Potting your pcbs?

[FiveseveN]

hot spare

Words of a true veteran (as if mr. Keen's credentials needed any more proof). Look at any "Guitar legend's" rig. Every one of them has a hot-swappable vital component, usually amp and/or wireless system or alternatives. And of course guitars. I sometimes wonder how much they would have saved by shipping a reasonable (YMMV) digital simulation platform rather than a spare tube amp. Some actually do that.
But we have veered offtopic. Though might be a good sales pitch: "You're actually safer buying a second unit than investing in a stronger single unit.".

[R.G.]

I ENJOY the effort of making things so they are less likely to break.  I read the NASA standards for fun, and while I certainly do not build anything that is actually up to the NASA Flight Hardware standard, I absolutely use some of the techniques they spec.

OK. Then think about the following:
1. Make up a potting box. This could be the actual box, but something smaller is better.
2. Harden the mounting and connection points. Figure where wires will go into and out of the hardened block and put something like screw tabs or 1/4" fastons there, to be held in place by the potting. Also consider embedding screw sleeves in the potting to mount the finished unit.
3. Soft-pot the circuit board in soft setting urethane gook. Look at someplace like Micro Mark or other modelmaking site for this stuff. Get a conformal coating of rubbery stuff on it first.
4. Now hard-pot it with an outer layer that holds the mounting inserts and wire connection points in place.

[G. Hoffman]

I ENJOY the effort of making things so they are less likely to break.  I read the NASA standards for fun, and while I certainly do not build anything that is actually up to the NASA Flight Hardware standard, I absolutely use some of the techniques they spec.

OK. Then think about the following:
1. Make up a potting box. This could be the actual box, but something smaller is better.
2. Harden the mounting and connection points. Figure where wires will go into and out of the hardened block and put something like screw tabs or 1/4" fastons there, to be held in place by the potting. Also consider embedding screw sleeves in the potting to mount the finished unit.
3. Soft-pot the circuit board in soft setting urethane gook. Look at someplace like Micro Mark or other modelmaking site for this stuff. Get a conformal coating of rubbery stuff on it first.
4. Now hard-pot it with an outer layer that holds the mounting inserts and wire connection points in place.

Yeah, I'm afraid I don't enjoy working with epoxy, or any of that squidgy crap, so I that isn't a part of it I would get into even if I thought it was a good idea (which I don't).    Plus, they make things unrepairable, which I don't much care for.  Which is why I said potting the board is a bad idea, but making the mechanical connections of your stuff as solid as possible is a much more reasonable action to take, I think, and much more fun.

I certainly wasn't saying you were WRONG, by the way, just pointing out that sometimes there are factors other than what is fiscally/practically best practice.  If I were doing any heavy gigging, I would certainly be carrying spares for most things.


Gabriel

[PRR]

> Repairablity is vastly more important than shock resistance.

_IF_ there is a repairperson around whenEVER needed.

Satellites can be "easily repaired" (access panels, snap-connectors) yet that does no good after the count-down hits "GO!" The arctic radars which warned us of evil Russian bombs were sometimes snowed-in for months. Heck, my electric company can't fix a wire break all that fast.... cost-cutting has left only a skeleton staff.

When a non-technical player is gigging in Podunk, and the essential pedal breaks, who is going to fix it NOW? Yes, some players know which end of the iron, and some can afford to carry a technician in the truck, but what about the other 99%?

> build two at once

To a point, that's an excellent idea. Even satellites do it: the Shuttle had, what, three antique computers working the same problems and voting on the answers? Military radio is normally serviced by replacing the deader with a spare radio. Long before stereo, broadcast mixers had two high-quality output boosters so that a failure in one could be quickly patched-around. (You think losing $75/hour is bad, broadcasters can lose $1000 a Minute if commercials don't air.)

Considering that many pedals are under $100 of stuff, and most of the "added value" (planning, thinking) does not double when you make two, and a few key spares is not THAT much more burden in the Econoline, an extra makes sense.

What is the Miracle Of Electronics? It is that one Pete Townshend and one Marshall can replace 100++ non-amplified guitarists. That's REAL economy! If part of the economic leverage is that the one guitarist has to bring most of two full rigs, that still saves 98 paid players. (Makes you think about the economics of amplification versus full employment for musicians.... 37-man swing bands replaced by 4-man bands, 33 musicians out of work.)

As a life-long gear abuser, IMHO potting is a Bad Idea. Most failures are Bad Soldering. After that, I think anybody using $1 pots and jacks is begging for trouble. Yes, it is amazing how well cheap parts do hold up; but $20 pots and jacks hold up better (usually).

[greaser_au]

Silicon sealant, like you put around your bath.  I use it to hold boards in place, and tack wires down, and all sorts.  And you can get a solution that breaks it down/removes it really well, so you can repair boards if necessary.

Google tells me dielectric insulation is 23000volt per millimeter and resistivity is 3 to the power 15 ohms per centimeter, so it shouldn't create any problems conduction-wise if you splurge it all over.

Apparently, the chemical reaction that takes place when it's curing can corrode copper (I forget, and never understood, the chemistry) but I've never had any problems using it.  But that doesn't mean anything.....

If it doesn't work for you, it wasn't my idea.

Yeah!  Don't use window/glass/bathroom/gutter sealant if you're potting, you want a sealant that does not mention acetic acid/vinegar smell on the label & says 72-hour cure time - use clear or maybe white (Dow Corning RTV738 is the 'real thing')  if you can get it. I made a few comments regarding  RTV (room temperature vulcanising) sealants a couple of years back:    http://www.diystompboxes.com/smfforum/index.php?topic=85406.msg716301#msg716301

david