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I'm working on a project that requires an ATSAMC21 microcontroller. I've got a reasonably good setup; air reflow system, USB microscope, ESD mat... the works.

I soldered the chip on the board using solder paste and I wanted to test to make sure the connections were good. When using my multimeter to do a continuity test, I keep getting a short between two non-adjacent pins. The pins are both SPI pins, and there are no other components on the board. The solder looks really good, so I'm kind of out of ideas. The chip itself could be damaged, but what are the odds that the two pins that happen to be damaged are two non-adjacent ones that I actually need (there are 100 pins on this microcontroller). I am measuring the short on all unpopulated pads that these pins are connected to. The weirdest thing is that the pin between them is not shorted to either of them. I also checked an unpopulated board, and these pins are not short on it. Does anyone have any ideas about what this could be?

enter image description here

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  • \$\begingroup\$ maybe the chip is incorrectly oriented \$\endgroup\$ – jsotola Apr 4 '20 at 6:05
  • \$\begingroup\$ Conductive flux or something? \$\endgroup\$ – DKNguyen Apr 4 '20 at 6:58
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If you are sure about chip then there are only 2 options left.

A) There is tiny hair that short these 2 pins. Looking the quality of your picture that I believe taken under microscope I cant see any hairs so this hypotheses is very unlikely. This is easy to check/fix. Just scratch between pins with lancet and test again.

B) More probable is that PCB etching was not done well. It may have defect or short under green paint that is not visible. If you try hard you may find the spot and try to fix it again with lancet. But this has to be hard cut at defected spot not just a scratch.
Other way to fix this without localising can be done with high current. First remove your chip, then increase slowly voltage with current protected PS. At some point (hopefully short is not too big) shorting jumper should blow away.

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