Epoxy Insulation (namely JB Welb) and Inductance?? Bad idea?
rwgast_logicdesign
Posts: 1,464
Ok so lets start by showing you what im up to! Ive made two little board meant to extend my motors cable and include EMF/Noise Filtering. The two heavy cables are for 18v +/- going to the diodes. The "cable" out the end has individuality shielded wires and a shielded core.. These are 5v +/-, Encoder channels A/B, And motor signal +/-
Now usually having the bottom of a PCB exposed is no biggie, but theres a few reasons it is in this case. First of all there are 18v and ground traces exposed here thats begging for a short that could cause some serious issues! Secondly these boards are goung to be mounted to metal. Thirdly they may see some water from rain or puddles due to there posistion.
Now I know most people use hot glue but I dont have any, nor do I have a gun plus hot glue isnt going to be as clean looking as what I would like to do! I was thinking I could clean the boards with acetone and alcohol really well and dry them to make sure all the joints are good and minimize risk of corrosion damage. Then I wanted to mold the board in a block of JB Weld, this will keep them well insulated and dry. Now I looked into JB weld and tested it with a meter it is advertised as an insulator and gives me an infinite resistance reading which is exactly what we want. What worrys me is that the product contains 10-20% Iron filling although this is non conductive and each particulate is encased and seperated by epoxy im worried about something else. Is there any possible way that the Iron particles would cause problems with inductance and just make a bunch of noise cancelling out the whole point of these boards??? It seems to me like its not possible as electricity would have to travel through the epoxy from one particle to the next. But I do know a nautrual earth magnet is strong enough to cause the iron in jb weld to do strange things, ive read about people trying to set hi powered magnets with it, and I also know that inductance is caused by magnatism. The reason i want to use JB weld instead of other epoxy is #1, I have it can get the job done right now, #2 I need to be able to sand and shape the set product and im not sure of any other epoxys not containing iron which can be formed like JB Weld
Now usually having the bottom of a PCB exposed is no biggie, but theres a few reasons it is in this case. First of all there are 18v and ground traces exposed here thats begging for a short that could cause some serious issues! Secondly these boards are goung to be mounted to metal. Thirdly they may see some water from rain or puddles due to there posistion.
Now I know most people use hot glue but I dont have any, nor do I have a gun plus hot glue isnt going to be as clean looking as what I would like to do! I was thinking I could clean the boards with acetone and alcohol really well and dry them to make sure all the joints are good and minimize risk of corrosion damage. Then I wanted to mold the board in a block of JB Weld, this will keep them well insulated and dry. Now I looked into JB weld and tested it with a meter it is advertised as an insulator and gives me an infinite resistance reading which is exactly what we want. What worrys me is that the product contains 10-20% Iron filling although this is non conductive and each particulate is encased and seperated by epoxy im worried about something else. Is there any possible way that the Iron particles would cause problems with inductance and just make a bunch of noise cancelling out the whole point of these boards??? It seems to me like its not possible as electricity would have to travel through the epoxy from one particle to the next. But I do know a nautrual earth magnet is strong enough to cause the iron in jb weld to do strange things, ive read about people trying to set hi powered magnets with it, and I also know that inductance is caused by magnatism. The reason i want to use JB weld instead of other epoxy is #1, I have it can get the job done right now, #2 I need to be able to sand and shape the set product and im not sure of any other epoxys not containing iron which can be formed like JB Weld
1024 x 1365 - 161K
Comments
Also, don't use acetone to clean the board. It's a very aggressive solvent and will attack stuff you don't want attacked.
-Phil
As far as the weld, the reson I wan to pot with it is becuase it can be filed and sanded, do you know another easy to find epoxy without iron that can be ground after setting?
I dont have a scope or anything but maybe in the name of science you could do me a favor, I just took a straight piece of wire and wound a 3 loop coil, I duplicated bot pieces. The first set I globed in JB weld the second is plane bare copper. The idea is to set up a prop LC meter which it looks like someone has written and test the inductance of the plain wire and coild and then the ones with jb weld all over. Im thinking this is a pretty easy thing to do if you have some weld on hand, and im sure you being Phil have tons of test and measurement stuff maybe you could conduct the same simple test more easily and accurately. Im very intrested in getting to the bottom of this wether I use JB weld to POT or not its just something I cant find anyone online acually testing its all here say.
http://www.mouser.com/ProductDetail/MG-Chemicals/832B-375ML/?qs=sGAEpiMZZMvJqaFk9BIiv6jhjM0Pk6Jz0ZBekZO8Zms%3d
data sheet says it ‘s: Optically clear epoxy that can be cured with or without heating. It is tough, water and chemical resistant, an electrical insulator, and has excellent machining properties.”
