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Back in the day when I was installing process control computers I soldered thousands of wires. I like the Kester thin wire (.031), resin core, 60/40 alloy. Good luck.
Dono
If your solder joint breaks or is brittle then you did something wrong. Probably a cold joint or you heated just the solder and not the wires. Many times this problem can be caused by using solder that is too thick for the job. Get the thin stuff. It will probably be about the same diameter as your wire. Then when soldering, put the iron on the wire joint and heat the wire, touch the solder to the wire and allow the heat in the wires to melt the solder. Allow the solder to flow into the joint. Remove the solder and the heat and don't move the joint or wires till the solder cools. Practice with some scrap wires a few times and you'll get the feel for how fast you can go. The trick is to get enough heat in the wire to melt the solder but not to melt the insulation next to the joint.
I've been using Radio Shack's 60/40 Rosin core for many years now. I also never solder anything without using soldering paste flux. My choice is alpha metals brand. It gives you a nice solder flow cause it cleans the surfaces for a good bond.