Fast soldering techniques

And inspect for and rework all the solder bridges.

John

A while ago I was working with an older engineer developing some boards for my old college. He kept saying how easy it would be if we had a large solder pot to dip the populated boards into to solder them. I know of reflow and wave soldering -- not ot mention manual soldering -- but how would soldering with a large solder pot work? I can see it being similar to wave soldering, the board being in direct contact with the molten solder. But i keep thinking that the dross would hinder the ability for the joints to form well, and what about fluxing?

Can anyone explain how static soldering works, and maybe a book about soldering techniques?

Thanks, Tom

I don't think you'll find a book on this subject -- this is definitely an inferior method of soldering. The most important flaw is that it will leave much more solder on the individual solder joints, which will really tend to cook your transistors and such. It was a fairly common method in the '60s, though, and it _is_ faster than soldering individual components by hand. So....

If you insist on doing this, and you have a fairly small board, you first have to look at which components are most susceptible to heat damage, and set them aside to do by hand later. Actually, this may not be so bad, especially if you're using sockets for your ICs. Basically, your biggest problems then will be with semiconductors, small signal diodes and electrolytic caps.

Once you've got that out of the way, give some serious consideration to clinching the leads down on the pads while you're cutting them. This will result in less solder in the solder joint, and less surface area of the lead in contact with the solder to channel heat up to the component. It also keeps the components from popping up when you don't want them to. You also might want to dab some soluble solder mask over the pads on the components you're reserving.

You obviously need a solder pot with a surface area larger than your circuit board. Make sure that solder pot has a good heat control (in most cases, the bimetallic switch type are too crude -- you don't want the solder any hotter than necessary. Try a setting of about 430 degrees to start, and see if you get good wetting of pads. You definitely need euctectic (63-37) solder for lowest solder temp, filled up past the top of the tub (the meniscus of the solder is over the level of the top).

Now, before you start, you need an infrared heating lamp, some liquid rosin-based flux (thicker is better), a paintbrush, and a fixture to hold the circuit board so your hands aren't too close to the heat (either that or wear thermal gloves). Try to use steel for the part of the fixture that clamps the board -- it's strong, the solder won't wet, and you can dump it in water to cool when you're not using it without worry. Also, ventilation is a must (you breathe, right?)

Now you can begin. Clean any dross off the surface of the solder. Clinch the leads on the components on the board. Apply solder mask. Place your small board in the hand-held fixture. Get your good rosin flux, and paint the flux onto the bottom of the board while holding the board so the solder side is down. Paint strokes left-right, and forward-reverse, so as to make sure the flux gets into every nook and cranny. Enjoy the dripping and slop. This is one place a syrupy flux comes in handy. Now hold the small board (solder side still down) under a heat lamp to increase the surface temperature of the traces and flux temperature (this activates the flux and starts the chemistry before it hits the solder). I guess you'll have to time this. If the components get too hot, or if the board starts to discolor, you've gone way too far.

Now immediately bring your handheld fixture over to the solder pot. Hold your eye (you ARE wearing safety glasses, right???) level with the surface of the pot, so you can see the surface of the solder. Not too close, OK? Now angle the board slightly, and then slowly set one edge of the bottom of the board barely touhing the solder. In about 1 second, pull the angle down so all of the bottom of the board is touching the solder. Now quickly push down slightly, so the meniscus of the solder is starting to come up on the edges of the board, but not enough to have the solder start pushing up through the holes. After a mystery time (you'll have to determine this yourself, use a count of four seconds as a start) leave the entire board in the solder. Now lift the end of the board opposite the one you put in, then take the whole board out of the solder. Hold the board steady and motionless once you've removed the board from the heat area immediately above the solder, to allow the solder joints to set. With a little practice, you won't have to eyeball the level of the solder -- you can play with the depth just by looking at the meniscus of the solder on the board. You can even let the level of the solder come down to the level of the top of the pot.

After this, clean using totally aggressive '60s organics which are now all blatantly illegal, hand solder the components you held back, and moan about your poor yield. If you haven't left the board in the solder too long, you don't have issues with solder bridges and with solder flecks sticking to the PC board. If you have, welcome to the rookie leagues. Get out your magnifier, pick and soldering iron, and get to work.

That is, if you insist. This is an ugly way to do real boards.

Chris