unintentional voltage on a floating pin

[Arunlal.G]

Hi,
I am facing a problem with one of the PCB from my production team. During the detailed analysis I found eventhough when i isolate one track, ie-: both ends of the track are floating, i can see some voltage on the track around 500mV. Actuaaly this track is a UART Rx line and the controller pin is disconnected from the track. The other end of this track is terminated on a connector and connector not installed. This problem i am getting on some PCB only and others are OK.
I have attached one image showing the waveform i am getting on the pin.
Can anybode help me on this ??????

 

[FvM]

It doesn't look like the line is actually floating. There's more than capacitive crosstalk.

I presume, the leakage resistance could be detected with a multimeter.

You didn't mention the probe impedance, is it 10 MOhm?

 

[FANT]

Also for me it is a crosstalk between two tracks.

Connect a 10 KOhm between track and ground and it will be OK, you should see only few spikes.

The 10 MOhm of the probe makes these tricks!!!

Mandi

 

[Arunlal.G]

It doesn't look like the line is actually floating. There's more than capacitive crosstalk.

I presume, the leakage resistance could be detected with a multimeter.

You didn't mention the probe impedance, is it 10 MOhm?

I am using a probe with 1M, 13pF. For other boards there is no issue. Is it a problem related with the PCB manufacturing or something happening in the process.

 

[FvM]

For other boards there is no issue. Is it a problem related with the PCB manufacturing or something happening in the process.

It looks to me like a trace short or a higher ohmic leakage. As said, you should be able to identify it by resistance measurements.

A leakage can be generated by a cavity inside the PCB and insufficient drill-to-copper spacing on inner layers.

 

[Arunlal.G]

It looks to me like a trace short or a higher ohmic leakage. As said, you should be able to identify it by resistance measurements.

A leakage can be generated by a cavity inside the PCB and insufficient drill-to-copper spacing on inner layers.

Please help me in finding the exact leakage resistance of the traack. Is this measured between ground and the track or with other tracks.

 

[FvM]

Ideally, you'll measure isolation against all circuit nets. It's presumed that no resistors or components that provide a DC current path are connected to the examined net. Then the net with the lowest isolation resistance can be expected to have an unintended short respectively leakage.