wire harness strength

We are soldering wire 26AWG to jack socket. However after went through washing, baking, coating, curing processes the wire breaks (at joining point with jack socket) during wire dressing. We have tried to increase and decrease the solder amount but rejects still happened. pull test of wire and jack socket is 40++ N. I found that the break wire has exposed copper on wire strands. what will be the possible root cause and what can I do to increase the wire strength?

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