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I'm having difficulty soldering components with 1 mm thick or larger wires. The solder melts fine, but it does not 'stick' and just falls off. Components with smaller prongs like regular LED's are no problem. Right now I'm trying to solder some diodes to make a bridge rectifier and the solder is just not sticking.

What could be the problem? I'm using a battery-powered RadioShack soldering iron that works fine for the small stuff, and lead-free flux solder. Any tips on how to solder the heavier stuff that seems to reject solder? I've looked all over for this issue and don't seem to be able to find an answer.

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    \$\begingroup\$ Is the solder actually melting? Are you trying to solder the leads to each other, or to a PCB? What surface finish is on the leads, and any PCB that may be present? \$\endgroup\$ – ThreePhaseEel Jan 1 '17 at 3:02
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    \$\begingroup\$ One problem could be insufficient heat (since you mention a battery powered soldering iron). A component with greater thermal mass will require a longer time to reach the temperature needed to melt solder. Be careful though, as a high thermal mass not only takes longer to heat up, but also takes longer to cool down. Using a "third hand" clamp to hold all the elements steady while the solder joint cools. \$\endgroup\$ – MarkU Jan 1 '17 at 3:03
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    \$\begingroup\$ A photo of what you're trying to solder might help us estimate how big it is, and whether there's some kind of surface contaminant issue. \$\endgroup\$ – MarkU Jan 1 '17 at 3:04
  • \$\begingroup\$ Please add the power rating of your soldering iron and a description (or better a photo) of your soldering tip. Please also add a photo of what you're trying to solder like MarkU said. \$\endgroup\$ – try-catch-finally Jan 1 '17 at 9:01
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Short answer: you need a better soldering iron.

Longer answer:

Your little battery-powered soldering iron doesn't have enough power to heat a large mass of heat-conducting metal up to the melting point of solder. All of the heating energy from your tiny iron is being conducted into the mass of metal that you are attempting to solder and being radiated away.

I would suggest that you need at least a 40 Watt iron to solder 1mm thick wire of any significant length.

Others might suggest that you would need more power than that but it all comes down to what soldering iron you use. A really inexpensive iron may not work well just because the design is such that there is significant thermal resistance between the heating element and the object being soldered. But better irons will do the job nicely.

A specific example of a soldering iron that works well at 40 Watts is the Metcal MX-500 series with a sttc-137 tip. This iron will solder the entire perimeter edge of a male DB-25 connector to a piece of copper-clad PCB material using 63/37 solder. It takes a while but it will get the job done. Your typical Radio-Shack 40W iron won't even attempt to work.

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    \$\begingroup\$ It's worth adding that a thermostatically controlled high powered iron will also handle the smallest soldering jobs better than a cheap low powered iron, because the bit is likely to be of better quality (e.g. iron-coated not copper) and less prone to oxidation. A good iron isn't cheap, but it will literally last you a lifetime - my 60W Weller is getting close to its 50th birthday, and still as good as new. \$\endgroup\$ – alephzero Jan 1 '17 at 4:42
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I would put this as a comment to Dwayne's answer, but I don't have the rep... in addition to what he says about having a better iron, I want to point out the following.

When soldering, the leads should melt the solder -- the iron should not do it directly. The iron heats the leads, the leads melt the solder. When you say "the solder melts fine, but does not 'stick'" I'm thinking that you are melting the solder with the iron, not with the diode leads. If the leads are not hot enough to melt the solder themselves, you'll get a dry joint.

(If you hadn't mentioned diodes, if it was say a heating element, I'd also suggest it might be the metal - some metals just don't wet to copper. But diode leads won't be made out of such metals.)

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There are generally three primary reasons for solder not wicking properly:

  1. Technique - as mentioned by others, heat the lead not the solder

  2. Iron temperature - again, mentioned by others

  3. Contaminants - make sure the wire is clean of contamination. Oxidized, dirty, or otherwise contaminated wire or leads will prevent the solder from wicking and will glob instead. A decent flux pen will help a lot.

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Maybe you're are using some Chinese manufactured component. I don't have that much experience or neither have I ever cared to look into what actually happens but as an hack, I have totally "assumed" that the terminals aren't enough rough to hold the solder to itself. Hence you could rub terminals with the mechanical instrument called "file" and it looks something like this:

File

Rub the terminals with the cutting edge side and then try to solder it.

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    \$\begingroup\$ Lack of "roughness" won't prevent soldering. Soldering is not like glue, it doesn't need a rough surface to hold on, rather it needs a clean surface free from dirt and oxide so that the solder can chemically bond with whatever is being soldered. \$\endgroup\$ – Tom Carpenter Jan 1 '17 at 10:24
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    \$\begingroup\$ @TomCarpenter May be file helps to do that. It is just an Hack that i use when things don't get solder. No an exactly answer with all the analysis \$\endgroup\$ – MaNyYaCk Jan 1 '17 at 10:27

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