This is interesting.........

[amitjagtap]

Hi all,
I encountered a very diff situation few days before. I was checking for short between VDD and ground tracks on our 12 layer PCB with no component mounted on it. lets say, point A when connected to +ve (Red) probe of multimeter and point B at common, it was showing some resistance of 150 ohms. When i reversed the connection (Pt A to common & Pf B to +ve ) then the resistance shown is 680 ohms.
I found this little bit different. this is may be capacitance that is formed due to this 12 layed PCB that what i thought :idea:. Dont konw if i m right.
Well, can anybody tell me the reason for this.....(different resistances when connection is reversed)
thanking you.

 

[FvM]

Obviously, PCB capacitances can't show as a DC resistance in a multimeter test (assuming the instrument isn't defective).

The observation sounds strange anyway. Multilayer PCB are usually undergoing an electrical test at the manufacturer, that shouldn't pass in this case. Returning the exemplar to the manufacturer for further examination would be the normal procedure, I think.

An explanation for a high-ohmic short with unsymmetrical resistance can be in a fluid filled cavity inside the PCB substrate connecting the nets. Although you can try to remove the fine-short by baking the PCB, long-term reliable operation can't be expected.

 

[kumarghz]

Pls try in this way.
1. Measure "point A-black-Black probe" point B-red probe" . check the value.

2. Use jumper wire and short Point A &B

3. Measure the other way around ,"point B-black-Black probe" point A-red probe". check the value.

if both shows the same value, meaning that there are capacitance.


regards,
kumar

 

[amitjagtap]

Thanks guys...
It is useful for me...