Process Capability and Cpk on an SMT Line

What Capability Measures

Process capability compares the spread of a process’s output with the limits that the specification allows. A process whose output fits comfortably inside the limits is capable; one whose output spreads to the limits or beyond is not, however good its average happens to be. The measure is expressed as an index, and its value tells you how many of the outputs will fall outside the specification, assuming the process is stable. The assumption of stability is important, because a capable but drifting process will produce defects over time even though its instantaneous index looks acceptable.

The Index and Its Interpretation

The capability index compares the distance from the average to the nearest limit with three standard deviations, and the commonly used versions also account for how well the process is centred. An index of one means that the spread just reaches the limits, which corresponds to a defect rate far higher than most products tolerate. An index of about one and a third is a common minimum requirement, and higher values are used where the consequence of a defect is severe or where the process is expected to drift. The index is a statement about the process, not about a particular unit, and a single measurement cannot produce one.

What to Measure on an SMT Line

The parameters worth a capability study are those whose variation produces a defect. A paste deposit’s volume, the placement’s position and rotation, the reflow’s peak and its time above liquidus, the solder joint’s fillet height and the plating’s thickness are the usual candidates, and their limits are defined by the process and the product. The measurement has to be repeatable and its own variation has to be known, because a measurement system that contributes a large part of the spread makes the process look worse than it is. The capability study therefore starts with the measurement system.

Sampling and the Assumption of Stability

A capability index is only valid if the process is in statistical control, which means its variation is caused by common causes rather than by a change or an intervention. The check is a control chart, which shows whether the data has a pattern, a shift or a trend. A process that is not in control should be stabilised before its capability is calculated, because the index of an unstable process describes a mixture rather than a process. The sampling should be frequent enough to detect a drift and spread across the shifts and the lots, since a process that is capable on one shift and not on another is not capable.

Acting on the Result

An inadequate index has two possible remedies, which are to centre the process on the specification and to reduce its spread. Centring is usually cheaper and is achieved by adjusting a setting; reducing the spread requires a change to the process, the material or the equipment, and it is the more durable answer. Where the index is adequate but the process drifts, the answer is tighter control rather than a process change. The response should follow the chart’s evidence, and the improvement should be confirmed by a further study rather than assumed from the change.

Capability and the Product’s Requirement

The requirement’s level should follow from the product. A consumer accessory can tolerate a lower index than a medical or an automotive product, and an automotive product may require a specific value for a designated special characteristic with a documented control. The requirement should be written into the specification so that the supplier and the customer agree on it, and the measurement method and the sampling should be part of the agreement. An index quoted without a stated method and sample size cannot be compared with another, which is why the two should be recorded together.

Beyond the Index

An index is a summary and it hides the shape of the distribution. A process that is bimodal, that has a tail, or that has a few outliers, can have an acceptable index and still produce defects, because the index assumes a normal distribution. The chart and the histogram should be examined alongside the number, and the outliers should be investigated rather than averaged away. Where a process produces a small number of extreme values, the cause is usually an assignable one, and removing it improves the process more than any adjustment of the average.

Capability and the Supplier

A purchased process has a capability too, and it should be demonstrated rather than assumed. A board’s plating thickness, a connector’s insertion force and a component’s dimensional conformance all come with a distribution, and the purchasing decision should consider whether the supplier’s spread fits the specification. Where the supplier’s index is marginal, the incoming inspection has to compensate, which is expensive, or the design has to be more tolerant. Asking for the data rather than for a certificate is the practical approach, and a supplier who cannot provide a distribution has not demonstrated a capability.

FAQ

What does a capability index measure? The spread of a process relative to the specification’s limits, and therefore the expected defect rate.

Why must the process be stable first? Because the index of an unstable process describes a mixture of conditions rather than a process.

What should be studied on an SMT line? The parameters whose variation produces a defect: paste volume, placement, profile and plating.

How do I improve an inadequate index? Centre the process first, then reduce the spread, and confirm the improvement with a further study.

Can a good index hide a problem? Yes, if the distribution is not normal, so the chart and the histogram should be examined with the number.

Conclusion

Capability compares the process’s spread with the specification, so confirm the measurement, confirm the stability and then act on the cause. Read the chart, not only the index. Process control belongs to quality management, the parameters it measures come from SMT PCB assembly, and the assembly context is described in PCB assembly. Capability requirements for a new product are agreed during prototype PCB assembly in 2026.