There are 3 facets of doing the job right:

• Proper Equipment/Supplies
• Proper Preparation
• Proper Technique

I will be assuming in my discussion that you have no experience whatsoever, and know nothing about solder or soldering technique. This will make the discussion lengthy, but hopefully you will pick up some background info that you didn't know before.

There are many different types of solder for many specialized applications. For electronics use, you need rosin core solder, usually 60/40. The “core”, in this case rosin, is the “flux”–a chemical that cleans the connection so that the solder can flow. NEVER use acid core, which is made for uses other than electronics. The numbers 60/40 refer to the ratio of tin to lead in the solder alloy. The 60/40 ratio is most common for general purpose electronic work. I actually prefer 63/37, which is also known as “eutectic” (spelling may be wrong). The alloy of 63/37 melts at the lowest possible temperature of all tin/lead solders. Also, most solder goes from molten, to plastic (partially solidified), to solid as the solder joint cools. The 63/37 alloy has a very small to nonexistent “plastic” region, which means that there is a much less likelihood of the joint “fracturing” due to movement of the component leads while the joint is cooling. My preference is for Ersin Multicore brand, which has 5 cores of the flux running through the solder. Choose the correct diameter of solder–larger diameters take more heat at the joint to allow the solder to melt and flow. I have 0.025“ diameter on my bench which works very good for fine work on Printed Circuit Boards (PCBs); perhaps up to 0.031” - 0.040“ may be appropriate for point-to-point work on tube sockets, power supplies, and such.

The next item to consider is the soldering iron. The wattage/heat range and tip size/shape must be matched to the job at hand. A 30 watt iron with a fine tip is great for PCBs, but probably wouldn't work well for repairing power supplies and other heavy stuff. The iron must produce enough heat to get the joint to the melting point of solder quickly or damage to the components will result. Consider trying to run your car on a lawnmower engine (or your lawnmower with a car engine!) Large diameter wire and component leads (or many leads at a common point) and large “lands” on PCB act as a heat “sink”–potentially drawing away heat from the iron tip faster than the iron can supply it. This means that you must keep the iron on the joint for a much longer period of time to reach melting temperature. On the other hand, too large an iron can damage delicate etched circuits on PCBs and overheat and destroy transistors and other semiconductor devices that are sensitive to heat. The best guide is “one size does NOT fit all!” Of course, some irons come with thermostatic tips where the iron will cycle on and off to maintain the proper temperature–and you can get tips with different temperatures for that one iron. Other models have a control to vary the temperature, as well, but these models cost $$$!! An example is the Weller WCC100 that I use at work which costs $120.

The most important step in a good solder joint is surface preparation. While cleanliness is next to godliness, we generally find that it is next to impossible! So we need to address proper preparation of both the soldering iron and the components to be soldered together.

Any residual oxidation or dirt on the leads of components, wires, PCB traces, solder terminals, or lugs will affect the quality of the solder joint. When the component leads and terminal/lug/solder pad is heated by the iron and the solder flows, there is a new alloy formed containing tin and lead from the solder, and probably mostly copper and perhaps tin, silver, or gold from the component. Some of the tin and lead atoms migrate into the copper of the component leads, and some of the copper migrates into the tin and lead.

The purpose of the “flux” core in the solder is to melt first and chemically clean oxidation from the joint before the solder flows. We can help this along by making sure that the component leads and solder point are as clean as possible as we begin. Sometimes, I use a pair of needle-nose pliers to lightly strip away the oxidized surface of resistor or capacitor leads. Likewise, a cotton tipped swab dipped in alcohol or acetone can clean dirty traces on a PCB. Sometimes, it is helpful to apply a little additional liquid or paste flux to the joint in addition to the flux in the solder. Don't overdo this–you'll need to remove the excess flux later!!!

It is also important to keep your iron tip clean and “tinned.” Applying solder to the end of a freshly-cleaned, heated iron tip is called “tinning.” Most iron sets come with a small sponge which you wet with water to clean the tip with. You can also get holders with wire brushes to clean the tips with. I use Kester SP-44 Rosin Paste Flux which comes in a container similar to shoe polish to stick a badly-oxidized tip into. The tip is cleaned as the flux boils away (but it sure stinks!!!).

The importance of this little ball of solder on the end of a properly-tinned iron tip cannot be understated–the heat from the iron is transferred to the component leads by the molten solder. Consider how food cooks faster with a little oil in the bottom of a frying pan to get the idea here… As you move the iron tip from joint to joint, it is usually necessary to re-apply a bit more solder to the tip to allow for the proper transfer of heat to the connection.

