The Soldering Process Can Be Easily Automated Offering The Possibility Of Inline Arrangements Of Soldering Machines With Other Equipment
Mass soldering by wave, drag, or dip machines has been the preferred method for making high-quality, reliable connections for many decades. Despite the appearance of new connecting systems, it still retains this position. Correctly controlled, soldering is one of the least expensive methods for fabricating electrical connections. Incorrectly controlled, it can be one of the most costly processes--not because of the initial cost, but because of the many far-reaching effects of poor workmanship.
Chronology of Soldering. Soldering is not a new process (Ref 2). It probably dates to the Bronze Age, when some metalworker discovered the affinity of a tin-lead mixture for a clean copper surface. As recorded in the writings of Pliny, the Romans used 60Sn-40Pb mixture to solder their lead water pipes.
During the late 19th and early 20th centuries, a great deal of work was devoted to the development and improvement of the soldering process and to the determination of the fundamental principles involved. Examination of the books, papers, and especially the patents of the era shows that soldering as a science was clearly understood long before the start of the electronics industry. There has certainly been a great improvement in the materials and machines used in the process, but the basics have changed little since the early part of this century.
Soldering in the electronics field has undergone many dramatic changes since the first radios were built. Between the 1920s and the 1940s, all interconnections were made using the point-to-point wiring method, and the solder joints were made using soldering irons. After World War II (Ref 3, 4), the demand for consumer electronics began to increase, and by the 1950s a mass market had developed. To meet this demand, the first mass soldering technique, dip soldering, was used with boards that were the precursors of printed wiring boards (PWBs). By the late 1950s, wave soldering had been developed in England and soon thereafter reached the United States. This development was necessitated by the introduction of PWBs. Wave soldering continued to be the method of choice through the 1980s and is still widely used.
The development and maturation of the multi-layer PWB and the first major effort toward miniaturization occurred in the 1970s. The dual inline integrated circuit package replaced many discrete components. By the early 1980s, the pressure to increase component densities had grown to such a degree that a new technique for attachment was needed, and surface mounting was developed. Surface mount technology in turn required new ways to make solder joints, prompting the development of vapor phase, infrared, hot gas, and other reflow soldering techniques.
Soldering remains the attachment method of choice in the evolving electronics industry. It is the only metallurgical joining technique suitable for the mass manufacture of electronic and electrical interconnections within the temperature restrictions of electronic components and substrates.
Solder Metals. The solder metal business has changed dramatically, having moved from merely producing wire to producing bulk, preform, and paste in order to meet the needs of soldering techniques. Solder can be applied in several ways (Ref 2, 3):
• A MIXTURE OF SOLDER POWDER AND FLUX, KNOWN AS SOLDER PASTE (REF 5), CAN BE APPLIED TO THE JOINTS BY SCREENING, STENCILING, OR USING ONE OF THE MACHINES THAT APPLIES DOTS OF THE PASTE BY A SIMPLE PNEUMATIC SYSTEM. THIS METHOD IS ESPECIALLY USEFUL WHERE SURFACE-MOUNTED COMPONENTS ARE TO BE SOLDERED AND CANNOT BE PASSED THROUGH THE SOLDER WAVE.
• PARTS CAN BE PRETINNED, THAT IS, COATED WITH A LAYER OF SOLDER BY FLUXING AND DIPPING INTO A BATH OF THE MOLTEN METAL.
• SOLDER CAN BE OBTAINED IN THE SHAPE OF RINGS, WASHERS, OR TUBES, WHICH ARE PLACED ON OR ADJACENT TO THE PARTS TO BE JOINED. THESE SOLDER PREFORMS CAN BE OBTAINED WITH OR WITHOUT A FLUX COATING.
The two latter methods of providing solder for the joint are used in many areas of the electronics industry where machine soldering is not possible. Tiny solder rings are placed over long wire wrap pins on a mother board, where the gold plating of the pins in the wrapping area must not be coated with solder. Solder washers are fitted under bolts that must be soldered to a ground plane. Wires and tinned before soldering into a pretinned wiring lug. The possibilities are seemingly limitless (Ref 6).
Solder filler metals have also changed. The first electronic soldering was done with the 50Sn-50Pb solder material used by plumbers. By the 1950s, the choice of materials had switched to eutectic tin-lead solders to counter the problems created by a vibrating wave solder machine conveyor. Today a wide range of filler materials is used in the electronics industry. With the spread of electronics technology into every facet of life, the environmental and materials demands have grown substantially, requiring the use of such specialized materials as indium and gold-tin fillers.
