Soldering is one method of joining two suitable metals and, in electronics, it is used mainly to ensure a good electrical connection. The process is similar to G1ueing which enables two dissimilar materials (e.g. fabric and wood) to be joined using a third material known as an adhesive.
The most important methods of metal-joining are:
a) Welding. In this two similar metals can be joined by heating them to near-melting and then either pressing/hammering them together or by running-in some of the same metal in the molten state.
The heated metal combines with the Oxygen in the air (it is said to oxidise and the resulting oxide forms a crust or scale which has to be chipped away. If such scale is allowed to form within the joint then failure is to be expected. Thus welding equipment sometimes floods the joint with an inert gas during welding to exclude air.
(b) Brazing. This method joins two metal pieces by encasing tile Contact line with Brass filler — hence the name. The filler/joiner is known as a brazing solder which is an alloy of Copper, Zinc, Silver and/or Nickel in which the exact combination is determined by the intended use. The inclusion of Zinc has lead to this material acquiring the generic name of Spe1ter which otherwise is the name used for commercial grades of Zinc.
The two metals are not themselves joined and so they may be different metals. It is necessary however that the two metals end the grade of brass have similar expansion properties with temperature else the join must crack as the piece cools.
However there is slightly more to this process than just covering the joint with brass. The spelter dissolves into the surfaces of the two metal-pieces to be joined in the same manner as glue is required to penetrate if a joint is to to be successful.
Oxidation problems ore dealt with by using a Flux; this melts at about the same temperature as the brass spelter and its job is to chemically remove the oxidation products mostly by rendering them fusible.
Brazing is included under the title "Hard soldering" which is described below.
(c) Soldering. This term usually is applied to joining two metals by use of a solder. Solders are alloys which come in two main types (i) hard solders (strictly these include brazing solders) which are alloys of Copper, Tin and Silver and (ii) soft solders which are alloys of Lead and Tin.
(i) Hard soldering is perhaps used mainly in the jewel1ery trade but it does have a place in joining Copper parts where strength is required rather then a good electrical connection. The presence of Silver in the solder has given the process the name Silver Soldering. The melting point of the solder is sufficiently high to necessitate taking the work piece to red-heat and often close to melting point and so do not try such jobs on valued pieces unless you have first acquired the skills and experience.
A flux is required and this is usually Borax which fuses at the temperature used in hard soldering.
Unless the work piece is large there is hazard in supplying sufficient heat because of a risk of complete melt-down. (I have witnessed such an embarrassing disaster during an attempt to solder new lugs on to a silver watch-case. For fine work a blow-pipe should be used to produce a fine jet of hot gases; a small tube with a fine hole is blown by mouth through a larger flame (such as that from a Bunsen Burner). It may be necessary to provide auxiliary heating to bring the job close to the operating temperature so enabling the blow-pipe jet to operate over a very-small area.
WARNING Take care not to breathe-in through the blow-pipe.
The two surfaces are cleaned, smeared with the Flux and fastened together with some kind of clamping (often improvised). Slivers of solder are cut and laid in/on the joint; experience is required here so that the correct amount of solder is used. Heat is applied and as the appropriate temperature is reached so the solder will be observed to melt and then be drawn into the joint by capillary action; remove the heat source immediately . When carried out correctly a silver-soldered joint can be totally invisible.
(ii) The most common use of soft soldering is in electrical and electronic work, where the requirement is to make good electrical connections. It is foolish to rely on the solder to also provide a mechanical joint because constantly stressing a soldered joint can lead to fracturing of the solder and so to bad electrical contact.
Soft soldering takes place at temperatures well below the melting point of the piece to be soldered and so it is necessary to ensure that the solder dissolves into the surfaces of the two materials to be joined. This requires a preliminary process known as tinning; the end of a wire is heated with a soldering iron and treated with flux+solder until a clean coating is achieved. When both components of a joint have been suitably tinned it is necessary only to place then in contact, heat them with the soldering-iron and apply a very-small quantity of fresh solder; as soon as solder is seen to run into the joint the heat should be removed.
It is important not to overheat solder because this oxidises the Tin content; reducing the percentage of Tin ultimately must reduce the supposed solder to little more than a blob of lead.
Again flux is necessary and this is usually a resin. For electronic work the solder is provided in the form of a drawn wire and the flux is formed within the wire (like seaside rock). Fine-gauge solder may have two such cores but the heavier gauges may have as many as five flux cores.
