A Few Basic Circuit Board, Solder Joint Questions
 

I have nice, GDL-rebuilt cruise on one of my cars....money well spent. Since I'm finding it hard to justify the cost of a proper rebuild on my second car, I thought I would deal with the solder joints on the amp myself and hope for the best. While I took an electronics course a long time ago, the only time I ever really used a soldering iron, I could use some help with some of the basics.

1. Is my goal to remove and then reapply the solder at each joint? Or is the goal to heat each joint to restore conductivity?

2. Assuming that the goal is to heat the joint, can someone quantify for me exactly what I need to do? I assume I need to touch each joint with x-watt iron for y-seconds and to make sure that the solder doesn't run. Do I heat the joint until it's shiny? molten? Some other criterion?

3. Is my 35-watt iron adequate or do I need something else?

I've been reading some related threads....my apologies if I haven't noticed answers to these particular questions.

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.

Thanks, Scott. This is helpful and interesting.

Question: why is it necessary to remove the conformal coating? I would have thought that the hot iron would break through the coating and that there would be nothing to prevent the reheated solder from forming a solid connection.

Do I touch the soldering iron to the lead or to the solder? Sounds like you're implying the lead, but just want to make sure.

Usually when soldering you apply heat to the lead. This will draw the solder into the join.

Heat the "work," not the solder.

It's best to remove the conformal coating as it can contaminate your solder joint. Once it's removed, the reheating of the solder joints goes quickly. The conformal coating can be removed with Isopropyl Alcohol but it takes a while. Methyl Ethyl Ketone (MEK) is much faster but use in a very well ventilated area! IIRC, it's available at Lowe's, Home Depot, etc... in the paint department.

+1 on what has been said about getting the conformal coat off. Critical step.

MEK works great and is what we use (Im an EE) in the lab. Its nasty though - we use it under fume hoods.