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paste release from stencil, and volume calculation

Steve Thomas


paste release from stencil, and volume calculation | 7 November, 1999

We're looking at redesigning our stencils to facilitate better (well, it's pretty much non-existant, so any improvement is a big leap) release from fine pitch apertures.

Currently we have perpendicular walled laser cut apertures. We'll be having them cut with trap. walls from now on, and possibly oval shaped. Is the oval necessary, and if so do I need to worry about loss of paste volume? Is the comment from the manufacturer that the laser with O2 introduction is so good that I don't need electropolishing possible, or is he blowing smoke? Can someone get me (or point me to) an equation for calculating paste volume requirements based on pad size, lead size, metal content, etc.? Can you tell I'm new to fine pitch?

Also, is there a preferred method (short of spending lots of money on a new printer, or adding load cells) for setting squeegee pressure on a semi-automated machine that only gives dimensional info. and nothing with respect to squeegee force on the board? All the industry lit. refers to lbs. per inch of blade. The tech. support guy from the machine manufacturer recommended feeling the pressure by hand. I'm afraid I'm not there yet. Even a guesstimation of how hard it is to pull out a piece of paper from between the board and the stencil would be better than what I have now.

Thanks in advance for any help you can give me. Maybe someday I'll be able to contribute, but for now I'm gonna be a sponge.

Steve Thomas

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Re: paste release from stencil, and volume calculation | 9 November, 1999

Steve: Addressing your points:


Electropolishing Laser Cut Stencils: Laser cut should not requiring polishing. Geesh, how much money do you have for stencils? ;-) Oval Shaped Apertures: Pass. What do you expect to gain from the shape change? Equation: There is no equation for calculating paste volume requirements based on pad size, lead size, metal content, etc. You are doing exactly what you should be doing ... looking at the SMTnet archives to understand others� "rules of thumb" for sizing apertures, so that you can get in the "ball park" when sizing your apertures.


Not a new topic on SMTnet. One example is: Squeegee Pressure - Bill Haynes 10:44:02 02/12/1999

You�re printing correctly when:

� Paste at least as big in diameter as your thumb is rolling across the stencil � Paste is filling the apertures � Squeegee pressure wipes the top of the stencil of clean � Paste releases from the stencil � Paste is on the board in nice bricks


It�s a bit concerning that you are getting poor paste release from laser cut stencils. Let's understand that situation better. Tell us about your paste, paste handling, and stencil aperture design.


Dave F

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Re: paste release from stencil, and volume calculation | 9 November, 1999

1 Money IS an object, and my boss will be thrilled. The stencil manufacturer recommended ovals to facilitate better release. Seems likely to be true, but whether or not it's necessary I don't know. We have perpendicular walls on these (6 mil) stencils, but will be going to traps. for the next generation. We don't know if this will solve all of our release problems.

2 I would swear I'd seen that formula somewhere, and my intent was to avoid drastic undersizing. If we don't go with ovals, I won't worry about it.

3 There is a veritable cornucupia of info. telling me how to tell if my printer is set up right. There is very little telling me what to do if it's not. As I have only been working on this process for a couple months (and only one day a week for fine pitch to boot), I have yet to see how it is SUPPOSED to work. I only see what happens when it's not going well.

I won't even get into how we handle paste here. It's a known issue that I'm doing what I can about, but one day a week on this process isn't getting much time to deal with it. I work Fri. through Mon., and they only run FP Mon. - Thurs.

Thanks for your input....I'll be back, I'm sure.


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Re: paste release from stencil, and volume calculation | 13 November, 1999

First, let me say what I always say: a good chem-etched stencil will print fine pitch. Chem-etch brings trap to the side walls because its a physical by-product of the process. Because they are less expensive you can add electropolish and still save money over laser cut. Chem-etch also rounds the corners of your apertures eliminating paste binding in the corners which can inhibit release, you don't even have to have the discuss about oval shaped apertures.

Regarding your formula, I'm kind of old school, test it, don't calculate it. Dave is right with his observation of good technique. Don't worry about what your squeegee pressure is, worry that once you find the right setting, that it is repeatable. A good auto or semi-auto printer will be repeatable. There are too many variables to dial in the process off the label on your paste jar. Add/subtract pressure until it prints, how many lbs/ is not relevant, which is why most printers don't measure it.

