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How to Solder a Bad Connection

August 1, 2002
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When a customer brings an electronic instrument into my shop, one of the first things that gets checked is the connections. Broken solder joints are by far the most common cause for the malfunction of electronic musical instruments.

Why are poor connections so pervasive? Partly because there are so many connections: a synthesizer or a mixer may have thousands of solder connections. Some integrated circuits commonly found in electronic instruments have dozens of pins, each with a solder connection to a printed-circuit board. Even the simplest electronic components have at least two connections.

One way to look at solder connections is as electronic components. A poor connection can have the characteristics of a resistor, a switch, or a semiconductor. Even connections of the highest quality have some resistance, which might be appreciable, depending on the application. A fractured connection often acts like a pressure-sensitive switch. If a connection has more resistance in one direction of current flow than in the other, the result is a crude semiconductor.

Bad solder connections have the effect of adding more components to the circuitry of an electronic instrument than its designers intended. Not surprisingly, this causes the instrument to go “tilt.”

CORE ISSUES

When you solder, always remember that you are working with molten metal, and apply commonsense safety rules accordingly. Protect your eyes by wearing goggles. Keep solder out of your nose and mouth, cover any sores or cuts before handling solder, and wash your hands when you finish your work. Avoid breathing solder fumes and make sure you have plenty of ventilation. Finally, keep food and drink out of the soldering area.

Electronics service techs work with solder every day, but not everyone knows what it is made of. The type used for electronics is called rosin-core solder and is composed primarily of a combination of tin and lead; the former is for conductivity, and the latter is for low melting point. Important: never use acid-core solder for electronics work; it is designed for building things such as gas tanks and will destroy electrical connections.

The most commonly used proportion of tin to lead is 60 to 40. This 60/40 mixture goes through three phases: its original solid state, a plastic puttylike stage when the solder is solidifying, and a molten-liquid stage. During its plastic phase, solder is highly susceptible to fracturing due to uneven cooling within the connection. For this reason, a 63/37 tin/lead combination, called eutectic, is preferable; it has no noticeable plastic phase and significantly reduces the incidence of internal, hidden fractures. It's especially important to use 63/37 solder for PC boards with plated-through holes.

You may have noticed the smell of pine trees while soldering. This is from the rosin contained in the hollow center of the solder. Rosin is the brittle, hard resin that remains after turpentine is distilled; when it is molten, rosin becomes a detergent. It flows rapidly and freely throughout a molten solder connection and attaches itself to contaminants.

FIG. 1: Some techs feel that multicore solder (above, right) ­provides more reliable flux activation because the ­multiple smaller cores respond quickly to heat. ­However, single-core solder (above, left) is more ­commonly used. Good-quality solder of either type should yield excellent results.

FIG. 1: Some techs feel that multicore solder (above, right) ­provides more reliable flux activation because the ­multiple smaller cores respond quickly to heat. ­However, single-core solder (above, left) is more ­commonly used. Good-quality solder of either type should yield excellent results.

Another name for the core is flux, which is suggestive of its rapid flow through the connection due to capillary action. When solder cools and solidifies, rosin is displaced to the surface of the connection, taking contaminants with it. The most common type of solder has a single core, but you also can get multicore solder, which responds more quickly to heat because each core is small (see Fig. 1).

The size of solder that you need to use depends on the type of project that you're working on. For IC and small-component work, and even for some larger components, 0.037-inch diameter solder should be fine. However, 0.025-inch solder is considered acceptable, and it's easier to handle.

CLEANLINESS

You must clean solder connections before and after soldering. Cleaning beforehand is a good policy because of the many contaminants present on the surfaces of the metals being bonded. Rosin's detergent action is not perfect and will not remove all contaminants. The best way to preclean connections is with a cotton-tipped swab and alcohol or acetone. You can prepare component leads for soldering by burnishing them with steel wool or braided wire to remove oxidation.

Cleaning connections after soldering is also important. Remember that rosin is merely a carrier, transporting contaminants to the surface of the connection. Few people realize that rosin is corrosive and, in time, will eat into a connection. Acetone is useful for removing rosin, and the best time to do so is soon after a connection has cooled and solidified.

THE RIGHT TOOL

You need a proper soldering iron for making good connections. Don't bother with anything less than a well-made 30W iron with a three-wire power cord; a cheap iron is a bad investment. The tip of the iron should look like a good solder connection, smooth and mirrorlike. A thin layer of solder should already be on the tip; the procedure for applying solder to the tip is called tinning the tip.

The tip size you need depends on the size of the work. For ICs and other small solid-state components, use a small (say, 1¼32-inch) tip to avoid creating solder bridges between components. For other types of components, such as resistors and capacitors, an intermediate-size tip is better. Soldering electromechanical components, such as audio jacks, calls for a larger tip, perhaps an ⅛-inch.

