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Effect of Process Changes and Flux on Mid-Chip Solder Balling
Effect of Process Changes and Flux on Mid-Chip Solder Balling
Paper documents experimental work performed to understand the impact on mid-chip solder balling from the manufacturing process and flux chemistry.
Production Floor

Authored By:
Katherine Wilkerson, Ian J. Wilding, Michael Carter, Daniel Buckland
Henkel Ltd
Hemel Hempstead, United Kingdom
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Summary
This paper documents the experimental work performed to further understand the impact on mid-chip solder balling from both the manufacturing process and the flux chemistry.

Mid-chip solder balling is a defect typically associated with solder paste exhibiting poor hot slump and/or insufficient wetting during the reflow soldering process, resulting in paste flowing under the component or onto the solder resist. Once molten, this solder is compressed and forced to the side of the component, causing mid-chip solder balling.

To increase the understanding of what factors can impact mid-chip balling, a study was undertaken to examine the effects of process variants and flux chemistry. Stencil thickness, aperture size and aperture shape were all identified as potentially significant factors with regards to process influence. Testing also revealed that the volume of paste was not necessarily proportional to the number of mid-chip balls, but was more influenced by the position of the paste relative to the pad. Comparative testing of a range of flux chemistries indicated that this also had a substantial effect on mid-chip ball occurrence.

The data suggested that mid-chip balling could be controlled by both process and flux design. New methods of quantifying the severity of mid-chip solder balling are currently being investigated.
Conclusions
Mid-chip balling can most effectively be reduced by implementing the following process changes:

Changing aperture shape
Reducing stencil thickness
Changing to a cooler reflow profile
Reflow in nitrogen if a hotter profile is required

The flux chemistry can also have an impact on the mid-chip balling, however:

There is considerable variation between different fluxes
Halogen appears to eliminate mid-chip balling
It is possible to get zero mid-chip balls with halogen-free when the process is right
Hot slump has only a minor impact on mid-chip balling when the process is right
Further work is required to identify potential contributors to mid-chip balling
Initially Published in the IPC Proceedings
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