Lifted Leads and Coplanarity Defects: Causes and Detection

A lifted lead is a component lead, typically a gull-wing lead on a QFP, SOIC or connector, that sits above its pad instead of resting in the solder, leaving an open or a weak, partly wetted joint. Coplanarity describes how closely all of a component's leads lie in one plane, and a coplanarity defect means one or more leads deviate far enough that they cannot all reach their pads at once.

What causes lifted leads and coplanarity defects?

Some lifted leads arrive on the part; others are created on the line by warpage or poor seating. The common causes are:

• Component condition: leads bent in handling, in damaged trays or tubes, or in the feeder, and supplier lead coplanarity outside the datasheet limit.
• Solder paste: a deposit too low to reach a slightly high lead, or uneven deposit heights across a row of pads.
• Stencil: a thin stencil chosen to prevent fine-pitch bridging leaves little margin for lead height variation.
• Reflow profile: board bow and twist during reflow lift leads at the corners of large packages, and wicking pulls solder up a lead that heats faster than its pad.
• Placement: low placement force, a tilted pick, or a nozzle that disturbs leads on placement.
• Pad design and finish: short pads, and uneven pad surfaces from finishes such as HASL.

A lifted lead often shows as a starved joint, so this defect overlaps with insufficient solder.

How are lifted leads detected?

Manual inspection needs a side view under magnification, which is slow on a board full of fine-pitch parts. SPI sees the paste deficit that makes a lift worse, but not the lead itself.

2D AOI judges a lifted lead from the shape and shading of the heel fillet in a top-down image. A small lift can look almost identical to a good joint from above, which is the classic weakness of 2D inspection for this defect. A lifted lead can also still touch its pad intermittently and pass electrical test, then fail in the field.

3D AOI changes the problem from interpreting an image to measuring a height. It measures each lead and the component body relative to the board, so a lifted lead, a tilted body or a coplanarity defect becomes a number checked against a limit. AXI is used for hidden joints, but gull-wing leads are visible optically.

In DaoAI's inspection coverage, item 07, Lead Condition, covers lifted, bent, damaged, missing or unsoldered leads. Item 04, Component Height & Coplanarity, uses 3D AOI to measure component height and detect lifted or tilted bodies and coplanarity defects.

The DaoAI P5 is the inline 3D machine for this work and is available for delivery now, with the P5D as its dual-track version. Its key 3D figures are below; specifications are reference values, not guaranteed limits.

Projecting from four directions reduces the shadowed areas a single projector leaves behind tall parts, which matters on connectors and large packages where lifted leads are most common.

How to prevent lifted leads

Have a connector or QFP that keeps escaping 2D inspection? Send us one of your own boards and we will test it for you, free of charge.

FAQ

What is lead coplanarity?

Coplanarity is how closely all the leads or terminations of a component lie in one plane. It is usually specified as the maximum distance between the lowest and highest lead, and parts outside the limit risk lifted leads after reflow.

Can 2D AOI detect lifted leads?

2D AOI can detect lifts large enough to change the fillet's appearance, but small lifts can look like good joints from above. 3D AOI measures lead height directly, so it is the more reliable method.

Will a lifted lead fail in-circuit test?

Not always, because a lead that touches its pad without a proper joint can still conduct during test. These joints can then fail intermittently in the field.

Which components are most prone to lifted leads?

Fine-pitch gull-wing packages such as QFPs and SOICs, and long connectors, are the most prone. Their thin leads bend easily, and board warpage affects their corner leads most.