SM stencils

From AS220Labs

Creating Surface Mount Soldering Stencils on the Laser Cutter

This is work in progress by Paul Badger on creating surface-mount solder stencils on the laser cutter

Facets of the problem

Materials

Some commercial stencil makers use a plastic called Kapton. This is somewhat expensive and hard to obtain. For finer pitch parts though, this might be the way to go. I have found that mylar is an acceptable material for stencils on medium pitch sm parts (eg TSSOP). The thickness of the material is still the subject of some experimentation. One commercial supplier of stencils (Pololu) suggests 3 mil material for finer pitch sm parts. I have successfully used 5 mil material, which has advantages and disadvantages, some of which I am still experimenting with.

My first experiments have been carried out on Duralar, which is described by the manufacturer as "Oriented Polyester Film". Mylar is also polyester film, so other brands of mylar (actually a trademarked brand name of Dupont) will probably work fine too. You don't want acetate though, which is brittle, and tears.

Duralar is available at Utrecht Art Supplies at 200 Wickenden Street, under the brand name of Grafix. I recommend the matt version. They sell 3, 4 & 5 mil, all of which will work, but will probably require some various tweaks. We are interested in your results if you do this and have success or problems to report.

Cutting the Stencil

There are two main problems in actually making the stencil

• Getting the Gerber file into the correct format for laser cutting
  • CAD programs (and pcb board houses use Gerber files)
  • The FabLab uses Inkscape (Linux) to drive the laser cutter

• Modifying the file to get the apertures to cut at appropriate sizes
  • Cutting on the Gerber outline will add half of the width of the laser beam (or more) to the desired aperature.

This is really quick version of the process, which I will expand more in the following weeks.

• Save your Gerber files in your preferred CAD program. I have only used Eagle files but others should work well too.

• Open the files in gerbview (Linux only) and save the files as .csv files
• Open the .csv files in Inkscape

To work with the laser printer driver the objects need a .5pt line and no fill, so do this: Select all objects, and add a .5 pt bright red line, while setting the fill to "none".

The "inset" feature of Inkscape does not work properly or your life would be easy now.

• As a workaround (also with more flexibility) I do this
  • Reduce the width of vertical IC pads from 50-70%
  • Reduce the height of horizontal IC pads from 50-70%
• Finally print to the laser cutter using the "mylar" setting