9kV CW multiplier potting process - Page 1
Forum section: electronics / projects

author: doktor pyta
author_rank: Frequent Contributor
timestamp: June 09, 2020, 10:00:51 pm
content: I've been asked to share some details of potting (moderately) high voltage assembly.So below You will find some subsequent stages and related photos.Sorry for not documenting all of the steps.If someone have any suggestions please share.Procedure:1. reflow soldering of components2. attaching wires3. ultrasound cleaning4. drying5. HV testing before potting (minimize corona discharge!)6. preparing special mould (I'm proud especially of this design from 2012)7. covering internals of the mould with 'release agent' (seems to be more less stearine in naphta)8. putting HV assemblies into the mould9. mixing two-component silicone10. degassing under vacuum (approx 30s)11. careful potting12. second degassing (approx 30s) to get rid of air trapped between components and PCB13. 24h curing at room temperature under atmospheric pressure14. HV testing after potting

author: doktor pyta
author_rank: Frequent Contributor
timestamp: June 09, 2020, 10:02:25 pm
content: p

author: doktor pyta
author_rank: Frequent Contributor
timestamp: June 09, 2020, 10:04:27 pm
content: p

author: doktor pyta
author_rank: Frequent Contributor
timestamp: June 09, 2020, 10:09:20 pm
content: p

author: coppercone2
author_rank: Super Contributor
timestamp: June 09, 2020, 10:11:59 pm
content: depending on the viscosity of your epoxy, you might want to mix it under vacuum. need specialized equipment. If its anything more then a light syrup, you only get the best results if you mix it under vacuum or mix it in a nozzle. If viscosity is low, there is no benefit.If you can't do this, I recommend getting a blank epoxy cartridge with a mixing nozzle and refill it with silicone epoxy.What is your epoxy type? I am interested. I assume its some kind of epoxy silicone.Also, for complex things, you need to do a vacuum pour, if you pour it in and pull a vacuum, you can make situation worse. But this depends all on viscosity. This is particularly relevant for inductor/transformer winding's.

author: doktor pyta
author_rank: Frequent Contributor
timestamp: June 09, 2020, 10:16:53 pm
content: I used low viscosity transparent silicone:https://www.tme.eu/en/details/zal-sil-011/encapsulating-materials/ag-termopasty/art-agt-219/

author: coppercone2
author_rank: Super Contributor
timestamp: June 09, 2020, 10:18:47 pm
content: https://abbess.com/wp-content/uploads/2016/03/Degassing-Mixing-V2-Secure.pdfyou need to control pressure. check the tables to see what your procedure should be.Based on your viscosity (2000 Cps), you can get away with atmospheric mixing and degas (so long pressure is controlled), and vacuum mold filling is either recommended or necessary depending on mold geometry.Read page 4.

author: coppercone2
author_rank: Super Contributor
timestamp: June 09, 2020, 10:23:37 pm
content: also be careful of ultrasonic cleaning, you should do a flush with clean solvent and bake it afterwards I think.Keep in mind any bubbles will cause corona damage possibly. I say this because HV potting is different then say, mechanical potting, which I think is less demanding, and there is alot of great info on vibration hardening that might not be as useful for reliable HV

author: ahbushnell
author_rank: Frequent Contributor
timestamp: June 09, 2020, 10:54:41 pm
content: Quote from: coppercone2 on June 09, 2020, 10:11:59 pmFilling under vacuum works very nice. No bubbles introduced when pouring.

author: texaspyro
author_rank: Super Contributor
timestamp: June 11, 2020, 04:39:54 am
content: When making composite rocket propellant. it is common practice to add a few drops per kg of thin silicone oil to help minimize bubbles and voids. Typically RC car "shock oil" is used. Shock oil is used to fill things like shock absorbers and steering dampers. You want to use the thinnest version you can find. The binder used in most propellent formulations is HTPB... basically a polyurethane.

author: floobydust
author_rank: Super Contributor
timestamp: June 11, 2020, 06:26:41 am
content: I add PC board holes and slots to ensure the potting compound flows in nooks and under parts. It also helps the compound bond to the board and not peel off a side, as a big slab.I get most air bubbles added during mixing and pouring.For HV multipliers, an output resistor is added to limit diode current if the load can ever arc.I run the boards in air in total darkness to see where the corona hot spots are, if the PCB layout is not so great you can see the glow. Sometimes a sharp edge or spacing is missed.Some HV rectifier diodes are sized to be potted i.e2CL71MTis SMA yet rated to 8kV and I estimate ~2.5mm gap between pads. You wouldn't use that in air.

author: ahbushnell
author_rank: Frequent Contributor
timestamp: June 11, 2020, 02:52:48 pm
content: Quote from: MasterTech on June 11, 2020, 12:42:15 amI agree. We always make slots under HV components.

author: doktor pyta
author_rank: Frequent Contributor
timestamp: June 13, 2020, 12:04:12 pm
content: Thank you for valuable feedback !