There are also special fluxes; one of these has an acid base and can be useful for tinning badly oxidised or corroded surfaces although care must be exercised to ensure that the corrosive action is terminated after the soldering is finished. Other fluxes have been formulated to assist in soldering difficult materials such as Iron and Aluminium.
(A useful technique for attaching electrical connections to Aluminium is to first melt a small pool of solder on to the surface and then to scratch that surface with the soldering-iron bit under the pool until tinning results. The idea is that the molten solder protects the metal from the air - Aluminium oxidises readily at temperatures below the melting-point of solder. Once tinning has been accomplished remove most or the solder and add fresh; the above protracted operation will have reduced the Tin content.)
A standard and constant hazard in electronic work is the so-called "dry joint"; this offers itself in forms ranging all the way from an open-circuit to an intermittent good contact. Its causes are many but, regrettably, the most common factor is lack of skill or attention on the part of the Operator who performs the original task.
A frequent path to producing a dry joint is the use of a too-hot soldering iron; as pointed out above this results in "burning-out” the Tin content. Tin is included to make the solder fuse properly at a temperature that will not cause damage either to the electronic components or to today’s circuit boards. Lead will not serve to join copper wires and, although the resulting join may look good, it be guaranteed to cause grief.
Another frequent cause is failure to clean the two surfaces properly before tinning — or indeed failure to tin the surfaces before soldering. A layer of dirt and/or oxide within the joint allows that joint to be peeled apart . Often an apparently soldered wire call be seen to rotate inside the joint.
Sometimes, after several years of service, equipment may develop intermittent contacts in the form of recurring crackles, hisses and temporary loss of gain; such are the result of dry joints which appear to arise purely from the passage of time. It does not happen in all equipment and so the inevitable conclusion in that the starting point is inadequate workmanship during construction.
DO NOT TRY TO REMOVE DRY JOINTS BY RE-SOLDERING EVERYTHING IN SIGHT. Most likely the problem will be removed but failure indicates that (a) yet more dry joints have been created (b) not all dry-joints were cured (c) it is now impossible to tell which joints are good and which need investigation.
Finding/removing dry joints is a slow and painfu1 process and it is more than worthwhile to take care at the start. Each joint must be tapped gently (both on it and around it to discover where the equipment is most sensitive; each joint that is diagnosed as faulty should be resoldered and, if necessary, marked. Re-soldering a joint may not remove the intermittent problem but that does not indicate a faulty diagnosis. Where there is one faulty joint it is more than probable that others exist. Sometimes it is necessary to withdraw a dry wire, clean and tin it before replacing it and re-soldering it; where solder is covering a dirty connection but is not making contact the hopeful application of lots of heat, lots of flux and lots of solder is guaranteed to damage the equipment while being unlikely to cure the fault or, at best, to achieve only a temporary cure. (My personal record in dry-joint removal is the certain cure of 32 dry joints in one elderly CD player and it was a frustrating labour of love. The cure took two days but three years have now passed without further problems.)
Solders come in various grades:
Low melting-point (LMP) solder has a composition 62% Tin, 36% Lead, 2% Silver and it melts at 179° C. It is useful for making joints where other local joints are not to be disturbed.
High Melting-point (HMP) solder has a composition 5% Tin, 93.5% Lead, 1.5% Silver and it melts at 296° C. It is useful for making joints where other work is to be undertaken using LMP solder.
6O/4O Standard Solder has a composition 60% Tin, 4O% Lead; this starts to melt around 183° C and is fully molten at l88° C.
40/60 Solder has the composition 4O% Tin, 6O% Lead; it starts to melt around 183° C and is fully molten at 234° C. It is used for general electrical repairs and for soldering/repairing copper and brass metalware.
"Savbit" Solder contains a small quantity of Copper the object being to reduce the leaching of Copper from a soldering-iron bit so as to extend bit life. It also enables fine wires to be soldered more easily.
An Aluminium Solder is also available with the composition 18% Tin, 8O% Lead, 2% Silver. This starts to melt at 178° C and is fully molten at 27O° C. It will joint Aluminium, Tin plate, Brass, Nickel, Copper and Stainless Steel.
A Lead-free Solder is also available which is 99% Tin and a very small amount of Copper which melts at 227° C.
Plumbers' Solder is 99.5% Tin and the rest Copper for use in joining Copper pipes in potable water supplies. Melts at 228° C.
