Cutting Mylar Solder Stencils from ExpressPCB and EagleCAD Files
In this post I’ll be going over how to cut out solder stencils for both ExpressPCB and EagleCAD designs on a laser cutter. I recently had a need to create some stencils from both of these programs and spent quite a bit of time at the Dallas Makerspace working with their laser cutter to get the settings just right. I’ll be cutting some designs that are mostly 0603/0805 package sizes, so don’t expect ultra-tight pitch stencils from this technique!
What You’ll Need
- ExpressPCB or EagleCAD Software (If you’re doing any sort of serious PCB fabrication, or design really, then please stick to EagleCAD/Altium/OrCAD)
- ViewMate Gerber Viewing Software
- PDF Printing Software (such as PDFCreator)
- SketchUp, AutoCAD, or some other program that can view and edit .dxf files
- Laser Cutter – The one at the space is a Full Spectrum 120W CO2 model.
- Mylar Sheets (I used 5 mil sheets purchased via Amazon)
The technique that I chose to cut these stencils is to use two sheets stacked on each other to create the stencil. The first sheet is your actual stencil, and the second is a sacrificial sheet which will catch the melted pads of the first sheet and allow you to pull them off cleanly when you separate them. The trick here is to expose the mylar to just enough heat so that the first sheet cuts clear through, but the second doesn’t. You also don’t want to apply too much heat since that will cause small sections of the mylar to melt.
Exporting Files From PCB Design Software
The first step in the process is to export your design from your CAD software. If using EagleCAD, use the cam file here to export your top and bottom cream layers (aptly named tcream and bcream) just as if you were exporting gerbers for manufacturing. If using ExpressPCB, use the “Export DXF Mechanical Drawing” option under the File menu.
Swelling Pads Using ViewMate
The next step only applies if you’re using software which exports gerbers. In order to compensate for melting that may occur during the cutting process, we need to reduce the size of the pads being cut out. Import your tpaste and bpaste gerbers into ViewMate by selecting File>Import>Gerber.. and choosing your files. I recommend only working with one layer at a time. Once opened, go to Setup>D Codes. In this menu, select all columns in the list by pressing Ctrl + A. Once selected, go to Operations>Swell… and type -0.008 in the text box. What this will do is make all pads 0.008 smaller! Feel free to adjust this value to suit your laser. Keep in mind that if you scale down, print, then try scaling down again, the final result will be a sum of the two scales (-0.008 -0.002 = 0.010). After you’re finished, print out your gerber to PDF and save it for future use. Use the following settings in ViewMate to make sure you don’t accidentally get the scale mixed up!
Swelling Pads with AutoCAD
If you’re using ExpressPCB, sometimes you have to use some sort of software which will open DXF files and let you scale them. I’ve found that there are instances where using a scale factor is not necessary, but in case you do, be sure to follow these steps. In AutoCAD, just select the entire drawing and then type SCALE. This will bring up a dialog asking for the scale factor. Type in 0.992 and press enter. This should result in a properly scaled drawing! Once the drawing is complete, it’s time to print out the final drawing to a PDF. The settings I used are:
Cutting the Stencils
The secret to cutting mylar stencils is to stack two 7mil sheets together and cut them at the same time! Unlike Kapton stencils, the Mylar will melt if exposed to large amounts of heat. I used a Full Spectrum 120W Laser Cutter available at the Dallas Makerspace to cut my stencils, so my settings will be specific to it. If you’re using a different laser, my settings should at least give you a starting point.
You’ll notice that my power setting is only 2% and Vector Current is only about 40%. This is because as I mentioned above, we want to cut through the top layer without making our way completely through the bottom layer. These settings worked exceptionally well for me, and hopefully they’ll work for you! Here are what some of my stencils look like:
I hope this tutorial was useful! Try out my settings and configuration and let me know if you find any issues!