Soldering is a process in which two or more metal items are joined together by melting and flowing a filler metal into the joint, the filler metal having a relatively low melting point.
Soldering is distinguished from brazing by use of a lower melting-temperature filler metal; it is distinguished from welding by the base metals not being melted during the joining process. In a soldering process, heat is applied to the parts to be joined, causing the solder to melt and be drawn into the joint by capillary action and to bond to the materials to be joined by wetting action. After the metal cools, the resulting joints are not as strong as the base metal, but have adequate strength, electrical conductivity, and water-tightness for many uses. Soldering is an ancient technique mentioned in the Bible and there is evidence that it was employed up to 5000 years ago in Mesopotamia.
A solder is a fusible metal alloy with a melting point or melting range of 90 to 450°C (200 to 840°F), used in a process called soldering where it is melted to join metallic surfaces. It is especially useful in electronics and plumbing. Alloys that melt between 180 and 190°C are the most commonly used.
Soft soldering is characterized by the melting point of the filler metal, which is below 400°C (752°F). The filler metal used in the process is called solder.
Different combinations of tin, lead and other metals are used to create solder. The combinations used depend on the desired properties. The most popular combination is 60% tin, 39% lead, and 1% alloys. This combination is strong, has a low melting range, and melts and sets quickly. A higher tin composition gives the solder higher corrosion resistances, but raises the melting point. Another common composition is 11% tin, 37% lead, 42% bismuth, and 10% cadmium. This combination has a low melting point and is useful for soldering components that are sensitive to heat.
As mentioned before, tin/lead solders (soft solders) are commercially available with tin concentrations between 5% and 70% by weight. The greater the tin concentration, the greater the solder’s tensile and shear strengths. At the retail level, the two most common alloys are 60/40 Sn/Pb which melts at 370°F or 188°C and 63/37 Sn/Pb used principally in electrical work.
The 63/37 ratio is notable in that it is a eutectic mixture, which means:
- It has the lowest melting point (183°C or 361°F) of all the tin/lead alloys; and the melting point is truly a point, not a range.
- At a eutectic composition, the liquid solder solidifies as a eutectic, which consists of fine grains of nearly pure lead and nearly pure tin phases, but in no way is it an intermetallic, since there are no tin/lead intermetallics, as can be seen from a tin/lead equilibrium diagram.
One of the most frequent applications of soldering is assembling electronic components to printed circuit boards. Another common application is making permanent but reversible connections between copper pipes in plumbing systems. Joints in sheet metal objects such as food cans, roof flashing, rain gutters and automobile radiators have also historically been soldered, and occasionally still are. Jewelry components are assembled and repaired by soldering. Small mechanical parts are often soldered as well. Soldering is also used to join lead came and copper foil in stained glass work. Soldering can also be used to affect a semi-permanent patch for a leak in a container cooking vessel.
There are all types of wastes generated from a soldering operation: solder, solder dross, wipes and packaging containers. Some have a recycling value and others have to be disposed of as hazardous waste. On the process side, there are effluent wastes during cleaning, where solder balls and some heavy metal salts are washed off.
In general, electronic manufacturing is a clean and safe environment to work in; however, governments still are targeting the removal of lead from solder, due to lead pollutants generated in other industries. The reason for the recent concern is that a great number of products are being disposed of in landfills, products such as televisions, radios, games and other products available to the consumer, and potential solder from these products is leaching into municipal water supplies.
In response to the new wave of regulations, the following list of desirable attributes has been compiled for lead free solder.
1. The selected element will have no negative environmental impact now or in the future
2. Sufficient quantities of base materials must be available now and in the future
3. Melting temperatures similar to 63/37 tin/lead, preferably below 200°C
4. Equal or similar thermal and electrical conductivity
5. Adequate joint strength and thermal fatigue resistance
6. Easy repairability
7. Low cost
8. Compatibility with existing processes.
"Hard soldering" or "silver soldering", performed with high-temperature solder containing up to 40% silver, is also often a form of brazing, since it involves filler materials with melting points in the vicinity of, or in excess of, 450°C. Although the term "silver soldering" is used much more often than "silver brazing", it may be technically incorrect depending on the exact melting point of the filler in use.
