Cosmetic Acceptance Criteria: Scratches, Dents and Laminate Defects

Cosmetic acceptance is decided by a person looking at a board, and it becomes repeatable only when the criteria are written as measurements rather than as adjectives. A scratch is accepted or rejected according to where it is, how deep it is and what it crosses, and all three have to be in the specification.

Why Cosmetic Criteria Have to Be Written

Every board has marks from handling, from fixture contact and from processing, and without a written limit the decision depends on the mood and the experience of the inspector. The same board is then accepted at one station and rejected at another, which is expensive and unfair.

A written criterion also protects the product, because an apparently minor mark can cross an impedance controlled trace or open a fine pitch link. Writing the acceptance criteria forces that assessment to be made by the people who understand the design rather than at the moment of inspection. The criteria are needed for incoming material too, because a supplier that ships a board carrying a mark the shop has never defined will argue, reasonably, that no requirement was stated.

Reference Standards and Their Limits

IPC-A-600 provides default limits for the most common conditions, and it defines three classes that correspond to increasing reliability requirements. The standard is a starting point rather than an answer, because it cannot know which trace on the board carries the critical signal.

The useful approach is to adopt the standard as the baseline and then write product-specific exceptions for the areas that matter. Those exceptions are what make the standard usable, and they are also what stops the customer and the shop from arguing about a photograph.

Scratch and Dent Criteria

A scratch is described by its length, its width, its depth and its location, and the location is usually the deciding factor. A mark on a non-conductive area of mask is cosmetic, while the same mark over a conductor raises the question of how much copper was removed.

The depth is judged by whether the mark exposes copper, by whether it crosses a conductor and by whether it reduces the conductor’s remaining width. A mark that crosses a trace and leaves less than the minimum width is a functional defect whatever it looks like, and it is measured rather than estimated. The measurement is taken at the narrowest point of the mark rather than at its centre, and it is compared against the minimum width for that layer.

Laminate Conditions

Measling is the white pattern where the resin and glass separate at a weave intersection, and haloing is the pale ring around a drilled hole where the resin has separated from the fibre. Both are structural, and both are judged by their size and by whether they reach a conductor or a plated hole.

Blistering and delamination are the more serious members of the same family, because the layers are separated rather than the fibres. Our fabrication notes describe the process conditions that produce them, which matters because the criterion and the cause are usually discussed in the same conversation; the laminate supplier’s own data gives the limits for the material. A halo that does not reach a conductor is usually acceptable, while one that touches a plated barrel is treated as a path for moisture and judged by the same rule as an exposed fibre.

Solder Mask Conditions

The mask conditions with a functional consequence are mask on a pad, mask that has lifted at an edge and mask that is thin enough to show the copper beneath. Cosmetic conditions include colour variation, scratches and the small blisters that form over a via.

Mask on a pad is a functional defect because it prevents the joint from forming, and it is usually caught earlier, at the printing or the assembly stage. Thin mask over a conductor is slower to appear, because the copper shows through as a change in colour long before anything fails, and it is judged by a measurement of the remaining thickness. A castellated edge is a good example of the two conditions meeting, because the plating left after the cut is what forms the connection, so the criterion for the cut is written with the assembly process in mind.

Edge, Burr and Finish Conditions

The edge conditions are burrs from routing or scoring, nicks in a copper edge and plating that has cracked where a castellated edge was cut. A burr is measured as a height above the surface and as a length, and a nick is measured as a reduction in the remaining width.

Finish conditions include pitting in the plating, discolouration and solder mask on a gold finger. A gold finger with mask on it will not mate, so the criterion is an absence rather than a limit, and it is checked before the connectors are assembled.

How the Measurement Is Made

The measurement depends on magnification and lighting, so both are stated with the criterion. A mark is examined at a defined magnification under a defined light, and its dimensions are read against a scale rather than estimated from memory.

Where a criterion depends on depth, the board is sectioned rather than judged by eye, because a scratch that looks shallow can pass through the copper. Sectioning is destructive and slow, so it is used for the borderline cases that matter rather than as a routine check; our board quality notes describe how a borderline board is escalated rather than decided at the bench.

Writing Product-Specific Criteria

The product criteria identify the areas where the default limits do not apply: an impedance controlled region, a fine pitch footprint, a high voltage gap and the mating surface of a connector. Each of those is written with its own limit and with the reason it differs.

The customer’s own drawing often carries requirements that are stricter than the standard, and those are transcribed into the work instructions rather than left in the drawing package. An inspector works from the instruction, and the instruction is what is audited.

Disposition, Records and Trends

A board that falls outside the criteria is dispositioned rather than discarded by default, and the disposition options are use as is, repair, rework or scrap. Each option has its own record and its own justification, and the justification is the point of the exercise.

The inspection results are also worth trending, because a rise in accepted marks usually means a process has changed rather than that the inspectors have become tolerant. Our board failure notes describe how those trends are followed into the process rather than recorded and forgotten. A trend that is acted on is worth more than one that is reported, and the entry that matters is the date the process changed rather than the date the chart moved.

FAQ

Are cosmetic defects ever functional? Often they are, and that is the difficulty. A mark that crosses a conductor and reduces its width is a functional defect that happens to look cosmetic, which is why the criteria are written in terms of what the mark crosses rather than only its size.

Can a scratch be repaired? A mark on the mask can be touched up in some cases, and a mark that has removed copper cannot be restored to its original thickness. Repair is dispositioned against the same criteria rather than assumed to be acceptable.

Who decides a borderline board? Someone other than the inspector, working from the written criteria and, where the design is at risk, from an engineering assessment. The escalation is part of the process rather than a sign that the inspector has failed.