Placement Accuracy Verification and Process Capability

A placement machine reports an accuracy figure, and the figure describes the machine rather than the process. What matters on a product is where the parts actually land, and whether the distribution fits inside the pad.

Accuracy and Repeatability

Accuracy is how close the mean placement is to the target. Repeatability is how tightly the placements cluster around the mean. A machine can be repeatable and offset, or accurate on average and scattered.

The offset can be corrected in the program; the scatter cannot, and it is the scatter that decides whether a fine pitch part is placed acceptably. Our nozzle notes describe one of the contributors to scatter.

Measuring on the Product

The measurement should be taken from the placed parts on real boards, using the vision system or a coordinate measuring machine, rather than from a machine calibration target.

The product measurement includes the board, the tooling, the paste and the component, so it describes the process as the product sees it. Our land pattern notes describe the pad that the distribution has to fit.

Process Capability

The capability index compares the spread of the placements with the tolerance allowed by the pad. A process that uses most of the tolerance has no margin for the drift that happens during a run.

The calculation should be performed on the smallest pad on the board, because that is where the tolerance is tightest. A high capability on a chip resistor says nothing about a fine pitch connector. Our paste inspection notes describe a related measurement on the deposit.

Sources of Variation

The sources are the machine, the nozzle, the feeder, the component in the package, the board in the fixture and the vision system. Each contributes, and the largest one should be identified rather than assumed.

The feeder contributes more than most people expect, because the part position in the pocket varies. Our yield notes describe how the contribution is detected from the defect data.

Correcting an Offset

An offset should be corrected in the program, and the correction should be verified with a fresh measurement rather than assumed to have worked.

Where the offset differs between machines or between heads, the correction is per machine and per head, and it should be transferred with the product only after verification. Our fabrication notes notes list the setup records that should be kept.

Vision and Verification

The vision system measures the part before placement, which removes the feeder and package variation from the result. Where the vision is disabled for speed, the variation returns and it does so invisibly.

The choice between vision and no vision should be made per component class rather than for the whole program. Our inspection notes describe the check that follows placement.

Monitoring During Production

The placement accuracy should be monitored at a defined interval during a run, because the drift is gradual and it becomes visible only when a part is misplaced.

Where the monitoring shows a trend, the cause should be found before the program is corrected again, or the correction will be repeated indefinitely.

Process Control and Verification

On a design of this kind, variation is the item that decides how the rest of the board is arranged. 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.

Running a first article through the same checks as the production panel confirms that the two agree, and that comparison is the cheapest form of process control available at prototype stage. 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.

Process Control and Verification

On a design of this kind, variation is the item that decides how the rest of the board is arranged. 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.

Running a first article through the same checks as the production panel confirms that the two agree, and that comparison is the cheapest form of process control available at prototype stage.

Process Control and Verification

On a design of this kind, variation is the item that decides how the rest of the board is arranged. 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.

Process Control and Verification

On a design of this kind, variation is the item that decides how the rest of the board is arranged. 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.

A first article check confirms that the process and the drawing agree on the points listed above, and that the coupon data supports the values used in the design.

Where a measurement falls outside the expected window, the sample is retained so that the cause can be established before the balance of the batch is released.

FAQ

Is the machine specification enough? It describes the machine under its own conditions. The product measurement is what the design depends on.

Can the paste compensate for placement error? Self-alignment corrects a small error during reflow and it does not correct a part that is placed outside the pad.

What does gopcb provide for placement capability? We provide product based measurement rather than calibration targets, capability calculated on the smallest pad, contribution analysis covering machine, nozzle, feeder and vision, offset correction per machine and head with verification, a defined monitoring interval during the run, and vision enabled by component class.