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To understand soldering, you have to understand some of the physics involved. Heat travels from a hotter material to a colder one. Heat is always trying to seek equilibrium. And for our purposes, solder behaves just the opposite. Solder flows from a colder area to a warmer one. This is why the joint you are soldering has to be hotter than the melting point of the solder. It is also why if you melt some solder on your iron, and dab it on a cold joint, it will not stick, or flow into the joint. This is, in fact, the definition of a Cold Solder Joint. The joint was not hot enough to cause the solder to flow into it, as well as all of it's nooks and crannys.

But, too much heat is also bad. It destroys components, lifts circuit traces, and, plain old messes things up. And, the longer you supply a heat source (energy) to a material, the larger is the area of the material that gets heated.

Soldering largely boils down to the art of heat control and manipulation.

The Materials

Besides the soldering iron you have selected from part one, you will need cleaning supplies, solder, and possibly some flux. A magnifying lamp is handy for inspecting the results, and it is highly recommended to use a commercial Flux Remover (Flux-off) to clean excess flux off your work after the soldering is done.

The solder you use should be a rosin flux core solder made for Electronics. The composition should be 60/40 (60% tin, 40% lead) or 67/43. Two common diameters are small - .031", and large - .062". I prefer the smaller size for most PCB work. Two very popular manufacturers are Kester ("44" solder), and Multicore. Never, ever, use solid, or acid core solder for electronics.


Two examples of Commercial Spray Flux remover.




Kester '44' Rosin core Solder. On the left is .031" diameter. On the right is .062" diameter.

There are also coming on the market lead free solders for electronics. I have no experience with these. Likewise, there are water soluble fluxes starting to become popular, especially in the manufacturing world. Again, I have only limited experience with these. My recommendation for now, is to stick with the older, more conventional materials, until you are confident of your abilities, then you can try the newer ones yourself.

If you get a separate flux, make sure it is Rosin core (For now, not water soluble). Again, Never, ever, use Acid core flux. The flux you find at the home center plumbing department is acid core. It will slowly eat your circuit board up!

And, a caveat. Soldering produces fumes. Among other toxins, the fumes may contain lead. While the old-timers, like me, savor the smell of 'burning solder', you are well advised to work in a well ventilated area. There are also fume hoods and such made to extract the fumes. Enough said.

The basics

So, The first lesson, is to heat the entire joint. By definition, a joint has more than one piece. A component lead going through a PCB through hole, two wires, a wire wrapped around a terminal. In order to make a proper solder connection you have to heat all the parts of the joint simultaneously, quickly, and to a temperature above the melting point of the solder. Quickly means you have to heat the parts you wish to solder hot enough, BEFORE the surrounding parts (I.E. the PCB) get hot enough to sustain damage. This requires an iron with enough capacity (wattage), a tip with enough mass (large enough), and a point that is large enough to contact all the different parts of the joint at the same time.

The second lesson is cleanliness. As addressed in part one, keep your soldering iron tip contaminant free, clean, and well tinned. Likewise, the parts you are soldering. Clean the circuit traces with a pencil eraser, a 'scotch brite' pad, or something similar. Make sure the leads of the component are clean. Especially if it has been in storage for a while. Clean component leads (resistors, capacitors, etc) with very fine sandpaper, emory cloth, or scrape them carefully with a knife. If the leads are plated, such as on many ICs, they should not need extensive cleaning. But check carefully. Solder will not adhere to a dirty, oxidized, or contaminated surface. Old components such as resistors or capacitors , or copper PCB Traces can be notoriously difficult to solder, because of the layer of oxidation which builds up on the surface of the leads. An unprotected copper printed circuit board will generally oxidize after a few months, especially if it has fingerprints.

One important point. Do not re-use solder. By this I mean, once cored solder is heated the rosin flux begins to vaporize. Once the flux is gone, there is nothing to clean the joint, or to prevent oxidation while doing the actual soldering. Do not melt a glob on the end of your iron, and then attempt to place it on a connection. Do not try to 'coax' already melted solder over to another connection. Always start with fresh solder. Flux cleans the metal surfaces as the solder melts. This is why you must melt the solder actually on the joint, not on the iron tip. Without flux most joints will fail because metals quickly oxidize and the solder itself will not flow properly onto a dirty, oxidized, metal surface. While there are exceptions, this is a good point to keep in mind.

Finally, remember this point. It is the mechanical contact that transfers the heat, not air. Many times it is not possible to make a solid mechanical contact between the iron and all the parts of the joint. For this reason, it may be necessary to add a little bit of solder to the tip of the iron while heating the joint in order to effect a proper heat transfer. Especially on an irregular joint. The liquid solder often serves as the heat path. Do not add very much, a little dab is all it takes for heating!

Note that most beginning solderers tend to use too much solder and heat the joint for too long.

How to do it.

