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I have a few types of solder. One of them is 0.4 mm and the other is around 0.8 (not very sure). Which one should I use for SMD, and which is good for general purpose?

I have heard that it is best to learn with a thinner wire. Is this true?

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    \$\begingroup\$ Small pads, tight pinout, narrow spaces could be though using thicker solder wire. It is easy to accidentally use too much and create shorts between pads, pins. On the other for example soldering the thermal pad of a TO-263 package would be easier if solder comes in bigger amounts. \$\endgroup\$ – Bence Kaulics Jul 20 '17 at 10:37
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The primary way of controlling how much solder you feed into a solder joint is by the length of solder you shove in.

Thus if you assume a certain "length accuracy" of your hand, the overall volume accuracy of a thinner wire is higher.

I personally use .5mm or less for most work, even non SMD, unless the solder joints are really big.

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Thinner solder allows you to control the amount of solder added more precisely. If you use too-large solder, then

  • you may have to remove excess solder, and try again after removing too much as desoldering is even less precise.

Thicker solder allows you to add more volume of solder per length. If you use too-small solder, then

  • you will have to move your grip on the solder more often (and doing it in the middle of a single joint is awkward and might contribute to overheating the joint or making mistakes), or

  • start with a long straightened piece of solder that is harder to handle precisely from the other end.

Still, I would choose a smaller size over a larger one.

I don't have a lot of experience with soldering, but .032" (0.8 mm) is the size I have mostly used, and it is good for normal through-hole joints, too thick for surface-mount ICs (usable, but expect to have to wick away solder bridges), and too thin for large mechanical joints.

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