SOLDERING IRONS
Originally solder work was performed using “solder bolts" which consisted of a shaped lump of copper riveted to an extension piece with a wooden handle. Such bolts still have occasional application today; they have to be repeatedly heated by means of an external source.
Modern irons are heated by means of an internal electric element and are tipped with variety of bits whose size and shape depend on the job for which they are intended. Usually the bits are interchangeable/replaceable. The physical-size of each iron arid its electrical power dissipation is determined by its intended use; they vary from around 15-watts to 120-watts.
Some irons are provided with built-in temperature-control systems but others depend on an external control-systems linked to some form of heat-sink-cum-iron-holder which senses the bit temperature . Clearly the most useful iron is the one in which a degree of control is provided over the bit operating temperature. The cheapest range completely lack any kind of control and, when not use, these should be either switched off or kept in contact with a suitable heat-sink to dissipate excess heat; this avoids destruction of the copper bit and of over-heating soldered joints.
A simple and effective system can be constructed by feeding the Iron via a simmerstat which are to be found in abundance on old discarded cookers. These switch the power on and off in a time-cycle determined by the setting or a small auxiliary heater with a bimetal-strip switch.
The usual soldering iron bit is made of copper but these suffer the disadvantage that they burn away partly through oxidation in the air and partly because copper is leached from them into each joint that is made. As a result the end of the bit becomes pitted and needs to be re-shaped with a File — followed by re-tinning. Their great advantage is ease in use and quick heating.
Bits are made also of iron and these are more resistant to oxidation and leaching and also (because of the greater “specific heat” of iron,) they can store greater amount of heat. As a consequence of course they take longer to reach operating temperature and, although they pack a greater punch they more readily damage circuit boards. Iron bits can be difficult to tin and usually they are provided with a special coating which should not be treated with a file. Special solders//fluxes are available for use in keeping these bits in good condition.
DE-SOLDERING
In repair work there is seldom a problem in removing components either for testing or for replacement but it is important not to apply excessive heat to printed-wiring boards or to strip-boards; overheating causes copper strip to part from the substrate. When this occurs a repair is made using normal wiring techniques.
Multi-pin integrated circuits (I.C's or “chips") pose a problem however in that it is difficult to melt the solder and withdraw all the pins simultaneously. So-called “de-soldering techniques have been developed to enable solder to be removed from the pins often leaving them completely free.
Solder Wick
This is a finely-woven copper braid which is impregnated with a flux and it
soaks-up molten solder. It is supplied in various sizes it roll form; it
is a sacrificial tool.
Hand Suction-Tool
These are spring-operated suction pumps with a heat-resistant nozzle. With
the spring set the nozzle is held against the soldered joint while an iron
is applied also to melt the solder; a button is pressed to release the plunger
when the molten solder is drawn into the body of the tool.
Soldering Station
These are grand and expensive pieces of equipment which offer both a temperature-controllable
soldering iron and a power-operated de-soldering tool. Continuous suction
is provided by a small pump that is operated from a foot switch.
De-soldering irons often provide a special head with which to de-solder all pins of a chip simultaneously. However, in confined spaces, crowded circuit boards or where the Operator lacks experience/expertise it is safer to remove a faulty chip by snipping the leads after which the pins can be removed one at a time if necessary with the aid of a de-soldering tool. (Be certain that the chip is defunct however !)
A de-soldering tool is useful when clearing old solder from a board in preparation for replacement work.
Wires
There can be a problem, especially for the inexperienced, when soldering insulated
wires into place; the plastic insulation responds to the soldering iron by
melting. The obvious cure is not to allow the wire to get too hot and
this involves speed; heat flows from the soldered joint back along the wire.
In turn this requires proper preparation so that the final application of
heat is as short as possible.
There are two main results of overheating insulated wire:
(i) Many plastic materials exhibit a "memory effect” in that deformation-strains are relieved by heat-softening and the material returns to the shape it held before it was deformed'.
Removal of the insulation from the end of a wire usually results in that insulation being stretched; when the wire is heated the insulation contracts again and a length of bare wire results next to the soldered joint.
The best cure for this fault is speed in the soldering operation but after baring the end of a wire, tin it first (this will shrink the insulation) and then cut it to the required length before soldering is attempted.
(ii) Where a wire has been bent the insulation is stretched around the outer circumference; when the insulation softens this stretch is relieved and the wire penetrates the covering on the bend. Where possible solder the wire into place and then bend it.
END