Phil
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?

Jim
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 thickness.

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 pitch QFPs.

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

Phil
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 mil.

Jim
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.

Perhaps it's something about the pressure, the squeegee or something that you're using that's causing the stencil to be damaged.

Phil
But either way, 3 mils is really pushing the envelope or in this case, pushing the aperture.

Jim
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 over rule.

Phil
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.

Jim
Don't solder like my brother.

Phil
Hey, don't solder like my brother.