2 Ohm Between VCC and Ground

[fagiano]

Hi,
I've just assembled a board I designed, is all smd that I soldered with solder paste and a hot air gun. I was testing for shorts and I found that ground and VCC are shorted with ~2ohms between them. I have an MCU+crystals a eeprom a regulator and few leds on the board. Is there a way to troubleshoot this? I already desoldered the decoupling cap that had the lowest resistance and it wasn't it. Any tip? or I just go component by component and desolder everything?

thank you for your time
ciao
Alberto

[MatCat]

A schematic / pics would be of help.

[electronics man]

check all the solder joints with a magnifying glass.

[fagiano]

Here's the board

[zapta]

A few shots in the dark

1. Have you measured a non populated board?

2. What is the resolution of your ohmmeter? Can you see fraction of a ohm? If so, you can track the resistance.

3. Apply a low voltage and track using voltmeter and voltage drops.

4. Apply a low voltage and examine with a thermal camera.

[tszaboo]

check all the solder joints with a magnifying glass.

Yes, visual inspection is maybe the best way. But it is possible, that the board has small errors. Unfortunately that happens sometimes, especially with PRC boards, or cheaper boards in general. If you feel like it, one of the debug mode is freezing the board, and turning it on, the short will heat up the frost. Or you can connect a 5A power supply to your VCC that will show the short also. Both can be destructive.

[fagiano]

I have an Agilent 1272A DMM, I tracked the lowest resistance to VCC_C3 and I desoldered it, the pads still give the lowest resistance . Could it be a toasted MCU? Maybe I cooked it. Unfortunately I haven't tested the board before assembling(lesson learned!). I wish I had a thermal camera. So, if apply power what should I look for? hottest component?

thank you

Alberto

[fagiano]

I applied 5V to the board, both the regulator and the MCU become really hot. The regulator I assume is because is sucking too much current, I tried up to ~1A, could it be the mcu?

Alberto

[fagiano]

I removed the MCU, and the short is gone. Does this prove anything?

thank you all for your suggestions

Alberto

[tszaboo]

It could be soldering problem, still. If you have a spare MCU, try with that. This time, maybe take care about ESD, if you didn't do it previously. And double check the schematic.

[MatCat]

A microscopic bit of solder can cause such a short and be very difficult to see, I have also had solder paste blob up under chips and you won't really know it until you desolder the chip to see.

[fagiano]

Ok, I think i found it. I generated the schematic symbol with a script I wrote that reads database files from ST MicroXplorer. There is a bug in the database file, one pin was wrong and I was grounding it instead of connecting it to VCC. Unfortunately I trashed 1 MCU, Argh. Hopefully the rest of the pins are corrent!

thank you all

ciao
Alberto

[electronics man]

so is the resistance between Vcc and gnd normal now?

[fagiano]

Yes, I also make the schematic symbols myself but for large components I usually write a script that reads manufacturer documents/files so I don't screw-up. Seems that assuming that all manufacturer files are correct was a mistake .
Yes the resistance is now normal.
I happened to have a unpopulated pad to VCC(R8) and I fixed the bug with a 0 ohm resistor. I have the board programmed and running!

Alberto

[amyk]

Note that although 2 ohms may look really low (and for low power applications it is) in low-voltage high-current circuits it might be perfectly fine - e.g. a typical laptop or desktop CPU will look like a complete short across the power rails.

[HackedFridgeMagnet]

Maybe first time you turn on the prototype use a current limited supply.

[zapta]

Ok, I think i found it. I generated the schematic symbol with a script I wrote that reads database files from ST MicroXplorer. There is a bug in the database file, one pin was wrong and I was grounding it instead of connecting it to VCC. Unfortunately I trashed 1 MCU, Argh. Hopefully the rest of the pins are corrent!

thank you all

ciao
Alberto

That's an happy end in my book. You found the problem.

[tszaboo]

I have the board programmed and running!

  glad to help