Two secrets to soldering ....
1) Absolute cleanliness.
2) Don't use modern lead free solder, its useless .... use the old fashioned lead/tin stuff.
Also, you can get crimp connectors where the shrinktube is lined with hot-melt glue.
This gives a reliable, sealed joint.
Or simply use the glue-lined shrinktube over soldered joints.
In either case, the glue gives added mechanical strength to the joint.
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