Smith-Kettlewell Soldering Series, Part 1
Table of Contents for Soldering Series
Soldering I
Soldering II
Soldering III: Tinning Stranded Wire
Soldering IV: Popular RF Connectors
Soldering V: RCA and Motorola Plugs
Soldering VI: Resistance Soldering
Soldering VII
JA3TBW Solder Guide
Soldering Kinks
Vinther Fingertip Soldering Iron
Introduction
This series, which spanned several years in the Smith-Kettlewell Technical Files, describes how blind people do electrical soldering. I intended this to be a book on soldering and the techniques used by blind technicians. There are as many different soldering systems as there are fabrication processes and materials. The compilation of such material would be a monumental undertaking.
I justify putting this series here by proposing that "we have to start somewhere." Arbitrarily, that "somewhere" will be with my own experience in soldering. Subsequent to my writings, other readers included suggestions which appear in the series.
--William Gerrey, April 2006
SOLDERING I
by Bill Gerrey
Editor's Note -- The rumor is not true! This discussion is not intended to promote the talking book edition over braille by way of injury to the readers.
For the last three years, I have intended to write a book on soldering and the techniques used by blind technicians. There are as many different soldering systems as there are fabrication processes and materials. The compilation of such material would be a monumental undertaking.
I justify this preliminary discussion by proposing that "we have to start somewhere." Arbitrarily, that "somewhere" will be with my own experience in soldering. You can rest assured that this is just the beginning of a series of articles regarding soldering as it is being done by blind and visually impaired people. (At some point, we will be including information on soldering under a closed circuit TV system). If you don't like this article, wait for the next one.
I will not burden you with a long bibliography; there are very few discussions of soldering that are high on science and low on popular myth. One such reference book is Soldering, Its Fundamentals and Usage by Kester Solder Co., Copyright 1961.
This 84 page booklet describes the fundamental scientific principles and the practical applications in a readable style.
Solder does not just "stick" or adhere to the metals being soldered; it alloys with them. Liquid solder is actually a solvent which forms a solution with the surface metals being bonded. With the solder acting as a solvent, the surface metal need not melt for this chemical interaction to occur.
The solder of interest to us is a tin-lead alloy which melts at a much lower temperature than do tin or lead individually. If this alloy is composed of 63 per cent tin and 37 per cent lead, (said to be the "eutectic alloy"), the melting point (183oC, 361oF) is the lowest possible with any tin-lead combination. The eutectic alloy is unique in yet another way -- there is no "plastic" phase. The eutectic alloy turns directly into a liquid when brought to the eutectic temperature, and reverts back to a solid when allowed to cool below the eutectic temperature. (Eutectic solder can be purchased; it has the advantage of having the lowest melting point possible for a tin-lead composition solder. I find in using it that it is extremely sensitive to minor vibration while solidifying, and it can actually "run away" from the heated part of the work to solidify elsewhere.)
The most often used compromise solder alloy has 60 per cent tin and 40 per cent lead. This composition, like any other composition that deviates from the 63/37 eutectic alloy, does not abruptly change from a solid to a liquid; it goes through a "plastic" phase which is best described as gummy. We can understand how this can happen by considering that 60/40 solder is eutectic solder with extra lead added to it. When this composition cools to about 374oF, 189oC, crystals rich in lead (about 84 per cent lead) solidify and form little "dendrites" (branch-like structures). High-lead crystals continue to form until all that is left is eutectic alloy. At 361oF, the eutectic temperature, the solder changes from the "plastic" phase (containing the lead-rich precipitant) to the solid phase. The plastic phase comes in handy, the solder doesn't run away from the connection before it cools, and minor vibration, such as the natural tremor in your hands, will not weaken the connection as often as it does with eutectic solder.
In order for "soldering" to take place, the connection must be hot! hot! hot!, about 550oF (288oC). The solder must be completely liquefied, and the alloying temperature must be reached where the solder "dissolves" or forms an alloy with the metals being bonded. The formation of this alloy on the surface of the metals is called "wetting" of the surfaces by the molten solder. The joint which has been "wetted" properly does not have globs, balls, or other rounded sculptures of solder attached to it. A properly soldered joint over which good wetting has occurred is shaped just about like it was before it was soldered; it is covered with solder, but the contours of the joint are still present. The solder will not just terminate its edges in lumps, but will nicely "feather" down to the bare metal at its extreme edges. The joint will look as if a very elastic sheet has been draped over it and been tucked in nice and cozily.
We have an understanding of the metallurgic principles of soldering, but unless the cleaning and soldering operations can be done in an atmosphere of helium, we can't just mix tin, lead, and the surface metals in the face of a blow torch and expect to get passable results. We will proceed to the discussion of "flux."
