PCB Acceptance Classes 1, 2 And 3 Explained

Fabrication and assembly standards divide products into three classes, and the class is a statement about the consequences of failure rather than a general comment on quality. A class one product is one where a failure is not a hazard and the only cost is the loss of function. A class two product is one where a failure is inconvenient or expensive but not dangerous, and a class three product is one where a failure can injure someone or where the equipment must keep working in a critical application.

This article explains where the acceptance classes differ, how the difference shows up in the acceptance criteria for fabrication and assembly, and why stating the class on the drawing changes the cost of the part.

What The Classes Mean

Class one covers general electronic products, which includes consumer goods where the main requirement is that the product works. Class two covers dedicated service products, where a high level of performance and an extended life are expected but an interruption is not a safety issue, which describes most industrial, communications, and computing equipment. Class three covers products where continued performance is critical, and where the equipment must not fail during use, which covers medical life support, aerospace, military, and some transport systems.

The class applies to the product, but it is applied to individual features during inspection. A single board can carry a fine pitch area that is inspected to class three criteria because of the consequences of a failure there, and a coarse connector area that is judged to a lower class, provided the choice is documented. What is not acceptable is to leave the class unstated and let the fabricator apply a default, because the default is usually the lower of the two and the difference appears only when an inspector has to make a call on a marginal feature.

<img src="https://www.gopcba.com/wp-content/uploads/2024/10/pin11-min.jpg" alt="Inspector checking a plated hole and annular ring under magnification” />

Where The Classes Differ In Fabrication

The differences are concentrated in the hole and the edge. The copper thickness in the wall of a plated through hole is the clearest example: a class two board commonly requires an average of twenty micrometres and a class three board twenty five, which means a longer plating cycle and tighter bath control. The annular ring, which is the copper that surrounds the drilled hole, is also specified more tightly for class three, and the breakout of a hole from its pad is acceptable in a lower class in specific circumstances and not acceptable at all in the highest one.

Other features follow the same pattern. The conductor clearance to the board edge, the definition of an internal short or a nick in a trace, the acceptance of a scratch on a solder mask, and the allowance for a dent in a plane are all graded, and the class three criteria are written so that no single feature is left to interpretation. The class also changes the inspection plan, because the highest class calls for a coupon on every panel and for a level of documentation that a lower class does not require.

Where The Classes Differ In Assembly

In assembly the classes change the solder joint criteria. A fillet that wets to the top of a pad may be acceptable in one class and not in another, the amount of vertical fill required in a plated hole is graded, and the permitted voiding inside a ball grid array joint is different between the classes, with the highest class setting the tightest limit. Component damage, marking legibility, and the acceptability of a reworked joint are also treated differently.

The practical effect is on inspection cost. A lower class permits sampling and a visual inspection at a defined magnification, while the highest class expects a higher inspection rate, a trained operator certification, and records that can be produced years later. The same board built to the same drawings can therefore cost noticeably more simply because of the acceptance criteria it will be judged against, without any change to the artwork.

Choosing The Right Class

The class should be chosen from the consequence of failure and from the environment the product will see, not from a general wish for quality. A consumer device that is expected to last three years does not benefit from class three documentation, and a medical monitor does not become safe because the class was written on the drawing without the traceability that the class implies. The useful exercise is to identify the features whose failure would matter and to apply the higher class to those.

There is also a commercial dimension. A higher class raises the price through tighter process control, more inspection, and more records, and the increase is usually small compared with the cost of a field failure in a product that needs the higher class. Pricing the class into the project at the quotation stage avoids the situation where the class is raised during production and the parts have to be reworked or scrapped.

Documentation And Common Mistakes

The class belongs on the fabrication drawing and on the assembly documentation, with the standards and their revision called out, and it should appear on the purchase order so that the supplier does not have to infer it. Where the product is intended for a regulated market, the class is normally accompanied by the traceability requirement, the coupon requirement, and the certification that the supplier must provide.

The most common mistake is a mismatch between documents. A board specified as class three on the fabrication drawing and assembled under a class two purchase order will be accepted against the wrong criteria, and the difference is discovered only when a customer audit asks for the records. The second mistake is assuming that the class covers everything: features that are not graded by the class, such as the pad design and the layout rules, are controlled by separate design standards, which are described under pad design standards and via to trace clearance in multilayer boards.

Acceptance In Practice

Inspection against a class is a comparison between a measured feature and a written limit, and it only works when the limit exists for that feature. An inspector who is asked whether a small nick in a trace is acceptable needs the class, the location, and the limit, and the answer is different for a signal trace and for a power plane. Writing those limits into the drawing is what makes the acceptance decision reproducible between shifts and between suppliers.

Where a deviation is found, the class also determines the disposition. A feature that exceeds the limit in a class two product may be repairable or acceptable under a documented concession, while the same feature in a class three product may require the panel to be scrapped even though it would function. The wider quality framework that surrounds those decisions is described under PCB design quality characteristics.

FAQ

Is class three automatically better quality than class two? It is a stricter set of acceptance criteria, which costs more to meet. Whether it is better for a given product depends on the consequence of a failure, and applying it where it is not needed adds cost without adding value.

Can a board mix classes? It can, and it is common to apply the higher class to specific features and areas, provided the drawing identifies them. What is not workable is leaving the class unstated.

Does the class change the design rules? It changes the acceptance limits for features such as annular ring and hole wall copper, which in turn forces changes in the design. The layout rules for pads, spacing, and vias are separate documents.