Soldering can be easy and even fun, but not with dirty tips, soldering irons of the wrong Wattage, poor solder choices, and bad fluxes.
Good soldering is 80% preparation, and 20% execution.
You should have a soldering iron tip cleaner. Make a penny-sized hole in a common sponge. Cut the sponge to fit into a small flat jar, one with a solid cap. Wet the sponge with water. Cap the jar, when finished work.
Rosin flux is lame. Acid flux destroys wires and the copper runs on circuit boards. Use LA-CO paste flux (non-acid, and it washes away with water). Do not leave any flux on finished work, not ever. It just causes problems later.
Brushes: hardware stores have cheap "acid brushes;" get a few extra. Cut the metal handles to a good length for you. The long bristles are wimpy. You make the bristles stiffer by trimming the bristles shorter with scissors. The short bristles will be stiffer. A cheap artist brush is good for applying flux to the work. Stiffen those bristles as needed in the same way as done for the acid brushes.
Use temperature-controlled soldering irons. 100 Watt soldering irons will do most small-sized work. Bigger soldering irons do bigger jobs. This is a US$ 50 Soldering Station with temperature control, a handy On/Off switch, and a 70 Watt iron. It's okay for most small jobs. You will want a heavier Wattage station for bigger jobs.
Good visibility of the work you will do can be a game changer.
A good low-power binocular microscope will be a great help with small jobs on the bench. 10X to 30X magnification should be about right. Some microscopes will have separate low-power optics assemblies, so you can select what works best for you. Check Fleabay, the For-Sale websites, and the thrift shops (ask them to check the back rooms).
You may also try the eyeshade/headband magnifiers. Some of these allow using eyeglasses, and some will have multiple lenses that may be used singly or in combinations of lenses, for greater magnification. Some will have LED illumination, but if so, you will want some way to aim the lights to suit yourself.
If you can do the job with lead-free solder, then do so. If you must use a solder alloy, use 60-40 tin-lead alloy. Tin-lead solder carries all of the hazards of lead into your place, so it is getting scarce. I do not recommend lead-alloy solder, due to the health issues.
Soldering irons are really pure copper at the working end, most often. Some soldering tools have replaceable tips. Contact the company who makes the tool, if possible, for new tips. New soldering tool tips may be coated (plated) with soft iron. I like to preserve the iron plating which you may get on some new tips, so a soft wire brush starts the tip cleaning. That brush may be enough to get the iron clean, and ready to be tinned with new solder again. Once the plating gets hopelessly corroded, however, you will have to file down the end of the tip (when cold) to see the clean copper. All soldering tools were bare copper a few decades ago, so do not be concerned about any iron plating lost to the file. The tool will work well, with or without the iron plating. Iron plating just makes a soldering tip last longer, and easier to clean.
If necessary, you can refurbish the corroded end of the soldering iron when it is cold. First, use a soft fine-wire brush to clean away the crusty flakes. Use a flat file to shape the copper into a thick, blunt common screwdriver shape, or shape it like a large common nail, with three flat surfaces at the tip, having a triangular cross-section. /\ With only bright shiny copper showing, wrap the tip copper tightly with solder-wire, so the solder-wire looks like a tight coil spring on the tip. Add flux, and turn on the iron at a low setting, hot enough to melt the solder-wire on the tip, but not extremely hot. When the solder melts, wipe the tip clean in the hole in the damp sponge. That process should give you a good working tool. You will rarely need to do this filing process, but you can, when necessary.
Heat up the soldering iron. Clean the soldering tip, bright and shiny, by wiping it into the hole in the damp sponge. Add a little fresh solder to the tip. Clean the tip again, and add a little fresh solder, after every minute that you are working.
A soldering iron running too hot gets too dirty, too fast, just from the oxygen in the air. Besides that, too much heat can wreck your project. Adjust the soldering iron tip temperature as needed. The wet sponge can clean and cool the too-hot soldering iron, but you will need to clean the tip very often.
A too-small (weak) soldering iron will not get the work hot enough to take solder well.
Solder-Wick is a good accessory to have. You can make some or buy some. To make some, strip the insulation from foot-long electrical wire pieces that are made with fine copper strands. (For our Metric friends, a Foot is the length of that thing found at the end of a leg, normally.) Give the strands a few extra twists, but not too much. Regular Solder-Wick is fine-stranded, and has dry flux in it. No matter if it's DIY or regular Solder-Wick, apply flux, heat, and solder to the very end. When cool, bend the now-stiff end back towards the new material, forming a "bent corner" at the end of the Solder-Wick. To remove dirty old solder from the job, dip the "bent corner" into some flux (not much), and put this "bent corner" against the old solder connection. Put the soldering iron into the "bent corner," and press the Solder-Wick into the old solder, using the soldering iron tip. When the dirty old solder is hot, it will be absorbed into the Solder-Wick, so you can make a clean start on the work. Remove the Solder-Wick and iron from the work. Use pliers to pull the hot Solder-Wick out straight, let it cool, and make a new "bent corner" in the Solder Wick, for the next use. Repeat as needed, until the connection has no old solder on it.
Whenever possible, apply solder to both parts of the connection, before they are joined, then use Solder-Wick to clean away all of the excess solder from those parts. Make the connection needed, bending the wire around the part to be soldered, or running the wire into the hole on the circuit board, and bending the end over, on the opposite side of the board.
All set? To make a good solder connection: With all of the old solder gone, use a small brush to apply a little flux to the intended connection. Clean the soldering iron tip by wiping it off into the hole in the damp sponge. Add a tiny bit of new solder-wire to the tip. Set the iron tip into the corner of the clean metals to be soldered, where they meet. Apply solder-wire to the connection, but not touching solder-wire to the soldering iron. When the solder flows smoothly into the connection, pull away the solder wire and the iron. Important! Do not allow the connection to shift or move until the new solder has hardened on the connection. A disturbed solder joint makes a bad electrical connection.
When finished, the soldering you did should look smooth and shiny, like silver. The solder surface should be flat or concave, not convex. Use a small stiff brush and a liquid (water or a solvent, depending on the type of flux you used) to clean away the flux. Leaving flux on the job just causes problems later. Blow away the liquid from the work. Compressed air or canned air is good for that stage of the job. Good and bad connections:
To perfect your skills, make up some practice soldering jobs for yourself, and check that work. It may come out looking like a mud fence at first, but soon you will be doing some first-class soldering work, with results that sparkle like new silver.
A local plumbing supply house should have several good fluxes available; rosin is a minimally capable flux, and acid flux is not for electrical work. If you use acid flux for any solder-assembled metalwork, you will need sodium bicabornate, a stiff brush, and water to neutralize and remove the acid flux afterwards. Rosin flux will need isopropyl alcohol for clean-up. I suggest using LA-CO flux (or similar), if possible. After soldering, you can wash away this excellent flux with plain water, with no chemicals needed. A few ounces of this flux will do many, many jobs for you.
Resources:
LA-CO non-acid flux paste:
https://markal.com/collections/soldering-fluxes/products/regular-flux-paste?variant=9204786921519
Chemtronics Solder-Wick from Digi-Key ~US$ 6.50:
https://www.digikey.com/en/products/detail/chemtronics/80-5-5/306984
K-tronics 70 Watt variable temperature Soldering Station. Cone-shaped tip, not ideal, but okay. Retail ~US$ 50.00:
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