desoldering double sided boards

[soundguruman]

It's better to cut them and gently heat/ pull the lead thru the hole,
rather than try to de-solder a multi legged object...

a temperature controlled soldering station- bigger mass and bigger circuit tracks = more power required.
need more heat to de-solder than to solder.
This is why an ordinary soldering pencil iron is generally useless for de-soldering...

plenty of electronic grade liquid flux - this is a huge benefit and will make the job much easier
good solder wick

or a solder vacuum which is serious if you are serious...

clean the area to be DE-soldered thoroughly...( alcohol, Q tips, toothbrush)
Old flux and crud will block heat transfer and prevent solder from flowing.
Any dirt or flux or crud = bad results. Think clean.

On Both sides of the board:
put fresh solder in the joint before desoldering
put electronic grade flux in the joint before de-soldering

*put flux on the wick and use rosin fluxed wick.* A big help as you will find...
use only clean fresh solder wick. Old oxidized copper wick won't transfer the heat.

The soldering iron (station) tip is perfectly clean, use a tip cleaner instead of a sponge.
the tip is tinned with fresh solder before wicking, cleaned and re-tinned during wicking.

Don't expect good result unless
the tools and board are very clean
you have plenty of adequate heat
you have clean fresh solder on the tip, always.

Clean everything after you are done with alcohol toothbrush paper towels, and get all the flux off, before you solder the new part in.
The board should look almost brand new if you have done this correctly...

[R.G.]

This discussion highlights a philosophical point. In desoldering, you're trying to remove a part from a board (well, duuuuh, R.G. ) The difficulty in doing this depends on whether you have to wind up with a salvaged good part or a salvaged good board afterwards.

If you must have a good board, the simplest way to fix this is to destroy the part by clipping away all but the through-the-hole section of any and all leads. Removing a snippet of lead wire even inside a double-sided hole is easy. What makes double-sided hard is the heat being wicked away from the other side, and the amount of metal mass in the hole.

If you must have a good part, the simplest thing to do is to destroy the board around the part lead; a hot air gun or ... propane torch! ... will do this just dandy and heat the leads quickly so the heat doesn't have time to travel back up to the part.

If you have to have both a good board and a good part after the removal, it's much more difficult. The only good way is to have a massive heat reservoir in the soldering iron tip - big tip, and/or very agile temperature control - so you can apply a lot of heat (and note, this is different from temperature, in the same way that current is different from voltage) to get the temperature of the joint up to solder-melting quickly, remove the lead, and get out quickly.

In fact, now that I write that, the analogy is exact. You want a temperature (voltage) limited source of all the heat flow (current) that's needed. This will put out a lot of heat flow, but without over-temping. Unsoldering with a small jeweler's torch that puts out thousands of degrees but low heat flow is a recipe for disaster. Unsoldering with a 1-pound block of copper heated to 20C over soldering temp ought to be a piece of cake. I'll have to rig up something to try that.

[km6xz]

I find that Chip-Quik is the best for parts AND board salvage. I have hot air, vacuum station, solder suckers, wick, and a home made board heater. Each does best for somethings. For general audio work, where heavy ground planes are sinking a lot of heat, a jack, multi-pin ic's, or large ic's it is best for keeping heat to low levels on the part and board. I use well fluxed wick only to clean excess solder from pads or traces, but for some connections it can use used for component removal also.
A jack with supports soldered to the ground plane can be hard to get enough heat applied without damaging the board but it is a 10 second operation with Chip-Quik and the part falls off. You do not need much applied to combine with the lead solder to lower the melting point of the new alloy of lead of the board and the low temperature metal in ChipQuik to keep it molten down to 120-130F, so you can pick up the part afterwards with bare fingers. Be sure to remove all the old solder however or any remaining alloy will reduce the melting point of the new replacement joint also. That is a good use of solder wick.

Removing large fine pitch ICs without messing the leads is not always either but apply ChipQuik and tap the board upsidedown and it falls off. It is a must have if one wants to reuse an IC or fragile part. I recently had to remove and replace a fine pitch 208 pin IC which came of in seconds with the low temp solder but when I soldered in the new one, a couple pins got bent so I had to remove the chip and reinstall it. It was a $84 chip so I did not want to lose the new one. I used the same technique to get it off and was able to straighten the pins. The pins were bent because I had to solder them with my fine tip iron that touched the pins. So I went to Youtube to see how the experts were doing with if I had no solder paste, the usual way of soldering such a large part without touching the legs.
I came across a video of using a technique that used a large chisel tip instead and went out and found one. It really was effective and with a little practice, many times faster.
Professional SMT Soldering: Hand Soldering Techniques - Surface Mount - YouTube

If you have access to solder paste, use that with a hot air pencil with the correct sized nozzle.

