Component Lead Coplanarity and Placement Force

A component can only be soldered if its leads reach the paste. On a fine pitch part the difference between a lead that touches and one that floats above the deposit is a few tens of microns, and it is a property of the component rather than of the process.

Lead coplanarity is the measure of that property, and the placement machine is the tool that either compensates for it or makes it worse. Understanding both is what prevents the class of defects that look like process problems but are incoming material problems.

What Coplanarity Means for a Fine Pitch Part

Coplanarity describes how far the lowest and highest leads of a component deviate from a common plane when the part rests on a flat surface. A device with good coplanarity has all its leads within a small tolerance of the seating plane; one with poor coplanarity has a lead that sits above it.

The tolerance scales with the package. A lead that is fifty microns out of plane is negligible on a coarse pitch part and fatal on a fine pitch one, because the paste deposit it has to reach is thinner than the error. The component datasheet states the limit, and it should be checked rather than assumed.

How a Warped Lead Fails

A lead that never reaches the paste produces an open circuit, or a joint that forms on one side only and looks acceptable from above. The second case is more dangerous, because the connection passes a functional test and fails later under thermal cycling or vibration.

The failure is often intermittent and appears at a specific temperature, which makes it difficult to diagnose. The evidence is in the paste inspection data and in the X-ray image of the joint, both of which show a gap that the process alone would not have produced.

Placement Force and Its Limits

Placement force is the load the nozzle applies while the component is being seated. Enough force is needed to press the leads into the paste and to overcome any slight curvature of the board; too much force squeezes the paste out from under the leads and can damage the component.

The correct setting is found with the paste inspection data: a force that produces a consistent deposit height and a deposit that has spread slightly beyond the lead is usually right. A setting that produces no visible spread indicates that the part is resting on its leads rather than being pressed into the paste, and it may be leaving gaps.

Nozzle Condition and Pick Up

The nozzle is the other half of the placement. A worn or blocked nozzle picks the component up at an angle, and the part arrives at the board tilted. The tilt is tiny and it has the same effect as poor coplanarity: some leads press into the paste and others barely touch it.

Nozzle condition is checked as part of the setup, and the check should include the vacuum level rather than only the appearance of the tip. A nozzle that holds the part reliably at the pick position can still release it unevenly if the tip is worn, and the difference shows in the first article rather than in the machine alarm.

Self Alignment During Reflow

During reflow the surface tension of the molten solder pulls the component towards the centre of its pads, which is the effect known as self alignment. It can correct a placement error of a small fraction of a pad width, and it can also correct a slight rotation.

Self alignment is not a substitute for accurate placement. It works best where the paste volume is even and the pad geometry is symmetrical, and it cannot compensate for a lead that is not in contact with the paste at all. Where the pads are uneven, or where one side of a chip component has much more paste than the other, the force is unbalanced and the part can be pulled off centre.

Supplier Data and Incoming Checks

The coplanarity specification is part of the component datasheet, and the supplier should be able to state how it is measured. Where the property is critical, the measurement method matters: a value quoted for a part resting under its own weight is different from one measured under a small load.

An incoming check does not have to measure every reel. A simple inspection of a sample under a low angle microscope will show a lead that is clearly out of plane, and it is enough to catch a batch that is outside its specification. Where a problem is found, the whole reel should be isolated rather than sorted.

Damage in Handling and Storage

Coplanarity is not fixed at manufacture. Leads can be bent during transport if the packaging allows movement, and moisture absorbed by the package can cause the body to swell during reflow and lift the leads away from the paste.

The moisture requirement is why components are supplied in sealed bags with a humidity indicator, and why the baking instructions exist. Handling the material correctly is described alongside the other requirements in the notes on component handling and baking, and it is the cheapest way to keep the coplanarity that the supplier delivered.

When to Reject a Batch

A batch is rejected when the measured coplanarity is outside the datasheet limit, or when the defect rate at placement suddenly rises without any change to the process. In the second case the component should be checked before the machine settings are adjusted, because a change made to compensate for bad material will be wrong when good material arrives.

Keeping a sample of the suspect component and a record of the settings makes that decision defensible, and the sample should be kept with the reel label so that the batch can be identified rather than described. The same discipline of recording what was measured and what was changed is what makes a quality control record useful months later, when the same symptom appears on a different product.

Board Flatness and Coplanarity Together

The component is only half of the geometry. The board also has to be flat where the part is placed, and a bow of a few tens of microns across a large package has the same effect as a lead that sits above the paste.

Board flatness is set by the fabrication and by the assembly process. A stackup with unbalanced copper, a panel that was not supported in the oven and a fixture that does not hold the board flat all contribute, and the corrections are described alongside the manufacturing tolerances that the fabricator is asked to hold.

FAQ

Can placement force compensate for poor coplanarity? Only within a small range. Pressing a warped part into the paste deforms the leads and stores a stress that will be released later.

Is coplanarity checked on every reel? Usually only on a sample. Full measurement is done when a product is new or when a defect rate suggests that the material is the cause.

Does reflow flatten a bent lead? The solder can bridge a small gap, but a lead that is far out of plane will not be pulled down by surface tension alone.