Process Capability for PCB Features: Using CpK
What CpK Measures
CpK compares the spread of a process with the tolerance it has to meet, and it also takes account of how far the process mean sits from the centre of that tolerance. A process with a small spread but a mean that is off centre can produce rejects even though its variation is small, and CpK is the number that captures both. It is calculated as the smaller of two ratios: the distance from the mean to the upper limit divided by three standard deviations, and the distance from the mean to the lower limit divided by three standard deviations. A value of one means the process is three standard deviations from the nearer limit, which corresponds to a small but non-zero reject rate; higher values mean more margin.
Why the Mean Matters
Two processes can have identical spreads and very different CpK, because one is centred and the other is not. On a board this appears as a feature that is consistently slightly narrow or a hole that is consistently slightly small, which is a systematic offset rather than a random variation. A systematic offset is often easier to fix than a spread, since it usually comes from a compensable process setting, but it is also easy to miss when only the standard deviation is monitored. This is why a capability study starts by looking at the mean against the target, and only then at the spread.
Typical Values for PCB Features
The features that are studied most often are the ones whose tolerance directly affects the assembly or the electrical performance: the finished hole diameter, the trace width, the dielectric thickness, the impedance of a controlled line, the solder mask dam width and the pad size. The values that a shop can hold vary considerably, and a shop that can hold a high capability on trace width may be poorer on hole diameter, because the two processes are different. Asking a shop for a capability figure on a specific feature is more useful than asking for a general statement about quality, because the answer depends on the feature.
Who Owns the Specification
The designer sets the tolerance, and the tolerance is what the capability has to meet. Where a design specifies a tolerance that no process can hold at reasonable cost, the shop has three options: refuse, quote a high price, or accept and ship some proportion out of tolerance. The third outcome is the one that causes trouble, and it is prevented by discussing the tolerance with the shop before the design is released. A tolerance that is looser than the application needs is a waste of process capability; a tolerance that is tighter than the process can hold is a specification that will be met on average and not at the limits.
Using CpK in a Purchase
Where a product is critical, the purchase specification can require a capability figure for named features, with the data supplied on the certificate or available on request. The requirement is meaningful only if it names the feature, the tolerance and the sample basis, since a capability figure without those is not comparable between suppliers. Where the requirement is a screening condition rather than a design requirement, it should be treated as such: a shop that meets the requirement on one feature may not meet it on another, and the requirement does not by itself guarantee a good board.
Limitations
A capability figure describes the process that was measured. It says nothing about whether the process is stable, and a process that drifts can produce a good capability figure over a short sample and a poor one over a long production run. It also says nothing about features that were not measured, and it can be manipulated by measuring at the most favourable point in the process or on the most favourable panel position. The useful way to read a capability figure is as one input alongside the control charts, the coupon data and the history of the relationship with the shop.
Practical Use
Ask for capability on the three or four features that matter most to the product, and ask what the shop’s own internal target is, since a shop that targets a higher value than the specification has margin to absorb drift. Where a feature has a low capability, decide whether the design can accept a looser tolerance before asking the shop to improve, because changing the design is often cheaper. Finally, keep the data: a capability figure measured on a previous order is a useful reference when a new batch arrives, and a change in the numbers is an early signal that something in the process has moved.
Capability Data and a New Supplier
When a design moves to a second supplier, the capability figures are the bridge between the two processes. A shop that has been building the board for years has its own internal capability on each feature, and a new shop will have a different one, even if both meet the same specification. Asking the new shop for capability data on the features that matter and comparing it with the existing shop’s data shows where the margin is thinner, and it identifies the features that should be checked on the first delivery. Where a design has a feature that sits near the limit of one shop’s capability and well inside another’s, the second shop is the one to use for that product, regardless of the price difference, because the cost of a low-yield build is rarely captured in the quotation.
FAQ
What does CpK measure? The relationship between the process spread and the tolerance, adjusted for how far the process mean sits from the centre of the tolerance.
What value should I ask for? It depends on the feature and the product. Higher values mean more margin, and the requirement should name the feature and the tolerance.
What is the difference between Cp and CpK? Cp describes only the spread, assuming the process is centred. CpK also accounts for the offset of the mean.
Does a good capability mean the process is stable? No. A drifting process can show a good figure over a short sample.
What should I do if a feature has a low capability? Check whether the design can accept a looser tolerance first, since that is usually cheaper than improving the process.
Conclusion
CpK is a way of expressing whether a process can hold a tolerance, accounting for both the spread and the offset of the mean, and it is useful only when it names the feature, the tolerance and the sample. Ask for it on the features that matter, compare it with the shop’s internal target, and use it alongside the coupon data rather than as a guarantee. Manufacturing capability is described under PCB capabilities, the processes behind each figure in PCB manufacturing, and the tolerances that the design requests in PCB design and layout. Capability is normally confirmed through quality management reporting in 2026.
