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August 15, 2011

Stepped Stencil Limit

I've used stencils with 1 and 2 mil steps successfully, but recently I've seen a problem with a 3 mil step (8 mil foil stepped down over most of the printed area to a 5 mil thickness).

The problem I'm seeing is that the stencil tends to warp around apertures, particularly in areas like fine pitch QFPs, where there are long apertures close together, and the remaining material is a thin strip. This warp makes the stencil act thicker and allow more paste to be deposited, leading to bridges in the fine pitch QFPs.

Is there a rule of thumb regarding the amount of step that can be etched into a stencil?

B. B.

Experts Comments

IPC 7525 recommends 35 to 50 mils keep-out for each 1 mil of step.

Bill Coleman
Vice President Technology
Photo Stencil
For over 18 years, Dr. Coleman has been the vice president of technology for Photo Stencil, working closely with customers to understand their printing requirements. His efforts have resulted in several new stencil products.
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