These are fine for wire to wire connections where the wire will not be flexed. Hellosailor is correct; the problem is heating the joint to the 183 deg C to melt the solder, without damaging the insulation of the wire. Even if you manage to do it, just bear in mind the joint will not be a strong as the wire so protect it from stress. I have never used them, preferring to make my joints in a way that I can inspect them for good solder flow or proper crimp compression. Just remember it is a soldered joint and so is brittle.