Potting your pcbs?

[therecordingart]

Now that DIY has consumed my entire life, I'd like to start playing with making my projects indestructible. There are folks that pot their boards so they can't be reverse engineered, but I want to pot my boards to make them resistant to shock. I want my pedals to survive everything from beer spills to falls from a helicopter.

Any resources on DIY electronics potting?

[defaced]

I can't help you much, but I can give you a key word that might get you going: conformal coating.  It's a coating used on board that are in harsh environments (industrial setting with lots of dust for example) to prevent the board from failing due to foreign material. A couple random thoughts:  I do not know if there are conformal coatings out there to help prevent shock though.  I suspect that off board wires would be the most susceptible to shock failure. 

[artifus]

hot glue gun?

[John Lyons]

FIXED!
I want to pot my boards to make them resistant being repaired. 

[mattthegamer463]

John is right; being able to repair is probably better than being shock resistant.  How often is there going to be a part failure compared to how often will it fall out of a helicopter?  No die-cast aluminum case from hammond will survive that anyway. 

Board resin might be akin to body armor for your internal organs.  Protect the outside and you don't have to protect the inside.  Have a bulletproof enclosure and the board won't need any more protection.  Fill the excess space inside with some pieces of low-density packing foam and you're all set.

[Tony Forestiere]

Minimally expanding foam insulation?

[mattthegamer463]

Minimally expanding foam insulation?

I was thinking more just some soft foam pieces.  That way they are easily removed to make repairs or modifications. 

[markeebee]

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.

[boogietone]

Good mechanical connections to board.
Solid solder joints.
Strain relief for offboard wires.
Board not floating around in the box.

[PRR]

> Silicone sealant.... Google tells me dielectric insulation is 23000volt per millimeter

Maybe when FULLY cured.

One of my more memorable smoke-tests was a fire in a 600 Volt amplifier from silicone goop which was cured to the touch but apparently not 100% reacted. Ascetic acid (vinegar smell) conducts pretty good.

Not an issue at 9V, or if you wait some days before pouring in high voltage.

[Barcode80]

Minimally expanding foam insulation?

I was thinking more just some soft foam pieces.  That way they are easily removed to make repairs or modifications. 

MXR tried foam, which is why so many vintage MXR pedals have grime and deteriorated foam remnants everywhere on the inside...

[mattthegamer463]

Minimally expanding foam insulation?

I was thinking more just some soft foam pieces.  That way they are easily removed to make repairs or modifications. 

MXR tried foam, which is why so many vintage MXR pedals have grime and deteriorated foam remnants everywhere on the inside...

I suppose if you were looking for longevity on a scale of decades it may not be ideal.  Keep in mind, however, that perhaps foam in the '70's is not the foam that is available now.  I'm sure a company has a long lifetime foam that might work, if 50 years of service is required out of a pedal.

[G. Hoffman]

Repairablity is vastly more important than shock resistance.

But there are any number of things you can do to resist shock that are more important than potting your boards.  Don't mount any mechanical components (I.E., pots, jacks, and switches) to the PCB.  Make sure your wire runs have enough slack to cushion the solder joints.  Use two sided boards with plated through holes for better solder joints, but don't rely on solder for mechanical stability.  Redesign your boards so you have enough lead on resistors and caps to provide some shock resistance.  (e.g., 1/4W resistors should be on at least 4 mil spacing, not 3 or 3.5.  Bending the leads right at the body of the resistor puts strain on the resistor.)  When you are making solder joints, bend the component leads out for a bit of mechanical connection, and clip off the excess lead BEFORE you solder (clipping it after you solder can cause microscopic cracks in the solder joint).  Make sure the bends in your resistors and wires have enough radius to avoid damaging them (sharp 90 degree bends look pretty, but they aren't good workmanship).  Mount your circuit boards on standoffs that use screws.  Yes, the screws will be visible form the outside of the box.  Deal with it.

Mind you, most of those are a bit excessive for pedals.  But they will all be at least as helpful, if not more so, than potting your circuit.  Most of them also have the benefit of making your pedals easier to repair, which is more important in the long run.  But if you REALLY want to build your pedals as robustly as possible, go look up the NASA workmanship standards for electronics.  It will give you more ideas for making your pedals robust than you could ever need.


