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Short story: I want to solder cable to a pretty delicate looking board. There is not a lot of room to get at things so I need to be careful not to just screw everything up plus the PCB looks fragile (that could just be my lack of understanding). What should I do to avoid killing it while successfully attaching the new wires?

Longer story: I have two eGo-T vape batteries which have been physically busted for various reasons. The parts of the official one all work (I know this because I was careless taking it apart and burned my fingers). I want to build a mod or two with them. The last time I used a soldering iron was over half my life ago and so I figure I have probably forgotten anything useful. The board on the 510 thread head has the little board and button all in place but the wires are way too thin for the current that they were coping with (factory/design fault) so I want to put a thicker more durable cable on before I set the head into a box of some kind and rig up a battery (perhaps with some form of variable voltage regulator). Basically with the first build I am just wanting to make a new case and replace the wiring with something that is up to the job. What I am worried about is cooking the little board and making it useless.

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    \$\begingroup\$ It's almost impossible to answer this question because we don't know what the board actually looks like nor how it is constructed. I can only advise soldering something else for practice until you once again feel sufficiently comfortable tackling the project. Soldering is not difficult but you need to have a feel for it which varies depending on your soldering iron, size of wire or component, and other factors. \$\endgroup\$ – JYelton Sep 8 '14 at 17:05
  • \$\begingroup\$ Ah, ok. Is there any way to work out if the PCB is going to be particular sensitive to the heat I am going to give it while attaching the wires? One has to assume that it was possible at the factory stage, right? \$\endgroup\$ – Matthew Brown aka Lord Matt Sep 8 '14 at 17:08
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    \$\begingroup\$ In general, components and boards are subject to temperatures above 230°C (450°F) for short periods of time. If the PCB is reflowed, the entire board is heated briefly. If it is wave soldered, the bottom is temporarily exposed to molten solder. If it is done by hand, individual components are heated as the leads are soldered. The idea is to get a good solder joint without exposing any component to high temperatures for too long. Generally, the PCB itself is not going to be a problem, but small components heat up faster than larger ones. It is a time/temperature trade-off. \$\endgroup\$ – JYelton Sep 8 '14 at 18:21
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I actually can answer this, because I know the exact board he's talking about. It probably looks very similar to this:

enter image description here

They are very simple, cheaply made PCBs. One set of wires connects to a lion cell, the other end connects to the 510 threaded head (sometimes with 2 wires, other times one terminal is press fit or soldered right to the connector).

These boards are usually just a couple LEDs and a switch. Some have a transistor to control the actual power so the switch doesn't die from arcing.

Soldering new wires to them isn't very difficult with a bit of practice. Get a small vice or something to hold the board in place, then use desoldering wick or a solder sucker to remove the old wires and clean out the holes, insert new wires and solder them on. The most important part is not to heat the board up too much - if you do, it will delaminate and the traces will lift off.

If you have a hard time getting the holes clean enough to insert the new wires, there are two tricks I have used before (after removing as much solder as possible, and getting the wires out:

  1. Push a small thumbtack or needle into the hole while heating it up. Twist the needle as the solder is cooling so it doesn't stick.
  2. If that fails, use a tiny wire drill to drill the remaining solder out.
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  • \$\begingroup\$ Brilliant, you have helped me no end. \$\endgroup\$ – Matthew Brown aka Lord Matt Sep 9 '14 at 5:59
  • \$\begingroup\$ Smoking is a small vice. For holding the board, a small vise would probably work better :-) \$\endgroup\$ – Dave Tweed Sep 9 '14 at 13:33
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    \$\begingroup\$ @DaveTweed true for america, but not worldwide. Outside the US vice is more commonly used for both meanings. \$\endgroup\$ – Grant Sep 9 '14 at 14:10
  • \$\begingroup\$ Here's your vice: smile.amazon.com/gp/product/B00Q2TTQEE/… \$\endgroup\$ – SDsolar Apr 25 '17 at 5:31
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Grant has given very good advice. I would add ...

Thin tracks on PCBs are easy to damage by applying heat for too long; the track 'delaminates', the 'glue' melts and the tracks are then fragile and easy to break.

Before doing the fix, get some practice. It'll be useful to jog your 'muscle-memory' for soldering back into shape.

Buy yourself a piece of stripboard/veroboard, a packet of small 0.125W thru-hole resistors, a solder sucker, and maybe a flux pencil. Practice soldering the resistors in, unsoldering them (with the help of the solder sucker), then solder them back in (possibly with the aid of flux from the flux pencil) for an hour.

I would strongly recommend getting some lead solder as well as lead-free solder. Use the lead solder first because it is easier to work with. Once you feel comfortable try the lead-free. If this is for your own use only, then you can stay with lead solder. Remember to wash your hands after using solder.

Stripboard/veroboard tracks are quite wide, but delaminate quite quickly, so you'll get a feel for what you are doing.

If you damage the PCB track badly, consider a 'verowire' pencil which, with care, might let you repairing a track.

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