[O/T] "Depopulating" a PCB

[Leandro]

I recently won an online auction for the approximate dollar-equivalent value of $0.67.  It's a telephone/data multiplexer, which I bid on mainly for the nice chassis.  However, the PCB has a lot of juicy components (silver mica caps, trim pots, quad op amps, push buttons, large electrolytic caps, etc.).

The PCB is covered with a green coating, which flakes white when I take a soldering iron to it, and even though the pads seem to be exposed from this green substance, I can't desolder anything from it (the solder won't melt).

Any tips on how to desolder the components?  Do I need to remove the green coating?  How would I do that?

Thanks!

[b_rogers]

if you dont care about the pcb, take a propane torch to the solder side while holding the pcb with a pair of "vise grips". heat the board up and give it a smack on a board or on a floor surface you dont care about and the components will fall off pretty easy.

hope this helps

Brent

[Leandro]

if you dont care about the pcb, take a propane torch to the solder side while holding the pcb with a pair of "vise grips". heat the board up and give it a smack on a board or on a floor surface you dont care about and the components will fall off pretty easy.

hope this helps

Brent

...and if I do care about the PCB (or, more realistically, if I don't have a propane torch)?  

[black mariah]

Hairspray and a Bic. :lol:

[niftydog]

the solder won't melt

how much grunt does your iron have?

how's your technique?  

I wouldn't bother removing the coating... provided it succumbs to the heat of the soldering iron.


Try reflowing the old solder using a bit of new solder. Sometimes you need to make a "heat bridge" of molten solder to get enough thermal conductivity into the joint.

often, wave soldered boards have tiny amounts of solder and can be really hard to de-solder.

[Leandro]

how much grunt does your iron have?

I don't recall right now if it's a 30W or 40W iron.  Cheap in any case...

how's your technique?  

Definitely insufficient.   :lol:

I wouldn't bother removing the coating... provided it succumbs to the heat of the soldering iron.

I was afraid of somehow damaging the iron tip if the coating got on it, but what the hey...  Technique-related, I realize that my iron tip conducts heat much better on the sides of the shaft than on the tapered tip itself.  I guess that's why I'm not getting enough heat, but on the other hand, I can't get anything but the tip into such small spaces.

Try reflowing the old solder using a bit of new solder. Sometimes you need to make a "heat bridge" of molten solder to get enough thermal conductivity into the joint.

Yeah, I just read something to this effect.  I'll give it a try.  How exactly does it work?  Do I apply extra solder to each soldered pad, let it cool and then desolder it?  (Don't let it cool?)  Also, I'm using braided desoldering wick, which I now think is a waste of time, because between the heat dissipated by the copper wick and the tapered-tip-not-really-hot-enough issue, I think I'm going nowhere.  Any pointers?  Just heat-sink the component and ignore the desoldering wick?

Thanks, niftydog!  I hope to improve my technique with any pointers you can give me.

[Paul Perry (Frostwave)]

It isn't easy removing chips from boards that are double sided, with plated thru holes. Maybe borrow a paint stripping gun & see if that will do instead of a torch.
I only try to remove chips that are worth more than $5 or are out of production, using solder wick to soak up the solder. But, a roll of solder wick doesn't go far, and costs much more than your board!
I don't find solder suckers much use on double sided boards. But if you are using one, try to wiggle the leg while the solder is still molten so  it is free from the side of the hole. Sometimes you have to add solder to try again.... in any case it is valuable practice for when you have to fix something.

[Hal]

if its single sided, you might be able to get components out with a screwdriver/pliers.  I've stripped a bunch of stuff like that.  Slow and tedious, but gets easier once you've done it a cople times.


If you _are_ gunna flamethrow the solder, I recomend doing it outside.  That green stuff might be toxic!

[Leandro]

There are tracks on both sides of the board, but all the components are on the same side.

[Jason Stout]

Paul Perry wrote:

Maybe borrow a paint stripping gun & see if that will do instead of a torch.

I think this is your best bet.

[Alex C]

I recently got a heat gun (paint stripping gun- it's in the paint equipment aisle at Lowe's) and promptly salvaged hundreds of parts from an old Karaoke tape recorder machine, a computer monitor, and several other "junk" PCBs that were laying around.  It was somewhere around $15-20, and I say it was well worth it considering the parts I've gotten already and the thousands I have yet to "set free."  ICs, transistors, and  their respective sockets come out super-easy, because heat can be applied to all pins at once.

I set the PCB in a bench vise, hold the heat gun in my right hand on the solder side, and use a hemostat in my left hand to pick out the parts I want.  So I'd recommend having one of these around for future desoldering use.  You can always get an old VCR or some sort of electronic equipment from a thrift store for next to nothing, or stop someone from throwing away a broken one.   The gun can also be used for heat-shrink tubing, frozen pipes in the winter, and warming day-old food that's been at the workbench as long as you have.  

Alex

[niftydog]

I was afraid of somehow damaging the iron tip if the coating got on it

should be ok. Just keep cleaning and re-tinning it between each use.

How exactly does it work? Do I apply extra solder to each soldered pad, let it cool and then desolder it? (Don't let it cool?)

You tin the tip of your iron with some fresh solder, then apply the tip, with the solder inbetween the tip and the component lead. This forms a bridge of molten solder between the tip and the pad and allows more convection to the joint.

Forget about ICs, too hard without a desoldering station.

For two lead components, just try heating up each leg in turn and "walking" the part out. Or, if you can, heat both pads at the same time and just pull the part right out.

As for the space issue, if you're not worried about saving the PCB, just hack into it! Who gives a crap if you burn it!

Some times you can just snip the leads of the component close to the board and still have a useful length of lead left over.

Another option; if you happen to have a semi decent butane soldering iron, some of these come with "blower" tips that effectivly work like a hot air soldering station. These do the same trick as the heat gun, but much more localised and probably far hotter.

Keep the solder wick handy, it can be useful sometimes. But it would seem your iron is lacking in horsepower!

[Leandro]

Thanks, guys!

I already pulled quite a few parts: some silver mica caps, two 25k trimpots, some large-value axial electrolytic caps, and others.

I simply applied some extra solder to the bottom of the pads, and with the very iron, I just nudged each lead out through the holes.  Trimpots and other components with more than two leads took some "walking out", like niftydog said, but it worked out.

Tomorrow I'll try to pull some ICs, but I have the feeling I'm gonna toast them...   :?

[niftydog]

BTW, normally, electrolytics (particularly large ones) aren't worth re-using. They are the least reliable electronic components by far. They dry out, drop capacitance, series resistance goes up... basically bad news.

[Leandro]

BTW, normally, electrolytics (particularly large ones) aren't worth re-using. They are the least reliable electronic components by far. They dry out, drop capacitance, series resistance goes up... basically bad news.

OK, good to know.  Anyway, don't see much use for those large elctrolytics except for power supplies, and I'm not in need of any just yet.