Flux technology has also evolved (Ref 3, 5). As a result of the restraints of military and government specifications, the emphasis has been on the addition of new flux types rather than on the replacement of old ones. Extensive work has been done with organic acids, which enlarge the processing windows. Other categories include synthetic activated materials, which are non-rosin fluorocarbon solvent-removable types, and low solids fluxes for surface mounting that can be used instead of the classic fully activated rosin formulations.
Other materials, such as adhesives for the surface mounting of components intended for wave soldering, are new to soldering technology. These materials must be specially designed to avoid interference with the soldering process, yet must remain intact through the solvent action of fluxes and, often, flux removal.
Mass Soldering. With the introduction of mass soldering techniques (Ref 1, 6), the issue of solderability has become one of the major interest. Mass production processes are not as tolerant as the hand soldering process. In addition, modern components cannot withstand as much heat, as for example, a tube socket. Lower soldering temperatures and less mechanical working have made it necessary to attain the highest solderability to ensure a metallurgically sound and reliably solder joint.
Machine or mass soldering (Ref 2, 4, 6) can make thousands of joints in a few seconds, providing the electrical connections and simultaneously mechanically fastening the components. No other process has yet been developed that can so economically interconnect electronic circuitry.
Wave Soldering. From the original concept of hand dipping boards came the idea of pumping solder through a slot or nozzle to form a wave, or mound, of constantly moving, dross-free metal through which the board could be passed. This offered several advantages: the surface of the solder was always clean and did not require skimming, the pumping action maintained an even temperature, and the raised wave provided an easy form for adding a conveyor system for automatic movement of the board over the solder.
Once the idea of the solder wave had been born, it was logical to use a similar method to apply the flux, and the basic soldering machine appeared. These original machines were quite crude, consisting of individual modules with a freestanding conveyor (often added by the user) placed over them. There was little attempt to integrate the parts into one system, and the smoke and fumes were left to escape into the factory exhaust, aided by a makeshift canopy hung over the machine.
Much work was done on the development of the actual wave in an attempt to improve the speed of soldering and to reduce the incidence of shorts and icicles. The solder weir, the double wave, and the cascade all experienced brief popularity. The cascade wave, which consists of several waves in series, was one of the more interesting of these developments. It was used with an inclined conveyor and was the basis of machines built for in-house use by several manufacturers. However, it never achieved great popularity.
Another development was the use of ultrasonics to form a wave, with the intention of causing cavitation at the joint of avoiding the use of flux, or at least reducing the activity required. Some experimental machines were built, but the cost was excessive, the soldering results were no better than those produced with the conventional pumped wave, and the power of the ultrasonic energy had to be so high that the matching horns used to energize the pot were quickly eroded away. With the advent of the semiconductor, ulrasonics were found to cause occasional damage to the devices. Today the main use for ultrasonics in the soldering industry is to be found in the pots used for tinning components prior to assembly. This era also saw the introduction of oil into the solder wave. This idea worked extremely well and has been a feature of some soldering machines for many years (Fig. 1) (Ref 2, 3, 4, 5, 6, 7).
- FIG. 1 SCHEMATIC SHOWING SIMPLE OIL INJECTION OF A SOLDER WAVE
Drag Soldering. In a totally different direction, the static solder bath soldering process, or drag soldering, was developed, and machines were designed using this principle (Fig. 2). These machines have been developed to a high level of sophistication, initially in Europe and Japan and later in the United States. As has often been the case in the growth of the soldering machine industry, initial developments were prompted because of patent restrictions that at the time prevented the free use of the more conventional solder wave. These restrictions have often proved beneficial to the industry by forcing creative thinking that has eventually resulted in improved products (Ref 2, 3, 4, 5, 6).
- FIG. 2 SCHEMATIC SHOWING SEQUENCE OF OPERATIONS FOR A DRAG SOLDERING MACHINE
Current Technology. Today machines are available from many different manufacturers, each with its own particular features. These machines have various levels of sophistication and capability, ranging from small laboratory units to high-production systems, often complete with in-line cleaning and assembly and return conveyors. Machines in use today are generally self-contained, with a high degree of control, optional auxiliary systems, and exhaust venting. Coming onto the market are systems with computer-controlled operation and setup, which practically eliminate human error from the machine soldering process.
Soldering processes have evolved dramatically since the introduction of mass soldering techniques. Wave machines have become highly specialized, especially since the introduction of surface-mount technology. Reflow techniques (Ref 8), which were once limited to hybrid technology and leaded flatpacks, have grown remarkably. Vapor phase, infrared, and hot gas soldering equipment have initiated a new process of evolution.
The performance of a solder joint in its service environment is a strong consideration in modern electronics. Many applications, such as jet engines, dishwashers, and automobiles, impose a repeated cyclic thermal cycle on the equipment. This produces thermomechanical strains, which result in fatigue or creep failure if the joint design is poor or if life predictions have been incorrectly calculated. Each situation must be carefully analyzed and the correct design employed for the equipment to fulfill its intended service life.
