Or do you have like a magic NP0 brand and series, which is working?
Dunno about NP0, but mica capacitors are pretty stable and precise.
Wow, those start above a EUR each. Also they dont go below 100V rating, and up to 47nF. Not a really good deal.
FYI, I've heard that those parts can leak: the stack isn't sealed on the sides, and is pretty much impossible to wash.
I dont think the GOhm was really ever required, if just had to be better than ceramics or elcos. And there arent many technologies, so it was film. I also had a leakage guard trace below, where it was really nessesary. But the Panasonic is molded. In any way, after producing few thousand boards, I never had to measure the leakage of the capacitor.
The linked ones are not really good for a film cap, but with SMD there is not that much choice. Due to soldering temperatures the choice is even more limited here. So film caps are often better in THT. If at all good SMD caps would be G0G / NP0 type ceramics - they are not that bad.
At the time, being SMD was more important. I also think THT is a better choice, I just dont have any experience with them. Is the DC film a "premium part" that I can preatty much choose any brand and series, and it will work as intended?
Surface mount caps tend to have problems the leaded caps don't and if you need the best performance, forget SMD and no, ceramic caps are not on the approved list for this type of app.
I'm not so sure about that. 1206 NP0 capacitors have >10GOhm specified leakage currents. Granted, they will not come in high values, but for example a 10nF 63V capacitor. No excessive board stress, no vibrations for microphonics, used on room temperature... A NP0 might be a better choice.
RS-Components still has polystryene capacitors.
not cheap but still in production.
I was a bit puzzled by the price and performance. They it clicked, that I'm not looking at 500MOhm, but 500 GOhm leakage. That is certainly unique.