Whatever happened to...? - Page 1
Forum section: electronics / general

author: Fred Basset
author_rank: Regular Contributor
timestamp: July 10, 2020, 01:33:50 pm
content: Thought it might be fun (under general chat) to have question where other might know what happened to components / companies or whatever?In my case it was because I suddenly realised that as a kid, so many of the electronics kits I had (and even bargain packs of assorted components) would contain lots of polystyrene capacitors, while now they are a great deal less common than they were.I checked on eBay and they cost a fortune on there - a great deal of them being old stock from the 70s. Does anyone know why they fell out of favor so quickly? Certainly no one in China seems to be making them, which might be why they are relatively expensive?

author: TimFox
author_rank: Super Contributor
timestamp: July 10, 2020, 03:23:24 pm
content: Polystyrene capacitors have great electrical properties, but a low service temperature (85 C). This does not allow modern automated PCB soldering techniques, which can melt the dielectric. Around 1982, in the days of DIP and through-hole components, we used polystyrene capacitors in the sample-hold of a digitizing board, and they had to be hand-soldered, while the rest of the board was wave-soldered.Seehttps://www.electrocube.com/pages/white-paper-what-happened-to-polystyrene-capacitors-data-sheetfor a discussion from a capacitor manufacturer.

author: coppice
author_rank: Super Contributor
timestamp: July 10, 2020, 03:26:55 pm
content: Quote from: TimFox on July 10, 2020, 03:23:24 pmThat's strange. A well controlled wave soldering line doesn't usually get the component side of the board that hot. You can usually get away with using all sorts of low temperature plastics for connectors and other hardware items.

author: TimFox
author_rank: Super Contributor
timestamp: July 10, 2020, 03:29:09 pm
content: That was the recommendation of our board assembly house. Of course, maybe only a small fraction of the caps would be destroyed but that was not acceptable.Interesting note: a recent search for polystyrene on Mouser turned up a couple of Xicon units (conventional construction), but more of the items returned were actually polypropylene, which is far more popular now.

author: coppice
author_rank: Super Contributor
timestamp: July 10, 2020, 03:37:18 pm
content: Quote from: TimFox on July 10, 2020, 03:29:09 pmPolystyrene caps have become niche. Pretty much anything that isn't easily made as an SMD part has become niche. Polystyrene won't got away, though. Its properties are too interesting.Did you have capacitors near the edge of the board? Heat spilling up around the edges makes those areas hotter and the central area of the board.

author: TimFox
author_rank: Super Contributor
timestamp: July 10, 2020, 04:07:06 pm
content: If I remember correctly, the polystyrene caps were about 1.5 inches away from the edge of a roughly 5 by 14 inch 4-layer board.I have beautiful General Radio decade capacitor boxes and lab-standard single units. They made both polystyrene and silver-mica versions (as well as air-dielectric singles), and the catalog listed the pros and cons of each. Basically, they recommended polystyrene for low-frequency applications, since the "apparent capacitance" of the mica caps increased at low frequencies. Hermetically-sealed polystyrene caps hold their accuracy: I found no errors measuring them with more modern equipment.

author: Benta
author_rank: Super Contributor
timestamp: July 10, 2020, 05:23:16 pm
content: Polystyrene caps disappeared, because the only company making suitable polystyrene foil discontinued the production; IIRC DuPont.