Mark Curtin on Large-Board Stencil Printing and Metal Squeegees


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Transition Automation Inc. recently developed a large-area benchtop stencil printer targeted for high-mix low- or medium-volume production. Said to be one of the largest SMT printers available in the world, the PrinTEK AP-3224-V fine pitch SMT printer features a print area of 40" x 24" (101 cm by 61 cm) and uses lightweight composite materials in its design. According to Mark Curtin, president of Transition Automation, it is the first tool that enables users to do large-board production for under $75,000.

"It's nice to do development that's driven by the customer," said Curtin. "And I designed it to build more of them in the hopes that there will be other people that have difficult issues related to large-board production."

Curtin notes that there are just no available large-area stencil printers in the market. "You do a search, and you can’t find any machine. You can find large silk-screening machines, but that’s a different process. You may struggle with converting it to solder paste printing," he says.

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The PrinTEK AP-3224-V is dedicated for stencil printing for solder paste. "Because it is an open system, and because it uses composite materials, you can actually order it, receive it in a crate, and be running it within an hour of receiving it. Whereas with most automated machine, you will have a crew and a rig coming in, and it is going to take several days to get it set up," Curtin says. "Considering a solution for a large board production, it is not a huge machine. Even the footprint of this is the same as the footprint of just a standard 18-by-18 automated printer. But it’s giving the print area double that, so that's pretty unique."

Metal vs Rubber Squeegee

One of the key factors contributing to defects in PCB assembly is the paste printing process. Curtin agrees. He also highlights the squeegee as among the key factors causing the defects.

"The one main thing that we always have been promoting is the idea that the squeegee, even though it is a very small, almost infinitesimal item, has a disproportional burden of providing the quality for the SMT line. If you would take one item (rubber squeegee) and reshape it in a minor way, it will result in a complete dysfunction for your SMT line,” explains Curtin. “So, the squeegee is one of the key things for quality and therefore it should also be built tough, built accurate, and it should have some features on it."

He notes that rubber was always used because with liquid ink printing, you need rubber to form a liquid gasket. But when the silkscreen printing was modified to stencil printing, where a stencil is used instead of a silkscreen, there is now a smooth surface on top, not a calendared screen.

"So now, you don’t need a rubber squeegee; you use a metal squeegee. The main thing that it does is shear the solder paste clean on the top of the stencil. It doesn't fit into the openings of the stencil, and because you are not dipping into the aperture, you are not wearing it out," says Curtin. "Because when a rubber goes over the aperture, it dips in, and it can shear off. Then you get lots of rubber microparts ending up in the solder paste. That will definitely contribute to voiding and gas problems in reflow."

Which is why, according to Curtin, metal squeegee is the way to go.

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