To solder a joint on a PCB. Place the iron tip so that it contacts both the component lead and the circuit board trace. If necessary, add a very small amount of solder to the tip/component joint after they are in contact to ensure proper heat transfer. Heat the joint for about 1-2 seconds, and then touch your solder to a part of the joint. NOT the iron tip. The solder should immediately wick into the joint. Add just enough solder to fill the joint, then remove the solder, and then the iron. This whole process on a PCB should take no more than two to three seconds AT MOST. Let the joint cool. Do not disturb it for a few seconds until it is cooled enough that the solder is solid again. Inspect the joint, and move on. If you find you have made a bad joint on a PCB, you can try re-heating it, and adding a very, very tiny amount more solder to ensure heat transfer... The best course though is to remove the solder with one of the methods in the desoldering section, let everything cool completely, and then do it again. Never linger on a joint, and always let it cool completely between attempts. When soldering a multi-leaded part, like an I.C. stagger the pins you are soldering so as not to concentrate too much heat in one area. And remember, if you reheat a joint that is already warm, it will just heat up faster to a hotter temperature, and in a larger area. This causes many PCB trace failures!

At left, you can see the proper way to hold your iron on a PCB joint. Note that the iron is 'wedged in' between the lead, and the circuit trace, touching them both.

After allowing the joint to heat for 1-2 seconds, apply your solder to the opposite side. Just enough solder to cover the joint. Remove your solder supply, then the iron, and let cool.

Be sure to clean your iron tip constantly and regularly (See part one), in between uses. When you place the iron in it's stand, add a little solder to keep the tip wetted

Soldering other joints follows the same basic rules, although naturally, the larger the joint the longer the heat will need to be applied.

After soldering your circuit board, use your spray can of defluxer to remove any excess flux. Hold the board over a suitable place (Trash can or rag), and spritz it well. This will not only make it look a whole lot more professional, but in the case of RF, excess flux can have an effect on circuit tuning and operation.

That is all there is to it. Oh, that and a lot of practice. It is an art. Soon soldering PCBs will become second nature. The biggest cause of bad solder joints is putting the solder on the tip before the joint is hot enough to wick it, or before even touching the joint. The biggest cause of PCB trace failure is too much heat, I.E. keeping the iron on it for too long.

Some other specific soldering tasks

When attaching stranded wires to a circuit board, strip the wire, twist the strands together, and tin the end by heating it, and applying a little solder to keep the strands together. THEN put the wire through the PCB hole, and solder it as above.

When attaching wires to a solder pot style connector. Tin the wire as above, melt a little solder into the solder pot on the connector. Let it cool for a short while, then simply heat the solder pot back up, and push the tinned wire into it.

On the right are some typical Solder Pot connectors. In front is a DB-25 connector. At left is a 4-pin DIN along with the shell parts. At right is a panel mount SO-259 jack.





When attaching wires to a loop type terminal. Tin the wire as described above. Then, using needlenose pliers, loop the wire through the terminal, crimp it down solidly. Make a good mechanical connection first. Then solder the lug, using just enough solder to fill the hole in the terminal lug.

These pictures show a panel mount potentiometer with the wires attached. One lug has been soldered (bottom and left). The other two lugs show the proper way to make the mechanical connection. Note that there is just enough solder applied to fill the terminal hole and cover the wire.


Two semi-related tips that I want to share here. When soldering a wire to a terminal, or connector, the spot where the solder stops going up on the wire is known as a 'stress raiser'. This spot is very prone to breaking. Make sure the wires are not subject to bending at this point. This is especially true of connectors. Make sure you use the proper strain relief's to keep the wires from moving! If you have the option, when connecting wires to terminals, connectors, or to each other (splices), the most acceptable method is crimping, not soldering. I would never make such a solder connection in an area prone to vibration, such as an automobile.

And the second. While opinions differ, I like to clip the excess leads off of PCB mounted components after soldering. One reason is that it is easier to deal with the longer leads, and use them to help hold the component in place. The main reason though is shock. Note how far the cut-off flies when you clip a lead with your cutters! Well, thanks to Sir Newton, that same force, or shock, is also transmitted into the component. While this may not affect a component like a resistor, it can have devastating effects on the more delicate components. Specifically, clipping the lead of say, a transistor, sends a shock back into the substrate. This will either cause the component to fail immediately, or, more likely, will create a weak point that will be subject to failure in the near future. This is especially true of junction devices such as transistors and leds.


So, start practicing your soldering skills. If you are inexperienced, do not tackle that special project just yet. Many times you can find cheap kits, un-populated bare boards, and the like in garage sales, electronics oriented flea markets, and, of course, on Ebay. Also, manufacturers will often dispose of defective boards, if you can find a source (Search the internet for your area). Another, slightly more expensive source is bare experimenter PCBs. These can often be found on sale cheaply. Due to their variable quality, Radio Shack experimenter boards are a good way to practice. If you don't mess those up, you will be OK with a better PC board. Real Old Computer boards often have unpopulated areas, check out the used computer mart. Whatever you find, Get a few bare boards just for practice. Stuff them with some old parts, and practice. Did I mention practice? It is an important point! And try to do your practice on new boards, not ones that have been previously soldered. The two will behave differently, and the techniques will be slightly different.




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