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There's a small solder blob in the middle of this one-sided PCB (a switch-mode power supply), surrounded by unmasked board. The blob is 1.5mm in diameter, and the non-masked area is 4mm in diameter. There's nothing on the other side of the board, and there is no hole.

PCB with blob

What's the purpose of this blob? My guess is some kind of manufacturing artifact, but I'm not sure exactly.

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  • \$\begingroup\$ Possibly a marker put in as a drill guide before drilling and slot cutting begins. Often many boards are on a sheet with a stack of 5 or more sheets. There needs to be an absolute ref marker for drilling to be precise. That's my guess. \$\endgroup\$ – Sparky256 Oct 27 '18 at 1:35
  • \$\begingroup\$ @Sparky256 I don't believe this is the case--it's just a blob of solder on the bottom of the board, it doesn't pass all the way through. \$\endgroup\$ – user60561 Oct 27 '18 at 1:45
  • \$\begingroup\$ Then read Dave Tweeds more accurate answer. It is a ref maker for the ABS position of parts, one of several that can be on a board. This board has one as it is for through-hole parts, so great accuracy is not needed. \$\endgroup\$ – Sparky256 Oct 27 '18 at 1:52
  • \$\begingroup\$ @Sparky256 and I did accept that answer :). I appreciate how helpful you and everyone else are here, even though it's clear that I know nothing about the subject. \$\endgroup\$ – user60561 Oct 27 '18 at 1:58
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It's most likely a type of fiducial marker, used by a pick-and-place machine to correctly place the SMD parts on the copper side of the board.

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That is definitely a type of fiducial marker by the existence of the solder mask clearance. A solder blob forms on the exposed copper when the board is put through the wave solder (by this stage is has served its purpose of a reference point during etching or placement). Normally there are 2 at least 2 markers to allow for accurate X and Y placement.

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    \$\begingroup\$ And you can see that it has been wave soldered because: A) There's a lot of hole-mounted components. B) The SMD components are glued. C) Every piece of exposed copper has solder on it. \$\endgroup\$ – pipe Oct 27 '18 at 20:16
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    \$\begingroup\$ You don‘t need more than one per board if your boards are panelized. The machine will use the same fiducial on several boards to check the entire panel‘s alignment. \$\endgroup\$ – Crazor Oct 27 '18 at 20:46
  • \$\begingroup\$ @Crazor Ah, that explains why there is only one on this board! Thank you! \$\endgroup\$ – user60561 Oct 28 '18 at 4:06
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Its a fiducial. Seems like the designer forgot to remove solder paste marking of the fiducial in the Gerber files.

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  • \$\begingroup\$ That board was wave soldered. Solder mask is kind of irrelevant here. ;-) \$\endgroup\$ – madscientist159 Oct 28 '18 at 2:43

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