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Issue: Alignment of photomask to reference holes drilled

When making PCB in panel form, the reference holes are usually drilled at 4 corners of panel to allow alignment of photomask to the PCB board. If the smallest trace width and gap are 70um, holes size of 0.15mm, what would be the best approach to align the photomask onto the PCB panel?

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    \$\begingroup\$ Maybe he's an ambitious hobbyist? \$\endgroup\$
    – chwi
    Sep 27 '12 at 14:51
  • \$\begingroup\$ Holes a drilled in the corners of a PCB so that you can put posts on it and mount it in a chassis. \$\endgroup\$
    – Kaz
    Sep 27 '12 at 15:58
  • \$\begingroup\$ Which photomask are you aligning? The one that defines the copper pattern, or the one that defines the solder mask? Normally holes aren't drilled until after all the etching is completed and multiple layers are laminated together, so they don't even exist when registering the copper features. \$\endgroup\$
    – The Photon
    Sep 27 '12 at 17:10
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It seems like you are trying to align something to an existing PCB/PCA process.

As @Wilhelmsen says, you just need to put down alignment marks. I'd suggest using the ones used in semiconductor litho systems either the early aligners or the later stepper scanners. They are designed for a best fit statistically. Place them in copper not in teh drill holes as drills wander and wear. Fig 4 & 5 here give you an idea

There are targets ones for X, Y , X&Y and rotation.

By using one set of alignment marks for all processes you minimize the overlay error.

In chips we place them in each die corner and also in 6 global locations (which will be harder for you).

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I wouldn't align using the drill holes as reference, instead make reference marks outside the PCB. So, when printing a schematic, cut around the reference marks and they will be fairly easy to align. This is an often used reference mark, but it would be advantageous to use up to three circles, one with an extra cross, tilted 45 degrees to the original one.

an often used reference mark

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