THE ART AND SCIENCE OF SOLDERING

Very Little has been written in the general audiophile press on the subject of soldering. This is unfortunate, as many of us are required to perform the task from time to time, and should know how it is done. Like anything else, there is a right way and a wrong way to go about it. Since a poor solder joint can do much to undermine the quality of the signal attempting to pass through it, it is important to insure quality whenever we tackle a soldering job. I hope that this brief introduction will give you the information you need to make minor repairs or modifications. Or, in the case of the more ambitious hobbyist, become proficient at making ultra-high quality solder connections for more involved projects.

The object in soldering is to join two metal conductors by flowing molten metal across their surfaces. Sounds easier than it really is. In a proper joint, the solder molecules actually combine with those in the metals being joined. The term applied to this action is wetting.

Proper technique requires proper tools, and this begins with a good quality iron of the right temperature for the job. Fixed temperature irons come in various sizes or wattage's and cost between $15.00 and $50.00, depending on quality. Fixed wattage irons are the most common types, although variable heat devices are the choice of most professionals. These units allow the user to select a variety of temperatures, one that will be correct for the task at hand. Expect to pay $50 and up for a variable iron.

The next most important element of the equation is the solder itself. For audiophile quality work this will normally be a solder containing a small percentage of silver. Most all solders contain a small inner core of flux. Flux helps the solder adhere to the work by removing oxides from the surfaces of the metals. For electronics application, only resin core solder should be used.

To begin, be sure that the metal surfaces of the work are clean, free of oils and debris that could reduce the ability of the solder to adhere. Also be certain that the tip of the soldering iron is clean and properly tinned. Place the tip of the iron at the junction of the two pieces being joined, this will insure that even heat is applied to both. After the metal has been sufficiently heated, apply the solder at the junction of the two pieces. Do not apply solder to the tip of the iron itself. The solder will flow to the hottest part of the connection, the point of iron contact. Withdraw the iron as soon as possible and let the joint cool. It is important to heat the connection for as little time as possible to avoid undue stress to the conductor and/or insulation or dielectric. Avoid disturbing the completed joint during cool-down. Finally, de-flux the area with an appropriate solution to remove unwanted traces of the resin flux. Your labor should have resulted in a bright, shiny, smooth surface.

A dull, rough or grainy surface texture is indicative of a solder job gone wrong, a cold solder joint has been produced, and is unacceptable. Either too little heat was used or too rapid cooling has occurred. Reheat the joint to see if you can restore the connection. If that fails, remove the old solder and start over.

Certain more specialized jobs, such as soldering components on a printed circuit (PC) board, require additional knowledge and care. The component parts themselves (especially sensitive transistors and integrated circuits) can easily be damaged with excessive heat. When installing these sensitive parts, one must be sure that an iron of the proper heat (wattage) has been selected. Small heat sinks designed especially for the job (or alligator clips) may be attached to the parts themselves to help dissipate damaging heat.

If you need to repair a poor solder joint, or replace a component soldered to a PC board, you must first remove the old solder. This may be accomplished using a variety of methods. Solder removal wick or braid is the most common. This product consists of resin coated copper that attracts molten solder. After the old joint has been reheated, the wick is applied to draw the solder off the connection. This procedure requires a bit of practice to master. Another common means of removing solder employs a tool affectionately known as the "solder-sucker." This little device consists of a spring loaded plunger inside a tube. Pressure on the trigger releases the plunger, creating a suction or vacuum at the tip. Simply heat the connection, melt the solder, put your "solder-sucker" in place and viola, clean surface! Concerning costs, a roll of solder wick sells for a few Dollars, the vacuum tool commonly sells for about ten.

It takes practice to produce a good solder joint. I recommend that you spend some time honing your skills before attempting more important work.

Dr. A.J. van den Hul has put together what he calls "The Commandments for Optimal Soldering. " While these recommendations were originally targeted for implementation with van den Hul products, the general guidelines put forth aptly apply is most cases. Although some of the suggestions are a bit esoteric for the average hobbyist, much can be learned from the basic techniques. It may also help you to understand some of the reasons behind the high termination costs of some of the more exotic, difficult to work interconnects and speaker cables. What follows is an amended version of those recommendations.

1. Properly prepare the surfaces for soldering by thoroughly cleaning them with a non-residue product such as Freon TF. It is especially important that any oils or grease be removed from the surfaces to insure proper solder adhesion.
2. Stripping of the insulation or jacket, should be done with a thermal stripper or very exact mechanical unit. It is very important to prevent the all to common surface damage inflicted by careless stripping. Significant sonic degradation may result from this type of conductor damage.
3. Soldering should be done with a silver-saturated solder or soldering device with a silver tip. The silver will improve the conductivity of the connection and decrease the tendency to oxidize over time.
4. The temperature of the iron should be around 300'C (approximately 550° F). This range may vary depending on what you are soldering. Lower temperatures may be recommended for more delicate operations involving IC's and transistors. Our note: It may be difficult to find a temperature specification for some irons, especially less expensive models. More common is a wattage rating. As a general recommendation, I find that a 25 to 40 watt iron is ideal for most audio work.
5. A light flow of N2 or C02 during soldering is strongly recommended. The gas is to be blown over the joint being soldered through a small orifice. Our note: Obviously this step is not practical for the average audiophile, but is another reason I always recommend factory termination of the more exotic cables.
6. The use of a heat-sink during the soldering process is highly recommended. This will draw heat away from delicate parts or conductors. Our note: According to Mr. van den Hul, the signal transfer quality of the cable is preserved if excessive heat build-up in the conductor material is prevented.
7. De-flux the joint with a suitable solvent to remove all traces of resin. A cloth or small brush may be used to help remove stubborn resin deposits. The completed joint should exhibit a bright, clean appearance. A dull or uneven surface indicates a cold solder joint and will not be conducive to optimum signal transfer.
8. Cover the completed joint with a flexible coating of enamel (Our note: fingernail polish works well) to prevent airborne contamination or oxidation. Our note: While not necessary on most connections, this procedure is vitally important when terminating some cables to prevent air migration into the jacket opening. In these situations it is important to close this entry-way by sealing the jacket opening at the point where the conductor exits.
9. Conductive parts of the connector should be treated with an oxidation inhibitor. Silicone oil with a viscosity of between 150 and 200, diluted 1 to 25 with acetone, may also be used.

CARE AND FEEDING OF THE SOLDERING DEVICE -
 

• Always keep the tip properly tinned. A well tinned tip should exhibit a bright surface, with no dull or discolored areas.
• For maximum tip life, apply solder to the heated joint, not to the tip itself. Repeated application of solder directly to the tip will significantly reduce tip life expectancy.
• To properly tin the tip, proceed as follows:

1. Heat the iron to operating temperature.
2. Apply a liberal amount of solder to the tip and let stand for one minute.
3. Apply more solder to the tip, allow to stand for two minutes then wipe lightly on a dampened sponge. Wipe off only the excess and try to wipe melted solder onto non-tinned areas.
4. Always add additional solder to the tip before returning the iron to its rest.

I hope that the information we have provided here will further your understanding of the soldering process. Good luck, and don't burn your fingers!