Does anybody have a recent problem regarding to THT Zener Diode (especially 9V1 and 5V6) (They behave like short circuits)
Actually we faced with such a problem where we use 1.3W Zener diodes below their electrical ratings in our capacitive supplies (In our boards our worst case Zener condition is 600mW ).
We observe this problem in the first function test during manufacturing or in the life test also so the failure rate doesn’t depend on the working period of the electronic board.
Yes, we use our Zeners in a right way(correct polarities, correct usage in a capacitive supply, etc.) and controlled all diode batches for the weekly periods.
However we can't find the main reason which triggers the Zener diode short circuit problem.
1.3W is probably the absolute max the zener can dissipate in a nice cool room with free ventilation and with very short leads soldered to large-area copper runs on a pcb. Something in the operation of your zeners is different and is causing them to overheat.