I am reverse-engineering a board which provides a VGA output signal for connecting an oscilloscope to an external monitor. I have quite a bit of experience with PCB design but there is a feature on this particular board which I have never seen before. It looks like a few SMD resistor footprints which are not populated with components, though the space between the pads appears to be a dark gray color, almost like carbon. What confuses me is the fact that there is soldermask over the pads themselves:
These footprints are on the R, G, and B video signal pins, as well as Hsync and Vsync. I am reverse-engineering from scans of the PCB. I have the ability to take the actual device apart and probe the card, but it is a bit of a pain in the neck and I would prefer that to be a last-resort. I am hoping I can get an answer without having to crack the thing open.
Any idea what these features might be? My suspicion is that they are some sort of carbon printed resistor, but I don't believe I've seen anything quite like this before.
UPDATE: I bit the bullet and I ripped the thing apart (as much as I didn't want to). The "devices" on the R, G, and B lines measure about 75 ohms, the ones on the Hsync and Vsync lines measure about 72k ohms. 75 ohm values are consistent with the characteristic impedance of the R, G, and B lines, so my guess is they're some sort of termination resistors. Not sure about the Vsync and Hsync though. Pull-ups/pull-downs?
Best Answer
I'm pretty sure they are carbon printed resistors. I've seen similar ones on display adapters. They do it so save parts to make it cheaper if they produce it in a high enough volume, because they can produce it using a single screen print process instead of 5 pick and place parts. But if you want to get a precise value to reverse engineer the card i think you have no other choice but to measure the resistance.
Here are some more infos about the topic (eevblog video).