Cutting Mylar

[Scott Rise]

Does anybody have any tips on cutting Mylar (2 or 5 mil) with a CO2 laser? I'm trying to make stencils for putting solder paste on circuit boards. The holes are fairly small and when I try cutting the edges get all melty. There seem to be lots of other folks out there doing just what I'm trying to do so I'm sure there's some trick that I'm missing. Is it maybe because I don't have air assist?

Thanks,
Scott

[Mike Null]

Scott

Experiment with your settings including frequency. You should be able to get a smooth cut.

[Scott Shepherd]

I agree with Mike, we do them for a customer on a regular basis. It takes very low power and air assist. You don't have your laser brand listed in your signature, so I'm not sure what to recommend for you, but it'll work. The problem arises when the squares and rectangles get too small. When they are super tiny, then you almost can't stop the melt because all sides are so close, but we're talking sizes where the box isn't much larger than 2X beam width.

[Scott Rise]

I've got a Jinan Artsign JSM-40. It's pretty low end, so no air assist or frequency control. I can pretty much adjust power. Hmm... Maybe I can add air to it.

The squares are around 1mm.

[Scott Shepherd]

I don't know if it's possible without air, I've never tried it, but I've had trouble in the past with using too much power. My system power goes from 1-100, and I'm using .8 (point 8) for power, so it's at the very bottom end of my system.

[Robert Ray]

I cut it all the time, yesterday in fact. On my 30 watt system I use 200ppi, 6% power, and 100% speed, with air assist. This is for 5 mil.

The parts have no attachment points, but stay on the sheet, then they just pop off the sheet by poking them with tweezers. (Z Scale window glazing)

-Robert Ray

[David Harvey]

Does anybody have any tips on cutting Mylar (2 or 5 mil) with a CO2 laser? I'm trying to make stencils for putting solder paste on circuit boards. The holes are fairly small and when I try cutting the edges get all melty. There seem to be lots of other folks out there doing just what I'm trying to do so I'm sure there's some trick that I'm missing. Is it maybe because I don't have air assist?

Thanks,
Scott

Scott,
Raster instead of vector cutting. Use a high power, low speed & 600 dpi and it will give you sharp edges and no melting.

[Richard Rumancik]

. . . I'm trying to make stencils for putting solder paste on circuit boards. The holes are fairly small and when I try cutting the edges get all melty. . . .

Scott, I doubt that there are lots of people making solder paste stencils. Maybe other kinds of stencils, where the edge quality isn't quite such an issue.

As you realize, for a solder paste stencil you need to ensure there are no ridges, as that will interfere with the paste application. But I really don't think you can get decent openings using vector cutting. When I vector cut Mylar, I always get some amount of ridge - but for solder paste even a few thou is too much.

I agree with David; you really need to raster these to have a chance at making it work. That will avoid overheating and improve the corners - you must have some pretty ragged corners (at least thats what I experienced when trying to cut .03" x .06" rectangles.) Even with raster, you will experience some rounding on the corners but this should be tolerable for solder paste.

The other advantage about using a bitmap image instead of vector is that it is easier to tweak the sizes of the apertures. I used PhotoPaint to reduce the size of all apertures by a pixel or two all around, to compensate for the beam. This would be difficult to do in vector format with the software I have. You can take the vector image and convert it to bitmap in Corel. If you use 1000 dpi it makes for .001" per "dot" which is rather convenient. I think I designed with 1000 dpi but lasered at 500 (.002" per dot.) Keep in mind that your laser beam is probably > .003" diameter.

You may have to do some careful calibration of your laser to make sure that your stencil is in scale with the actual board. Probably good to ask the customer for a board to test with, until you get the procedure down pat. I have found that my x axis plots 99.78% of what is should be. Sounds like not a big deal, but it is for many of my projects. On a 24" part I am almost .060" short. Depending on your laser and software you have to figure out where you want to compensate, if needed. In my case I create a laser machine file that is stretched to compensate. (i.e. stretched vector image 100.22% then convert to bitmap)

I tried to develop a process that worked but I did not find a lot of interest from my customers and eventually dropped work on this.

The people at pololu.com seem to be making mylar stencils successfully . . . one of my recent customers referred me to them, and I had to say right off that I could not make a solder paste stencil for $25. That would pay for only 15-20 minutes of work including front end artwork preparation. I suppose if you had enough continuous volume and a streamlined process with appropriate software, you might be able to batch several jobs and do this. But it does not sound lucrative at $25.

There are companies out there making small proto PC boards for $50 so I suppose the designers are expecting very low cost stencils as well.

[Scott Rise]

I've pretty much been trying vector. I'll give raster a shot.

Fortunately I'm the customer here (they're my boards; I'm just trying to speed up production) so I don't need to worry about profitability.

Thanks for all the suggestions. I'll go play around some more and let you know how it goes.

[Scott Rise]

You guys rule. Doing raster worked pretty good (in my brain vector made more sense but I can't argue with results).

Thanks again!

[Andrea Weissenseel]

On my 30 watt system I use 200ppi, 6% power, and 100% speed, with air assist. This is for 5 mil.

The parts have no attachment points, but stay on the sheet, then they just pop off the sheet by poking them with tweezers. (Z Scale window glazing)

-Robert Ray

I just tried 0.127 mm (5 mil) Mylar and got a pretty good cut at 45 power, 100 speed with 200 ppi on my 40W laser. Can settings differ that much ?

Andrea

[Rodne Gold]

Andrea , it depends on the model , the speeds and power figures on various models of the GCC machines , like mercury , explorer , explorer II , spirit etc do not directly translate.