You don't need to solder splices in servo leads, in fact you don't want to if you follow along with MILSPEC and aerospace standards for field wiring. They did away with solder type connections in the wiring harnesses back in the early 1970s. Too many Cobra gunships bit the dust due to soldered connections work hardening and finally breaking loose. Same goes with other aircraft/machinery. It took too long for the old school types to finally admit that soldering was an inferior technique to a simple mechanical [union] splice. Some guys needlessly died or got maimed as a result. My soldering teacher in the Armys' electronics school was one of those pilots. Soldering is only done at the component level on circuit boards.
A good twist job and heat shrinking is all you will ever need. The only part of the system where soldering is a must is in the RF section.....for our purposes that means the antenna wire.
I'm sure that 99% of the guys in this hobby who extend their servo leads with soldered connections never have any problems, but I'm just here to tell you that it isn't neccessary plus give you some background as to why that is so.