³³Á¢(Soldering)

II. Materials
Soldering is a method of making a permanent electrical and mechanical connection between metals. Unlike glue, which forms a solely physical adhesive bond, solder chemically reacts with other metals to form a different alloy. While there are many different processes utilized in soldering, virtually all of them involve four basic elements: base metals, flux, solder, and heat.

A. Base metals
A base metal is any metal that contacts the solder and forms an intermediate alloy. When attaching electronic components to a printed circuit board, the component's leads or pins and board's metallic circuitry are the base metals that will contact the solder. Many metals, such as copper, bronze, silver, brass, and some steels, readily react with solder to form strong chemical and physical bonds. Other metals, such as aluminum, high alloy steels, cast iron, and titanium, range from very difficult to impossible to solder. The fact that there are metals that do not react with solder is important; these materials are used in the construction of soldering machinery. These metals can also be used as temporary covers for components that are not to be soldered. Also of importance to the electronic industry is the fact that ceramics do not react with solder. This allows a manufacturer to draw liquid solder over a ceramic circuit board and not have any chemical reaction between the solder and the board itself.
There is a direct relationship with the level of surface oxidation on the base metal and how readily solder will react with it. The more oxidation is present, the weaker the solder bond will be. The fact that most metals oxidize at a very accelerated rate when heated creates a particular problem, since the chemical reactions associated with soldering require high temperatures. Flux is the primary material used to overcome problems caused by oxidation.

B. Flux
Flux is often applied as a liquid to the surface of the base metals prior to soldering. Though flux actually has a number of purposes, the first and primary purpose of flux is to stop the base metals from oxidizing while they are being heated to the soldering temperature [2]. The flux covers the surface to be soldered, shielding it from oxygen and thereby preventing oxidation during heating. Most fluxes also have an acidic element that is used to remove the oxidation already present on the base metal. Using a strong acid, it would be possible to virtually completely clean off the oxidation layer. However, the use of strong acids presents a serious problem. The corrosiveness of acids desirable to remove oxidation layers is not limited to the oxidation layer. Very strong acids can be damaging to electronic components, and even mild acids leave a residue that continues to corrode after the soldering process is complete, leading to future failure. There is a definite trade off between using a flux with a strong acid that removes a lot of oxidation and is very corrosive, and using a flux with a mild acid that is not as corrosive, but does not do as good of a job removing the oxidation layer. In any case, most fluxes in common use are corrosive enough that their residue must be cleaned off after soldering.
When the liquid solder is applied, the flux must readily move out of the way so the solder can come into direct contact with the base metal. During this process some of the flux inevitably combines with the solder. Flux designers typically take advantage of this fact and design the flux to lower the surface tension of the solder upon contact, thereby allowing a more efficient wetting.
Fluxes can be divided into two basic parts, chemicals and solvents. The chemical portion includes the active components, while the solvent is primarily the carrying medium. The solvent determines the cleaning method that must be employed to remove the flux residue. While some fluxes can be removed with simple water treatments, many require other cleaning agents such as organic solvents, alcohol, terpenes, and chlorinated fluorocarbons. (Note: it is no longer legal to use chlorinated fluorocarbons due to environmental concerns).

C. Solder
There are many different metals and metal alloys that can be used as solder. The decision to use a particular material is largely based on its properties. Is it ductile or brittle? How well does it conduct heat? Does it expand rapidly at high temperatures? How much electrical resistance does it have? What is its tensile strength? Is it toxic? What materials will it wet? And perhaps most importantly, how much does it cost? Although it is by no means the perfect alloy for soldering, the material most commonly used in the electronics industry is a tin-lead alloy. Tin-lead alloys have a relatively low melting point and can be produced at a low cost in comparison with other alloys with similar properties. Lead is a very cheap and abundant metal, so the cost of a tin-lead solder is primarily controlled by the cost of the tin.
When an alloy is heated it typically goes thorough multiple phases. It goes from a solid state to what is known as a pasty stage, sort of halfway between a liquid and a solid, and then to a liquid state. In soldering it is difficult to work with a substance that goes through a pasty stage. Eutectic solder is often used for this reason. A eutectic alloy is one that goes directly from a solid state to a liquid state without a pasty stage. The eutectic tin-lead alloy is made up of 63% tin and 37% lead. Eutectic tin-lead solder can be applied as a liquid just above the melting point, and then as it cools it will transform directly into a solid. This makes it possible to form solid solder joints very quickly. Sometimes a 60% tin and 40% lead alloy is used. This alloy exhibits a nearly eutectic change from solid state to a liquid state and can be produced at a lower cost.
It is very important to keep the solder free of impurities. Not only is it important to produce a pure solder alloy, but it is equally important to use a process that prevents metals from the electronic components or circuitry from entering the solder pot. The presence of even slight concentrations of other metals in a tin-lead alloy results in drastic changes in surface tension. Poor wetting of the base metals leading to a poor solder joint is often the result. In addition, metal impurities often change the melting temperature of the solder. Dust, oil, vapors, and other non-metal impurities tend to weaken solder bonds.
Solder is typically transported and sold in solid form. Common forms of solder include chips, bars, and wire (often with a core of flux), each of which has advantages in different soldering processes. A common process called reflow soldering calls for a solder paste. Solder paste is a substance with a cream-like consistency made up of solder, flux, and some carrying medium. While putting the flux and solder together in the same mixture has an obvious advantage when it comes to applying the substance to the base metals, it also presents a problem. Highly corrosive flux cannot be used in solder paste. The flux by nature is acidic and corrosive to the metal solder, which means that solder paste is inherently unstable. The shelf life of unused solder paste ranges form about three weeks to three months. The time between when the solder paste is applied to the base metal and when the final heating is completed is limited to a maximum of a few hours.

