Component Lead Coplanarity and Placement Control

Coplanarity is the distance between the lowest and the highest lead of a component when it rests on a flat surface. A lead that is out of plane does not touch its pad, and no amount of paste will bridge the gap.

How It Is Specified

The supplier states a maximum deviation, usually as a small figure across the span of the leads. The figure has to be compared with the paste deposit height, because that is what has to bridge the gap.

A lead that is out by more than the deposit height cannot form a joint. Our paste volume notes describe the deposit.

Where the Deviation Comes From

The lead frame, the forming operation, the body material and the storage conditions all contribute. A plastic body that has absorbed moisture can change shape during reflow.

The storage contribution is avoidable. Our floor life notes describe the control.

Placement Force and Its Limits

A placement force can push a lead down onto the paste, and it cannot correct a lead that is significantly out of plane without deforming the component or the pad.

The force should be set for the package. Our placement notes describe the setup.

Pad and Paste Design

A slightly thicker deposit compensates for a small deviation, and it risks bridging on a fine pitch part. The balance is set by the component specification rather than chosen freely.

The paste volume is the adjustable term. Our SPI notes describe the measurement.

Detecting a Lifted Lead

The joint looks present from above because the fillet at the toe can form without the lead being in contact. An oblique view or a microsection shows the gap.

The inspection should include an oblique view on fine pitch parts. Our inspection notes describe the techniques.

Storage and Handling

A component stored outside its conditions can deform. The bag, the desiccant and the floor life all contribute to keeping the lead frame flat.

Our packaging notes describe the practice.

Verification

The verification is a coplanarity figure compared with the paste deposit height, a placement force set for the package, an oblique inspection on fine pitch parts, and a storage record that supports the lead frame condition.

Our quality notes describe how the records are kept.

Process Control and Verification

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.

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

Can a lifted lead be reworked? It can where the lead can be brought into contact, and the rework applies the same heat to a joint that has already been formed once.

Does more paste solve it? It compensates for a small deviation and it creates bridging risk on adjacent leads.

What does gopcb provide regarding coplanarity? We provide a coplanarity figure compared against the paste deposit height, a placement force set for the package, an oblique inspection on fine pitch parts, and a storage and floor life record that supports the lead frame condition.