How to resolder a cold solder connection?

"micky"

** Yep.

** Wicking ( or using a de-soldering gun) to get the old solder off is needed to look at the component leads or pins before re-soldering

If the lead looks shiny and tinned all over - fine.

If the lead looks dark and dry of solder - not fine.

Also, there is now the issue of mixing dissimilar solders - ie leaded and unleaded.

I use Multicore "Savbit" solder for all repairs as it will not absorb copper, has higher than usual strength and a very good flux core.

.... Phil

I think "it depends".

There may be some advantages to clearing off the old solder and replacing it entirely. Two possible such:

(1) Some years ago I had a problem with an audio amplifier... intermittent solder joints. The manufacturer informed me that their contract assembly house had "use the wrong solder" in the wave-solder machine, and that this had resulted in a poor joint on the transistor connections. I'm not sure whether it was a poor bond due to the alloy used, or whether there was a difference in thermal expansion rates which caused the joint to suffer more stress due to power-on-heat/power-off-cold cycling. The manufactuer specifically recommended that I strip out the old solder and replace it. (2) In some cases, the problem with a bad solder joint might be due to the contact materials being soldered - oxidation, other contamination, or an alloy which didn't "wet" well with the solder/flus combination originally used. In this case, stripping off the old solder, perhaps cleaning or lightly scraping the metal, applying some proper flux to the metal, and re-soldering would give you the best chance of a good solder bond. Simply re-flowing the existing connection would not necessarily get enough flux into the joint area to address any problems on the contact surfaces. Issue (2) might hold, even if the joint was originally OK. If the solder bond has cracked away from the contact metal, then the metal surface has had the oppotunity to oxidize... and so getting some good flus into the contact surface when re-soldering is probably a good idea.

--
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Yes. Especially when the old solder is leaded, and the new solder is un-leaded. If I'm not sure, I clean the joint with a solder sucker and maybe braid just to be sure.

RoHS unleaded solder which is not very strong. If you want strength, use silver solder. My guess is that the engine control board gets quite a pounding under the hood, and may have top heavy components trying to flex the leads. If this is a problem, you might get a big improvement if you can support the top heavy components with RTV, silicon rubber, or hot melt glue. That will take the load off the solder connection.

I do both. When I know that I'm using the same type of solder, I just reheat and add more solder (with flux). If I don't know the solder type, I remove as much of the old solder as possible.

I've never heard of that term.

--
Jeff Liebermann     jeffl@cruzio.com
150 Felker St #D    http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann     AE6KS    831-336-2558

Yes. Re-flowing is *possible* but risky, because it generally results in a oxide-polluted 'cold' joint.

In either event, it is critical to apply separate flux to the joint to float oxides out during the soldering process. The improvement in quality is substantial.

Honestly, I dab flux on the joint and reheat. It is quick and effective. For joints that appear cold after that operation, I remove the old solder, re-flux and use good quality leaded solder. (Then *always* clean with a 50/50 mix of alcohol and naphtha). That hasn't failed yet.

After your first 10 repairs it goes a lot faster. :)

He meant to say: ""Cold" solder joints (a frosted appearance),"

Here is a classic example. Note the frosted appearance of the solder and the lack of 'wetting' on the pin:

--Winston

"micky"

** In fact he said " Dry solder joint " - a very common term.

The word dry refers to a lack of " wetting " of one or both the metal surfaces to be soldered.

If an otherwise solderable metal surface is contaminated with oxides or similar, hot solder will simply refuse to alloy with it and there will be no or only minimal electrical connection and no mechanical strength.

A frosted appearance on the joint is generally due to movement while cooling, insufficient heat or the use of lead free solder.

.... Phil

What the other say is true but I wanted to add, those pictures of the 'good' soldering job is substandard soldering. After cleaning off the old solder with a solder sucker and/or wick, the old flux should be removed and again after the new soldering.

G=B2

What the other say is true but I wanted to add, those pictures of the 'good' soldering job is substandard soldering. After cleaning off the old solder with a solder sucker and/or wick, the old flux should be removed and again after the new soldering.

** What for ?

Solder flux is normally harmless and routinely left behind by many manufacturers.

.... Phil

There's me thinking all those decades that cold solder joint meant lack of sweating, ie lack of wetting (melding) to the metal surface due to lack of heating

I think that is a left-ponder/right-ponder 'distinction'.

(...)

It can.

A poor joint can result from lots of different things. An overheated joint can also exhibit 'dewetting' because of rapid corrosion growth.

In casual Googling just now, I see some really misleading information about proper electronic soldering technique. That's a pity, because each article I saw contains several good ideas with a real stinker thrown in.

In one case, the user is cautioned (and I am not making this up): "The use of separate acid flux paste (e.g. as used by plumbers) should NEVER be necessary in normal electronics applications because electronics-grade solder already contains the correct grade of flux!"

Well, of course one would never use *acid* flux on an electrical or electronic joint. One uses a flux compatible with one's properly selected solder.

In another case, the poor reader is bombarded with information about 5 different kinds of solder,

3 of which should never be used on electronics. They go on to imply that added flux is *always* a bad idea. This is nonsense. Added flux compatible with your properly selected solder is *always* a good idea.

This video is difficult to watch for folks that already know how to do electronic soldering because it illustrates some really bad technique, initially.

It does go on to illustrate good technique but still does not emphasize fluxing and joint prep properly IMHO.

--Winston

Naaah. I see that 'dry solder joint' is a real term. It is a variety of 'cold solder' joint with insufficiently wetted parts.

Avoid 'dry' joints by using careful joint preparation and fluxing, proper soldering tools and techniques.

--Winston

Well, using Google:

and a few others. It seems like it's a real term, but not very common.

--
Jeff Liebermann     jeffl@cruzio.com
150 Felker St #D    http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann     AE6KS    831-336-2558

I agree. I had to look it up too.

--Winston

Dry Joint = UK Cold solder = US

Thus it ever shall be

Ron(UK)

My favorite flux (water-soluble) isn't really compatible with resin-type no-clean fluxes. So, cleaning off the old flux helps me redo the joint. It's possible, too, that 'hot enough' iron for my flux is not suitable to fully activate the (unknown, foreign) original flux residue.

It's also possible that what looks like flux is actually someone's candlewax drip. Cleaning first is cheap insurance.

Not the good ones. So, it's OK to be a slob just because you can? Bollocks.

G=B2

Obviously, Phil has never done NASA or NATO approved soldering.

--
You can't have a sense of humor, if you have no sense.

"Phil Allison"

Not the good ones.

** Circular argument.

Solder flux IS routinely left behind by many major manufacturers ( both now and for decades into the past) and clearly does no harm.

So, it's OK to be a slob just because you can?

** Seems to be the way you go about YOUR thinking.

Fuck off - imbecile.

both

back at ya doofus

G=B2