III. Forming solder bonds
When tin-lead solder is used, the tin reacts with the base metal to form an intermetallic alloy. This intermediate layer of metal ranges from a very high concentration of tin on the solder side to a very high concentration of the base metal on the other side. These intermetallic alloys are typically both brittle and weak. This means that the solder joint is the "weak link", and is susceptible to mechanical failure due to stress or vibration.
In order to reduce the likelihood of failure, the intermetallic alloy is made to be as thin as possible. The intermetallic layer grows at a negligible rate at room temperature, and its growth rate accelerates as the temperature is increased. Therefore it is advantageous to solder at the lowest possible temperature, typically just above the solder melting point. A shorter time of contact between the solder and base metal at an elevated temperature results in a thinner intermetallic layer. This means that soldering should be done as quickly as possible. Another way to get a thinner intermetallic layer is to slow its growth. A lower tin content in the solder results in slower growth [5]. This creates another trade off; when the tin content is lowered from its eutectic concentration, the advantages of having a eutectic phase transition are lost.

IV. Wave soldering
Early attempts at soldering large numbers of electronic contacts at the same time involved dipping the whole printed circuit board into a pot of solder, or dragging the board across the upper surface of solder in a pot [6]. These soldering techniques were vastly improved when an Englishman by the name of Strauss came up with the idea of wave soldering. He modeled wave soldering on a method used to coat cookies with chocolate [7]. Liquid solder is pumped up through a nozzle and out the end. Gravity then causes the solder to fall back down, creating a parabola shaped "wave." The printed circuit board, with the electronic components already inserted, travels over the apex of the wave. As the wave of solder comes in contact with the bottom side of the board, the already fluxed metals chemically bond with the solder.
One of the advantages of wave soldering is that the process is easily automated with the use of a conveyor system to move the boards. Conveyor systems can move boards on flat pallets or by the use of finger conveyors. In either case, the conveyor system is constructed of a material that does not bond with solder. The conveyor must move the board into the fluxing area, from the fluxing area through a preheating process, and then over the solder wave.

A. Applying flux
Flux can be applied in a number of ways. Some early methods involved dipping the board into a liquid flux or brushing the flux onto the board with rotating bristles, similar to those used in a car wash. The greatest problem with these methods was that the amount of flux that is applied could not be controlled. Large quantities of flux are very harsh on electronic components. In addition, the brittle brushes had a tendency to dislodge the components before they even arrived at the solder wave.
Another way of applying the flux is to have a flux wave. A flux wave operates very similarly to a solder wave. The nozzle that the flux is pumped through contains a mesh to help eliminate ripples as the flux is propelled upwards. Flux is applied as the board travels over the apex of the wave. An air knife is then used to blow excess flux off as the board leaves the wave. While this method is a very effective way of applying flux, it requires continual cleaning and maintenance in order to keep operating.
Yet another way to apply flux is to spray it on. While spraying has the inherent problem of depositing flux all around the target area, the amount of flux applied can be very precisely controlled. Flux can be sprayed by using a compressed gas process similar that of industrial paint sprayers. However, most of the excess flux is not recoverable when a compressed gas sprayer is used. This can partly be overcome by using an "airless sprayer." Though not widely used, this device compresses the liquid flux and then sprays it out a nozzle without ever introducing a compressed gas. Another type of flux sprayer utilizes a revolving drum and compressed air. As the drum revolves, the underside comes into contact with a tank of flux. The drum has a mesh like outer surface which allows it to pick up small amounts of flux as it passes through the liquid. Air then blows particles of the liquid flux off the upper side of the drum onto the circuit board.
The most common way of applying flux in a wave soldering process is foam fluxing. The flux is aerated with extremely fine bubbles of compressed gasses, causing it to foam up. This foam is allowed to climb up a chimney and spill out over the top, creating a foam head over the chimney. The printed circuit board then moves across the top of the foam head, picking us some of the flux. An air knife is then used to help remove the excess foam as the board exits.