In the course of repair, we frequently find that solder joints have degraded over a period of time. We find the “cold” solder joint, or the “fractured” solder joint. Sometimes these defects are caused by faulty technique on the original build, and sometimes caused by vibration or moving the instrument around in the back of trucks, etc.

A proper solder joint should have a smooth and bright surface. In point-to-point work, such as resistors or capacitors soldered to tube socket lugs and such, you should still be able to see individual strands of wire (if the interconnecting wire is stranded) and the component leads–that is not so much solder that all you see is a blob. There should be *enough* solder, too, to form a “fillet” around the component leads. This is difficult to put into words, but essentially there should be solder 360 degrees around the lead where it sticks through the pad on a PCB, or where it sticks through the solder lug. For point- to-point, the lead should be bent or “hooked” around the lug such that there is a firm mechanical joint before the solder is applied. We should not be relying on solder alone to physically hold components in place.

A “cold” solder joint looks dull and crystalline. This is usually caused by not enough heat at the junction of the component where the solder is applied.

A “fractured” solder joint is where the component lead was disturbed while the solder was in the “plastic” state– partially solidified, but not yet solid as the joint cooled. Sometimes, if there is heavy oxidation on the component lead, the solder doesn't even “stick” to it. Remember that properly done, the solder forms a new alloy with the tin and lead of the solder and the metal of the component wire where atoms of each metal migrate into the crystalline lattice structure of the other (you DO remember High School chemistry, don't you??)

So as we do our repair work, sometimes the first step is to clean things up a bit. I frequently use an old toothbrush dipped in alcohol or acetone to clean away old built-up flux [that sticky orange-brownish stuff]. You might start out with a bit of compressed air or a vacuum or a dust brush, or a combination of all of the above. I've even taken an amplifier chassis into the shower with me to literally wash it clean with a bit of spray detergent and water [just make sure that it is really dry before plugging it back in!!!!] You may not wish to try this method with motors and transformers–but most PCB equipment cleans up fairly well with this technique!

The next step is a detailed visual inspection. We're looking for burnt or charred components, deteriorated wire insulation, broken wires, and defective solder joints. Repair the obvious stuff. If a solder joint looks suspect, then remove the existing solder before adding more. Some of my coworkers use the theory if little is good, then more is obviously better. With solder, this just isn't so. The correct amount is the correct amount, and adding new solder on top of a poor solder joint won't accomplish very much for very long.

There are a number of techniques for removing old solder, ranging from the cheap and dirty to the exotic. Since I do quite a bit of PCB repair, I own a vacuum pump driven repair station made by PACE which I bought used for $300 [They start at about $600 and go way up from there!]. The end of the desolder tip is hollow, and the molten solder is collected in a glass tube when the connection is heated by the desolder tip and the vacuum pump is actuated. These are also made by APE, Weller, Ungar, Hakko, and others. There are also suction bulbs rather like an ear syringe, and spring-loaded devices like the Sold-A-Pullit, and “desoldering wick” which is braided copper wire impregnated with flux that “wicks up” molten solder when heated with an iron. There is a place for each one of these methods, and the one you use is primarily determined by which one(s) you have access to and your preference for the job at hand!

After the old solder is removed, make sure that you've cleaned the connection before applying new solder. Again, I suggest an old toothbrush and a bit of acetone (although there are special flux solvents that you can buy). Apply a bit of solder to the tip of your iron [“tin” the tip], having first selected the proper heat range and tip size for the job. Make sure that the tip is touching all of the wires or component leads that are to be soldered, as well as the trace of the PCB or the lug or terminal. I prefer a “chisel” or “screwdriver” tip over the conical tip for most applications because this shape makes it easier for me to do this, YMMV! Apply the solder to the component lead, NOT the tip. When enough heat has transferred to the components, the solder will flow. If you practice this several times, you will soon get a feel for how big a tip and how much wattage (heat) is required under various conditions.

After the solder joint has cooled, remove the excess flux with a bit of acetone or flux solvent and a cotton-tipped swab or toothbrush. This will make your complete job look more professional and prevent dust and dirt from sticking to the flux residue.

If you have a specific question that I didn't answer, please e-mail me back, and I'll try to answer as best as I can. There are also commercial companies [Claude Michael comes to mind] that produce excellent (and expensive!!) videos and other course materials on soldering equipment and techniques.

Email the author: Robert Hayton

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