(Reprinted from issue 61 of UHF Magazine. To purchase the issue, click here. Or click here to subscribe to UHF)
State of the Art
by Gerard Rejskind
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Who first invented soldering? I suppose its origin is unknown. After all, the wonderful stained glass windows of medieval times were soldered, and the realization that metal could be melted to form a bond may well have followed closely upon the discovery of fire. What is amazing is that this ages-old invention is still a key element of the "high tech" amplifiers, speakers and digital components of today. Is soldering so perfect that it deserves technological immortality?
In fact soldering ought perhaps to be relegated to handicrafts fairs. Solder joints are notably unreliable. Indeed, the aerospace industry, which prizes predictable performance, largely gave up its soldering irons decades ago. In hi-fi, however, soldering lives on to a surprising extent, and as we shall see it is the enemy of more than just reliability.
Most solder is a mixture of tin and lead. Neither is as good a conductor of electricity as copper, silver or gold, but both can be melted at reasonable temperatures -- 232°C and 327°C respectively. Both are toxic, especially lead, whose destructive effects on the brain are tragically documented. We can't make solder from copper, because copper's high melting point (1085°C) would require a soldering iron hot enough to destroy the parts we were trying to connect.
And silver also has a high melting point -- a whopping 962°C. "Silver solder" is used for both jewelry and high quality electronics, but the silver content of these solders is well below 5%. The rest is the usual metals.
Metals used in solders have another undesirable characteristics: extremely high surface tension. A drop of tin or lead will bead up like water on a freshly waxed car, and it won't spread onto an electrical joint we are attempting to secure. To break down the surface tension, we need to add what can only be called an impurity, one which does not conduct electricity at all...and at this point soldering begins to look more and more like a quaint anachronism.
Some acids will reduce surface tension, and they are used in solder intended to connect plumbing. However acids are so corrosive that an electrical joint so made would fail within weeks. Electrical solder uses rosin, also spelled resin, the sticky sap of pine trees. As the tin, lead and/or silver melt, the rosin breaks the surface tension so that the molten metal flows to cover the joint. The technician must then continue to heat the joint just long enough to burn off all of the rosin, but not so long that the components are damaged (a delicate balancing act!). Will you be surprised if I tell you that no two solder joints are alike?
Bad joints come in several flavors. The cold solder joint wasn't heated enough for the rosin and the metal to mix, and the result is a round bead of shiny metal that looks nice but doesn't conduct electricity. The burned joint was heated too much. Not far from it you may find resistors and capacitors that have "sweated": you can see tiny droplets on their surface. If these components have not actually failed, they are unlikely to meet their design specifications. The partial joint results when the solder perfectly fuses two conductors...but not a third which was to have been connected to the first two. A rosin joint is one which has not been heated long enough to burn off all the (non-conductive) rosin. Last and least is the crystallized joint, with its dull surface. The joint was moved slightly before it cooled, and the resulting crystal structure is a terrible conductor. It is also mechanically weak.
You can see why soldering is not popular in the space shuttle, with its billions of electrical joints, but it causes problems in hi-fi as well. A solder joint is not a perfect conductor, which becomes a serious problem in low impedance circuits such as amplifier outputs and loudspeakers. Its actual resistance may actually vary somewhat with mechanical stress, which makes it microphonic -- it is vulnerable to vibration. Unburned rosin can allow a joint to act like the diode in a crystal radio. Hello Top 40!
The alternative to soldering is well known: crimping. A crimped joint is held together mechanically under pressure, with no impurities or semiconducting substances. It is more reliable by an order of magnitude, and all else being equal it will sound better too. I'm often asked whether crimping is better than soldering. Unless you are creating stained glass, the answer is evident.
In fact soldering ought perhaps to be relegated to handicrafts fairs. Solder joints are notably unreliable. Indeed, the aerospace industry, which prizes predictable performance, largely gave up its soldering irons decades ago. In hi-fi, however, soldering lives on to a surprising extent, and as we shall see it is the enemy of more than just reliability.
Most solder is a mixture of tin and lead. Neither is as good a conductor of electricity as copper, silver or gold, but both can be melted at reasonable temperatures -- 232°C and 327°C respectively. Both are toxic, especially lead, whose destructive effects on the brain are tragically documented. We can't make solder from copper, because copper's high melting point (1085°C) would require a soldering iron hot enough to destroy the parts we were trying to connect.
And silver also has a high melting point -- a whopping 962°C. "Silver solder" is used for both jewelry and high quality electronics, but the silver content of these solders is well below 5%. The rest is the usual metals.
Metals used in solders have another undesirable characteristics: extremely high surface tension. A drop of tin or lead will bead up like water on a freshly waxed car, and it won't spread onto an electrical joint we are attempting to secure. To break down the surface tension, we need to add what can only be called an impurity, one which does not conduct electricity at all...and at this point soldering begins to look more and more like a quaint anachronism.
Some acids will reduce surface tension, and they are used in solder intended to connect plumbing. However acids are so corrosive that an electrical joint so made would fail within weeks. Electrical solder uses rosin, also spelled resin, the sticky sap of pine trees. As the tin, lead and/or silver melt, the rosin breaks the surface tension so that the molten metal flows to cover the joint. The technician must then continue to heat the joint just long enough to burn off all of the rosin, but not so long that the components are damaged (a delicate balancing act!). Will you be surprised if I tell you that no two solder joints are alike?
Bad joints come in several flavors. The cold solder joint wasn't heated enough for the rosin and the metal to mix, and the result is a round bead of shiny metal that looks nice but doesn't conduct electricity. The burned joint was heated too much. Not far from it you may find resistors and capacitors that have "sweated": you can see tiny droplets on their surface. If these components have not actually failed, they are unlikely to meet their design specifications. The partial joint results when the solder perfectly fuses two conductors...but not a third which was to have been connected to the first two. A rosin joint is one which has not been heated long enough to burn off all the (non-conductive) rosin. Last and least is the crystallized joint, with its dull surface. The joint was moved slightly before it cooled, and the resulting crystal structure is a terrible conductor. It is also mechanically weak.
You can see why soldering is not popular in the space shuttle, with its billions of electrical joints, but it causes problems in hi-fi as well. A solder joint is not a perfect conductor, which becomes a serious problem in low impedance circuits such as amplifier outputs and loudspeakers. Its actual resistance may actually vary somewhat with mechanical stress, which makes it microphonic -- it is vulnerable to vibration. Unburned rosin can allow a joint to act like the diode in a crystal radio. Hello Top 40!
The alternative to soldering is well known: crimping. A crimped joint is held together mechanically under pressure, with no impurities or semiconducting substances. It is more reliable by an order of magnitude, and all else being equal it will sound better too. I'm often asked whether crimping is better than soldering. Unless you are creating stained glass, the answer is evident.
(To read all of this issue of UHF Magazine, just order issue 61 at our secure server.)
Complete articles from this issue:
The Battle of the Super Discs, Cambridge Isomagic Converter, Soundcare Superspikes, State of the Art
Excerpted articles from this issue:
New Surround Formats, Defeating DVD Zoning, Vegas 2001, Audiomat Tempo & Vecteur D-2, Audio Refinement Pre 5 Preamp, Osborn Mini Tower Speakers, Mirage OM-9 Speakers