Rodney,
I agree with Don and Jay on the type of solder to use. Silver solder is mostly for mechanical joints like soldering flex cable. I also use 60/40 electronics solder on all my boats. I do use a liquid rosin and pre-in the wire ends in a solder pot before soldering to a plug. When soldering to a plug, I heat the brass plug with the soldering iron, not the wire. When solder melts and flows nicely into the socket I then move the wire into position letting it warm up against the cup. When the solder in the wire starts to melt I fully seat the wire into the cup adding a little more solder. I will let the iron sit on the plug for enough time till the connection reflows and is fully wetted. On large 8mm plugs I believe an 80W iron is marginal. I use a 200W chisel tip iron which quickly transfers the heat. The issue with an under sized iron is the heat will flow quickly out the wire and/or the connector depending on what is supporting it.
Second point: It may not be the solder joint that is causing the problem. It can be the connector to connector resistance. If you have a weak mechanical connection between the male and female plugs, it will self heat when high current passes through. So the connector heats up and reflows the solder leading to the wire slipping out. If this is happening you need to improve the connection by increasing the spring force on the fingers, lighting sanding any oxidations off the contact surfaces or replacing the plugs all together with better ones. Bigger does not always mean more current. I have seen plenty of 8mm connectors with poor fits or thin walls.
-Tyler
Tyler Garrard
NAMBA 639/IMPBA 20525
T-Hydro @ 142.94mph former WR
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