How to Eliminate Formation of Solder Beads and Solder Balls?

Definitions

Solder beads are large solder spheres which form aside components during reﬂow. They are most common with low stand-off resistors and capacitors.

Solder balls are small, widely scattered solder particles found within the ﬂux pool.

Process Impact

Solder beads and balls may cause electrical shorts immediately or in the future if the balls roll around the PCB. Beading and balling could also cause insufﬁcient solder joint volume.

General Causes for Beading

✅Paste printed or pressed onto solder mask

✅Stencil aperture design

✅Large overlap between pad and component

✅Excessive paste volume due to thick stencil

✅Switch from rubber to metal squeegee

✅High component placement pressure

✅High ﬂux activation temperature

✅Flux and moisture outgassing

✅Fast ramp rate

✅Excessive solder powder oxides or contaminates

✅Low metal load

General Causes for Balling

✅Print smearing

✅Stencil aperture design

✅Large number of fines in powder

✅Excessive humidity

✅Insufﬁcient ﬂux activity

✅Excessive solder powder oxides or contaminates

✅Solder mask and paste interaction

✅High ﬂux activation temperature

✅Flux and moisture outgassing

✅Fast ramp rate

✅Component outgassing

✅Low metal load

Solutions

✅Modify the stencil apertures and thickness. Reduced apertures are recommended.

✅Different pad designs or metallizations may also reduce beading and balling.

✅Evaluate component and board metallizations for solderability and contaminates.

✅Adjust the reﬂ ow profile. A slow steady ramp (≤1°C/sec) permits moisture and solvents to evaporate gradually prior to rosin/resin softening. Hot slump is minimized. Pastes with long stencil life and tack time generally require a slow ramp so environmentally stable solvents can evaporate.

Conclusion

PCBA boards are more sophisticated product components and sensitive to conductive objects and ESD static electricity.In the manufacturing process of PCBA, the manager of the factory needs to improve the management level (at least IPC-A-610E Class II is recommended), strengthen the quality awareness of the operators and quality team, implement from two aspects of process control and ideological awareness to minimize the solder beads and solder balls on the PCBA board to the greatest extent.