In silver soldering ("hard soldering"), the goal is generally to give a nice, structurally sound joint, especially in the field of jewelry. Thus, the temperatures involved, and the usual use of a torch rather than an iron, would seem to indicate that the process should be referred to as "brazing" rather than "soldering", but the endurance of the term "soldering" serves to indicate the arbitrary nature of the distinction between the two processes.
In silvers smithing or jewelry making, special hard solders are used that will pass assay. They contain a high proportion of the metal being soldered and lead is not used in these alloys. These solders also come in a variety of hardnesses, known as 'enamelling', 'hard', 'medium' and 'easy'. Enamelling solder has a high melting point, close to that of the material itself, to prevent the joint desoldering during firing in the enamelling process. The remaining solder types are used in decreasing order of hardness during the process of making an item, to prevent a previously soldered seam or joint desoldering while soldering a new joint. Easy solder is also often used for repair work for the same reason.
Mechanical and Aluminum Soldering
A number of solder materials, primarily zinc alloys, are used for soldering aluminum metal and alloys and to some lesser extent steel and zinc. This mechanical soldering is similar to a low temperature brazing operation, in that the mechanical characteristics of the joint are reasonably good and it can be used for structural repairs of those materials.
As mentioned above, although there are many different types of solder that can be used in wave soldering, tin/lead-based solders are the most common. However, these types of solder are quickly being replaced by lead-free solder. Tin/lead-based solder is highly toxic, and government regulations regarding all lead products are becoming increasingly strict. Workers dealing with tin/lead-based solder can be contaminated at work or carry it home on their clothes, where it can cause contamination. Lead-free solder provides a solution to this problem, but it also has its own set of problems that must be taken into consideration.
Solder adheres best to copper. The next best metals to use are nickel, brass, aluminum, tungsten, and lastly steel.
Lead-Free Solder Problems
In addition to higher material costs, lead-free solder has a higher melting point, slower flow rates, and can sometimes cause leeching in the iron components. Higher melting points and decreased flow rates require a longer contact period between the solder and the connections in order for the solder to completely fill the holes. Higher melting points also require a melting pot made of more stable material. The melting pot and ducts also need to be made from material that will prevent dissolution.
On July 1, 2006 the European Union Waste Electrical and Electronic Equipment Directive (WEEE) and Restriction of Hazardous Substances Directive (RoHS) came into effect prohibiting the intentional addition of lead to most consumer electronics produced in the EU. California recently adopted a RoHS law and China has a version as well. Manufacturers in the U.S. may receive tax benefits by reducing the use of lead-based solder. Lead-free solders in commercial use may contain tin, copper, silver, bismuth, indium, zinc, antimony, and traces of other metals. Most lead-free replacements for conventional Sn60/Pb40 and Sn63/Pb37 solder have melting points from 5–20°C higher, though solders with much lower melting points are available.
Different elements serve different roles in the solder alloy:
- Silver provides mechanical strength, but has worse ductility than lead. In the absence of lead, it improves resistance to fatigue from thermal cycles.
- Copper lowers the melting point, improves resistance to thermal cycle fatigue, and improves wetting properties of the molten solder. It also slows down the rate of dissolution of copper from the board and part leads in the liquid solder.
- Bismuth significantly lowers the melting point and improves wettability. In presence of sufficient lead and tin, bismuth forms crystals of Sn16Pb32Bi52 with melting point of only 95°C, which diffuse along the grain boundaries and may cause a joint failure at relatively low temperatures. A high-power part pre-tinned with an alloy of lead can therefore desolder under load when soldered with a bismuth-containing solder.
- Indium lowers the melting point and improves ductility. In the presence of lead it forms a ternary compound that undergoes phase change at 114°C.
- Zinc lowers the melting point and is low-cost. However it is highly susceptible to corrosion and oxidation in air, therefore zinc-containing alloys are unsuitable for some purposes, e.g. wave soldering, and zinc-containing solder pastes have shorter shelf life than zinc-free.
- Antimony is added to increase strength without affecting wettability.