B. Preheating
During preheating the printed circuit board, with its components, is heated in order to raise the base metals to their soldering temperature. If these metals are already at or near the required temperature when the solder is introduced, the amount of time that the solder, in its liquid form, must be in contact with the board is minimized. This results in a much stronger intermetallic layer. Heating the board up at a slow steady rate also works to minimize thermal shock to the board and its components. Simply subjecting a room temperature circuit board to a wave of liquid solder without any preheating can cause extensive warping and cracking.
There are three different ways to preheat that are in common use. Electric heaters that work on the same principle of a household toaster are used, as are convection and infrared heating processes. Infrared heating has the advantage of being able to heat up the assembly quickly, but the equipment employed is more expensive than that used in other types of heaters. While there are some inherent advantages and disadvantages of each method, each of them accomplishes the desired result of preheating the board. Therefore, the decision as to which one to employ is largely based on cost, space, and personal preference.

V. Re-flow soldering,br> The development and increased use of surface mount technology has led to the use of other soldering methods. While surface mount components can be wave soldered, they must first be attached to the circuit board with some type of adhesive or cement in order to keep them in place during soldering. This introduces another step in the assembly process. Since the whole surface-mount component is immersed in the wave, it must be constructed in such a manner to withstand the high temperature of the liquid solder. There is also a problem of gasses being trapped between the component and the board [10]. Because of these difficulties with wave soldering, re-flow soldering is the preferred method of soldering surface mount components.
In a re-flow process, solder paste is put on the component sites of the printed circuit board, and then the components are put on the board on top of the solder paste. Often a separate adhesive is used to hold the device in place until soldering takes place. The board and attached components are then heated to activate the flux, elevate the temperature of the base metals, and melt (or "re-flow") the solder.

A. Applying solder paste
There is more than one method in use to apply the solder paste to the circuit board. One way of doing this is to dispense a slightly pressurized solder paste through the end of a tube. This is very similar to the operation of a syringe. This type of application has several advantages over other methods. First, it employs a closed tank of solder paste that allows little opportunity for solder contamination. The syringe can reach into odd shaped places, which is of particular use in re-flow soldering surface-mount components onto a board after through-hole components have already been wave soldered on. Utilizing disposable syringes is also a relatively inexpensive process. Notwithstanding these advantages over other techniques, it is very difficult to control the precise amount of solder paste that is applied to the board. Adequate control can only be achieved with complicated, and expensive, high tech control systems. Additionally, dispensing takes place on only one pad at a time, making this method relatively slow.

Screen printing is a more common way of dispensing the solder paste onto the circuit board. This is essentially the same process that is used in applying paint to clothing and street signs. A screen stencil is placed slightly above the board, and a squeegee is manually drawn over the stencil, forcing solder paste through the screen onto the board. The amount of solder deposited can be quite accurately controlled with the density of the screen and the shape of the squeegee. The alignment between the board and screen is very important. If the either of them move even slightly, or if the alignment is even slightly off, the solder will not be deposited in the right place.
Another technique used is to dip dull pins into the solder paste, and then dab the end of the pin onto the board. The amount of solder applied is directly related to the size and shape of the pin. An advantage to this method is that it is fast. A whole array of pins can be lowered onto the board at the same time.

B. Heating
The assembly, the board and its components with solder sandwiched between, is uniformly heated to a predetermined temperature. It is then held at this temperature to give the solvents in the solder paste time to evaporate and dry. This is the same temperature that the flux becomes active, and begins to clean the base metals. After a sufficient time at this temperature, the temperature is raised above the melting point of the solder and held for a time, usually between thirty and sixty seconds. The board is then slowly cooled at a continuous rate. There is a trade off here; cooling the board quickly results in a very strong solder bond, but it also introduces stresses into the board.
Early heating devices used conduction heating similar to that used in hand held soldering irons. As the component leads were heated, energy was conducted to the solder. The heating device never came in contact with the solder as in hand held soldering irons. Today heating is accomplished with the use of either a convection or radiation heating process or a combination of the two.
A common type of convection heating is vapor phase heating. A liquid is boiled, causing some of it to vaporize and saturate the air within the vapor chamber. When the board is inserted into the chamber, the vapor condenses onto it. Energy is transferred from the vapor to the board during condensation, causing the board to heat up. The temperature that the board is heated to is the boiling point of the liquid. The fact that this is a very fast heating method and that the increase in temperature is very uniform makes it advantageous. Another advantage of this method is that it is performed in a closed vapor chamber where no oxygen is present. When oxygen is not present during the heating process, oxidation does not occur. This allows the use of a very mild flux in the solder paste. Sometimes the flux that is used is so mild that cleaning is not required.
Radiation heating allows the assembly to be heated using electromagnetic waves. Just as in a household microwave, these waves do not heat up the air in-between their source and the board. Either infrared or laser light is used. These processes allow precise control of the amount and placement of the transferred energy. The drawback of these methods is the slow rates at which they heat up the boards.