Journal of SMT Article

OPTIMIZING STENCIL DESIGN FOR LEAD-FREE SMT PROCESSING

Authors: Chrys Shea and Ranjit S Pandher
Company: Cookson Electronics
Date Published: 1/1/2005   Volume: 18-1

StenTech

Abstract: As any new technology emerges, increasing levels of refinement are required to facilitate the mainstream implementation and continual improvement processes. In the case of lead-free processing, the initial hurdles of alloy and chemistry selection are cleared on the first level, providing a base process. The understanding gained from early work on the base process leads to the next level of refinement in optimizing the primary factors that influence yield. These factors may include thermal profiles, PWB surface finishes, component metallization, solder mask selection, or stencil design.

Because stencil printing has such a large influence on first pass yield, and lead-free alloys exhibit different wetting properties from their lead-bearing counterparts, the authors have undertaken a study to identify stencil aperture geometries that optimize desired SMT characteristics. The concern of the lower spread of lead-free alloys on some alternate surface finishes is explored. Aperture designs that maximize pad coverage while minimizing defects like midchip solder balls are offered. In addition to aperture design guidelines, methods of optimizing the overall stencil design will be reviewed.

Key words: lead-free, stencil printing, aperture design, process control.



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