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I was a design engineer at NASA for 8 years. All engineering, techs and assemblers had to undergo their 3 day soldering and mechanical assembly course. A good solder joint starts with clean surfaces. Whether you are soldering wire, connectors, circuit boards, etc. all surfaces need to be cleaned of oxidation and oils from fingers and hands or packaging. Denatured alcohol was used with an acid brush to clean all surfaces prior to soldering. Tin and clean your iron as usual, but clean your surfaces with alcohol and then apply solder. You will be happy with the results.