Homemade PCBS: Potting, Conformal Coatings, Tinning

[therockdemon]

For those of you into etching and drilling your own PCBs from copper clad board... I have some questions as well as food for thought.  Comments are encouraged.

Tinning the board
I noticed some individuals will etch their boards, drill, and then immediately after will tin all the remaining copper.  Does covering all the exposed copper with solder have any benefits to it?  I suppose this will prevent the copper from oxidizing before you've added all the components to the board.  I suppose heat from the soldering iron causes this premature oxidization, which could theoretically result in poorer connections?

Pottings and/or Conformal Coatings
Has anyone considered coating their finished, populated boards with wax or a commercial conformal coating for durability and (a little) noise reduction?  Conformal coatings sold by silicon manufacturers are quite pricy, but I guess an 80% parafin wax 20% beeswax mix is cheap and effective.  Submurge your component into the 150 degree molten wax mix for 15-20 minutes in a double boiler, and wallah.  One individual here (click) recommends the following for potting pickups with.  If interested, click that link, read the article, and comment back here please.

I have always used readily available canning wax (parafin) available at grocery stores.  Recently, I've heard that some manufacturers add a little beeswax to keep the wax soft and prevent it from becoming brittle and flaking or cracking with age.  [Ed. note – John hadn't tried this yet and didn't know what percentage of beeswax manufacturer's use.  I'm guessing about 10%-20% beeswax based on other applications I've seen for wax.] [Later Ed. note– that wasn't a bad guess – I recently stumbled across a Fralin article at the StewMac website where he recommends 20% beeswax.] [Still later – I (editor) recently potted some single coil pickups using these instructions.  I couldn't find beeswax at WalMart so I bought some Kiwi "Camp Dry Beeswax Water Proofer" intended for sealing boots and such.  This stuff contains beeswax and a small amount of lanolin.  A seven ounce container was two bucks and worked beautifully with two pounds of parafin.]

[R.G.]

Tinning the board
I noticed some individuals will etch their boards, drill, and then immediately after will tin all the remaining copper.  Does covering all the exposed copper with solder have any benefits to it?  I suppose this will prevent the copper from oxidizing before you've added all the components to the board.  I suppose heat from the soldering iron causes this premature oxidization, which could theoretically result in poorer connections?

It does prevent oxidation. Oxidized copper is ugly, but I've never had a board malfunction in a way I could pin on copper oxidation, though. Makes initial soldering a lot easier if you tin the whole board. Also makes getting solder threads between traces easier if you have poor soldering skills.

Back when tinning the entire board was common in commercial houses, they would paint the board with liquid flux, then run it over a roller which was rolled slowly in a solder pot. The (tinned steel) roller picked up a thin layer of solder and transferred it thinly and evenly to the copper. I like a similar process where one uses a brass pipe of about 25-50mm/1-2" diameter coated with solder and heated inside with a torch. The pipe is passed over the fluxed surface of the etched board. If there's a proper amount of solder on the pipe, the board gets a thin solder coating.

Electro-less tin plating solutions do much the same thing, making a few micons of tin over the whole copper surface. Looks neat, prevents copper oxidation.

Has anyone considered coating their finished, populated boards with wax or a commercial conformal coating for durability and (a little) noise reduction?

Many have. I'm not sure what noise reduction you get, other than perhaps damping microphonic ceramic caps.

Be aware though that wax is not guaranteed to be either a perfect insulator, free from contamination, or innocuous in use. If you do this, use pure stuff. The real issue is that MOS and some JFET devices have really, really, really sensitive inputs, and anything that conducts a voltage, no matter how small, can be picked up. Also, any solid material has a dielectric constant, where it multiplies the capacitance between any two conductive surfaces. The wax will increase capacitive crosstalk by some multiple. This may or may not be an issue, depending on the circuit.

Of course, collected dust and generic grunge will have the same issues, so good wax coatings are going to help with that. It's a tradeoff. The military used to use waxes until they found out that some tropical microorganisms eat beeswax/paraffin mixtures, so they went to synthetics long ago.

One individual... recommends the following for potting pickups with.  If interested, click that link, read the article, and comment back here please.

Potting wound pickups is a good thing. Wound pickups are not subject to the same high impedance issues as FET circuits, so it keeps (most) things from eating the wires. The biggie for pickups is that a wax impregnation is good for keeping the wires in the same place and damping any mechanical issues, which is really good in pickups.

For just keeping contamination off and passivating against oxidation, spraying finished boards with clear lacquer is common as well. Same comments as wax.

[R O Tiree]

I use a non-electro-process tinning solution.  Dead easy (a little smelly, though) and a very neat finish.

I used to use a thin layer of solder in the past.  Use plenty of flux and ensure that the traces get good and hot.  Obviously, if you over-do it, then the traces might start to lift, so there's a "Goldilocks" temperature which you'll find very quickly.

[defaced]

Electro-less tin plating solutions do much the same thing, making a few micons of tin over the whole copper surface. Looks neat, prevents copper oxidation.

