I have a couple of boards with an SSOP-28 package. The pads are 0.48 wide with 0.65 spacing.
For one boards I got a steel stencil from the PCB manufacturer, 0.12mm thick. This works nicely and lays down thin strips of solder on the pads. I can get it right first time and every time.
For the other board I got a 5mil (0.127mm) polyamide stencil from OSHStencils. With this I get too much paste on the pads, trying over and over different amounts of angle and pressure with no improvement.
Comparing the two stencils there is a small but noticeable difference in the gaps between the pad apertures with the steel stencil having a slightly larger gap.
I like the lower cost of the polyamide stencils but how can I handle this apparent difference? Should I be post-processing the paste Gerber file before sending it to OSHStencils to reduce the aperture sizes?
Have a look at: Pcbnew / File / Board Setup… / Design Rules / Solder Mask/Paste Solder paste absolute clearance
If you change these values, then KiCad may complain a bit, but you can ignore that, and it should do what you want.
What do you mean with:
I guess that with “gap” you mean the steel (or polyaminde) that is left between the pads, and then the aperture holes in the steel stencil are smaller because the gaps are larger?
I’ve never ordered a stencil myself, but my guess is there are no rules of thumbs for this and it is very dependent on where you order your stencils.
The best you probably can do is to make a guess of how much paste you have too much on the pads, and then calculate some clearance from that, and then order your next stencil from the same manufacturer.