Metals that are electrically active (in the molecular sense) love to combine with oxygen to form oxides. No matter how well you clean the surface of metals such as copper, oxides will form as soon as you stop cleaning. Furthermore, the formation of oxides is tremendously enhanced with the application of heat to the metal. These oxides prevent the metallurgical process of soldering; the molten solder cannot contact the bare metal surface to alloy with it (no wetting occurs). The oxides which rapidly build up at the point of heat transfer (between the hot iron and the work) insulate the iron from the work so that poor heat transfer occurs.
There are agents which remove the oxides as the work is being heated and soldered. For those of you who are chemists, these compounds (which are called "fluxes") perform a reducing action which separates the oxides from the surface metal. The flux residue, containing the oxides, floats to the top of the boiling solder and allows the metal-to-metal alloy to form underneath.
Rosin, as used in soldering, is an inert polymer (complex chain of molecules) which resists forming chemical bonds when at temperatures below those require for soldering. When the resin flux is heated, the polymers break up, leaving active molecules to serve as the reducing agent. Upon cooling, the resin and oxide compounds are once again inert, and harmlessly remain on the surface of the joint.
Resin is about the only flux which is inert, the others are conductive corrosive nasty substances which work wonders during soldering, but which must be washed away with water or other polar solvents after use. Why have to bathe your projects? Use resin flux. There are a few other resin based fluxes whose residues are inert, but they are not as chemically sable as resin at high temperatures. I use resin flux myself.
In hand soldering operations, thin tubular solder is used, in the center of which the flux has been premeasured and stored. In electronics work which is on the scale of the projects described in this issue, solder of about 0.050 inches in diameter is suitably sized (Ersin Multi-core solder comes by the gauge number; No 20 through 22 are suitable). I prefer this size of solder because it is rigid enough to be used as a "feeler," and it is small enough in diameter to permit accurate gauging of the volume applied.
The metals being soldered must be in contact with each other, so that they will heat up together. In order for efficient soldering to be accomplished the iron should be applied to the point of maximum heat capacity, i.e., the largest piece of metal permitting the largest contact area. (All the surfaces being bonded should be in contact with the iron if possible). Solder which is applied to the iron alone will not bond to the connection; its flux will be used up cleaning the iron and not the work. Solder should be applied at the point of contact of the work and the soldering iron. Once solder has melted at this point of contact the flux will attack the oxides cooking away between the iron and the work, and at the same time, molten metal will flow between the iron and the work, causing very efficient heat transfer. After that, solder applied just about anywhere on the connection will be involved in the formation of the surface alloys, because the connection itself is hot enough to melt the solder. When the connection is hot enough to melt the solder, you are just about guaranteed to have made a "good" solder connection.
The essential properties of the soldering iron warrant discussion here. The tip of the soldering iron must be made of a metal whose thermal conductivity is high. In addition, the tip of the iron must be able to accept and to retain a surface alloy of tin or solder (known as "tinning") so as to afford the formation of a completely continuous metallic path between the iron and the work, which is necessary for efficient heat transfer. Finally, the iron must be powerful and efficient enough to heat up the localized area of interest faster than heat can be dissipated or transferred away from the iron.
Because of its high thermal conductivity, copper is widely used as a base metal in soldering tips. The use of bare copper tips is a long-standing tradition. Bare copper is highly soluble in solder, however, so that these tips actually wear away and require frequent reshaping and/or replacing. Because of the high degree of maintenance required on bare copper tips, another type of soldering iron tip has become very popular; it is relatively maintenance free. This type uses copper as a base metal, to take advantage of the thermal conductivity of copper, and has a plating or cladding of ferrous metal (iron or steel).
"Tinning" the iron specifically refers to applying a coat of solder or tin to the tip. This process retards the buildup of oxides on the exposed tip metal. It also assures that fresh solder, when applied to the point of heat transfer on the joint, will be able to wet the soldering iron tip and establish a complete continuity of metals from the iron to the work. Bare copper tips can be tinned by applying fresh solder to them on a regular basis. (Periodically, the copper tip must be filed down to a smooth new finish and tinned again.) Tips which are clad with ferrous metals are tinned by the manufacturer, because the cladding does not readily dissolve in solder. After a coating of tin is alloyed with the tip by the manufacturer, solder can alloy with the surface tin. Keeping fresh solder on the tip will help prevent oxides from separating the tin from the cladding. Once the factory tinning has become flawed, the tip must be replaced.
A common fallacy is that the iron must be small enough not to damage the work. (I made this mistake for years.) Used inappropriately, low-powered irons do more damage than soldering. Their intended application is in cases where a larger iron cannot be maneuvered into position for soldering. In general purpose soldering, low-powered irons do not heat up the localized area of the connection quickly, allowing considerable heat to be transferred to and absorbed by components of the work. In most cases, an iron of 50 watts or more can heat up the connection quickly and efficiently, allowing soldering to be accomplished without overheating the entire collection of circuit elements. The iron must be "big enough," not "small enough."