Tip temperature should be electronically controlled at 600 to 700 degrees Fahrenheit, depending on the application. Lower temperatures are preferable for semiconductors and other heat-sensitive components. Tip temperature is critical and should not be allowed to fluctuate.

Keep the soldering iron in a solder station when not in use. The solder station should have a heat-resistant sponge on which you should periodically wipe the soldering-iron tip clean. Wet the sponge with distilled water to avoid contaminating the soldering-iron tip with the minerals that are normally present in tap water.

To clean and tin the tip, apply solder liberally to it, then wipe it gently on the sponge. The same detergent action that cleans solder connections also keeps soldering-iron tips clean. Replace tips regularly, because they are constantly subject to the corrosive effects of rosin.

HEAT THE CONNECTION

When making solder connections, heat the connection first, not the solder; then, apply solder to the connection, not the soldering iron. Heating the connection thoroughly ensures proper detergent action and allows rosin to do its work. If a connection is hard to heat, apply a small amount of solder to the soldering iron tip to act as a catalyst to heat the connection, then apply solder to the connection.

Inspect all solder connections carefully. There are several telltale visual clues you can use to spot a bad, or “cold,” connection. This type of connection was not heated properly to begin with, and detergent action never occurred. A cold connection is usually obvious: it has a pitted, dull appearance and is spherical in shape, indicating poor capillary action. A good connection is shiny and smooth and has a graceful, gentle slope from the center of the connection to its perimeter. You can see your face in it.

From the perspective of the service technician, a bad connection often requires resoldering. Dirty or fractured soldering connections can be found in instruments built by the most reputable manufacturers. You may find several poor connections. Fracturing can occur because of manufacturing flaws or mechanical stresses caused by users, such as dropping.

Resoldering is sometimes known as reflowing. This means that the connection is heated until the old solder is molten, and then new solder is applied to clean the connection.

Resoldering several strategic connections in an instrument may completely solve the problem. Resoldering is also a highly effective preventive measure. Learning how to decide which connections to rework out of hundreds in a given instrument comes only with experience; connections that look good may contain hidden flaws.

A good place for you to start looking for fractured connections is with components that are subject to mechanical force, such as connectors mounted on printed-circuit boards. Some of the worst culprits are ¼-inch phone connectors. Due to their length, they exert powerful forces on connections to printed-circuit boards. Phone connectors were designed for use with telephone switchboards, in which ease of insertion and removal are paramount, rather than for the rigors of musical applications.

Another common source of fractured connections is heavy components that are not mechanically attached to the circuit board. Large electrolytic capacitors or power resistors fall into this category.

OUT WITH THE OLD

Some connections are so bad that the old solder must be removed and replaced with new. Removing old solder is sometimes accomplished with a braided copper wire coated liberally with rosin, known as solder wick. Solder wick is pressed gently against a molten connection, and the old solder is then drawn into the braid through capillary action. Although inexpensive, braid can present the danger of overheating the work and can leave solder in the hole, making safe removal of the lead component problematic. Still, with practice you can learn to avoid most of these problems.

Some people use a simple vacuum bulb to remove solder, but the results are unpredictable, and I don't recommend this approach. Another unreliable method involves a plunger-type vacuum device with an internal spring. You place the suction tip over the solder joint, heat the joint with the soldering iron, and trigger the plunger to create suction. This method brings about some of the same problems as using a solder wick.

FIG. 2: This Weller DS 800 electronic desoldering station provides a closed-loop, temperature-controlled hollow tip through which solder is sucked into a reservoir by an internal pump.

FIG. 2: This Weller DS 800 electronic desoldering station provides a closed-loop, temperature-controlled hollow tip through which solder is sucked into a reservoir by an internal pump.

The most professional way to remove solder is to use a desoldering station that has a closed-loop, temperature-controlled tip with an axial bore, through which the solder can be sucked into a reservoir (see Fig. 2). An internal (or, in some models, external) pump provides the suction that draws molten solder from the connection. The suction is controlled by a trigger in the grip of the desoldering assembly.

When you use a desoldering station, be sure to select a tip that matches the diameter of the lead to be desoldered. Proper tip maintenance is crucial to obtaining good results. For example, unlike soldering tips, desoldering tips should be lightly filed to remove debris. Do not use a conventional file, however, or you will damage the tip; a light-duty file is usually provided with the unit. You must clean the bore of the desoldering device at regular intervals with a large-diameter rod, empty the solder receptacle, and replace the filters.


Peter Miller has specialized in the repair of electronic musical instruments for more than 30 years. He has owned and operated CAE Sound (located in San Mateo, California) since 1980 and has designed custom audio electronics for groups such as Tuck and Patti, Counting Crows, and the Grateful Dead. Paul Howard is a former service tech at CAE Sound.

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