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Is There a Limit to the Step in a Step Stencil?



Is There a Limit to the Step in a Step Stencil?
I've seen a problem with the 3-mil step. Is there a rule of thumb regarding the amount of step that can used in a step stencil? Phil Zarrow and Jim Hall, The Assembly Brothers, share their own experiences.
Board Talk

Transcript


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.

Comments

I'd say that Frederick Cox, Metal Etching Technology Associates Inc., is right on the money, when using a isolated step up don't chem etch the rest of the foil, use laser welded inserts, there are several large stencil houses with this technology, and leave room for the squeegee transition, about 50 mils for a 2 mil step up, 75 mils - 100 mils for a 3 mil step up if your design allows for it.
Jim Dowsey, SANMINA
Depending on your screen printer, you may consider utilizing on-board dispense capability if available. This solution will allow you to utilize a single thickness stencil optimized for the smaller components and supplement areas where additional paste volume is required. These systems are cost effective and, in our case, offer the flexibility of single or dual dispense heads therefore paste and glue can be applied to the same board if necessary. We dispense dots as well as lines for fencing applications.
Mark Brawley, Speedprint Technology
We have consistently produced stencils with 1 through 10 mil step depths using the appropriate alloy metal. Customers doing two print applications to print fencing often do a 10 mil step. Stencil vendors who are unable to chem-etch the multi-levels are the ones saying you don't need them - or they say they can't be done.
Mark Devereaux, Photo Etch Technology
We laser weld our steps into the foil, as described by Fraser above. Larger steps can be made quite easily. Printing successfully today really depends a lot on your spacing. As long as you have enough space between the large step areas the squeegee blade should be able to transition to the secondary thickness.
Frederick Cox, Metal Etching Technology Associates Inc.
Years ago I worked with Great Lakes on a process to obtain a tapered stencil. essentially this is a step stencil with incremental steps then polished to obtain the thickness without the harsh edge. For a paste in hole connector, with 16 mil pitch QFNs 4 inches away, this worked well for me.

You do need considerable clearance between any fine pitch and the component that requires the thicker solder, but it can be done, and No, it is not cheap. But if you need it, see what your Stencil house can do, it might surprise you.
Bruce Webster, Iridium Communications
We have an application where we go from a 5 mil overall stencil thickness and step up to 12 mils for a transformer (bell shaped pegs for leads). The stencil is etched. There is a 20-mil pitch IC (thus the 5 mil overall stencil thickness). We have had good success with this.
Doug Miller, Miller Electric
You hit the nail on the head with stencil Fab technology. The truth is that there is tolerance on most step foil fabrication methods. That tolerance can be as much as 20%. Obviously the thicker the step the more effect this has. Tannlin Multi Level doesn't suffer from this as we laser weld inserts into the blank. This means we can deliver 2% thickness control. Since we have introduced this product we have seen step foils get more and more radical as our customers see that we are actually in control of step stencil aperture volumes. Another advantage is that we can ramp up and down to allow the squeegee or the print head a smooth transition. 4mil steps are not uncommon now.
Fraser Shaw, Tannlin
Best practice is to try avoiding step stencils, there are better technologies in the market like foil types and coating technology which allow releasing more solder volume with the same thickness. This would help when there is a mix of large and small components. But when it's not an option and step is needed, then enlarging the apertures for the bigger parts in order to reduce step thickness to lowest possible would be the best. It's not recommended to work on thickness only and end up with big step thickness.
Labib Saleh, FCT Assembly
I agree with Jim 3 mils is off the chart, I would look into fab and Gerber to customize opening of apertures to make sure it matches and prevent solder bridges, you never know I've seen a lot of fab that doesn't match up with G editing plays a big part. Skies the limit.
Tuan Nguyen, Radian Smt Technology, USA
With regards to step stencils the warp within the stencil, limits and the print performance depends on the substrate material being used. There are specific materials on the market that are designed to decrease this warp and also generate consistent printing. Datum Fine Grain (FG) is a stainless steel specifically designed to eliminate these issues.
Peter Anniss, Datum Alloys, UK
We successfully produce 3 mil step stencils using a special stainless steel alloy. 1 and 2 mil step up and downs are very common - but we do see a 3 mil step once in a while.
Mark, Photo Etch Technology, USA

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