Guide pins on smd solder mask.. #pcbgcode

peterg1000
 

In order to properly locate a stencil over an etched circuit board I have placed circular smd pads at appropriate places on the board.  Holes to accommodate guide pins are placed centrally on this pad.  When the matching stencil is generated it is  square and slightly larger than the original smd pad.  Two questions arise, firstly - is it possible to generate circular holes in a stencil, and secondly - is the oversize intended?

After a lot of checking it would seem that in the case of my circular pad, the final hole ends up half a tool diameter larger than the original pad size.  Viewing the cutouts for other devices on the board, they would appear to be similarly oversized as well.

For example I placed an smd pad 0.05" diameter on the circuit board and the coordinates on the stencil for the sides of the matching square hole Gcode turn out to be 0.04413 in length.  With a tool diameter of 0.01182" (3mm ) I would have expected this to be 0.050" -  0.01182" (i.e.0.03818" ).  Curiously enough this is almost exactly half a tool diameter smaller than the actual value. Is this intended, an error in calculation or just co-incidence ?

Peter Harrison

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