Wire termination for screw terminals
Q: Should stranded copper wires be tinned with solder for connecting in screw terminals.
A: Industrial best practice is to NOT tin wires that are to be terminated with screw terminals.
First the reason this is so is that the differing thermal expansion rates of copper and solder cause a problem. The solder in the tinned wire end expands more than the copper (brass) terminal and a big peg in small hole causes the softer metal to yield (it gets softer with increasing heat as well). When the temperature drops later the parts contract again but now the tinned wire end is smaller than it was and causes a bad connection.
I have come across this symptom in practice a number of times so know that it is a real problem. It usually only shows up on high current or hot environment connections, traditionally in things like heaters so could become an issue in the occasional RepRap machine on the main power, extruder and hot bed terminals.
While screw terminals are ubiquitous and used all over for all sorts of things they usually get used without wire tinning because of cost saving issues so the problem does not surface as often as it might.
In professional wiring cabinets and distribution panels you would not find tinned wires under screw terminals. The ways of avoiding the need to tin have been solved and are pretty practical.
- Firstly most professional screw terminals these days have a flat pressure pad or tongue that protects the wire from the turning screw end and keeps the wire end strands in order.
- Often solid wire conductors are used in alarm and telephone applications.
- In factory automation (PLC) and distribution boards the wires are systematically terminated with bootlace ferrules (little thin wall copper (thin tin plated, not solder dipped) tubes with a plastic funnel) that are carefully crimped (or just squashed with a pair of pliers) onto each wire end before termination starts. These ferrules allow for multiple re-connections without undue damage to the wire end. 
There are also pre-insulated and traditional uninsulated wire end terminals that are available from any good electrical wholesaler (like the bootlace ferrules) that have a flat tongue instead of a round/forked lug or blade terminal. The extra high reliability terminals for aviation and aerospace use may have a crimp and internal solder so they can be reflowed with a hot gun after crimping to flow the solder and shrink the strain relief/insulation boot to seal the wire end from moisture and gas while making better electrical contact after an already sound mechanical connection.
Now for the correct DIY way to terminate wires for screw terminals, there are three that are practical and might be worth mention.
- Firstly I recommend that you strip the wire about 8mm, twist the end tight with your fingers only and then fold the end double neatly, this has twice the number of strands and can suffer a screw terminal a few times, leave a bit more slack in the wires (16mm more) so you can re-terminate the end if the strands all break off from repeated reinsertion. Use screw terminals with the protective tongue where ever possible.
- Secondly what I have seen in some consumer appliances that were sold in the past without moulded plugs, to keep the pre-stripped strands in place but avoid the solder under the terminal screw the supplier would tin just the last 1 to 2mm of the wire and the area where the screw would bear down was plain copper, many times untwisted as this survives repeated termination more times. If I have a soldering iron nearby when I am working on mains wires this is the technique I often use but the solder can wick up the wire pretty easily unless you are careful, liquid flux and a tiny solder pot are likely the best production technique for this type of preparation.
- Thirdly if using pre-tinned wire ends, the easy fix for the problem (if you don't want to cut and strip) is to very gently torque up the screw on the main power and heater wires after some hundred hours of use. After a few times all the solder that can yield has flowed out of the way and you have a good copper to brass mechanical contact. You do not want to over tighten the screws and you also don't want to loosen them when doing this maintenance, just a careful torquing it up to take up the slack without repositioning it in a new orientation.