A Few Basic Circuit Board, Solder Joint Questions

[scottmcphee]

I would scrub the solder side of the board first, with a nylon brush and warm soapy water, then rinse with water, yes water. Let dry for day then commence work.

Some of these boards have been coated with electronics conformal coatings (http://en.wikipedia.org/wiki/Conformal_coating) that will need to come off before applying solder iron. These coatings vary in material, and you have to deal with each type differently I suppose. Rubbery silicon, shellac, hard set epoxy, ... you never know. You gotta get this off so look for the right solvent.

Then clean the board again, really well.

If all you have is an iron, then you've got the right idea about touching up every solder point on the board. Get some liquid resin to apply to each joint as you do this to encourage flow and adhesion. The idea is to get the part lead hot enough that it flows the solder around it and re-bonds with the solder. It's almost always the part lead and not the solder pad on the PCB where the cold joint occurs. But as you resolder each point the pad is going to reheat and bond again as well so you're covered.

Beware of overheating any component. A couple of seconds of 30 watt heat should be all that it takes. For any semiconductor component consider using clip-on heat sinks on the leads on the component side of the board. Any metal clip that you can get on a lead as you solder it will help.

On analog boards capacitors are usually suspect of falling out of spec over time due to drying electrolyte inside them. Even with no visible clue a (tin can looking) capacitor can be bad. As they age, internal resistance grows so they can heat up, and sometimes they will swell or bulge so look for that.

When done, and working, re-coat with a suitable conformal coating, for the original reasons of doing so in an automotive environment.