...it is very very well known...
so where is the link? It should be easy...
I did not find something useful for: "SMPS malfunction metal case"
not telling anyone thats how they got the PCB working!!!
Its a secret of the trade
Sorry, this sounds like a modern myth. (My opinion without prove)
Like: Many others have found a way to use free energy from space. But they hide it as a secret. Some say the governmet has stolen the invention (And now they can´t remember how they did it !!??) ..... The internet is full of such (beep).
All microwave engineers know this.
First: your headline is focussed on "SMPS" now you talk about microwave, which starts at about 300MHz.
On the other hand: How does this work that all MW engineers know this? Does this happen at birth? Or are there documents. .... one could provide a link to?
I know that usually
microwave circuits have their metal case "HF wise shorted to GND". But they don´t use "Y capacitors" (your headline).
The "Y" in Y capacitors referes to safety standards for mains voltage. But MW devices only in rare cases involve mains voltage.
And I doubt that Y capacitors are useful for 300+ MHz.
A TDK Application Note talks about an effective frequency range up to 8..10MHz.There surely may be other Y capacitors for higher frequency.
(Again: ON MICROWAVE circuits, then HF wise connection to the case may have influence on it´s function. Mainly stability of amplifiers and frequency flatness)
My personal opinion: If your SMPS PCB is that bad that the function get´s confused by a metal case (HF wise connected to GND or not), then you should change your job.
(correctly designed regarding all other design and safety rules)
Klaus