EEVblog Electronics Community Forum
Electronics => Projects, Designs, and Technical Stuff => Topic started by: electricForce on January 30, 2021, 05:06:44 pm
-
Does anyone have a feel for the repeatability of a leakage current measurement through flux residue left behind on a PCB?
I have some PCBs which I've discovered have left over flux left on them (oops! it was hidden, came as a suprise to me). When I force a voltage onto a node to check the leakage (by checking the smu current), the measurement can vary by 200nA or so. In total magnitude I've seen a range from 200nA to 400nA when forcing a 5V potential onto the nodes in question.
All things being equal (temperature, humidity, particulate contamination) - is there a physical reason why the leakage measurement could have such a large variation? I'm trying to find a physical understanding of why this could be happening. I know that if I apply 5V to a known resistor, say 1kOhms, I will get close to 5mA every time I apply that voltage. I do realize that the resistance of flux residue and PCB material is a far cry from a 1kohm resistor (several orders of magnitude!) but I'm just wondering if anyone has any ideas about that resistance, and why measurement to measurement it could vary so much.
Thanks!
ElectricForce
-
Imho -the leakage current depends on the chemical composition of the flux, the physical dimensions of the flux residue, contacts area and pcb mechanical stress and humidity and temperature..
-
What is it that's soldered onto the board and could that be causing your leakage? Is the PCB trace large in area, directly opposite ground plane, does it get very close to other traces etc? Is there a solder resist?
If I had a circuit with resistances in the ~10M range, I would anticipate solder flux to be the cause of occasional problems, not so much below that, and above that I would go to special lengths to try and control the problem. But ofcourse, its all very geometry dependent, especially if there's anything fine-pitch involved, or its high layer count.
I would expect a slightly more consistent measurement (assuming you are talking about measuring the same node multiple time on the same board etc) If you're getting inconsistent results measuring ~100nA, make sure all your cables are kept separate from one another and absolutely was far as possible from anything at an appreciable voltage. Could the SMU be set in a range not quite suited to such a low current? Make sure there's nothing light sensitive on the circuit also. Its well worth repeating measurements and checking and different voltages to help work out where your inconsistency is coming from.
-
Flux can certainly have a noticeable resistance on high-resistance circuits, flux is not an ideal dielectric / insulator. Normal manufacturers normalize and specify this parameter. Moreover, atmospheric moisture on the PCB can also increase the resistance. To combat this, I think it is necessary to varnish the PCB.