Schottky Diode Help

Thread Starter

DBTech7

Joined Apr 21, 2022
30
I recently ordered from JLCPCB and after my order arrived I realized a few flyback diodes were in the wrong orientation.
I used my soldering iron at 350c with a small chisel tip and was able to flip each diode to the proper position.
My problem now is each diode is conducting about 2.775v in the reverse bias direction, and all diodes show an almost identical reading of about 2.775v +- 0.005v
If I de solder a diode and test it off the board, my multimeter indicates its working as it should, but as soon as I re solder to the board it gives me reverse bias voltage again

Boards have not been powered at all yet
All diodes are same part number (SS54)
The iron wasn't contacting any diode for more than 5 sec or so
There is no solder bridged underneath the diodes that I can see
PCB Traces are not touching anywhere (verified with gerber and visual inspection of board)
Multimeter indicates diodes are good when tested individually away from the board, even after multiple times soldering/de soldering
Multimeter didn't show these readings before flipping each diode

I'm at a loss as to why, if anyone has any ideas where this odd behavior is coming from it would be greatly appreciated!!
 

Ian0

Joined Aug 7, 2020
5,510
It takes less than 1 second to solder or desolder the tiny diode. Your 5 seconds was too long.
But it will withstand minutes in the infrared reflow oven. 5 seconds on a soldering iron doesn’t kill a diode, provided it isn’t powered up before it has cooled down.
I suspect the reading is caused by something else in the circuit.
 

BobTPH

Joined Jun 5, 2013
5,230
My problem now is each diode is conducting about 2.775v in the reverse bias direction, and all diodes show an almost identical reading of about 2.775v +- 0.005v
Please clarify what measurement you took. Components do not conduct voltage, they conduct current. I cannot even guess what you are measuring.

Bob
 

KeithWalker

Joined Jul 10, 2017
2,403
How did you decide that some diodes were in circuit backwards? Do you have a schematic for the board? You may be mistakenly causing your own problem.
 

Thread Starter

DBTech7

Joined Apr 21, 2022
30
Please clarify what measurement you took. Components do not conduct voltage, they conduct current. I cannot even guess what you are measuring.

Bob
Diode test measurement on my multimeter, I also did an ohm test and they all read about the same including the unaffected ones, 170kohm in reverse bias, and 7.5kohm in forward bias
 

Thread Starter

DBTech7

Joined Apr 21, 2022
30
How did you decide that some diodes were in circuit backwards? Do you have a schematic for the board? You may be mistakenly causing your own problem.
Are you going off the pcb printing to verify the diodes are wrong way round, or do you have the circuit diagram to prove it?
The dfm manufacturing file showing the polarity in the wrong direction, I didnt catch it until after the boards were in production
 

Thread Starter

DBTech7

Joined Apr 21, 2022
30
Does the circuit work with the load/ coils ok, the diodes are just for back Emf?
I havent tested anything besides the buck converter and ESP yet, buck converter powers up and ESP runs stable with a blank sketch. Thats why I asked this question because I wanted some insight before I put the mosfets under a load
But yes thats correct they are simply flyback diodes to protect the mosfets
 
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