Have you though about using a large piece of Heat shrink to cover the boards? I've used that methos with good results and I know that Parallax used that method on the SX-Key and SX-Blitz programming modules they used to sell. That gives you the option of cutting off the heat shrink to make repairs/changes later on. A nice thing if you need to re-attach a wire or something.
If you really want to encapsulate the board then you may want to check out the PLASTI DIP coatings. It comes in a can and you can dip the whole board in. Repeated dippings will give a thick coating.
JB weld is great stuff but it is probably not something I would use to encapsulate your board.
Robert
Mineral spirts (known as paint thinner) will work with some plastics, but bad for your liver in prolonged exposure
Alcohol may not disolve plastic, but it is an acid and will corrode lead solder joints. You will see white power that is lead oxide.
Benzene (which is known a white gasoline) is much more aggressive with plastics, even worse for your liver in prolonged exposure.
Touluene is agressive with some plastics and not with others, but it is harder to get. The EPA say it is bad for the evironmnet, but it is a key ingredient in making TNT.
Ammonia glass cleaner is a handy solvent to clean plastics, but I am not sure how it is around electronics. Household ammonia has been harder to get. Apparently one can make explosives with it.
If I must be absolutely safe around plastic electronics, I use a spray can of TV tuner cleaner, but this stuff is refrigerator coolant that has been outlawed for years as it is bad for the ozone layer. This is the stuff that wash the inside of my notebook computer with.
So pick your poison. It is all better living through chemistry.
-Phil
The epoxy at mouser says its black... If I could set an optically cleay epoxy that looked like acrylic when finished and polished that would be totally cool.
If you cover the whole thing in silicon first then what is the point of going over the top of that with JB weld? Is there some advantage? It would just add to the cost and it sounds like it would end up being messy.
It is suitable for insulating 1000 volts or less at 194 degrees F.
I get it at Home Depotk
-Phil
Pure ethyl alcohol is not acidic (or more correctly is less acidic than water) and will completely evaporate - I think you must mean impure alcohol like meths?
Unless you are bound and determined to actually see the boards, why not just buy some small project boxes from Radio Shack and make some small cutouts near the lid for the wires to protrude from, just throw the circuit boards inside and put the lid back on. Then seal the holes with duct seal, which is available at any hardware store or electrical contractor supply.
Duane J
EDIT: And yoy can file and sand it too!!!
Polymorph wouldn't be a good choice for anything that will get hot but anything that will get hot probably shouldn't be encased anyway.
I don't think Polymorph sands or files well though.
You win, wires are vinyl insulated. But I was referring to polyester resins and fiberglass. I believe that printed circuit boards are epoxy resins and fiberglass.
My main point was that it is very hard to generalize. My second point is that it is almost impossible to find a solvent other than dish soap and water that is easy to buy. Acetone is used in nail polish remover because it is significantly less of a hazard to humans.
If wires and vinyl insulation are not part of the board, acetone will do fine. But I would not also use alcohol as the lead oxide can build up and create resistive links where none are wanted.
In general, acetone is not very fast at dissolving vinyl. I have used it to clean surface discoloration off of vinyl covered seating. One just has to be careful not to scrub hard. It is a quick wipe off and that's it.
The winning combination is to use a water soluble soldering flux with solid solder wire instead of a paste flux or a rosin core solder. Then just was with soap and water. Forget all the fancy stuff.
I'd prefer to pot in silicon any day as one can cut the stuff out with a knife for repair. Potting in epoxy is great for high voltage, high heat, and extreme filth - but nothing is really to be gained by doing it with 5 volt and 3.3 volt powered devices.
All clear plastics yellow with age and exposure to UV. So having something that is clear to look at may be rather short lived. Clear epoxy that I used on some tiny h-bridges turned brown with exposure to heat - mostly caramel colored now.
Get it at Walmart cheap by the quart.
Duane J
-Phil
FF
One of Johnson and Johnsons greatest inventions IMHO!!!!