Thermally induced fatigue and creep failures, virtually unknown in the 1950s but common today, are illustrative of the added stresses and strains placed on joints. It should be emphasized that miniaturization of electronic parts, particularly current paths, results in a dramatic increase in current density, which generates strong thermal and mechanical shocks upon switching. Proper design can minimize these strains and make the joints more highly producible. Design has become even more critical with the introduction of surface-mount technology.
Quality Control. Once a joint is made, its quality must be assessed through inspection. Advanced technology has produced some electronic assembly designs that are impossible to inspect visually, and an entire group of automated inspection tools has thus evolved. These tools include radiography, infrared signature, and two- or three-dimensional vision systems. Most of these are aided with very sophisticated computer software. The role of inspection, which historically was to locate those joints that needed rework, is changing along with the technology. Inspection is now used as a process-control audit to locate those portions of the process that need improvement or should returned to a position within the control limits. Therefore, the emphasis has shifted from merely shipping an acceptable product to manufacturing it correctly the first time.
Future Trends. Several other technologies and materials for soldering are being developed and tested (Ref 2, 3, 4, 5). One process that is becoming widely used is condensation, or vapor phase, soldering. In this process, the vapor from a liquid with a high-temperature boiling point is allowed to condense on the items to be soldered and, in giving up the latent heat of vaporization, quickly and evenly raises their temperature and produces a sound soldered joint. The method is simple, clean, and extremely well controlled. It requires-special equipment, and, of course, the parts are all exposed to the same temperature.
In another method, the parts are dipped into an oil heated above the melting point of the solder. Peanut oil can be used, although several excellent synthetic products are available. Here again, the component parts are subjected to the same temperature as the joint. This is a messy process, and the parts must be cleaned after soldering to remove the oil.
Infrared heating is a useful method for producing the temperature necessary for soldering. The shorter wavelengths can be focused onto very small areas, allowing selective heating of the joints to be soldered, without subjecting nearby components to the soldering temperature. Equipment and tooling are expensive if complex assemblies have to be soldered, and the relative emissivity of the joint metals and the adjacent materials can cause processing problems. For example, the base laminate of a PWB can be burned before the joint to be soldered has even reached the soldering temperature. This process thus requires very careful control.
Resistance heating can be used for soldering in two ways. The first method passes a current through the parts of the joint, causing their temperature to rise by virtue of the resistance existing in the joint structure. This is not a very controllable process and is primarily used for less critical applications on large connections. The second method passes a current through a wire that is shaped to touch the parts to be heated. Despite the lack of precise control, the process is often used to solder the leads on surface-mounted components.
Lasers have been used to provide the heat for soldering, although this is not yet a common method. A laser can provide a very high-energy, small-area beam, and heating is therefore extremely fast, localized, and limited only by the ability of the joint materials to absorb this energy. However, the shiny metal surfaces in the joint reflect most of the energy of the commonly used lasers, and coupling the laser and the joint is a major problem. The soldering flux is often used as a coupling medium. When correctly set up, laser soldering offers a fast, clean method of making joints. The cost of the equipment is quite high.
Since 1980, significant improvements and innovations in the soldering arena with respect to materials, equipment, and processes have occurred. However, two main tasks remain: developing environmentally friendly solder materials and establishing failure-free solder joint technology (Ref 2, 3, 4, 6, 7, 8).
For solders and soldering, two issues that are directly related to environmental concerns are the replacement of chlorofluorocarbon (CFCs) as the primary cleaning agent for electronic assemblies and the feasibility of using alternatives to lead-containing solders. For electronic assembly solders (Ref 4), in addition to using CFC substitutes and other solvents (e.g., hydrocarbon-based systems), aqueous cleaning and "no-clean" setups are two viable solutions.
The other task to be accomplished is the development of failure-free solder joint technology. Strengthened solder materials are expected to possess the properties that impede the occurrence of undesirable metallurgical phenomena during their service life under various in-circuit and external conditions, particularly temperature and induced stresses. Three approaches are being examined: macroscopic blends of selected fillers in a tin-rich matrix (composite solder), microscopic incorporation of doping additives to tin-rich base material, and atomic-level interaction. Preliminary experiments have been carried out to accomplish the first approach. Figure 3 demonstrates the increased yield strength of the 63Sn-37Pb composite solder with various amounts of filler (Ref 9).
- FIG. 3 EFFECT OF FILLER REINFORCEMENT ON THE YIELD STRENGTH OF 63SN-37PB COMPOSITE SOLDER. SOURCE: REF 9
The intrinsic properties of the new materials are expected to possess the following characteristics, while maintaining thermal and electrical conductivities:
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