Gabriel

[therecordingart]

Repairablity is vastly more important than shock resistance.

But there are any number of things you can do to resist shock that are more important than potting your boards.  Don't mount any mechanical components (I.E., pots, jacks, and switches) to the PCB.  Make sure your wire runs have enough slack to cushion the solder joints.  Use two sided boards with plated through holes for better solder joints, but don't rely on solder for mechanical stability.  Redesign your boards so you have enough lead on resistors and caps to provide some shock resistance.  (e.g., 1/4W resistors should be on at least 4 mil spacing, not 3 or 3.5.  Bending the leads right at the body of the resistor puts strain on the resistor.)  When you are making solder joints, bend the component leads out for a bit of mechanical connection, and clip off the excess lead BEFORE you solder (clipping it after you solder can cause microscopic cracks in the solder joint).  Make sure the bends in your resistors and wires have enough radius to avoid damaging them (sharp 90 degree bends look pretty, but they aren't good workmanship).  Mount your circuit boards on standoffs that use screws.  Yes, the screws will be visible form the outside of the box.  Deal with it.

Mind you, most of those are a bit excessive for pedals.  But they will all be at least as helpful, if not more so, than potting your circuit.  Most of them also have the benefit of making your pedals easier to repair, which is more important in the long run.  But if you REALLY want to build your pedals as robustly as possible, go look up the NASA workmanship standards for electronics.  It will give you more ideas for making your pedals robust than you could ever need.


Gabriel

Thank you very much for all of this. It's a sickness that I have to go overboard with everything I do.

[FiveseveN]

No die-cast aluminum case from hammond will survive that anyway.

How high is the helicopter flying? Terminal velocity for a "typical" stompbox isn't terribly high, nor is its mass. And Al is quite soft, so even if it landed on diamond, I bet it would not disintegrate.

Protect the outside and you don't have to protect the inside.

Not against shock, unless you have somehow stolen inertial dampeners from a Federation Starship. PCB standoffs can be used as shock absorbers. That's one argument against PCB-mounted pots and jacks. In fact, I've seen more loose jacks and lifted jack and pot solder joints than any other form of shock-related injury. The usual parts (R ,C, Q, U) usually have such low mass that acceleration is not a problem. But yeah, the NASA papers which have been referenced here a couple of times are a great standard.

PS: it's a shame how much people protect their rack equipment and how much they abuse stompboxes, isnt't it?!

[R.G.]

Its simpler and cheaper to build two at once. One gets used, the second is the "hot spare", tested and ready to be used while the primary is being repaired.

If ever.

Just as I hit send, it occurred to me that having a hot spare is especially important if you pot up the whole thing because potting makes it essentially irreparable, unless very special precautions are taken, notably hardening the input/output positions into potted-in pins so the whole thing becomes an "integrated circuit".

Just as I was about to hit send, I remembered that the process of potting all by itself can be lethal to electronics. If you use stuff that evolves gasses or chemicals (acetic acid, notably) then the evolute can cause corrosion even at low or no volts through electrochemical corrosion. And hard-setting materials frequently shrink on setting, so they can literally crack leads, joints and components.

Potting in beeswax is probably most chemically and mechanically benign, but even then, the relative permittivity of beeswax is higher than air, so it does affect the inter-electrode capacitances of everything - not an issue for some things, but on FET-input whatsits, it's enough to compromise stability or AC leakage.

[G. Hoffman]

Thank you very much for all of this. It's a sickness that I have to go overboard with everything I do.

I'm a fellow sufferer.


Gabriel

[G. Hoffman]

Its simpler and cheaper to build two at once. One gets used, the second is the "hot spare", tested and ready to be used while the primary is being repaired.

If ever.

For a production line, absolutely; for a one (or two) off, maybe yes, maybe no.  But there is also the fun aspect to this, since we are talking DIY here, and that operates on an economy which is completely different.  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.


Gabriel

[artifus]

hot glue.

it is strong yet flexible. maybe applied selectively. can be easily peeled off. what am i missing here?

[Minion]

The Military and nasa still use a lot of through hole boards in Fighter jets , rockets and other High G applications because through hole is more durrable and less prone to failure than SMD so i think your pedals should hold up pretty good at up to 10 g"s .........