How to CNC Mill a Solder Paste Stencil
Sales have been good the last few months and so we needed to step-up production. Sure we could have purchased a solder paste stencil. However, we are makers and we like to be creative. So we thought we would show you How To CNC Mill a sheet of 5mil brass to make your own solder paste stencil. The stencil we are milling is for the TrueRNG V2 Hardware Random Number Generator.
We purchased the 5mil brass from Amazon but you can also pick it up from McMaster. We also picked up a piece of Corian ® (countertop material) which we milled into a jig to hold the PCBs. Then we used Duct Tape as a hinge which provided unexpectedly consistent results.
Since we were using a V bit, we milled the brass from the back side so that the side that faces the PCB is slightly wider than the top of the stencil. This allows the stencil to be lifted without trapping the paste. We made the holes about 3 mils smaller than the pad size. We were a little concerned about the pads on the SOT23 parts being a little too small but it actually wasn’t a problem at all.
Although the 5mil brass got the job done we would feel better with a little more paste on the board. However, the brass is hard to find in thicknesses other than 5mil and 10mil. SO, we’ll be experimenting with other material such as aluminum. We hope to find some off the shelf material that is about 8mils thick from one of the local hardware store.
Hi, thanks for the video. Do you mind to explain how you generated the gcode? I tried to invert the gerber file via the %IPNEG*% command right at the beginning. Gerbv shows it nicely inverted but FlatCAM seems to ignore the command. So I can’t use it to generate the outline (or better inline) paths. Any help/advice welcome
In tool diameter use a negative value. it will then cut it from the inside
You can also make the holes slightly larger if you want more paste. The paste does a good job of flowing together onto the pad, so even if your paste is larger than the pad it will all still flow onto the pad. Plus the larger your hole the less likely the paste is to stick. 5mil should work fine.