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Soldering PCBs with a Solder Pot


Solder pots are simple things and can be used to solder PCBs with through-hole/leaded components quickly and cheaply. Be sure to get one with setable temperature. It costs more, but is worth it. A simple solder pot and a 600W lamp dimmer works well as a power controller if you want to save money, but the temperature regulator is you.

The 'pot' at the top should be rectangular or square, large as you can afford. Turn the heat on to max, then add the bar solder, 63/37% tin/lead is best, to the pot. Keep adding solder until the molten metal mounds up above the lip, brimming. You want it to be just on the verge of spilling over. Now it's time to set the temperature, get a strip of newspaper and touch one end to the solder. Perfect temp is when the paper just turns a light tan. (Remember Ray Bradbury's Fahrenheit 451? Same temp!) Black or flaming is bad, so is no change. Keep at it, until you get it right, it's really, really important to set the temp right.

By this time the solder has probably got a dull or corroded surface, this is 'dross'. Lightly scrape it off with the newspaper strip, leaving a bright shiny surface that looks like a pool of mercury. Add more solder if the surface is below or even with the pot lip... you want it brimming.

In a separate glass tray, pour some liquid rosin flux, Kester makes a good one. It's a reddish color and very fluid. About a 1/4" depth is fine. Put the tray next to the solder pot.

Ok, now it's time to solder the PCB. Make sure the leads of all the parts on the PCB are trimmed to less than 3/16 of an inch, but more than 1/16". The copper PCB pads should be shiny and clear of any tarnish or corrosion.

Pick the PCB by the sides up with a pair of steel tongs or very long nose pliers. Lightly 'skim' the bottom of the PCB flat onto the surface of the flux, don't drown it! You just want to wet the all the bottom of the board. Spilling it over on the top won't hurt anything, just wastes flux and makes the PCB look ugly later.

Then skim the PCB, flat, across the brimming solder surface, slow enough that it takes three to five seconds to make a complete pass. If the board is wider than your solder pot, make several passes in strips. Try not to overlap too much.

Once you've made a pass, hold the PCB in air, still, over a piece of tin foil for a few more seconds. (The tin foil will catch any drips.) This stillness is important, any parts moving about will increase the odds of bad solder joints. After 20 or 30 seconds, carefully inspect the bottom of the PCB for solder joint quality. If you have a lot of bad joints, check the rosin level in the tray and the solder temp again. Then run the PCB through the same steps again.

After a few boards, you'll likely need to add more solder and flux. If you have to add a lot of solder wait a few minutes afterwards before soldering more boards. This allows the solder temp to come back up. Scrape the dross off too. (Check the temp again! It's worth it!)

Ok, after you've done all your boards, time to shut down. Turn off the solder pot, clean off any dross that's accumulated with a scrap of newspaper, then leave the solder in the pot. It won't go anywhere once it cools. Pour the unused flux back into it's bottle then seal tightly. It'll all be ready for your next run.

Done right, this works great and doesn't cost an arm and a leg. I did this for small PCBs years ago and never had a problem, once I was told how to set the temp right by a kindly old timer. I would have never thought to use newspaper as a temp indicator, but it does work.

Also:

"We use 63/37 solder at 250-260 degrees Celsius and Kester 951 "no-clean" flux, which is a clear thin liquid (mainly alcohol). For soldering small, double-sided, tinned circuit boards dip the whole board, parts and all, just into the surface of the flux so the entire top and bottom of the board is wetted. Don't get flux onto the components beyond their leads. Make sure you use sealed capacitors, switches, etc. Drain for a few seconds by tipping up the board (watch the parts!) or by applying an air knife. Then preheat the board over a radiant heater for about 1 minute to evaporate the flux solvent. The board should reach around 150-180 degrees while in preheat. Then dip the board onto the solder for 3 to 4 seconds. Don't just drop it straight down or many of the components will float up - slide it into position."

"After soldering there should be almost no visible flux residue. If there is you are not draining properly or your flux is too strong. You can thin it with Kester 110 flux thinner. The flux gradually increases in strength as it is used (evaporation of the solvent) so you'll need to thin it periodically. We try to keep it as thin as possible. You'll know your flux is too thin when you start getting unsoldered joints. Just out of the bottle, Kester 951 seems to be a bit too strong - thin it by about 5% initially. If that seems OK, try thinning by another 5%. You have to watch the solder temperature carefully. If it gets too hot you can damage components. If it gets too cool, the solder will form ugly blobs on the bottom surface of the board. We use a Fluke high temperature thermocouple probe to monitor the solder temperature."

"Our soldering is done in a vented fume hood - it's smelly business. We skim the dross after every dipping - we haven't tried dross reducing agent. Don't throw the dross in the garbage. Save it and donate it to a PCB assembly house - they know what to do with it. Wash your hands carefully when you are done or you'll get bits of lead and its compounds in your food."

See also:


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