3D Printing Solder Paste Stencils from Gerber Files with Go

When assembling surface-mount (SMD) prototype printed circuit boards at home, applying solder paste by hand with a syringe is tedious and messy. Laser-cut stainless steel stencils from PCB manufacturers are high quality, but they take 4 to 7 days to arrive and cost extra shipping for what might be a single throwaway test board.

With modern 3D printers and a 0.2mm nozzle, you can print a high-precision 0.16mm thin solder stencil on a smooth PEI sheet in about 12 minutes.

However, CAD tools output PCB paste masks in RS-274X Gerber format, not 3D meshes. Most existing conversion scripts rely on heavy Python dependencies, OpenSCAD rendering (which takes minutes to compile CSG booleans), or clunky online tools.

I wrote pcb-to-stencil in pure Go to generate slice-ready binary STL meshes directly from Gerber files in milliseconds.

How Gerber Files Work

Gerber RS-274X is a vector format developed in the 1980s for photoplotters. It defines:

• Apertures: Shapes like circles (C), rectangles (R), obrounds (O), and complex macros (AM).
• Commands: D01 (draw/interpolate with shutter open), D02 (move with shutter closed), and D03 (flash current aperture at X,Y coordinates).

For a solder paste mask layer (.gtp or .gbp), almost every SMD component pad is a discrete flash (D03) of a specific aperture dimension.

The Conversion Pipeline

Instead of running heavy constructive solid geometry (CSG) boolean subtractions in 3D, pcb-to-stencil uses a raster-to-mesh optimization:

1. Parser: Reads RS-274X commands, handles coordinate scaling (metric / imperial), and parses standard apertures and custom macro definitions.
2. High-Resolution Rasterizer: Renders the stencil frame as solid material and subtracts pad flashes onto an internal 2D grid at 1200 DPI (approx 21 microns per pixel).
3. RLE Mesh Generation: Converts solid pixel spans into 3D rectangular voxels using Run-Length Encoding (RLE) to drastically merge adjacent triangles along scanlines.
4. Boundary Walls: Adds optional alignment walls around the PCB boundary so the stencil snaps directly over the board without sliding.
5. Binary STL Export: Writes standard binary STL triangles directly to disk.

// Fast binary STL triangle writer
func writeSTLTriangle(w io.Writer, normal, v1, v2, v3 [3]float32) error {
    var buf [50]byte
    // Normal vector
    binary.LittleEndian.PutUint32(buf[0:4], math.Float32bits(normal[0]))
    binary.LittleEndian.PutUint32(buf[4:8], math.Float32bits(normal[1]))
    binary.LittleEndian.PutUint32(buf[8:12], math.Float32bits(normal[2]))
    // Vertices
    binary.LittleEndian.PutUint32(buf[12:16], math.Float32bits(v1[0]))
    binary.LittleEndian.PutUint32(buf[16:20], math.Float32bits(v1[1]))
    binary.LittleEndian.PutUint32(buf[20:24], math.Float32bits(v1[2]))
    // ... v2, v3, and 2-byte attribute byte count
    _, err := w.Write(buf[:])
    return err
}

3D Printing Recommendations

To get clean pad cutouts for tiny components (down to 0603 and SOIC packages):

• Nozzle: 0.2mm (essential for crisp rectangular corners).
• Total Height: 0.16mm (Layer 1: 0.10mm, Layer 2: 0.06mm).
• Bed: Smooth PEI sheet cleaned with IPA for a mirror-flat bottom surface.

Run the tool:

go run main.go gerber.go -height=0.16 my_board_paste_top.gbr

Slice the resulting STL, print, and you have a ready-to-use stencil on your bench in 15 minutes.