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This is the "Automatic Ink Detection” PCB from an Epson inkjet printer. According to the service manual, it either detects or cleans clogged nozzles (this is not clearly explained). The service manual does say that high voltages at a few microamps are present on this board. The board is connected to the main control board through a flat flexible cable and to a wire grid on top of the waste ink pad/nozzle cleaner assembly through a silicone insulated high voltage wire. The smaller board attached to it with the Matsushita logo and the transformer seems to be some kind of self-contained flyback converter but I could not find a datasheet.

In terms of design, I’m primarily wondering about the irregularly patterned via stitching all over the board and the loops in the PCB traces areas marked L1 and L2. Since nothing on the board seems high-current, the via stitching is probably not for heat dissipation. If anyone knows what exactly the “Automatic Ink Detection” feature does, that would be helpful too.

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    \$\begingroup\$ I'd guess that the "L1" and "L2" text are just layer designations for the PCB artwork. They are copper, not white silkscreen like the other text. \$\endgroup\$ – Peter Bennett Jun 5 '18 at 21:34
  • \$\begingroup\$ No idea of how it works but L1, L2 could simply be the layer designators. The clearance around the thin trace to the 10M (?) R80 resistor is interesting. Maybe the copper pours are part of some kind of capacitive sensor. \$\endgroup\$ – Dejvid_no1 Jun 5 '18 at 21:49
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    \$\begingroup\$ Via stitching serves two purposes: 1) inductance reduction, 2) EMI blocking. \$\endgroup\$ – Ignacio Vazquez-Abrams Jun 5 '18 at 21:51
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Via stitching is used for impedance and noise control. It lowers the resistance and inductance between two parallel planes. The three contact areas do not have plated-through holes, so in those areas it probably is for current sharing between the top and bottom layer contact areas.

You also can use via stitching to create pseudo-coaxial cable around a trace. For example, layers 1 and 3 are GND, and layer 2 has a trace for a delicate signal. Rows of vias along both sides of the trace create a 3-d GND shield around the trace. The closer the vias, the better the high frequency performance.

Via stitching around the edge of a board can prevent noise or signal radiation outward through the edge of a board. This can work well in TEMPEST applications.

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