Electronics Assembly Knowledge, Vision & Wisdom
Stencil Printing Yield Improvements
Stencil Printing Yield Improvements
In this paper two methods were studied to improve solder paste release. A nano-scale hydrophobic, oleophobic and adhesion promoting coating.
Production Floor

Authored By:
Mike Bixenman D.B.A, Debbie Carboni
Kyzen, Nashville, TN, USA

Jason Chan
Kyzen Asia
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Transcript
Understencil wiping has gained increased interest over the last several years. Changes in circuit design due to miniaturized components and highly dense interconnects have increased the importance of stencils being free of solder paste deposits in the wall of the aperture.

In most stencil printing processes, dry wiping has been followed by vacuum assist in an effort to clean solder paste from aperture walls. As stencil apertures reduce in size, more frequent wiping is needed to assure that stencils are free of solder paste deposits.

To improve solder paste release, two technology approaches were studied with higher levels of frequency.

The first technology is a nano-scale coating. The objective is to treat the metal stencil surface with a nano-coating to prevent solder paste from sticking to aperture walls.

The second technology is to wet the under-stencil wipe with a solvent based cleaning agent. The cleaning agent dissolves the flux component within the solder paste to improve release of solder balls from aperture walls.

Seven under-stencil wipe cleaning agents were selected as the research solvents. The control solvent was IPA @ 100% concentration. One of the cleaning agents was a water solvent azeotropic mixture, which allows the material to evaporate at a common rate.

Five of the cleaning agents were solvent mixtures. Nine solder pastes were selected for the study. Two of the solder pastes were tin-lead no-clean. Five of the solder pastes were lead-free no-clean. Two of the solder pastes were lead-free water soluble.

So what were the conclusions?

Of all the cleaning agents tested, one of the solvent blends and a solvent water Azeotrope provided the highest potential for under-stencil wipe cleaning solvents that can replace IPA.

Summary
Understencil wiping has gained increased interest over the last several years. Changes in circuit design due to miniaturized components and highly dense interconnects have increased the importance of stencils being free of solder paste deposits in the wall of the aperture.

In most stencil printing processes, dry wiping has been followed by vacuum assist in an effort to clean solder paste from aperture walls. As stencil apertures reduce in size, more frequent wiping is needed to assure that stencils are free of solder paste deposits.

To improve solder paste release, two technology approaches were studied with higher levels of frequency.

The first technology is a nano-scale coating. The objective is to treat the metal stencil surface with a nano-coating to prevent solder paste from sticking to aperture walls.

The second technology is to wet the under-stencil wipe with a solvent based cleaning agent. The
cleaning agent dissolves the flux component within the solder paste to improve release of solder balls from aperture walls.

Seven under-stencil wipe cleaning agents were selected as the research solvents. The control solvent was IPA @ 100% concentration. One of the cleaning agents was a water solvent azeotropic mixture, which allows the material to evaporate at a common rate.

Five of the cleaning agents were solvent mixtures. Nine solder pastes were selected for the study. Two of the solder pastes were tin-lead no-clean. Five of the solder pastes were lead-free no-clean. Two of the solder pastes were lead-free water soluble.
Conclusions
Understencil wiping has gained an increase in interest over the last several years. Changes in circuit designs, such as miniaturized components, increased density of components, and new stencil technology need to decrease print defects as well as changes in and increased attention to employee safety and environmental regulations have driven renewed interest.

New understencil wipe solvents have been introduced recently to address these issues. Of all the cleaning agents tested, Solvent Blend #5 and Solvent Water Azeotrope provided the highest potential for understencil wipe cleaning solvents that replace IPA.
Initially Published in the SMTA Proceedings
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