What would cause a 4 Ohm Short on a PCB

[ste2006]

Hi,

I have a 4 layer board I am working on and it was perfect, In the design and testing stages though I had to cut a track as it was causing me issues. After cutting the track the board was fine but then when I pulled a battery pack out of it I started to get a 4 Ohm Short between the Ground and Vdd on the board. Whether I dug too deep cutting the track and shorted the power planes in the middle two layers I do not think so but possibly or whether it was just coincidence I am not sure.

I ended up drilling a clean 2mm hole through the whole board where I wanted to cut the track just to try to make sure that none of the inner layers were shorting but still a 4.1 Ohm short.

I would imagine if it was the inner layers shorting or a stray piece of copper I would be getting less than one ohm short but perhaps not??

Would 4 ohms sound like a bad component??

Any bright ideas for helping to track down this short?

I have board on a current limited supply and it keeps maxing out the DC-DC on board keeps screaming also as I assume its being overloaded by the short.

Any ideas would be brill as the boards only came back from the PCB house two days ago :sad:

Thanks,

Stephen

 

[123jack]

How many boards did you get made - do any other show the problem before you carry out the procedure?
Do you have enough to carefully repeat the same procedure on 1 more to see if the result is the same?

How did you notice you had this problem? You say you cut a track - have you looked closely at the
consequence of cutting the track (sounds obvious I know but oversights are easy) to the components on
and around it and what they do? Presumably you powered up the board after cutting the track - this could
have caused a problem which then manifested as a short.
Without the full circuit and board details only you can know this.
If you suspect a design error have you contacted your PCB manufacturer to ask for any comment or advice
they could give as to the physical layout?

My best guess would be a design error of some sort as the most likely cause but dont rule out failures.
(And panic when a new design fails can make your head explode - try to stay logical ;-) )

 

[ste2006]

Hi,

Thanks for the reply, Got two boards made and stuffed and both were perfect s design is fine. Initially after cutting the track there was no issue (everything worked as it should do). System was powered from a bench power supply and also has a small backup battery. I pulled out the battery (2 pin plug) and then suddenly the power supply current limiter kicked in and the DC-DC on the board starting making a noise (overloaded I assume)

Ever since the short is there, As I say the short is about 4.1 Ohms between 2.8V and Gnd

Could it be a component has failed and is bridging the supply rails, If so what is most likely to fail, Caps I assume??

Thanks,

 

[spudboy488]

I'm assuming that plugging the battery back in does not remove the "short". Look in the area of the schematic affected by the battery. Analyze the circuit and look for what might have happened where there was once a voltage and then there was none (with the removal of the battery). Feel around for a component that is warm/hot that maybe shouldn't be.

 

[ste2006]

No plugging the pack back in does not remove the short, Battery just goes through a FET and then straight into a 2.8V Regulator with a few bypass caps etc,

I guess I would have assumed an actual short to be less than 1 ohm even if it was only a hairline of copper but the fact that I have 4.1 ohms leads me to believe its a component causing the issue but maybe im way off here??

What does anyone think about the actual resistance between Gnd and Vdd being 4.1 ohms. The meter by the way shows around 0.1 ohms ground to ground so the meter is ok.

 

[123jack]

I think we'd need to see the cirtcuit to get more of an idea ste
Short of suggesting you start cutting more track or component pins to isolate the supply from the load and just see hat happens I can't think of any other suggestions.
as for 4 ohms ... I= v/r does it sound ok to you for your circuit?

 

[ste2006]

Ok, thanks for your help guys,

I found the problem, cut tracks had nothing to do with it. Somehow when I pulled the battery I damaged the DC-DC converter which in turn delivered too much power to the rest of the system and then in turn blew the GSM module and caused it to short out, GSM module removed = short gone,

Anyone and ideas how pulling the battery could have damaged the DC-DC??

Thanks,

 

[mtwieg]

Depends on the overall design of the system's power supply. I've seen designs where removing the power source causes the load of the voltage regulator to immediately decrease (via UVLO), while regulator itself is not shut off, leading to a large positive swing on the regulator output. However doing UVLO in such a way is very rare (and doesn't have any real advantage) so I doubt that's you're issue. More specific info on the DC-DC converter and control method (especially UVLO/OVLO on the input/output) would help.

 

[GUMY]

Damage can be caused by charged capacitors on secondary side of DC/DC, causing that output voltage is higher then input voltage. But it depends on actual design.