PCB Tolerances: Where They Come From and What They Cost

An engineering drawing is a contract, and the tolerances on it are the terms. A tolerance that is stated without a reason is either too tight, which costs money, or too loose, which costs yield, and both are avoidable.

What a Tolerance Communicates

A tolerance tells the shop how much variation the design can accept. It does not tell the shop how to achieve it, and it should not be used as a substitute for a specification: a dimension without a nominal value and a tolerance is incomplete in both directions.

The tolerance also defines what is measured. A flatness tolerance on a board is measured against a reference plane with the board supported at three points, and a thickness tolerance is measured at defined locations. Our warpage notes describe the measurement convention.

Tolerances That Come From the Process

Some tolerances are properties of the process rather than requirements of the design. The registration between layers, the etch tolerance on a conductor width and the hole position tolerance all fall into that category.

Asking for a tighter value than the process can deliver does not improve the board; it either raises the price or produces a non-conforming batch. The process limits are published as capability figures, and the design should be checked against them. Our data review notes describe the check.

Tolerances That Come From the Function

Other tolerances come from what the product has to do: the impedance of a line, the fit of a connector, the thickness that determines the spring rate of a contact, the spacing that determines a breakdown voltage.

Those should be derived rather than assumed. The derivation is usually short, and it produces a number that can be defended when the shop asks whether it can be relaxed.

The Cost of Tightening

A tighter tolerance costs in three ways: a slower process, more inspection and a lower yield. The first two are visible in the price and the third is hidden in it.

The cost is not linear. Moving from a routine tolerance to a tight one may double the cost, while moving from a tight one to a tighter one may double it again. That is why the derivation matters more than the habit.

Relaxing a Tolerance Safely

A tolerance can be relaxed when the function permits it, and the analysis is a worst case calculation rather than an average one. The question is what happens at the extreme, not what happens typically.

Where the analysis shows no effect at the extreme, the tolerance can be relaxed. Where it shows an effect, the tolerance is doing work and should be kept.

A dimension shown without a tolerance is governed by the general tolerance block, which is why the block has to be appropriate to the work. A general tolerance intended for machined parts is far too tight for a laminate.

Where a dimension is critical and the general block is loose, the tolerance should be stated on the dimension. The alternative is a dimension that is critical in the design and uncontrolled in the drawing.

Geometric Tolerances and Their Use

Geometric tolerances control form, orientation and position rather than size, and they are the right tool for the features that must align: a connector pattern, a mounting hole group, a set of contacts.

A position tolerance applied to a hole group is more useful than a set of plus-minus dimensions, because it describes the requirement in the way the assembly experiences it. The datum system it references should be stated and should correspond to how the board is located.

Documenting the Reasoning

The reasoning behind a tolerance belongs in the design record, not on the drawing. The drawing carries the requirement; the record carries why it is what it is.

Where the reasoning is recorded, a later request to relax a tolerance can be answered with an analysis rather than with a preference. That is what makes the discussion short. Our design release checklist lists the records to keep.

Process Control and Verification

Keeping a sample from the panel turns a dispute into a measurement, because the same coupon can be re-examined by both parties without rebuilding the batch. Reviewing the design before the data is released is cheaper than correcting it after the panel is in the tank, because every step downstream inherits the decision made at the front end.

Documenting the assumption is part of the design work, and a short note on the drawing prevents a question that would otherwise arrive a day later and cost a day of schedule. The process window is set by the narrowest step in the flow, so an improvement anywhere else shows up as margin rather than as yield until that step is addressed.

Checks Before Release

The checks that matter are the ones performed on the product rather than on a sample kept for the purpose, because a coupon that travels with the panel is the only evidence about that panel. Where a requirement can be measured, it should be measured at the point of manufacture and recorded against the board or the lot it applies to.

A parameter that is set once and never re verified drifts, and the drift is usually discovered by a defect rather than by the record. The tooling, the material and the profile form one system, and a change to any of them should be assessed against the other two before it is released.

Where the process window is narrow, the measurement resolution has to be better than the window, or the data cannot distinguish a good part from a marginal one.

Verification and Records

A record that identifies the operator, the date and the settings is worth more than a record that identifies only the result. The acceptance criteria should be written before the work starts, so that the decision is made by the specification rather than by the person inspecting.

Points to Confirm at First Article

Where an operation cannot be verified afterwards, it has to be controlled during the operation, and that control has to be visible in the record.

Documented Process Control

Sampling is a compromise between cost and confidence, and the sample size should follow from the failure rate that has to be detected.

Inspection and Traceability Notes

The first article confirms that the setup matches the intent, and it is the cheapest point at which a wrong setup can still be corrected. Handling between operations is part of the process, and the damage it causes is often attributed to the operation that preceded it.

FAQ

Which tolerance is most often specified too tightly? The impedance tolerance, because it is quoted from a system budget that assumed an ideal line. Half of the designs that specify a tight figure would work with a looser one after the etch and stack tolerances are combined.

Should every dimension be toleranced? Only the dimensions that matter, with the rest covered by a general block appropriate to the material. Tolerancing everything increases inspection time without adding control where it is needed.

What does gopcb do with a tolerance it cannot hold? We say so before the panel is released, with the figure we can achieve, and we ask whether the design can accept it. We do not accept an order we cannot meet and then deliver a deviation without agreement.