New PCBs: Multimeter reads a short if Potentiometers are maxed but are isolated

F

Fizzbopgalore

Member
Hello,

I just got some new PCBs in and have run into an odd scenario that I have not experienced before. I have tried uploading photos of the annotated boards, but it keeps saying the upload has failed.

Upon receiving the PCBs, I checked all of the through hole connections to make sure that that ground planes on the front and back were correctly going to the appropriate Teensy pins and that there were no shorts. Everything checked out as good to go.

I have since soldered on some potentiometers (10k) and have run into something I have not before, and it isnt consistent either.

If the potentiometers are at full resistance, my fluke multimeter will sometimes beep as if there is a short, although there is no connection or solder bridges between the potentiometers. They are physically far enough away that a solder bridge of that type is impossible for the data traces, and where the traces lead to eventually connect to the teensy are devoid of all solder and components as they have not been installed yet: it is just the raw throughole that previously checked out as OK, so there isnt a bridge on that side of the run.

I got the boards from JLCPCB, which was a first for me, so I was initially concerned that potentially the temperature of my soldering iron ~275-285c may have melted something away (I still have not found a lead free solder that I like, although I am trying a new brand that has some silver and rosin core which has been behaving more consistently at sub 300c temps); however, if this were due to thermal damage, this would cause a short with the ground fill that surrounds the components, not the individual data traces. The traces are all separated by at least 5 inches until they converge at the teensy's location, and I have not even touched that area yet on the boards.

Is this is most likely due to the combined resistance of the potentiometers triggering the multimeter, or is there a problem that needs to be addressed? Again, it doesnt always happen if they are maxed out either, which is throwing me for a loop.

I have done multiple passes to check under my microscope, and while the flux has melted a bit between each potentiometer's mounting hole, there are not any visible bridges that I can see.
 
Photos would be indeed of help to get possible ideas.

I wouldn't expect that any damage from heat could cause a short. Worst thing is that something is getting loose on the board. I am using lead free solder on the cheapest JLCPCB boards with no problems. Soldering iron at 340°C. The change from solder with lead to lead free took several weeks for me to really get used to it. Of course it needs some practice and you have to be relatively quick.

How exactly are the potentiometers connected and at which points exactly are you measuring?
 
OK. Got the files small enough!

Back View, without pots connected. Indicates the through-holes and ground fill connection for back at the teensy area. (No shorts or anything when checking the pcb from JLCPCB).

PCB_1_Back_TeensyTraces.jpg

Back View, with pots connected. The soldering job isnt pretty, but I can confirm there are no shorts between each pot's ground and data pins. Nor are there any shorts between the data pins with the ground planes (which is where I would expect one to be if the board got damaged).

PCB_3_Back_Potentiometer.jpg

Front View Shows how the traces on the top look like.


PCB_2_Front_NoPotentiometer.jpg


Where the multimeter has been throwing a fit is if Pot 1 is maxed out with any of the other potentiometers. I have been testing by going to each potentiometer's data pin and then testing with the currently empty/corresponding through-holes at the teensy's location.

For example, if Pot1 is at max resist (10k) and pot 6 is as well, I will sometimes get an indication that there is a short (one end of the multi meter at Pot1's data pin, other end at Teensy's Pot 6 hole); however, they are the furthest removed physically, and nothing shorts with the ground fill. If the potentiometers are not both at max resistance, the multi-meter stays silent—as expected.

Otherwise, they are pretty isolated, and from checking the empty boards, the traces behave as expected.
 
Fizzbopgalore:
Are these pots wired as variable resistors to GND or are they wired as voltage dividers providing a variable voltage between Vcc at one end and GND at the other? If they are voltage dividers, then with both at full resistance to GND, the voltage tap is at minimum end resistance to Vcc. This means that from one pot output to another the resistance is a very small value and your meter may be reading a "short" from one pot to another back through the Vcc path.
 
Ah, I forgot to annotate that the pots are connected to a 3.3v trace up above. It got cropped when I was trying to decrease the image file size.

Right now Vcc isnt even hooked up, but that's what I was wondering as well. What's triggering a "short" is a very small Ohms value, like 10-25.

Otherwise the Pots' resistance all appear to be correct amongst themselves. I also just did a check checking each pot to the ground plane. That looks fine as well. Not sure how much of a difference this will make once the pots reach the Teensy's analog pins. I'm not dealing with super accurate stuff here, it's just a simple midi controller, so there isnt anything additional in circuit aside from some encoders (mostly all on the front plane) going to the digital pins.
 
Fizzbopgalore said:
my fluke multimeter will sometimes beep as if there is a short
Click to expand...
When you do these measurements, is the multimeter perhaps in the conductivity/diode mode? I'm asking because my mulitmeter only beeps in that mode, not in resistance mode. You should measure in resistance mode.
You mentioned JLCPCB; did you perhaps use their CAD tool EasyEDA? If so, are you willing to share your project [you can export it as a .ZIP]. That way it probably becomes more clear to me where and how you are measuring.

Paul
 
Pots have 3 terminals, your PCB shows 4. Schematics are very useful to have for understanding any circuit issue.
 
And you've checked the fader pin-out is what you expect?

By the way with lead-free solder be sure to use the 4% silver sort, the other kind is not eutectic and not very workable, certainly not for re-work. Check the resistance value is sensible, don't just listen for the beeps...
 
Update.

Mark, I ended up ordering new solder that uses silver in it. The previous one just would not create a solid connection, regardless of the temperature or flux used. Even on a separate perf board, I just couldnt get that other spool to work well.

After changing solder, the problem ended up being wonky solder joints. They looked correct from above, but when I angled the board under the microscope, it looked like the solder beaded on the pin and wasnt actually making full contact with the trace. I re-flowed those few outliers and everything works like a charm.

Once that was sorted, all the resistance and continuity readings were what I was expecting from the start.

In hindsight, I should have suspected that from the get-go, as the PCB and whole circuit is not exactly complicated, and there arent any sensitive components on this board; however, between it being my first time ordering from JLCPCB and using solder I'm not familiar with, I was thrown for a loop.
 
If you are new to soldering the basic rule is " the solder will go to where the heat is ". So if you need to solder a pin on a device to a pad and plated thru hole, ensure that both the pin AND the pad are heated, then apply the solder.
 
Also its vital that flux is present when heating, or you may just oxidize the surfaces before the solder can wet them. More flux, less solder, is often useful advice for beginners - and don't apply heat for more than a couple of seconds if the solder hasn't wetted the surfaces - remove iron, wait a little, start again - otherwise you can overheat stuff.

If the joint can't be made in 3 seconds or less, there's a problem - iron too cold? No flux? Unclean/oxidized surfaces?

Having said that small through-holes in thick PCBs are always a problem as the heat won't get all the way though to the other side readily.
 
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