Lead Coplanarity and Placement for Fine Pitch Components

A fine pitch package relies on every lead touching its pad at the moment the alloy melts. If one lead sits higher than the others, it may still form a joint by reaching down into the paste, or it may float on top of a deposit and form a joint that is weak, intermittent or absent. Lead coplanarity is the property that determines which of those happens, and it is a supplier characteristic that the assembly process can only partially compensate for. This article covers how it is specified, measured and managed.

What Coplanarity Means

Coplanarity is the deviation of the lead tips from a common plane. It is expressed as the maximum distance between the highest and lowest lead, measured with the package seated on a reference plane. A package with good coplanarity rests on all its leads; one with a single low lead rocks on the others and leaves that joint without contact pressure.

The property is a result of the package construction, the lead forming process and the material. Thermal expansion during reflow changes it slightly, and the change depends on the package body material and on how the leads are attached, which is why a specification that is met at room temperature is not automatically met at reflow temperature.

Specifications and Incoming Inspection

The specification should state a maximum deviation, and it should apply to the package as received. Incoming inspection verifies it on a sample, using a measurement system that is appropriate to the tolerance. An optical system that measures to a few micrometres is usually necessary, since the tolerance for a fine pitch package is a small fraction of the lead pitch.

The inspection plan should include a sampling rule and a disposition for a lot that fails. Rejecting a whole lot is expensive, but accepting it means the assembly line will see the defects instead. Where the failure is marginal, the more useful action is to compare the result with the paste volume and placement settings used on that product, because a slightly non coplanar lot can be assembled successfully with an adjusted process. The general approach to incoming checks is described in judging PCB quality.

<img src="https://www.gopcba.com/wp-content/uploads/2026/04/EMI.jpg" alt="Fine pitch component leads resting on printed solder paste pads” />

How Placement Force Interacts

The placement force presses the leads into the paste, and it partly determines how much tolerance the process has. A force that is sufficient to seat the package on its leads reduces the effect of small coplanarity errors, while a force that is too low leaves the package resting on the highest few leads.

The force is limited by the paste. Too much force spreads the deposit, closes the gap between adjacent pads and causes bridging, and it can also push the package down until the leads contact the pad directly, which displaces almost all the paste. The usable range is therefore narrow, and it should be established by measuring the deposit after placement rather than by adjusting the machine setting alone. Placement capability in general is covered in placement accuracy capability.

Paste Volume as a Compensation

Paste volume is the main process lever for a marginal coplanarity lot. A slightly higher deposit gives a high lead something to reach into, and it provides enough alloy to form a fillet even when the lead does not seat fully. The limit is bridging, which is the failure mode that a higher volume creates.

The balance is product specific. A lead pitch with generous spacing tolerates more volume, while a dense package does not. The volume should be set for the worst realistic coplanarity in the product, not for the best, and the measurement method described in solder paste inspection should be used to confirm that the intended volume is actually being printed.

Reflow Profile and Self Centring

The reflow profile affects how well the package self centres. A slow, gradual melt allows the surface tension of the molten alloy to pull the package into alignment before the joints freeze, while a fast, hot profile freezes the joints before the package has time to move. Soak time and peak duration therefore have as much influence as the peak temperature.

The profile also determines whether a high lead can be drawn down. As the alloy melts, the wetting forces pull the lead into the deposit, and that movement requires time. A profile that spends longer above liquidus gives the geometry more opportunity to correct itself, at the cost of more intermetallic growth and a higher thermal load on the component. The trade offs are the same as those covered in mixed thermal mass reflow.

Defect Symptoms and Their Diagnosis

An open joint on a single lead of a fine pitch package, with all other joints complete, is the classic coplanarity symptom. A joint that is present but has a fillet only on one side, a package that appears tilted and a lead that is visibly not touching its paste before reflow all point the same way.

Distinguishing a coplanarity problem from a paste problem requires the data from both. If the deposit volume on the affected pad is normal and the package shows a low lead, the package is the cause. If the deposit is short or missing, the paste process is the cause. Recording both measurements together is what makes the diagnosis possible, and the defect patterns are catalogued alongside other joint failures in solder defects and board failures.

Handling and Storage Effects

Coplanarity can be created after the package is manufactured. A reel that is wound too tightly, a tube that is overfilled, a package stored under load in a shallow tray or a part handled with a tool that presses on the leads all introduce a permanent deformation. The damage is usually small and invisible, and it is discovered as a single lead defect on an otherwise correct assembly.

Handling instructions should therefore cover the package as well as the board. Packages should be kept in their original packaging until use, trays should not be overfilled or stacked beyond the design limit, and manual handling should avoid the leads entirely. Where a part must be reworked, the leads should be inspected for coplanarity before it is placed again, because a package that was pressed off a board is very likely to have at least one bent lead. Checking the placement result against first pass yield data by package type shows whether handling is a systemic issue.

Working With the Supplier

The most effective control is at the source. A supplier that measures coplanarity and ships within a defined limit removes the problem from the assembly line, and the requirement should be stated in the purchase specification rather than assumed. Where a lot fails, the data should be shared rather than simply rejected, because the supplier can only correct a process if they know which characteristic drifted.

The gopcb assembly team records coplanarity results and correlates them with joint defects by package type. That record is what supports a specification change when a particular package proves to be consistently at the edge of the tolerance, and it turns an assembly problem into a supplier requirement with evidence behind it.

FAQ

What coplanarity limit is typical for fine pitch packages? It is normally a small fraction of the lead pitch, and the exact figure comes from the component specification. Measure the actual lot rather than relying on the catalogue value.

Can a non coplanar lot be used? Sometimes, with a higher paste volume and a longer time above liquidus, if the pitch allows. The decision should be based on a first article inspection rather than an assumption.

Does placement force solve coplanarity? Partly. It seats the package on its leads, but excessive force displaces the paste and causes bridging, so it is not a substitute for a compliant package.