Solder Balls on PCBA: Causes, Detection and Prevention

Solder balls are small spheres of solder that separate from the joint during reflow or wave soldering and end up on the solder mask, the laminate or beside a component. A loose ball can move in service and short two conductors, so workmanship standards such as IPC-A-610 limit solder balls by size, location and whether they are held in place.

What causes solder balls?

Solder balls form when some paste never joins the main joint: it was printed in the wrong place, spattered by outgassing, or too oxidized to coalesce. The common causes are:

• Solder paste: paste that has absorbed moisture spatters as the water boils off, and expired or over-exposed paste has oxidized powder that does not coalesce into the joint.
• Stencil: paste printed onto the solder mask through poor gasketing, stencil misalignment or a dirty underside. On chip components, a full rectangular aperture puts paste under the body, where it gets squeezed out as a bead.
• Reflow profile: a fast preheat boils off solvent violently, and a long or hot soak can exhaust the flux before the solder melts.
• Placement: excessive placement force pushes paste out from under the part and onto the mask.
• Component and pad design: pads much larger than the terminations, and moisture in the bare board, which outgasses during reflow.

Excess or slumped paste is a shared root cause with solder bridging, so a line seeing both should look at the print process first.

How are solder balls detected?

Manual inspection misses small balls easily, especially on dense or dark boards. SPI cannot see balls, since they form in reflow, but it flags their causes: smeared deposits, paste on the mask and offset prints.

2D AOI can see a solder ball as a small, bright, specular sphere. The limitation is where it looks: a conventional program inspects a region around each component, and a ball sitting on bare laminate between components is inside none of those regions. 3D AOI adds height, which helps separate a real ball from a flat residue or reflection. AXI is needed for balls trapped under a component body.

DaoAI handles this with a whole-board inspection pass that looks at the board between the components. In DaoAI's inspection coverage, item 14, Solder Balls & Particles, finds solder balls or particles anywhere on the assembly, not only adjacent to a solder joint. Available checks depend on the machine and configuration.

What about foreign object debris (FOD)?

The same gap in coverage applies to anything else that lands on the board. Wire clippings from through-hole trimming, a chip component dropped by the placement machine, or a fragment from depaneling can sit on open laminate where no component inspection region reaches.

Item 15 in the same whole-board pass, Foreign Object Debris, covers debris, wire clippings, dropped components and other foreign material. A dropped component is the case that matters most, because it is both a missing part somewhere and a potential short somewhere else. Surface contamination such as flux residue and stains is listed as item 16 and marked coming soon.

How to prevent solder balls

Most solder balling is fixed in paste handling, stencil design and preheat. Check profile changes against the paste datasheet.

Tell us your board finish and the defect you are chasing, and we will show what the whole-board pass finds. Send us one of your own boards and we will test it for you, free of charge.

FAQ

Are solder balls acceptable under IPC-A-610?

It depends on the product class and on whether the ball is entrapped or encapsulated so it cannot move, and whether it violates minimum electrical clearance. Check the current revision of the standard and your customer's requirements for the exact acceptance criteria.

What is the difference between solder balls and solder beads?

Solder beads, or mid-chip balls, are larger balls that form beside the body of a chip component, usually from paste pushed under the part. Random solder balls are smaller and can appear anywhere, often from spatter or paste printed on the mask.

Can AOI find solder balls away from components?

Only if it inspects the open board, not just regions around components. DaoAI's whole-board inspection looks for solder balls and particles anywhere on the assembly, including bare laminate between parts.

Is foreign object debris a solder defect?

No, FOD is any foreign material on the assembly, such as wire clippings, dropped components or other debris. It is often inspected in the same pass as solder balls because both can sit anywhere on the board.