Insulating plastics - Page 1
Forum section: electronics / general

author: john_c
author_rank: Contributor
timestamp: May 12, 2017, 07:55:39 pm
content: Hi all!I have a rather boring question about plastics. Heh!You know how there's often a thin plastic sheet used as an insulator where components might possibly short out against each other (especially, e.g., around the power transistor heat sinks in power supplies)? I have an application where a board is going to be sitting right up against a heat sink - can't really be helped, given my chassis. So, what material should I use? I'm thinking I need a plastic that doesn't start to soften until > 100 C. Can I just use ABS sheet?Thanks,John

author: jmelson
author_rank: Super Contributor
timestamp: May 12, 2017, 09:30:11 pm
content: Those things on power transistors are thermal conductors. Bergquist makes a wide range of them. It sounds like you want something totally different, though.Jon

author: Gyro
author_rank: Super Contributor
timestamp: May 12, 2017, 09:47:35 pm
content: He means the insulation used to provide reinforced isolation between primary and secondary parts of the circuit.I think mylar is probably the material of choice, that's (IIRC) what the yellow adhesive tape is made of, that is used for SMPS transformer insulation. It has a higher melting /softening point than ABS. I've seen yellow adhesive mylar tape wrapped in several layers around exposed metalwork to provide additional insulation from PCB mounted components but I'd be worried about mechanical abrasion or puncture damage.You can get thicker mylar sheet that you could probably retain by a combination of folding, fitting around spacers, and gluing.

author: mc172
author_rank: Frequent Contributor
timestamp: May 12, 2017, 09:50:12 pm
content: Quote from: jmelson on May 12, 2017, 09:30:11 pmThey are not "thermal conductors". The thermal conductivity of these "sil pads" is of secondary importance. They really are not all that thermally conductive relative, they are just thin.

author: MagicSmoker
author_rank: Super Contributor
timestamp: May 12, 2017, 09:53:14 pm
content: Polyester/Mylar/PET could work depending on just how how the heatsink really gets and whether it will be subjected to any force or pressure. The gold standard in this application, however, is polyimide (kapton).

author: john_c
author_rank: Contributor
timestamp: May 12, 2017, 10:03:45 pm
content: Quote from: Gyro on May 12, 2017, 09:47:35 pmYes, that's what I meant. Thanks.QuoteOk, great. I think I'll go with this, then:https://www.mcmaster.com/#85815k11/=17lka4pQuote from: MagicSmoker on May 12, 2017, 09:53:14 pmI can't find it on Mcmaster-Carr, or I'd consider.Anyway, the heat sink is pretty well rated for the application (it connects to heat pipes that go to large fins), so I think PET sounds alright.Thanks!

author: Cerebus
author_rank: Super Contributor
timestamp: May 12, 2017, 10:04:49 pm
content: Quote from: Gyro on May 12, 2017, 09:47:35 pmMylar is a proprietary name for PET (polyethyleneterepthalate), a polyester; the yellow tape is polyimide (Kapton). Polyesters, depending on specific polymer and grade, are good to about 180 C, polyimides are good to 450 C continuously and higher for short periods. I have yet to get polyimide tape to even show a hint of thermal damage, no matter how badly I abuse it.

author: john_c
author_rank: Contributor
timestamp: May 12, 2017, 10:08:28 pm
content: Is the PETG sheet that I linked to equivalent in this application to PET? What is "glycol modified"?Ack, plastics are so complicated! This is worse than dealing with my 3D printer.