You ask, "Is he, the author, ever going to get down to business?" Yes, yes, let us descend from the ivory tower of conceptual cognition and get our fingers warm.
The remainder of "Soldering" will focus on soldering with "instant heat" soldering irons or guns, which have proven to be well suited for use by blind technicians. The next article will discuss the techniques that I use when soldering with a continuous-heat soldering iron, along with a survey of readily available tools and accessories used in soldering.
A variety of soldering irons and soldering guns are available whose features include "instant heat" capability; they warm up to soldering temperature within a few seconds of being turned on. However, only a particular type of these "instant heat" irons is of interest to us. We are interested in the irons and guns whose tips have very low mass, and consequently have low heat storage capacity. These not only heat up quickly, they cool down quickly after being turned off.
Unlike conventional soldering iron designs in which a large heating element heats the tip by thermal conduction, the irons to be described here use high current electricity to heat a small low-mass tip. The tip itself is the heating element. Within 60 seconds after the current through the tip has been turned off, the tip is cool enough to be touched.
These instant-heat, fast-cooling irons have two major disadvantages in comparison to conventional irons. The first is that the small surface area of the tip, which is the heating element, does not permit efficient heat dissipation in free air. If the current through the tip is left on while the tip is not in good thermal contact with the metals being soldered, the tip can reach an extremely high temperature which will quickly oxidize its surface and ruin the tinning job. The second disadvantage is that because very high current is necessary to heat the tip, a physically heavy high-current power source must be incorporated into the handle of the iron or gun. These disadvantages are something that can be lived with, and they are more than offset by the convenience offered to the user who wants to guide the iron into position with his fingers while the tip is cool to the touch. For the hobbyist who occasionally rolls up his sleeves and solders a project together, these irons eliminate the need for constant practice necessary to safely use a continuously hot iron. Two kinds of these "instant heat" fast-cooling irons are commercially available. Actually, they differ only in size. The first is the transformer type soldering gun, and the second is a battery operated "cordless" soldering iron.
The tip of the transformer type soldering gun is a simple elongated loop of wire. The body of the gun contains a power transformer to match the high current load of the tip to the a.c. power line. These guns are available from a couple of manufacturers; however, in my opinion, the best is the Weller Model 8200 dual-heat soldering gun (also marketed by Radio Shack as the Archer Professional Dual-Heat Gun, stock No 64-2190, for $16.00). Its bare copper tip is cheap and simple; a piece of 12 gauge can be fashioned into a tip in a pinch. The binding posts for the tip are good solid construction so that the ends of the tip can be secured firmly (these binding posts need to be tightened occasionally.) The two position trigger switch provides two power ratings, 100 and 140 Watts; one for small jobs and one for larger work. The main limitation of these guns is that the tip is too large to be used on integrated circuits and other crowded assemblies.
With the advent of sealed rechargeable batteries, a junior member of this family of instant heat irons has become available. These units are said to be "cordless." Instead of using a power transformer to supply current to the tip, a nickel-cadmium battery is incorporated into the handle. For practical reasons (not the least of which is that the high tip current must be controlled directly by a push-button switch), these cordless irons are very low power and are only good for very small work. In my opinion, the best of the cordless irons is available from Radio Shack as the Archer Cordless Iron, stock No. 64-2075, for $20.00.
They work fine for circuit board work (either printed circuits or point-to-point wiring on perforated boards), but they are not husky enough for soldering terminal lugs such as those found on plugs and jacks.
With a complement of two irons, a cordless iron and a transformer type gun, those who wish to use instant-heat soldering irons can cover the full range of electronic assembly work by using the appropriate tool.
Tinning these irons and preparing them for use is a simple matter if you carefully monitor what is happening by holding on to the solder. Spool off a couple of feet of solder and wrap about three inches of it around the tip of the iron or gun. Hold on to the solder about an inch away from the tip. Turn on the iron and wait for the solder to melt, which will disconnect the iron from the solder in your hand. Immediately and simultaneously release the button or trigger and give the iron a quick little shake in a direction away from you. After a minute, feel the tip and inspect your tinning job. The tip should feel smooth and perhaps a little gummy from the flux. If the tip feels rough in spots or if it has a glob or an "icicle" of solder that might get in your way during the first connection, repeat the process.
Some thought should be given to the preparation of the work area. Since splattering and dripping of the solder is inevitable, choose a work surface on which marring is of no concern.
Because the instant heat guns and irons are characteristically heavy, a collection of blocks, books, or heavy transformers against which your hands can be braced may help slipping of the iron off of the connection.
