Intermittent Low Resistance Between Adjacent Signal Lines on PCB With 74LVC541 ICs

Has anyone ever seen a problems where resistance between two adjacent singal lines on a PCB beome low resistance, but later returns to normal high resistance? Whe ave a PCB that seems to get s low impedance between two signal lines, one is a 74LVC541 output that pulls an LED low thoruhg 82 ohms and the other is to a seocnd 74LVC541 input with 10k pullup. these singal go to adjacent pins on a connector that goes to a front panel overlay with switches and LEDs in it. Both IC's are powered by 3.3V.

When we power up the board, the output LVC541 pulls low to turn on a LED, then within seconds, the pin next to it that is wired to a different LVC541 input with 10k pullup goes low as well. When we turn off the board and disconnect the overlay we read low resistance with a DMM, around 30 to 300 ohms. After some time of being off the condition seems to heal itself, but re-occurs when the baord is powered up.

I suspect that it is the 74LVC541. You can always test the PCB by removing the components and then testing the PCB traces. Are there any capacitors on those traces?

Hi Cody,

Thanks for the advice. We have a factory support perosn who has removed the parts to show the PCB is probably not the problem. I am an HW design engineer trying to assist, so I am not sure I have heard all the facts. The factorys support person also claims that simply removing the LVC541 and re-soldering cleared the probelm on one board.

There are not any capacitors, just 10k pullups and a connector that goes to the front panel overlay which has some LED's and snap dome switches. The most common occurance we are seeing is that after power is applied, one LVC541 output goes low to turn on a power LED and the connector pin next to it, which is an overlay switch input to a second LVC541 (one 541 is outputs, one is inputs for software control) starts out high and goes low and stays there. It is then when it reads low impedance between the the two signals, which are not electracilly connec ted. I have looked over the PCB layout and do not see anything alrming or unusual about it. I think it was also reported that this problem will occur when the overlay is not connected on a board that shows the problem.

The foctroy support person