VCC-GND short

[mkeith]

I ordered my kit through Make Magazine. I assembled it, and it seems I have a short from VCC (5V) to GND. (Ohmmeter reads 0.2 Ohms, which is the same reading you get if you touch the leads together).

First of all, did you supply kits to Make magazine (and might therefore support me) or did they just make a bunch themselves? I would understand if you were unwilling to support kits you didn't put together. FYI, the board has black soldermask and says nootropic design on it.

I visually inspected the assembled board, and did not see any shorts.

I removed the 7805 and the electrolytic output cap, and that didn't fix the problem (still 0.2 Ohms). I pulled the processor out of the socket because that is easy to do. Still short. I took out one of the bypass caps and did a little bit of damage to the PCB in the process. Now I'm afraid if I keep going I will destroy the board. I'm an EE, but I usually have technicians do this kind of thing. ;-) I can't ask a tech to do this, though, because my techs are busy with real work. I'm doing it on my free time.

A couple of things:
1) I did put the socket on backwards, but that shouldn't matter (right?) The uProcessor pin one location is the same as shown in the silkscreen. During assembly, I didn't notice that the socket had a polarity indicator.

2) Even though the ohmmeter shows a dead short from VCC to GND, the 7805 doesn't get warm. Also, the power supply (not yours) makes a faint sound at something like 2 Hz. I'm thinking maybe the power supply is cycling in and out of a fault protection mode, and that is why I hear the sound (and is also why the 7805 doesn't get hot). Two different power supplies do this, and they work fine on other products.

I'm not sure what to do now. Any chance of sending me a freebie? I can't confirm that the board or a component is defective. There could be a short somewhere that is not visible (under a component, for example). Or maybe the soldermask got scraped off of the GND fill somewhere, and a little solder from one of the VCC pads bridged over to it. I don't have a microscope at home, and I wouldn't be able to see that without one. I could send this board back to you if you want to try to figure out what is wrong with it. If you do send me a new one, I will check the bare board for power-ground shorts before starting assembly, and will proceed in a more logical fashion to avoid this kind of problem.

--McKenzie

[Michael]

Yes, I supplied the kits to Maker Shed, so I support them technically. I'm not exactly sure what their policy is regarding replacement, though, so I'd like you to at least ask them whether they replace kits that have defects. They were never clear with me on this policy.

Since you're an EE, I assume your soldering is good -- I've found that the most common cause of failure is horrible soldering. You wouldn't believe some of the soldering I've seen. Backwards socket won't matter. All my boards undergo electrical testing at the manufacturer, but a defect is still possible I guess. I think the most likely explanation is that a solder joint is shorted against the ground plane (bottom of board).

If Maker Shed won't replace the kit and if you can't find the short, I can replace the board and components you need. Maybe with some close inspection and reheating of solder joints you can find the problem. Would you need all the parts except for the MCU? I need to keep my replacement costs down...

[mkeith]

"Since you're an EE, I assume your soldering is good..." Haha! All our boards are mostly SMT, and assembled by machines. If they need rework, other people (who are good at it) do it. But you are probably right that my soldering is better than people who are doing their first project or first few projects.

Anyway, I've been looking around the forum. You are doing an awesome job of supporting your projects. I will keep looking for the problem, and if I track it to root cause I'll let you know. Otherwise I'll just order a new kit from you (and pay). Thanks for your advice and please keep up the good work!

Believe it or not, my son actually asked for "fake TNT" for Christmas. Something he saw in a cartoon. So I HAD to get this kit when I saw it on make.com. I built the fake TNT with wood dowlings painted red. Bundled with black electrical tape. I left the ends unpainted because otherwise it looks way too real. I assembled it and discovered that it was not working on Christmas eve. Ugh.


--McKenzie

[Michael]

Below is an image that shows which pins are connected to VCC. These would be the ones to check for a short against the ground plane (which is the bottom of the board). Sometimes the solder mask gets burned away a bit, and a tiny amount of excess solder shorts against the ground pour.

[mkeith]

Awesome! Thanks. I don't have eagle, so this is very helpful!

--McKenzie