I need some guidance on how to begin troubleshooting a specific problem with a bareboard.
Board Info-
Size-22"x26"
Laminate-ITEQ
Layers-30
In addition to standard bareboard ET (10v with 10Mohms isolation threshold), we also do HV test @ 500v (with isolation threshold of 500Mohms). It passes standard test, but fails HV test for pwr/gnd shorts. The only way it passes HV test is when we use voltage ramp up delay. Using Fluke 8845A to measure resistance, it will detect resistance between pwr and gnd (that should be isolated from each other), but quickly dissipates within 1 second. I then pick up probes and touchdown on the same pins and it no longer detects any resistance. I then switch polarity (switch probes) on the same pins and it picks up resistance again. What is causing this discharge?
Cu pour (not thieving) was added on all signal layers (.5oz. striplines with .008" dielectric to gnd) and on empty areas on power layers (1oz. with .004" dielectric to gnd). Cu pour have airgaps no less than .050" to nearest features.
Blindly, we released another lot w/o the Cu pour to see if it resolves the issue. If we do find that removal of Cu pour fixes this issue, can someone detail what exactly is happening with the board; how is Cu pour creating the problem (which is TBD)?
Best Answer
Capacitance.
You will need to wait (to charge the capacitance) before making leakage measurements. As long as the "leakage" stops, this is just basic physics and in no way a defect on the board.
Removing the copper pour will reduce the capacitance, but in many applications that capacitance is a good thing, rather than a problem! Indeed, about a quarter of the components fitted to the board later, are only there to increase the capacitance between power and ground planes!
Discuss this "problem" with the customer : he will probably be glad to have it!