A properly soldered joint is very strong and also electrically elcellent but if making terminations for a boat it is very wise to crimp first then solder. Never solder then crimp.
Here's what NASA has to say on that:
"4.3.4 Crimping. Stranded wire shall be used for crimping (Requirement). Crimping of solid wire is prohibited. Crimping of solder tinned wire is prohibited."
Soldering after a crimp has been made is fine provided you:
A) Know what you are doing (most don't).
B) Use a heat sink to prevent solder creep up the wire
C) Use the correct solder.
D) Use proper strain relief/adhesive lined heat shrink.
E) Use the proper crimp terminals for soldering which are DIFFERENT, and non-insulated and rarely as robustly built as the insulated ones or heat shrink ones.
F) Use the proper crimp tool for the solder/crimp terminals which most don't own or have if they have been using a crimper designed for insulated crimps.
In short doing crimp then solder requires different tools, proper training and more labor and will not really yield much if any better end result for what you need it for.
The ABYC actually specifically bans solder as the sole means of connection and this includes Western Union Splices or any other form of wire twisting in an attempt to to make a "mechanical connection".
The problem with solder is usually with folks who don't know how to do it and in my experience that is perhaps 90% of folks. I have rarely come across a properly soldered termination in boats. Sure you fidn them but they are about as rare as a Nun in a g-string bikini.
When soldering you should always use a proper heat sink to prevent solder creep up the wire and also make sure you are using the right solder with no acid. Soldering old dirty wire is a futile effort and the solder usually won't even stick to it. Cold solder joints are very common in DIY applications. I found many on my own boat as it had been owned by an EE who knew his stuff book wise but had no clue how to solder. The actual panel made at the factory was soldered on the back side and there were three failed solder joints.
The reason the ABYC bans solder as a primary termination tool is due to a massive case history in the industry of solder failures. ABYC is evidence based and the evidence is overwhelmingly against solder in the marine industry.
Crimping with the "proper tools" is also part of the ABYC E-11 standard and when done in this manner crimps are far more repeatable and reliable even in the hands of a DIY. The failures are usually where the solder crept up the wire and created a hard spot and the wire fractured. UL 1426 wire is finely stranded for a very good reason, vibration. Soldering and allowing creep causes a hard spot that can fracture over time and use. I have witnessed this and come across it many times. I have owned two boats owned by EE's and they were the absolute worst solderers in the world. Nearly 20-25% of the soldered connections on my old C-30 were faulty to the point of not even working. I was forced to rip it out and start from scratch.
Here's Calder's take:
Nigel Calder from Boat Owners Mechanical & Electrical Manual Pg. 122 said:
Soldering is a controversial subject. A properly soldered connection creates the best electrical connection, but all too often the soldering is not done properly. In any case, ABYC regulations require that every joint have a mechanical means of connection other than solder. The reason for this is that if the joint gets hot (through excessive resistance or a high current flow) the solder may melt and the joint fall apart. So solder often becomes just an adjunct to a crimped connection, but in this case the solder wicking up the cable creates a hard spot, which is then liable to fail from vibration. The concensus among professionals is that a properly made crimp, done with the proper tools, is frequently a more reliable termination than soldering.
There really is no need to solder a well crimped connection. Doing so and doing it properly requires a whole different set of tools and terminals. Nearly every wire termination in both the aircraft and auto industry is crimped.
Using standard yellow, blue & red terminals is not advised for solder/crimp because to do it right you would first have to make the crimp with the yellow, blue or red insulation on the terminal and then cut off the insulation and solder it. The insulation is a critical part of the tolerance for the crimped terminal and cutting it off first will yield a poorly crimped terminal.
After all that you'd then need to get some adhesive lined heat shrink and HOPE you make a good seal. It is FAR easier to use the right tool and pre-made heat shrink crimp terminals. Good ones are 100% water tight and DIY heat shrinking is often not, especially on ring terminals.
The other concern is that with boats corrosion is a big problem and corroded terminals can cause resistance, resistance causes heat and heat can melt solder. If the terminal is not crimped on it can literally fall out of the terminal and you'll have a live hot dangling wire. Of course by that time you would have hopefully smelled the meting insulation...
You can crimp and solder, but with the right tools it is a lot more work for little to no gain and only the added risk of creating hard spot that could possibly fracture.
So how good are ashesive lined crimps? As one who lives in the North East, where nearly every home has a deep well, with LONG wire runs and fairly high amp draws, we just don't see failures. Boat interiors usually only get humid. Consider how "humid" the wires are that heat shrink sealed 200+ feet below the surface, that live underwater for their entire working life span and that last for 15, 20 years or more or until the well pump fails? Almost every industry uses crimps reliably, without solder, including aerospace, and have been for years and years and years with very high reliability and repeatability.
The well at our old house was 245 feet deep and a true artesian. It over flowed out the well cap almost all year. The crimps were about 240 feet below ground and totally submerged. The three wires connecting the pumps were simply crimped and heat shrinked with what are called Stakons. Stakons are nothing more than adhesive lined butt splice crimps. These bare copper, non-tinned, wires had been under water for over 13 years only crimped and heat shrinked. Something to consider..
The ABYC standards are a very good guide to use. Here are a few good points from E-11:
"Conductors shall be at least 16 AWG. EXCEPTIONS: 1. 18 AWG conductors may be used if included with other conductors in a sheath and do not extend more than 30 inches (760mm) outside the sheath.” 11.16.1.1.2."
"Conductors used for panelboard or switchboard main feeders, bilge blowers, electronic equipment, navigation lights, and other circuits where voltage drop must be kept to a minimum, shall be sized for a voltage drop not to exceed three percent”.
"Conductors used for lighting, other than navigation lights, and other circuits where voltage drop is not critical, shall be sized for a voltage drop not to exceed 10 percent.”
"Solderless crimp on connectors shall be attached with the type of crimping tools designed for the connector used, and that will produce a connection meeting the requirements of E-11.16.3.3.” 11.16.3.8."
"Current-carrying conductors shall be routed as high as practicable above the bilge water level and other areas where water may accumulate. If conductors must be routed in the bilge or other areas where water may accumulate, the connections shall be watertight.”11.16.4.1.6."
"Terminal connectors shall be the ring or captive spade types.” 11.16.3.4."
"Twist on connectors, i.e., wire nuts, shall not be used.” 11.16.3.6."
"Ring and captive spade type terminal connectors shall be the same nominal size as the stud.” 11.16.4.1.12."
"The construction of insulated cables and conductors shall conform with the requirements of: 11.16.1.2.2.1. UL 1426, Cables for Boats…”11.16.1.2.2.”
"Solder shall not be the sole means of mechanical connection in any circuit.”11.16.3.7."
If you use poor crimp tools than moisture could find it's way into the terminal as seen here after being cut open. Notice the individual wire strands still visible.
In a properly formed crimp there will be no room for moisture creep:
I made these crimps during the re-wire of my own boat ago and heat shrunk them. To make all 20 took all of about 6-7 minutes. I pay about 0.38 ea for these FTZ 10-12ga ring terminals, so they don't have to be expensive either.
Crimped & Shrunk - Top
Crimped - Bottom
You can see the pile of crimped wires in the back ground.
Perhaps no one let them know that adhesive lined butt splices have been keeping well pump wires BONE DRY for over 30 years..... The FTZ butt splices I use are already 100% water tight and the whole process takes less than 60 seconds......