Step Stencil Setup

Phil
Welcome to Board Talk. You're here with Phil Zarrow and Jim Hall, the Assembly Brothers, talking about SMT process materials and all of that fun stuff.

Jim
Today's question is from M.K. When shifting from standard solder paste stencils to step stencils, should we change the pressure or attack angle?

I assume that would mean squeegee angle.

Phil
So, some shifty guy's stepping on somebody's stencils. That's not a very good best practice, is it? Well the answer is maybe yes, maybe no. As in the case of anything you're going to be basically setting up the to do, you have to do a design of experiment to ascertain the optimum settings on your printer.

And whether that's going to be any different or not depends. That sounds like somebody who's not doing their DOE properly. Doesn't it, Jim? One setup does all type of thing.

Jim
Phil, I think it's somebody else looking for the silver bullet, and in this situation, when using step stencil, just do this, and everything will be perfect. Wouldn't it be wonderful if the world was like that, but unfortunately, it ain't.

You should be checking pressure and attack angle, speed, and everything with every different stencil you use. Few people do that because their stencils are very similar in terms of geometries and size of apertures and so forth, but technically, you should optimize the printing by adjusting those parameters for every stencil and certainly, when you make a significant change, as M.K. is indicating, going from standard one thickness stencils to step stencils.

Phil
Usually attack angle is fixed, based on the type of machine you're using. But everything else, including your squeegee speed, pressure and everything else is part of the proper setup.

Do your due diligence.

Jim
And with all due diligence, Phil, there are newer printer machines that do advocate using adjustment of squeegee angle, and some of them, I believe, at one point, would actually allow you to adjust it in the middle of the stroke.

I certainly have never worked with one or never seen one, but I believe that I have heard a claim and maybe a tech paper advocating the desirability for changing squeegee angle, and dynamic squeegee.

Good luck, optimize your stencil printing processes as much as you can, and whatever you do, don't solder like my brother.

Phil
And don't solder like my brother whether you're doing step stencils or not.

A note on pressure. For me, it is a rare stencil that is not stepped, up and down. I approach each one in a similar fashion as I do a non stepped stencil. I adjust the pressure down to a point where is leaves a thin film, thin bring it up just enough to wipe clean. Keeping in mind you need to look into step downs to make sure they a wiping clean.

Of course you need to work on all the other variables, and check every thing with SPI. Just don't change too much at once.

If this is a reactive change of stencil, i.e. you have had issues applying paste to this product using a non-stepped stencil, then yes you will need to change pressure. Currently you would have the pressure and speed set to compensate for the incorrect thickness at the area which has been stepped.



If this is a new PCB, and the stencil has been stepped pre-emptively, then you will need to run trials as stated above.

Depending on your screen printer, you may consider utilizing on-board dispense capability if available. This solution will allow you to use a single thickness stencil 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 so paste and glue can be applied to the same board if necessary. This eliminates complexity in the stencil design, programming and print head.

The Yamaha YSP printer does allow you to adjust the Squeegee angle from 65-45 degrees (3-S Print Head). Printing experiments have shown that by decreasing the angle you can apply more downward force to the material thus helping fill the aperture. Combine this with metal blades that are half etched along the edge and you will get a better clearing (shearing) of the paste out of the pocket stabilizing the variation in volume from print to print. The angle is controlled by servo-motors and is set in the software.

Many of our customers will switch to polyurethane squeegee blades when printing with stepped stencils. I believe that 80 durometer is the hardness that has the most success. The polyurethane blade will contour to the shape of the step, getting down inside recessed areas of a stepped stencil to wipe it clean so that solder & flux residue won't build up and cause problems on subsequent print cycles.

We have recently had several customers come to us who are using "stepped up" stencils, and the polyurethane blades work very well in these applications as well. The poly blades will also contour to the shape of the raised step, and provide a better wipe in these situations. Squeegee speed and pressure adjustments are generally needed when switching from metal to polyurethane blades. Hopefully the board/stencil designer took into consideration the real estate that is necessary surrounding the stepped stencil area so that adjacent components and their stencil printing requirements won't be affected by the stepped stencil. When printing with a stepped stencil, the squeegee blades needs a certain amount of time and space to react and adjust back to the non stepped portion of the stencil.