Stubborn Copper!!

[Govmnt_Lacky]

Soliciting advice.

I am currently soldering up some etched PCBs and I am having one hell of a time getting the solder to stick to the copper pads. I did an initial cleaning of the boards before starting and everything flowed good. My problem is that I took a break between soldering on these boards and I think a film or some sort of oxidation has happened and it is impeding the solder flow.

What do you folks do to clean partially populated PCBs to accept new solder?

[Haberdasher]

I've never tried to solder on partially populated boards before, but in general I use acetone to clean them.  One thing you could try is roughing the copper up with those fake steel wool pads they sell at places like home depot (then clean with acetone).

Of course, another thing that helps a lot is using 63/37 solder, but maybe you are already using it.

[mremic01]

I actually don't have much of an issue getting solder to stick to oxidized pads, but I probably run my iron hotter than necessary. Generally, once the board is etched and drilled, I use cotton balls with acetone to take off the toner, then get the copper nice and shiny with some steel wool, then go over it again with cotton balls and acetone to clean it up. The solder sticks incredibly well for the first hour or so after that. Then it still sticks well, just not like it did before. I can come back to it a week later and still get decent joints though.

[somnif]

Lots of flux. I use a needle tip dropper bottle and give ever pad a dose.

[davent]

A preventative measure, I was getting into this bottleneck from time to time as well and found that a mist coat of lacquer after the initial cleaning solved any issues down the line. Also tried Liquid Tin but in the end it's was no better then the unprotected copper, same issues when  trying to solder  to pads that had sat exposed for any length of time. You can solder right through the lacquer without having to make any accommodations.

dave

[somnif]

Now that I'm at a real keyboard:

I've had a few tricks that have worked. A plastic dishpan scrub pad for an initial "reveal" helps in the short term, but tends to make oxidation worse if you leave things to sit for a while.

Acetone works, but can also strip silk screen, and can make keeping track of things a bit annoying. Also, fire hazard, never fun.

Weak acids will take off copper oxide, so vinegar, citric acid, etc, will do the job. Always clean up with a some rubbing alcohol or distilled water though, as acid residues are conductive.

And again, my personal method of choice is just brute forcing the issue with copious flooding of rosin flux. Kester 1544 in a needle tip squeeze bottle (2oz size), a little dab on each pad once I've loaded a component (so both the copper and lead get a taste). Its literally just pine rosin dissolved in alcohol.

You'll want a fairly hot iron, and the iron will smoke quite a bit, and you'll want a tip cleaner (brass sponge, etc) handy, but it will make even dull brown copper wet fairly nicely. Only thing I've ever really see defeat it are some 70s vintage diodes I was trying to solder, those littler bastards were stubborn.

Only annoyance I have is the flux tends to solidify in the needle between uses, so I have to warm it up by holding near my iron to get it to flow each time.

[EBK]

My avatar image tells you my secret to trouble-free soldering. 

[alanp]

A preventative measure, I was getting into this bottleneck from time to time as well and found that a mist coat of lacquer after the initial cleaning solved any issues down the line.

I heard from a guy in Rotorua that this works well, and Rotorua is famous (possibly world famous) for the air throughout the whole town being permeated with sulphur.

Discl: never tried it myself.

[somnif]

 Only thing to be careful of with lacquer is the fumes/smoke can occasionally be annoying. I had to re-solder the brains (ECU) of an old British car for a friend, and it has a lacquer soaked board. By the end of it I could barely see straight, could barely stand upright, and everything tasted funny.

Maintain airflow and ventilation! (unlike me... cough...)

[Willybomb]

Rotorua is famous (possibly world famous) for the air throughout the whole town being permeated with sulphur.

Yeah, that place stinks.

[peterc]

After etching, I like to use this stuff:
http://www.mantech.co.za/ProductInfo.aspx?Item=90M0056

Protects the copper and makes soldering a breeze.

[m-Kresol]

After etching, I like to use this stuff:
http://www.mantech.co.za/ProductInfo.aspx?Item=90M0056

Protects the copper and makes soldering a breeze.

I agree. there are some protective coatings for this. Copper is a quite precious metal, but residues of the etchant, soldering adjacent pads (=heat) and so on can lead to oxidation.