Extruding a paste layer 0.4 mm thick takes ten lines of code. What comes off the printer is in pieces, with walls the nozzle never laid down and openings closed up. Here is what sits in between.
A row of pins at 0.5 mm pitch
A QFP, a connector, any fine-pitch part.
The file
Five openings, with 0.2 mm of plate between them.
Extruded as drawn
does not printOnly 0.10 mm of plate is left between two openings. Your nozzle is 0.40: the slicer erases those walls and the five deposits become one.
What I punch
printsA single opening, narrowed to carry the same amount of paste. No wall left to print, and reflow separates the deposits.
A ring of paste around a connector
The design deliberately leaves the middle without paste.
The file
An open ring, and a disc of plate in the middle.
Extruded as drawn
does not printThat disc is attached to nothing. The stencil comes out in two pieces, and the second one stays on the bed.
What I punch
printsFour bridges the width of your nozzle hold the disc. They cost 10% of the paste on that joint, and the stencil comes off in one piece.
Every one of these breaks a naively generated stencil. None of them shows up on screen before the print. I handle them all, without asking you anything.
Before there is any geometry to speak of.
There is no universal convention. Some exports name nothing at all. Someone has to decide which layer is the paste, and be right.
layer detectionNothing says which one is current, and mixing two of them produces a stencil that matches no board at all.
mixed archivesSome tools do not place an opening, they draw it stroke by stroke. Miss that and the whole face comes out empty.
stroked exportsA file can place the same opening twice, in two different forms. That alone defeats everything that reasons about spacing.
duplicatesWhat the outline says, and what it leaves out.
Exports leave micro-gaps and T-junctions at the corners. An outline that does not close has no inside, and the frame that holds your stencil cannot be built from it.
board outlineA panel is not one big board. Treated as one, it gets a single frame and no alignment where it matters. I build one plate per board, each with its own shoulders.
panelsAn export can carry pads beyond the outline, left over from a panel or a stray footprint. Punched as they are, they would open holes in the frame that holds the stencil. I clip them and say so.
outside the outlineSome exports carry no paste layer at all. Rather than hand you nothing, I derive the openings from the solder mask and tell you I did: it is an approximation you should check before printing.
solder mask fallbackWhat becomes a hole, and how big.
A 0402 pad is barely wider than the bead your nozzle lays down. Printed as drawn, it closes up and takes no paste at all. I widen the small ones just enough, and leave the rest alone.
nozzle compensationBetween two neighbouring pads there is sometimes a tenth of a millimetre left. Your slicer drops it silently, and the two pads become one puddle.
fine-pitch rowsA fine-pitch BGA has no printable wall between its balls. Shipping its openings as they are means shipping a short circuit.
2D gridsA thermal pad is often drawn as nine squares with thin gaps between them. Those gaps are thinner than your nozzle, so the print smears them and the pad comes out ragged. I replace the checkerboard with a single, smaller opening carrying the same paste.
thermal padsWhat decides whether the print comes off in one piece.
A ring of paste around a via is deliberate: it keeps the paste away from the hole. Several passes would happily fill it back in. I keep it, and warn you when a pass has closed one anyway.
intentional holesA ring of paste leaves a disc in the middle attached to nothing. The stencil comes out in two pieces and the second one stays on the plate.
detached islandsThe back of a board is seen from underneath. A back stencil that is not mirrored lays every deposit in the wrong place, and nothing on screen tells you before the print.
back faceLining it up by eye means laying the paste next to the pads. Alignment has to come with the part, fitted to the real thickness of your board.
registrationEvery decision in the engine was settled against real boards, not textbook cases. A rule that works on a test case and breaks on a real board does not get in.
real board files replayed on every change to the engine
families of traps handled before you ever see the result
settings to know for it to work. You drop your file.
You have no use for this page. Drop your export, pick your thickness, take your file. Everything above happens during the few seconds of the progress bar.