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Optimization of Stencil Apertures to Compensate for Scooping
Optimization of Stencil Apertures to Compensate for Scooping
This study investigates the scooping effect during solder paste printing as a function of aperture width, aperture length and squeegee pressure.
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Authored By:
Gabriel Briceno, Ph.D. and Miguel Sepulveda
Qual-Pro Corporation

Summary
This study investigates the scooping effect during solder paste printing as a function of aperture width, aperture length and squeegee pressure. The percent of the theoretical volume deposited depends on the PWB topography. A typical bimodal percent volume distribution is attributed to poor release apertures and large apertures, where scooping takes place, yielding percent volumes <100%, while SMD apertures and apertures near PWB features that raise the stencil yield percent volumes >100%. This printing experiment is done with a concomitant validation of the printing process using standard 3D Solder Paste Inspection (SPI) equipment.

The data collected from the SPI equipment included the solder paste volume, printed area, solder paste height and x-y offset printing. The volume data for each aperture width exhibits a Gaussian distribution, with the mean and standard deviation changing as a function of aperture width. For small apertures poor release is observed, while the reduction of the solder paste volume for large apertures is attributed to the appearance of the scooping effect at 0.070" aperture width. The Gaussian distributions, when analyzed separately, indicate that the printing process for each aperture width is under control yielding C-pk greater than 1.33 (DPMO <63), with USL and LSL set at plus or minus 20% from the mean volume.

We also investigated the release of apertures with and without round corners. The former exhibited better solder paste release.

Conclusions
The SPI volume data should be analyzed by individual pads. However, the analysis can be simplified by grouping the data into different groups (e.g., low percent theoretical volume pads and high percent theoretical volume pads) and then fitting each group of data to a Gaussian distribution to determine its mean and standard deviation. The solder paste printing process is bimodal by nature. The LSL and USL for the SPI equipment should be determined from specified absolute solder paste volumes that yield reliable solder joints. The limits can be tighter for critical components and should be analyzed separately.

The stencils apertures should be designed to be in the 90%-100% solder paste percent release volume, whenever possible. If apertures smaller than 0.025"X0.025" are present, they should be oversized to compensate for their lower solder paste release and be able to meet their absolute specified solder paste volume. Small apertures will shift the overall mean percent volume below 100%, making the LSL lower.

Similarly, apertures that are 0.060"X0.60" or larger will shift the overall mean percent volume below 100%, as the onset of solder paste scooping was observed at around aperture widths of 0.060". These large apertures should be avoided. A good Rule of Thumb from this observation is to split the apertures whenever a pad is greater wider than 0.060" to avoid the scooping effect. For example, if a pad is 0.115"X0.250", it would be wise to have a 2X3 window pane with a 0.010"-0.015" gap between apertures.

Initially Published in the IPC Proceedings

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