Another key to good soldering is proper flux for the job at hand. The solder with flux core is not enough.

[salvarsan]

... Chip-Quik ...

Fascinating stuff. The manufacturor specs it as melting at 136F. Add that it must alloy with lead/tin solder and you can find it by searching on "low melting alloys". It's about 50% bismuth with the remainder roughly split between lead, tin, and indium, the last accounting for most of the high price.
One ingot vendor sells Roto136, a Bi49/Pb18/Sn12/In21 mix.

[bsco]

FWIW...........I used to have a hard time with this as well....always cutting leads off and heating up the joints in order to get them out...then cleaning up the hole...I finally bought a vacuum desolder....couldn't afford a Pace unit but managed to find one at MCM on sale...best thing I ever bought...as long as you keep it CLEAN.....I use it when needed.....if I can do the job with a manual sucker..the one with the plunger type I'll use that...I also use solder wick when it will work....so I use a few different methods depending on what I am doing...For heavily populated boards with a lot of SMD, I have a couple of 30 and 40 W irons which I have made copper tips out of 12 gauge electrical wire..(12/2 house electrical wire).....I file it down into a chisel shape and tin it..It is secured into the iron with a set screw on the side..I can bend this copper tip into any angle I need and when the tip is no longer useful, I just toss it and make another one.....works great when used for the proper soldering job..***.Note**** that when using these vacuum assist de-soldering stations,(or any de-soldering device for that matter), it is sometimes necessary to apply heat to the other side of the joint in order to heat it up enough to pull the solder away....if on a large copper trace....but with a little practice it isn`t too bad to do...Haven`t tried the quik chip stuff but I will as soon as I pick some up...anyway, that`s my take on it....If you can afford it...get a de-soldering station...you won`t regret it.....

[Mick Bailey]

Some boards are getting really tricky - large ground planes coupled with combined through-hole and SMD components don't make for an easy life. A trick I've recently discovered with ground planes or similar large areas of copper is to use an SMD board pre-heater - whether the board has SMDs or not. Works a treat, just keep the temperature down a little. For every 10 degrees above ambient you get the board, that's 10 degrees less differential between the joint you're working on and the surrounding area.

Say, Soundguruman - what's your technique with SMDs?

[Jazz P Bass]

Say, Soundguruman - what's your technique with SMDs?

[Jazz P Bass]

I have used a solder sucker for a long time & it has never failed to work.

My issue with desoldering stations is they the maintenance.
They must be kept clean or they will clog.
Plus, the added heat is not good for cheap pcb pads.

[olddawg]

I used a Pace desoldering station for years. The trick with them is to use the right tip. To big of a hole and you will not get the proper suction. There are a lot of different tip sizes. You don't need to grind the tip into the board. And.. of course do the ritual maintenance religiously. The maintenance is only a big deal if you neglect it. Then it becomes a nightmare. For many SMDs I used a Hako hot air unit. Again you have to have an assortment of nozzles. (The single nozzle is great for re flowing large ICs) I spent many years regularly repairing multilayer boards and often replacing 250+ pin ICs. Use flux. Take your time. Use a magnifying device. Do not use too much heat. And... chip quick is magic for some applications.

[km6xz]

I think the reason so many people have problems with some aspects of soldering/unsoldering is not having equipment that makes life simple. An inspection scope is a must for me, I can't see the details of some of these fine pitch LSIs so would be lost without a scope or at least a x5 mag light. Rework tools used to be really expensive but now are cheap and anyone who works on electronics for a living really has no excuse for not having hot air, vacuum desoldering, desoldering tweezers, solder paste, controlled temperature solder station, a selection of flux types, and the most useful of all, a supply of Chip Quick. 20 years ago this would cost $5000 but now you can have all this for $500. It is a minor cost of doing business but returns value in faster repairs, less damage to customer owned units and more shop throughput.

[bsco]

Hey olddwag...my last employer had 7 or 8 Pace stations....WE used them all the time and like you said....as long as you maintain the stuff it works great...these stations are still expensive but I managed to get a station from MCM at a decent price that I could afford and it is great..just keep it clean!!! very simple when you think about it...I have an IR thermoneter...a heat gun...but I need some Chip Quik and a hot air station...which will be purchased this coming year....I intend to do this for quite some time so you have got to have the gear...i also need a powerful magnetfier...have a few here which are good for some things but for some SMD inspection I need a better one...
km6xz...you are right... the stuff used to be very expensive but these days this equipment is very reasonable priced....and will provide years of faithful service as long as you look after it...