can support that review through PCB design and layout before the data reaches the line.

Conductor width and spacing are measured against the approved design data and manufacturing tolerances. Width affects current capacity, impedance, and yield, so evaluate it together with copper thickness and stackup. Spacing affects the risk of shorts, arcing, and solder bridging. Check the minimum spacing against design rules, voltage needs, and process capability. A supplier that measures both during fabrication and final inspection gives the buyer evidence rather than assurance.

Deciding Accept or Reject

A visible imperfection does not make a board unusable. The sound sequence is observation, measurement, review against IPC and customer specification, risk assessment, then a decision. This order prevents scrap driven by a first impression. It also gives the customer a written reason for every rejection. When the same defect appears across a lot, the record shows whether the cause is design, material, or process. That feedback loop is where quality actually improves.

Rejection or rework is justified when a defect affects function, reliability, or an agreed limit. Cosmetic variation outside a cosmetic spec still counts, but it should be judged against the drawing rather than a preference. Train inspectors on real samples so their judgment stays consistent between shifts. Keep the acceptance criteria with the lot record, and revisit them when the design or supplier changes. Consistency across lots is what buyers actually pay for.

From Fabrication to Build

Inspection does not end at the fabricator. Flatness, pad condition, mask quality, and dimensional accuracy all influence paste printing and placement downstream. For projects that move straight into turnkey PCB assembly, incoming checks catch problems before they reach the printer. Early detection protects the whole build, because a warped panel or a damaged pad costs far more once components are attached. Ask how incoming boards are verified and what happens when a lot fails.

Storage and handling also affect quality before assembly. Control humidity, keep boards in sealed packaging, and respect shelf life for surface finishes. Handle boards by the edges and avoid stacking that presses on components or pads. Requirements vary with material, finish, and packaging, so follow the material maker’s guidance and the customer’s notes. A board that passed inspection can still fail at assembly if storage was careless. Include storage in the audit, not just the line.

For buyers comparing suppliers, the useful questions are about records. Ask for dimensional data, flatness results, plating evidence, and the acceptance class used. Ask who approves a deviation and how it is documented. Suppliers serving high-volume PCB assembly should show that these checks run on every lot, not only on the first article. Records that are easy to produce on request are usually kept honestly during production. That habit separates a controlled process from a hopeful one.

The practical takeaway is simple. Agree the class and the customer limits before the first panel is built, then measure every disputed feature instead of arguing about it. Keep the numbers with the lot, and use them to correct the process when a trend appears. A board accepted on evidence is easier to defend than one accepted on opinion, and the same evidence supports component procurement decisions when a supplier change is proposed.