Tips & Tricks: Generating Stencil Tooling

Estimated reading time: 2 minutes

Many engineers are leaving the editing up to the stencil fabricator these days, describing the PCB array, aperture undersize/oversize, shape conversions, etc., for the fabricator to then edit. From the outside, this may appear as a time saver for all of us overworked process engineers, but considering how many stencil redos have been required and how many processes that have run “sort of OK,” there’s a tremendous amount of scrap and rework that could be saved from just a little more attention paid to stencil tooling. I prefer to make my own mistakes rather that receiving a stencil (with the project due tomorrow) and finding that someone else didn’t do something right, or misunderstood my design intent.

It’s a huge advantage for the experienced engineer to be able to produce good, first run results most, if not all, of the time. You can learn what works best for your equipment in your environment. There are several Gerber editing tools available that will let you create a library and apply changes semiautomatically or automatically to your data.

One of the most common edits used for resistors and capacitors is the “home plate” design. The left aperture below (magenta) is a typical aperture reduction. The right aperture below (cyan) is the home plate pattern. Black being the component outline and terminals, and red being the original pads.

Figure 1: Home plate design.

This is a very useful aperture design for mitigating solder balling and tombstoning. It can also be a difficult shape to describe, and convey where to apply it, to a stencil fabricator. Some large connector pins may benefit from oversizing the aperture to improve mechanical interconnect and ease inspection. Large heatsinks, such as those for DPAKs (Decawatt Package), can be divided into multiple apertures to reduce squeegee scooping and voids. Bowtie patterns can reduce solder balling on large gull wing pins.

I position the print image within my foil perimeter in the data, depending on the assembly type, and indicate the location for the stencil ID. My operators like the image forward of center for easier access, a better visual in process, and less ambiguous installation. We like to put both images of a double-sided board on the same stencil to reduce change over time. The distance between the two images needs to match your printer’s blade span and over travel distance.

Editing your own data also allows you to eliminate the check plot, further reducing your design cycle time. It also reduces the fabricator’s editing time, further improving their turn time.

Rather than spending time generating a document to describe the edits to your stencil fabricator and hoping they’ll get it right, make the edits yourself to get what you intended and improve your process.