Placement Offset Control and Vision Correction

A placement offset is a systematic displacement of every component, or of one component, from where it should be. A systematic offset is corrected once and a random one is investigated, and the two look similar in a single image.

Where the Offset Comes From

The nozzle, the head, the feeder position, the board location and the vision alignment all contribute. A constant offset in one direction across the board usually points at the board location.

An offset on one feeder points at the feeder. Our feeder setup notes describe the check.

Vision Alignment and Fiducials

The machine locates the fiducials and calculates the board position, rotation and scale before placing. Where the fiducials are missing or poorly contrasted, the machine falls back on nominal values and the offset appears.

The fiducials have to be usable by the vision system. Our fiducial notes describe the requirements.

Board Scaling and Drift

A board that expands during reflow of the first side will have moved by the second pass, and the machine compensates by scaling. Where the scaling is not corrected, the offset grows toward the board edges.

The growth pattern is the clue: an offset that increases with distance from the centre is a scaling error. Our fine pitch notes describe the consequence.

Measuring the Offset

The offset is measured by comparing placed positions with the design positions, usually from an optical measurement of a sample board. A single component does not describe the pattern.

The measurement should be repeated after any change to the setup. Our yield analysis notes describe the record.

Correction and Its Limits

An offset can be corrected in the program up to the point where the placement is outside the pad. Beyond that the cause has to be found, because a program correction hides a wandering feeder or a worn nozzle.

A drifting offset should be trended rather than re corrected each time. Our capability notes describe the measurement that supports it.

Process Capability

The capability index compares the placement distribution with the pad tolerance. A process that is centred but wide will produce defects at the tails even though the average looks good.

The capability should be established on the narrowest pitch on the board. Our component tolerance notes describe the pad that sets the limit.

Verification

The verification is a position measurement against the design, a growth pattern checked for scaling rather than offset, a trend recorded rather than a series of independent corrections, and a capability figure for the narrowest pitch.

Our quality notes describe how the records are kept.

Process Control and Verification

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.

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

Should an offset be corrected in the program? A constant offset can be, provided the cause is understood. A changing one should be corrected at the source.

Why does the offset grow toward the edge? That pattern indicates a scale error, usually from board expansion or from a fiducial used at the wrong separation.

What does gopcb provide for placement control? We provide fiducials that the vision system can locate, a position measurement against the design, a check that separates offset from scaling, a trend rather than repeated program corrections, and a capability figure at the narrowest pitch on the board.