Solder Beading on Chip Components After Reflow
A solder bead is a small sphere of alloy that has formed beside a chip component, usually attached to the termination or to the mask. It is a defect because it can move during later handling, because it can bridge to a neighbouring feature and because many customers reject it on sight. The mechanism is a small amount of paste or molten alloy that has been separated from the main fillet, and the separation has a cause that can be corrected.
How a Bead Forms
The common mechanism is squeeze out. When the component is placed, the paste under the termination is displaced sideways, and a small amount ends up beside the pad on the mask. During reflow this material melts but does not merge with the fillet because it is separated by the mask edge, and it forms a sphere that stays where it landed.
A second mechanism is paste that was printed outside the pad area in the first place, either because the aperture is oversized or because the print smeared. The material is present before placement, and its position determines where the bead appears.
Stencil Aperture and Paste Volume
The aperture size relative to the pad determines how much paste can be displaced. An aperture that is larger than the pad leaves paste around the pad edge where it cannot merge, and it also increases the total volume that has to be squeezed. The usual practice is to keep the aperture equal to or slightly smaller than the pad, and to reduce the volume rather than increase it on a component that beads.
The aperture shape also matters. A shape with rounded corners releases more consistently and leaves less material at the corners than a sharp rectangle, and a slight reduction at the ends of a chip pad reduces the amount that escapes toward the mask. The volume relationships are the same ones described in solder paste volume and stencil design.
Placement Force and Component Standoff
Placement force pushes the component into the paste and displaces it. Too much force spreads the paste beyond the pad, and the amount that escapes increases with the volume. A machine that is set with a high force for a heavier package will over compress a small chip component unless the force is set per package type.
The standoff of the component also matters. A chip with a very small gap between the termination and the board pushes the paste out more readily than one with a taller termination. Where a component is known to bead, a small reduction in placement force and in paste volume is usually more effective than any change to the reflow profile.
Mask Surface and Pad Geometry
The mask surface energy determines whether the displaced paste stays where it landed or flows back to the joint. A mask with a low surface energy repels the paste, so the bead stays separate; one with a higher surface energy can allow it to be drawn back. The mask chemistry and the cure therefore influence the defect, and a change of mask supplier can change the beading rate.
The pad geometry and the mask opening also decide where the paste can go. A mask opening that is much larger than the pad leaves a wide ledge for the paste to sit on, while a tight opening restricts the escape to a narrow band where it is more likely to rejoin the joint. The registration between the opening and the copper is part of the same decision, and the considerations are the same as those in PCB quality assessment.
Reflow Profile and Atmosphere
The profile determines how long the separated material stays molten and whether it reflows into the joint. A long time above liquidus gives the sphere an opportunity to be drawn toward the fillet by surface tension, while a short, hot profile freezes it in place. Increasing the peak duration is therefore sometimes enough to eliminate a marginal beading problem.
The atmosphere contributes in a similar way. In nitrogen the oxide on the small sphere is reduced, so it can merge with the joint rather than remaining a separate ball. The effect is not a substitute for correcting the print, but it explains why a process moved into nitrogen often shows a lower beading rate.
Cleaning and Detection
Beads that are not attached to anything are the most dangerous because they can migrate. Cleaning after reflow may remove the loose ones, but a bead that is fused to the mask or to a termination will not be removed and has to be found by inspection. The inspection should include the area beside each chip component rather than only the joint itself.
The gopcb assembly group classifies beading as a print related defect and traces the rate against the stencil and the placement settings. Where the rate rises after a stencil change, the aperture design is reviewed first, and where it rises without a tooling change, the paste condition and the placement force are checked. That order reflects the frequency of the causes and keeps the investigation short.
Acceptance and Its Evidence
The sequence of operations is part of the specification, because a different order produces a different result from the same steps. 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.
Where two operations share a tolerance, the allocation between them should be explicit rather than left to whichever is measured first. The narrowest feature on the board usually sets the process window for the whole product, so it deserves the closest attention at review.
Related reading: our fabrication notes, board quality and design release notes cover the same ground.
FAQ
Is a solder bead always a defect? A loose sphere is always a defect because it can move. A small bead fused to the mask beside a joint is often rejected as well, depending on the customer criteria.
Does a smaller aperture always help? Reducing the volume reduces the amount that can escape, but an aperture that is too small starves the joint. The aim is an aperture that matches the pad.
Can cleaning remove the beads? Loose ones, yes. Beads fused to a surface have to be found by inspection and removed mechanically, which is a rework operation.
