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The Knowledge and Know-how Connection
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June 18, 2013
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Views: 6171
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I had an application where we had a nineties designed board with mixed technology, including 4 fine-pitch QFP's as well as 8 .050" pitch J-leaded devices. We used a 5 mil stencil and constantly got shorting on the fine-pitch and insufficient solder on the J-leads (quad 16-pin).
I ordered a 6 mil stencil etched down to 4 mil, leaving islands of 6 mil for the J-leads. I also designed laser cut squeegees that would lift over the J-lead islands. The squeegees had .003" slits that terminated at the top, just under the holder with .100" holes to prevent stress risers.
We suspected that the squeegees would leave lines of paste from the splits, but the splits clogged with paste and never left any features. It cured the shorting of the fine pitch as well as the insufficient J-leads. Both components came from PhotoEtch in Lowell, Ma.
It's worth it to use dedicated squeegee holders in this case, but the blades don't work-harden and distort or crack like they would if they weren't laser cut.
George Garneau, MKS Instruments, USA
Either a rubber or metal squeegee can be used with a stepped solder paste stencil, but most use metal squeegee blades. However, the usable life of a metal squeegee blade is reduced due to accelerated wear caused by the metal squeegee blade impacting the edges of the step areas.
The proper depth of a step is determined by the stencil aperture sizes within the step areas. The foil thickness within the step areas must be thin enough to meet, or exceed, the minimum aspect ratio and/or surface area ratio requirements for the stencil aperture sizes. Most stepped solder paste stencils have step depths in 0.001" increments.
Mike Scimeca, FCT Assembly