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I'm still really new to electronics. I'm trying to get used to reading PCBs and what not. I just got a new multimeter and I wanted to test it out so I was using the continuity test on the board's copper traces. But what's really weird is it was saying that a point on the trace isn't connected to another point on the same trace. To clear up what I'm trying to say, here's a pic:

enter image description here

The multimeter's continuity test says the connection between A and B is open. I also measured resistance and it said it was infinite. To be sure it wasn't the multimeter I tested it on the below strips of solder and the continuity test said it was connected as it should.

enter image description here

So am I completely misunderstanding how these PCBs are made? Is there some kind of coating on the top of the traces that I can't see?

Thanks a lot, mikfig

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The copper is covered with solder-mask, a sort of resin. You need to scrape it off, or use sharp pointed probes.

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  • \$\begingroup\$ Heh ok thanks, I always thought that the solder mask was the less shiny dark green area that occupies most of the PCB. I.e. the non-trace areas. So if I understand now how PCBs are made, each layer is copper then a semiconductor substrate over the areas where there's no traces, then solder mask over the traces, and silk screen to write on the board. Then the various layers are connected by vias. Am I correct on this? \$\endgroup\$
    – mikfig
    Dec 4, 2011 at 23:56
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    \$\begingroup\$ Almost. The board is fibreglass or paper and epoxy. On top of that is a solid layer of copper. The copper is etched off using acid (usually Ferric Chloride) in the areas where the traces aren't. These are the darker areas. Then the board is covered with solder resist except where the solder is needed (component pins). On top of all that is an "ident" layer - a silk-screen printed (usually white) layout of the components and their numbers. Then the board is soldered - either with Reflow (SMD) or wave soldering, or more rarely, by hand. \$\endgroup\$
    – Majenko
    Dec 5, 2011 at 0:09
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    \$\begingroup\$ The solder resist is green. Without it the board would be cream/yellowish (dark green bits) with raw copper coloured traces (lighter green bits). \$\endgroup\$
    – Majenko
    Dec 5, 2011 at 0:10
  • \$\begingroup\$ Oh ok so most of what you see on the board is solder resist, its like the "background" of the PCB with the solder masked traces, vias, and soldering joints being the "foreground". As for those tan boards you see, those don't have solder resist on them then I'm guessing. What about the substrate then? What is that/where is it? \$\endgroup\$
    – mikfig
    Dec 5, 2011 at 0:20
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    \$\begingroup\$ The "substrate" is the board itself. Substrate, n: A substance or layer that underlies something Usually either FR-3 (paper and epoxy) or FR-4 (Woven glass and epoxy) \$\endgroup\$
    – Majenko
    Dec 5, 2011 at 0:38

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