Welcome to Board Talk. It's Jim Hall and Phil Zarrow, the Assembly
Brothers, who by day go as ITM Consulting. So what is today's question?
It comes from B.B. The question is
about step stencils.
"I've used step stencils with one and two
mil steps successfully. But recently,
I've seen a problem with the 3-mil step.
This is an 8-mil foil step down over most of the printed area to a 5 mil
The problem I am seeing is
that the stencil tends to warp around apertures, particularly in areas like
fine pitch where there are long apertures close together and the remaining
material is a thin strip. The warp makes
the stencil act thicker, allows more paste to be deposited leading to bridges on fine
Is there a rule of thumb
regarding the amount of step that can be etched into the stencil?"
Well, yes there is. It's been speculated for many, many
years. I know back, years ago, we used
to use about 2-mil step, but now I think the common wisdom is one about 1.5
My experience is that most everybody who
does step stencils feels comfortable with the 1-mil step. A lot of people use 1.5 mil. Above that, very few people like to go
more than a 1.5 mil to get repeatable printing.
But this raises other questions. This is really a step-up stencil. Where most of the area of the stencil is
the lower thickness and you are stepping up for a few thicker areas. I don't know that much
about the specifics of the stencil fab, but this seems like a common issue
that you should discuss with your stencil manufacturer.
Because if you're getting that problem,
you're paying for the stencil and you're probably paying extra get all that extra material etched off or whatever milling process they use to reduce thickness.
something about the pressure, the squeegee or something that you're using
that's causing the stencil to be damaged.
But either way, 3 mils is really pushing
the envelope or in this case, pushing the aperture.
Although we should say stencil manufacturers have successfully stepped
5, 7, 8 mils, but I honestly have never seen an application. The question - is this a viable robust process to put into a
manufacturing operation. So our
general rule is 1.5 mils, maybe to 2 mils, but 3 mils definitely goes
Well, good, I think we answered that question. Regardless of whatever you are stepping down on your solder
paste, when you go to reflow it.
Don't solder like my brother.
Hey, don't solder like my brother.