Sounds to me like your paste is the problem. Get a couple of fine pitch paste formulations from your current vendor and his competitors and try them out. Use a metal squeegee blade.

Do a design of experiment, printer perameters and paste types. Find a measureable criteria for a successful result, weight, volume, deposit geometry, solder filet characteristics, whatever you use for your coarse pitch should hold true to the fine pitch as well.

Do the testing, design the process and your done. Good Luck. Dan.

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Re: paste release from stencil, and volume calculation | 13 November, 1999

Thanks for the input, Dan.

The reason I'm asking for so many specifics (instead of just jumping in and testing) is that the process was wildly out of control when I inherited it and I'm trying to find some sensible starting points (hence looking for pressure setting info since the industry lit. refers to it as such) just so I know I'm not way off base. I appreciate your input, though. Everything you've said makes good sense. Not that I have the knowledge yet to refute any of it, you understand. ;-)

I thought chem etch gave you an hourglass( >< ) sort of shape. Does the electropolish pretty much eliminate the pinch point? I just figured it was another possible cause of our problems, and since we needed a new stencil anyway.......

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Re: paste release from stencil, and volume calculation | 15 November, 1999

Steve: Continuing with Dan�s thinking, generally, "poor release" (paste hanging-up in the stencil holes) could have three main (or combination of) causes:

1 Equipment: Stencil is too thick. Specifically, your 0.006 should be OK, providing you don�t violate the ratios in another post on this thread too wildly. 2 Process: Snap-off distance is too small. 3 Material:

� Solder paste viscosity is too high � Solder paste is too tacky � Solder paste particle size is too large

Good luck

Dave F

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Re: paste release from stencil, and volume calculation | 15 November, 1999

Steve: There are guidelines for determining if your aperture has the correct proportions to the foil thickness. Maybe that�s what you�re looking for:

1 Aspect ratio = aperture width/foil thickness Chemically etched SB GT 1.5 Laser cut SB GT 1.2 Electroformed SB GT 1.1 2 Area ratio = (aperture length x aperture width)/(2x(aperture length + aperture width) x foil thickness) SB GT 0.66.

Good luck

Dave F

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Re: paste release from stencil - recommendations | 19 November, 1999

Steve, your comments from your stencil vendor are very interesting. Any decent laser cutter automatically cuts a trapezoidal aperture. This is just the nature of the cutting. We have a bunch of lasers (and chem-etch). You have to polish your laser cut stencils. No matter what kind of laser or whether it is air or oxygen fed makes no difference (we do both and see that polishing is critical). We include polishing with all stencils (as do most vendors now).

The first thing you have to look at is the aperture widths for your fine pitch components and the metal thickness on your stencils. Your stencil vendor should be making these recommendations to you.

A sample recommendation would be to use 6 mil laser cut stainless steel (electropolished) and have the apertures 12 mil wide on the board contact side of stencil.

There are a number of "standard mods" that should be done for fine pitch stencils. If you make all the modifications and are still having problems, I recommend looking at your room temp and humidity and talk to you paste vendor.

Good luck.


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Re: paste release from stencil - recommendations | 21 November, 1999

We're going to 5 mil foil from 6 mil, with a 10 mil aperture, radiused corners. Supposedly going to traps. from right angles, but who knows?

We've got temp. controlled at ~73F, but no RH control. We are going to also try out a more humidity tolerant paste.

I'll pass along your comments to my stencil rep., who'll be here Monday to observe the process. That'll be interesting.

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Re: paste release from stencil - results | 27 November, 1999

The new design resulted in much better release characteristics. We are quite satisfied. By the way, he says he can cut dead nuts 90 degree walls on the apertures, or traps. I didn't ask what the wall angle was, nor have I seen any recommendations, but I'd be interested in knowing if there is a suggested best case.

He says he uses a proprietary mixture in his gas introduction, and the plume is much larger than what is normally seen due to the increased vaporization of the waste products. Whether it's really pertinent or not, I sure like the results.

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