This is what I do, and soley to easy soldering by not starting out with oxidized copper.  This has worked well in practice and consider the extra step worth it.  I use MG Chemical's Liquid Tin.  My board making process at the moment (it's still being developed, applying the photo resist is a bit of an art) is tarnex (remove oxidization), scrub if needed, apply photo resist, expose, develop, etch, acetone to remove the resist, liquid tin, drill, store/use as needed.  The liquid tin has impressed me at how well it prevents oxidization, but it is easily removed by the Tarnex which *shouldn't* be an issue, but stuff happens. 

Having spent the past few days on Yahoo's PCB making group, there seem to be alot of different ways to protect a board/solder mask, some are more suited for production (duh) while others are pretty OK for DIY.  I plan to attempt some of the DIY solder mask approaches out there.  The UV films/paints appeal to me most at the moment, but that could change with the wind. 

[therockdemon]

I'll do the tinning, and not bother with conformal coating.  For interference/noise reduction, I already use shielded wire and ground everything properly.

I think I'll go with this (click).  I should be able to just go over the copper, and not the spaces between the tracks.

I also read that the liquid tin stuff, much like the conformal coatings by Dow Corning I was reading on, have a limited shelf life.  I need stuff I can throw in a drawer and expect to be just fine 2 years from now.  I don't make pedals often, but I do have an interest in quality when I do make 'em, because after all, it's part of my rig.  =]

Any other useful/interesting PCB improvements people do?

[defaced]

I also read that the liquid tin stuff, much like the conformal coatings by Dow Corning I was reading on, have a limited shelf life.

I keep my go to tinning solution in an unsealed lidded tray.  It's yellow, has precipitated some white stuff, and still tins just as it did when it was new.  It's been there easily a year - I'd have to look at my old Mouser orders to see when I ordered it.  Spec sheets are for production, DIY can get away with alot of abuse that would be ditched instantly in a real operation. 

[R O Tiree]

> I also read that the liquid tin stuff, much like the conformal coatings by Dow Corning I was reading on, have a limited shelf life.

There's a lot of stuff printed in datasheets that is largely an @rse-covering exercise.  Of course, there is also stuff that you must pay attention to.  Bitter experience will enable you to decide which is which.  As defaced says, this particular solution lasts a lot longer, usually.  Of course, you don't have to make up the whole batch instantly... you can just make up a small quantity and keep the rest in powder form, in which case it has a vastly extended shelf-life.

[therockdemon]

Wanna make a specific recommendation?  I don't really want to pay more than I would for the Chemtronics CW8200 Rosin Flux Dispensing Pen linked above.

[davent]

Simply spraying on a coat of clear lacquer, whether waterbased or based on nasty solvents, will prevent oxidation of the copper traces. I use the same stuff i use to finish the enclosure. To solder you just proceed as normal, touch the hot iron to the pad and the lacquer's gone and everything solders up just fine. For me this works better then Liquid Tin, which i find if left unsoldered for any length of time, is about as easy to solder as the bare copper would have been when left exposed for the same length of time.

dave

[Mustachio]

I use the Mg chemicals liquid tin. I Usually tin it right after I etch and clean off the photoresist, before I drill. Its quick and simple, and it barely smells ( but it does smell a lil like burning hair, maybe its my nose hairs haha). It seems to make soldering easier but I usually never have much of a problem soldering. It looks pretty.

I haven't read that it has a short shelf life , in fact I thought I heard it lasts forever. I just pour back the small amount I use into the bottle. I usually solder my board within a day or 2 of etching/tinning so I cant comment on how it is in the long run.

I would imagine tinning the entire board with solder starting with a flux pen to make it flow better could have problems like lifting traces from heat and just being ugly from the solder not being even. As well as taking a lot longer and being more tedious. the MG chemicals tin takes about 5 seconds turns silver almost instant. And the small bottle costs about 10 bucks same as the flux pen. Id say experiment and see what you like best. As for a solder mask Ive never done it. I don't see it practical for what I do but it does look nice when done right.

The flux pen would be good for smd stuff though. I just used the flux goop, sorta helps keep them lil buggers down on the board. But I've seen some videos of people soldering smd using a flux pen to prime the pads.

The few times I've seen a pedal with conformal coating I believe was the klon centaur, and I believe that was done to hide the circuit from prying eyes.

[R O Tiree]

Here's the stuff I use: Scroll down to "Immerse Tin Plating".  It looks expensive, but I've had a solution made up for over 6 months and the smaller bottle contains enough powder to make up 2 litres (just under 4 pints).  That's way too much for normal "hobbyist" needs at one go, so just make up half and then make up the other half when it runs out of "oomph".  The whole 2ltrs is enough to tin about 5-600 stomp-box sized boards, which works out at about £0.03 ($0.05?)per board.  It smells a little sulphurous (burning hair? perhaps?), but it's not wildly unpleasant so, as long as you open the back door to vent the kitchen afterwards, her indoors shouldn't get too @rsy with you.

As to tinning with solder, I said in another thread about this that you do, indeed, have to be careful about overheating the traces, but it makes quite a nice finish if you use 63/37 solder and cover the traces with flux beforehand.  Thinking about it, I probably wasted money in flux and solder before I found out about this tinning solution.

[CodeMonk]

I've been using the MG Chemicals Liquid Tine for about 6 months now,
Seems to do the job well enough from what I can tell, so far anyway.