Your paraphernalia should include good solid holding devices which can rigidly support the work. A small table vise with a swivel is good for most applications. (More on this in Part II.) In designing the layout for your projects, try to arrange for the soldering to be done in accessible places. When stringing connecting wires around your project, make them long enough so that they can be gotten out of the way during soldering.
When fashioning the connection to be soldered, make sure that all the metals being joined are in contact with each other so that they will all heat up together. It has been said, in fact overstated that the connection should be mechanically self-supporting before soldering. This philosophy can get you into trouble; if leads are wrapped around and around terminals to make them mechanically rigid, the solder may not flow around all the surfaces leaving portions of the connection unsoldered. Maximizing the area of metal to-metal contact will minimize the susceptibility of the solder bond to shear stress, but the small weight and size of modern electronic components has outdated the wrapping practice. Some specific examples of good practice are:
When soldering a wire to a round terminal post, bend the wire three quarters of the way around the post.
When soldering a wire to a flat terminal lug which has a hole through it, pass the wire through the hole and bend it over so that it lays against the flat surface of the lug.
When components are installed on a printed circuit board, lean the wire over at an angle before soldering it. In the military code, the wire must be bent right down against the printing on the board, but this makes component replacement difficult.
When attaching components to terminals, always leave at least a distance of one-eighth inch between the terminal and the component. This practice will permit the attachment of a clip-on heat sink when appropriate, and will prevent direct heat from the hot iron on the body of the component.
Identify the item of largest heat capacity on the connection so that the iron can be put in good contact with it. In the ideal situation, the iron should be in contact with all the metals being soldered; however, this is not always possible. To be specific, when a wire is being soldered to a terminal lug, you must at least apply heat to the lug. When a comparatively heavy component lead is being soldered to a very small terminal on a socket for an IC (integrated circuit), you must at least heat the wire, because it is massive compared to the socket terminal and can absorb much more heat energy.
Using the above guide lines, clamp a project in your vise and fashion a connection to be soldered. Place your iron or gun against the work and brace your hands as necessary so that you are in a comfortable holding position. Check to see that the "barrel" of your iron does not run close to wires or components that could be damaged by the heat. With your free hand, hold the solder at a point about three-quarters of an inch back from the end, and place the end of the solder up against the connection and the tip of the iron never against the iron alone. Make sure that the three-quarter inch piece of solder between your fingers and the connection is straight, so that you know which direction to move in feeding the solder once it melts. Turn on the iron and wait for the solder to melt.
When the melting occurs, the solder will "disappear" off the end of the piece in your hand. Depending on the size of the connection, you must supply one-eighth to one-half inch of solder onto the joint. When feeding the solder after the initial melting, apply it to the connection and not to the iron. After the desired quantity of solder has been applied, leave the iron against the joint for one or two more seconds. Then, slide the iron off the connection with a smooth deliberate motion. Sliding the iron off the connection rather than jumping straight away from it will help to break the surface tension of the liquid solder without leaving sharp "icicles" in the direction of your departure.
Any of all of three indications can be monitored to affirm that good wetting and soldering has taken place. First will be if you can get the solder to melt when applied to the connection at a point that is not in direct contact with the iron; this means that the entire connection has reached soldering temperature. The second indication is that very small motion of the iron or of the components will feel "squeaky;" the flux has done its cleaning job and the solder-wet surfaces are "squeaky clean" like dishes in soapy water. (My thanks to Dennis Bernier, Vice President of Research and Development at Kester Solder Co. for explaining this effect to me.) The third sign is that heat transfer to all the component leads becomes very efficient; the temperature of the connecting leads will rise sharply.
After the connection has been allowed to cool (and don't allow components to move in relation to each other until the solder has completely solidified), a systematic inspection of the joint is in order. Five indications of a faulty joint should be looked for: If no solder has made it on to the connection, the terminals and leads will feel "rusty" i.e., they will have corroded from being heated without the presence of flux and solder to protect them.
If good wetting has occurred, the connection will not have lost its characteristic shape, but will be smoothly covered with solder. If, however, solder has melted but not wetted onto the connection, round pieces can be picked off, because they have not formed any surface alloy with the metals being joined.
Gently wiggling each of the connected components should cause no rattling or relative motion with respect to the others. If they are soldered together, the connection is one piece of metal.
If too much solder has been applied to the connection, heavy droplets may be hanging down from the underside of the joint which could short something out or break off and rattle around in the project until they bridge two other connections. Turning the connection upside down and reheating it can often take care of this problem.
Finally, it is often difficult to keep connections from "bridging," i.e., adjacent connections may become soldered together accidentally. This can happen if too much solder is applied, or if adjacent connections are inadvertently heated simultaneously by the iron. A small probe, such as a braille stylus or small screwdriver, should be passed between closely spaced terminals to check for bridges. If a bridge is found, reheat the connections separately and clear the bridge with your probe as the solder melts.