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Streaking at Screen Printing

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Hello, I am seeing something new to me and I am hoping so... - Dec 15, 2006 by kvanzill  

#46181

Streaking at Screen Printing | 15 December, 2006

Hello,

I am seeing something new to me and I am hoping some one some where may have resolved this before. I am running a Panasonic SPF63 screen printer with Tamura (RMA-012-FPL) solder paste. Now this problem is hard to describe but I will do my best. I also am using a metal squeegee blade. The problem is after say about 20 prints I begin to get a build up of what looks like a metal on the blades which is the solder paste but it is become a harder solder ball and some what shiney. What this does is cause streaking where it is on the blade keeping the blade from contacting the stencil evenly. We original chased this as a machine issue and replaced cylinders and pressure regulators. I also changed the parameters in the beginning and which it would make a difference for maybe an hour and it would return. Also I am able to rule out the paste since we have 9 reflow lines and run the same paste on 4 other lines without the problem. Has anyone ever seen this type of an issue? I know I have not. I have taken these deposits and run it through the reflow oven and in which it does reflow so I am certain it is the paste.... Please help if you can.

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#46182

Streaking at Screen Printing | 15 December, 2006

Try changing blades. If still a problem, move job to another printer and cross feed to your existing line. If all else fails, try buying a new stencil. You could get new by Monday.

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#46183

Streaking at Screen Printing | 15 December, 2006

Thank you for the suggestions,

I did change blades and we did try another printer, problem still exists and it is across several different products but only that line. One thought is aperature size since we over print for other reasons due to part design, but my question is have you ever seen a metal looking ball form on squeegee blades in your experience?

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dougs

#46185

Streaking at Screen Printing | 15 December, 2006

is the blade angle the same on the problem printer as on the others?

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#46191

Streaking at Screen Printing | 15 December, 2006

Yes we see this also.......

It is what they call "cold welding" of the solder paste to the blades......

The solder spheres actually "weld" together......

It leaves streaks on the stencil, not a clean wipe of the stencil.......

We have never found out WHY this occurs........

If the steaking gets to bad, we have the operators "scrape" the blade to get the metal particals off and continue printing......

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#46206

Streaking at Screen Printing | 16 December, 2006

"I did change blades and we did try another printer, problem still exists and it is across several different products but only that line."

Not sure what that means since you reference "only that line" in the same phrase as "did try another printer". Do you mean that no matter which printer you run the job on, the results are the same? Are you moving the blades with the job too?

Regardless, I'd look at blade and stencil condition and thickness. If your blade is thick (stiff) but worn (no edge), you're at risk of it skidding/skimming and not shearing the paste from the stencil surface. I would think squeegee speed, might also have an effect. If your stencil is worn/striated or dented it may leave room for powder to get under the blade edge.

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#46211

Streaking at Screen Printing | 17 December, 2006

While you use this paste on several lines without problem, what is it about the paste used on this line that's different?

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#46212

Streaking at Screen Printing | 17 December, 2006

Paul

We've seen "cold welding" of solder paste when dispensed improperly. It occurs in the narrow confinds of the nozzle when someone tries to pipe too much paste, too quickly. The friction of all those little solder balls and paste juice fighting to be the next one to squirt out of the nozzle just becomes too great and "ploop" everything stops.

That aside, it sure sounds like cold welding, as you say. If that's so, it would be interesting to understand the cause better.

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#46213

Streaking at Screen Printing | 17 December, 2006

Well,

We have several lines some products run Kester 256 and others run the Timura. I have changed the product to print on another printer and it follows. We have several products that use the same paste and similiar board design that has a problem on the same line(we only run key pads on that one line). I have changed blades and same results on a new blade. The print angle is the same on all lines so im not sure that would have anything to do with it. We do not change angles. I just never have seen it before and we think maybe due to the board having large areas open in the board(openings where no pcb is) that maybe the stencil flexes in that area. We do not automatically depense it is actually in jars to it is placed on the screen using manually.

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#46253

Streaking at Screen Printing | 18 December, 2006

What happens when you take a jar of paste from a line that doesn't have this problem and use this paste on the line that has the problem?

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#46255

Streaking at Screen Printing | 18 December, 2006

We still have the same problem, I also tried a different lot number of paste as well.

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#46272

Streaking at Screen Printing | 19 December, 2006

I'm not familiar with that printer so not sure of the support config., but can you plug the openings in the product with some pieces of scrap board (not likely on support pins but workable on a plate) and look for a difference in performance?

Does the location of the buildup coincide with the voids in the product or occur adjacent to them?

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#46273

Streaking at Screen Printing | 19 December, 2006

Oh sh*t, we have the same "problem". But it's not really a problem. Depending on how thick your stencil is, and at what speed you run your squeegee blades at, you will get this deflection of the stencil. Age of the stencil is also a factor.

You can minimize this by lowering you squeegee pressure, but again, depending on how thin/thick your stencil is it may never go away.

Now when your operator gets naked and goes streaking at the screen printer - that's a problem.

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#46276

Streaking at Screen Printing | 19 December, 2006

Totally dependant upon the operator.

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#46284

Streaking at Screen Printing | 19 December, 2006

The streaking or the streaking?

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#46285

Streaking at Screen Printing | 19 December, 2006

As to whether or not your pass/fail criteria have been met.

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James

#46286

Streaking at Screen Printing | 19 December, 2006

Let me get this straight. You have large apertures for some reason? If this is the case you could try making the aperture like a screen, many small apertures to support the blade, rather than one large aperture that allows the blade to deflect.

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