Soldering Basics

Solder is an alloy (mixture) of metals. The type that I am familiar with and use is comprised of 63% tin and 37% lead. This ratio melts to a liquid state at 361 degrees Fahrenheit (183 C.) and solidifies at that same temperature. This is called “eutectic”, having no semi-solid plastic state, and makes soldering easier and more reliable. Some solders that we use are 60/40 and, although the solidifying temperature remains at 361 degrees, it melts at 375 degrees and extra care should be exercised to insure that the joint remains static until the solder solidifies completely. Little changes make a big difference.

Working temperatures (the temperature of the tip of the iron while idle) is largely a matter of the nature of the soldering equipment, the device being worked on, and the preferences of the operator. Personally, I use the Weller WTCP irons which have a high specific temperature. They hold their heat pretty well when transferring energy to the soldering joint. The 60 watt heating element is a contributing factor, lower power would cool off faster, and the choice of soldering tips also contributes. A long, tapered conical tip will cool faster than a shorter “chisel” shape.

Flux

All of the solders that I use have flux embedded in core, which makes work easier. But if you are re-working a circuit, remember that the solder joint that you are re-heating should not have any flux and we will need to add it. We can do this by adding just a touch of fresh solder, “sweetening” the joint by adding more flux, or by adding some flux paste using the point of a pin or a toothpick.

Flux has two main purposes: Removing oxides, and keeping oxygen away from the liquid solder while it solidifies. As soon as we have tin or lead oxide present the character of the solder changes rapidly. The almost magical surface tension disappears so that we don’t have the wetting of the target material, and the solder itself becomes chalky and unworkable. A touch of flux brings it back to life.

Ever get hold of some old or cheap de-soldering braid? Like just plain coaxial cable braid, it sorta kinda works but not what you expected? Spread a thin film of flux on that braid and watch it come to life. A “snuff can” of flux will last a life time, a syringe full will last me a decade.

While on the subject of flux; I’m sure that all the readers are aware of the dangers of acid flux, but there are many varieties of “rosin” fluxes, too. Keep in mind that even if the manufacturer may call theirs “no-clean” that all fluxes should be removed after the work is complete. Even if the flux didn’t continue to corrode the metals that you have joined, it will probably attract moisture over the years. You may find that you begin to have intermittent problems. So a good spritz of flux remover and some scrubbing might save you that trouble.

Flux is dangerous stuff. Solder does not smoke, that is impurities, coatings and the flux. That smoke does contain solder (including lead) as well as a mixture of organic materials. Use fans, use filters, use anything to keep it from your lungs. This is serious stuff, in my opinion more dangerous than electrocution and burns. Necessary but dangerous. Like driving.

Keep the iron tip clean and tinned (coated with solder). A very wet sponge works well. When the iron is up to operating temperature, drag the tip through one of the slots cut in the sponge for a quick steam cleaning. Be sure to periodically dump the waste solder bits into a separate container when re-wetting the sponge. Eventually that container will go to a hazardous waste site but it will take years to collect enough.

The new “brass sponge” cleaners do not cool off the tip as much as the wet sponge and it cleans off the excess solder and breaks up the oxides. A quick dip into the sponge just before applying the iron to the connection does the trick. Personally, I have only been using these for the last three years and so I cannot honestly say that they do not affect the longevity of the plated tips more than the wet sponges. The tips have lasted many years when using the water soaked sponges. I’ll know in a few more years how the tips hold up to the brass cleaners. Probably the best method is to use both!

Tricks and Tools

When re-working or repairing commercially built gear you will certainly encounter lead-free and specialized solder alloys. I find that adding a bit of my own (leaded) solder makes it easier to work with. Not only does it add flux, but introducing some lead drops the melting temperature to where I can remove or re-work it. Do not feel guilty using lead. Electronic hobbyists keep more electronic waste out of land fills than commercial recyclers and the amounts that we deal with are negligible.

Any soldering work needs more than just an iron. Good needle-nosed pliers and wire cutters certainly, but good de-soldering braid, some tools for prying, cleaning, and bending makes for a satisfying experience. Braid comes in several widths, buy several. Order a couple of different soldering tool sets. They have little pry-bars, brass brushes, round and square files for cleaning through-board holes, and probes for holding down SMT devices for soldering. Never use your soldering iron tip to pry wires off a terminal or out of a board! Not only does that not work well, it destroys the iron! The little hooks are perfect for lifting leads and removing wires from terminals. The insulated handles are easy on the hands and soldering tools are cheaper than screwdrivers and such. If nothing else, they make you look like you might know what you are doing!

For through-hole printed circuit boards, a “solder-sucker” is good for removing most of the solder on a joint. The manual cylinder type is fine for non-sensitive circuits, especially if they are going into the trash. Just keep in mind that they can build up a hefty static voltage upon actuation. Most of the time that doesn’t matter. Follow up with braid, clean out the holes, and you are ready to replace components. These tools also generate a shower of tiny solder balls as they slurp up the molten solder. When you are done removing the solder invert the board and tap several times to dislodge these balls and then do a thorough search with a microscope of some sort. Insidious little trouble-makers.

After the leads are removed some solder usually remains in the holes, making it impossible to re-insert the new component. Use the hole file to clean it out but try to do it sparingly so as to avoid removing any plating that may connect the top and bottom layers. The hole file has a round end, try it first. Gently insert the file while twisting it back and forth. Turn the board over and do the same from the other side. The square end is more aggressive so use it with care. Lacking a hole file, use a wire clipping: Hold one end of a component wire clipping with small-nosed pliers, heat the hole with the iron, and draw the wire through the hole to plate it with that reluctant solder to clean it out. If the clipping becomes loaded, discard it for another and repeat until the wire slips easily through the hole.

Some circuit boards have components that are glued to the board before they were soldered. Although that makes removal more difficult, a bit of time and a sharp pry tool will get it done. Remove all the solder possible then heat the pins while prying the body of the part until it comes loose. As a last resort, you can break up the part or cut off the pins and then carefully remove each pin. Just be careful not to over heat the board so that the trace lifts. If it lifts, try to glue it back down. If it gets really bad and you are that desperate then you can run a wire jumper to replace the damaged trace.

de ND6T



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