unintentional voltage on a floating pin

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 ??????

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?

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

FvM said:

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?

Click to expand...

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.

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

Click to expand...

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.

FvM said:

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.

Click to expand...

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

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.