Assembly Process Capability and Control Charts

A capability index compares the spread of a process with the tolerance it has to meet. A control chart shows whether the process has moved. The two answer different questions and both are needed.

What Capability Answers

The index compares the process spread with the specification, so it answers whether the process can meet the requirement at all. A value below one means defects are expected even when nothing has changed.

The specification comes from the design. Our component tolerance notes describe where the tightest requirement usually sits.

What a Control Chart Answers

The chart shows whether the process is stable, by plotting the measurement against limits derived from the process itself. It detects a shift long before the specification is reached.

The limits are not the specification. Our yield analysis notes describe how the data is collected.

Why a Capable Process Still Drifts

A process that is capable of meeting the tolerance can still move out of it over time, through wear, material change or ambient conditions. The chart is what detects the movement.

The drift is often gradual and it is found after a defect. Our maintenance notes describe one common cause.

Choosing What to Measure

The measurement should act on the outcome rather than on the input. A print volume is more useful than a squeegee pressure because it is the volume that decides the joint.

The measurement also has to be capable. Our measurement notes describe the study that confirms it.

Setting the Limits

The control limits are calculated from the process data, which means they describe the process rather than the requirement. Adding a limit because it looks reasonable defeats the purpose.

The calculation should be documented. Our quality notes describe how the records are held.

Acting on a Signal

A signal on the chart calls for a cause investigation rather than an adjustment. Adjusting a process that has not moved adds variation rather than removing it.

The response should be written into the procedure. Our audit notes describe the follow-up.

Verification

The verification is a capability figure against the tolerance, a control chart with limits calculated from the process, a measurement chosen at the outcome, and a written response to a signal.

Our quality notes describe how the records are kept.

Process Control and Verification

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. A short note on the drawing about handling, storage or packaging is often worth more than an extra decimal place on a tolerance.

Where a value sits close to a process limit, the drawing should say so, since the shop can then open the process window rather than working to a nominal figure that carries no tolerance. A stack-up that is drawn rather than described removes most of the ambiguity from a quotation, and it lets the fabricator price the board against the dielectric and copper weights that will actually be used.

Where the requirement is not written down, the shop supplies its own default, and the default is chosen for the process rather than for the design. The measurements that matter are the repeatable ones: conductor width and spacing, annular ring, finished hole size, plating thickness and surface finish are all verifiable on a coupon that travels with the panel.

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. 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.

FAQ

Is a capability index of one good enough? It means defects are expected at the tails. Most processes aim higher, and the target depends on the consequence of a failure.

Can a chart replace inspection? It reduces the inspection needed when the process is stable, and it depends on the measurement being capable.

What does gopcb provide for process control? We provide a capability figure against the tolerance, control limits calculated from the process itself, measurements taken at the outcome rather than at the input, and a written response to a signal that calls for a cause rather than an adjustment.