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Stencil, squeegee blades affect aperture fill sub-process

Posted: 17 Jul 2006 ?? ?Print Version ?Bookmark and Share

Keywords:Michael Burgess? Photo Stencil? green engineering? design talk? spot light?

By now, global European electronic suppliers have selected alternative Pb-free alloys and tested materials in production. Engineers are being challenged to optimize process variables around the new Pb-free requirements.

Recent studies have analyzed the effects of Pb-free solder paste implementation on a multitude of SMT processes including solder joint strength, wetability of SMD leads and pads on the PCB, solder paste alloys/flux compositions, and stencils used to print Pb-free solder pastes.

This article reviews the impact of the stencil and squeegee blade on the aperture fill sub-process of Pb-free printing.

The first step in printing solder paste is to fill the stencil aperture with solder paste. This is normally achieved with a metal squeegee blade. Several factors can contribute to the aperture fill process. The orientation of the aperture with respect to the squeegee blade has an effect on the fill process. An aperture oriented with its long axis in the same direction as the blade stroke doesn't fit as good as an aperture oriented with its short axis to the blade stroke.

Squeegee speed also has an influence on aperture fill. Normally, the squeegee speed must be reduced to fill the aperture, with its long axis oriented parallel to the squeegee stroke. The squeegee blade edge influences how well the paste fills a stencil aperture. The rule of thumb is to print at the minimum squeegee pressure while still maintaining a clean wipe of the solder paste on the stencil surface.

If the squeegee pressure is too high, both the squeegee blade and the stencil may be damaged. If the pressure is too low, leaving paste on the stencil surface may lead to one of two possible bad events. Paste left on the squeegee side of a small aperture will hold the paste, preventing its release to the PCB pad, which will result in insufficient solder. Paste left on the squeegee side over large apertures will be pulled down through the aperture, resulting in excess solder.

The squeegee blade smoothness is a factor in the printing process. A smooth blade prevents paste from clinging to the blade. If excess paste accumulates on the blade, it can dry and contaminate the paste being applied to the stencil, causing aperture clogging. Surface roughness measured in a recent study using a Pocket Surf showed that the E-Blade had a surface roughness of 1inch Ra compared to a surface roughness of 3inches for the Teflon/nickel-coated blade.

Minimum squeegee pressure is also a function of the blade type. In the same study, the minimum squeegee pressure for the E-Blade was 0.31Kg/cm and 0.82Kg/cm for the Teflon/nickel-coated blade for Pb-free solder paste.

Tests have confirmed that Pb-free solder pastes typically require higher squeegee pressure than tin-lead pastes. The increase is usually in the neighborhood of 25 percent additional pressure. An additional factor affecting squeegee pressure is the surface roughness of the squeegee side of the stencil. Studies have shown that squeegee blade pressure needs to be increased by 70 percent for step stencil surfaces with surface roughness of 15inches Ra compared to E-FAB stencils with an Ra of 1inch.

- Michael Burgess
Stencil Product Manager, Photo Stencil

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