While I start work on the power transformers, I also start rinsing/cleaning the PCBs. I'm not sure anyone does this anymore, but I've been told that stray flux will corrode over time - or at least collect loads of dirt and stain - and eventually get so bad that it will conduct electrical current. So, I was always taught to clean up a soldering job.
Many people recommend flux - the more, the better. Generally, that is true, especially if you are soldering metal-to-metal for strength joints. I go way back (an old fart, now) and first started soldering to build brass slot car chassis in the late 60's. I was still only a teenager. Back then, the order of the day was Kester rosin-core solder, with a hockey-puck sized tub of Kester soldering paste. Soldering irons for brass were best with an Ungar. You'd dip the end of the solder into the tub of flux, wait for the iron to melt/boil the flux paste on the end of the solder, then apply the solder. it worked well and was even fairly easy to clean up on the brass chassis that was built.
Electronics soldering is different. The parts are small and delicate. Too much flux and it becomes a nightmare to clean, and damaging parts becomes more likely. So, I like to use flux only when something gives me trouble soldering - or with SMD. I always use a flux pen with SMD. The flux quickly partially dries into almost a glue. So, it helps to stabilize the part while soldering. In a lot of cases, a full flush of alcohol works quite well, so cleanup is not so bad - even with the additional flux - in using SMD. Without leads, the parts are not so delicate. There are always exceptions, of course.
The solder I use is still Kester rosin-core, but is eutectic 63-37. The rosin core, 99 times out of 100, is sufficient flux to make a great joint. It still leaves quite a bit of flux residue, however.
So … I rinse and clean every PCB with alcohol. I do this until most, if not all, of the flux is removed and the PCB is nice and shiny. It's not rocket science and very rudimentary, but I wouldn't feel right selling an electronic product without doing it. The pic below shows the basic tools (very complicated!
Walmart 91% isopropyl alcohol, a used butter tub, old toothbrush, and paper towels. (The Pug soap dispenser is optional.). I also use Q-tips, but more on that later on.
On the other side of the sink, the PCB is placed top down on a set of paper towels:
Looking at the pic below, you can see why this is necessary. The area around the tube sockets are worst, because of all the solder I use to make a good
mechanical joint:
Notice the large clear bubbles of solder flux residue around the tube socket pins and all the other leads, too. This is what we want to remove, so it won't cause issues years later. If you've ever opened up vintage electronic equipment, you'll know what I mean. Even stuff made in the 80's and 90's can look pretty rough if the flux was not cleaned off.
It generally takes about six rinses of alcohol, applied with the toothbrush by dipping into the butter tub, coating the PCB with the brush, and rubbing/brushing at the leads and pins where the flux collects. Then I pat up the dissolved alcohol/flux with paper towels, before the alcohol completely evaporates. As said, it's far from rocket science and very tedious, but necessary.
I use the simple Walmart 91% alcohol because it works best. I purchased some commercial flux remover solution years ago, but just using a little almost knocked me out. It also didn't remove the flux as well as simple alcohol. I've never used it since. Sometimes the alcohol fumes get strong, but TELs are quite high for alcohol and the coating is quite thin. I think it works the best.
Here's what it looks like after the first rinse. One might think it's not worth the trouble, because this looks ten times worse than before we started, with streaks and gunk everywhere:
This shows the paper towels absorbing the dissolved flux/alcohol rinse. When it's thick, the paper towels will turn a yellow-brown where it picks up the mixture:
Believe it or not, this pic is very, very close to being completely clean. When it looks like this - dissolved white powder everywhere - it means that the hard, dried flux bubbles have all been dissolved and are drying as a fine rosin powder all over the PCB. From this point, it only takes about two more rinses:
Here is a PCB completely rinsed, nice and shiny:
The Q-tips at bottom are used for the top of the PCB, to clean off the flux-pen residue from the SMD transistors.
Here's a closeup of the tube socket area that we pictured above - big difference, now!
Note that the slight bulging around the tube socket pins is NOT flux, but the FRP of the PCB material, bulging at the edges of the holes because they are so large for the tube sockets. It is not flux.
And the rinsed top of the PCB:
The Q-tips make it easy with the flux-pen residue: one tip to apply the alcohol, the other tip to wipe it up. It's a bit harder than that sometimes, but not too much.
Thanks for your patience! I know this is not flashy stuff, but a necessary part of the building process.
