|
Wire soldering operations are still widely accepted in the electronics assembly industry. When a product or the parts within cannot withstand the heat of oven reflow, the localized heat provided by a soldering iron has been the traditional solution. In high volume operations, this can often require large labor pools to keep up with more automated assembly processes upstream or downstream. Soldering with wire and iron also leaves process judgments up to individual operators, and can produce a wide variety of defects, scrap, or long term quality issues. Despite the many improvements in automated soldering technology through the years, many soldering operations are best suited to manual production methods, which produce inconsistent results. Whether in low volume custom operations or large-scale manufacturing processes, the quality of hand soldered joints will exhibit a high degree of variation. Defect, scrap, and rework rates can be excessive, even when using skilled employees. Higher temperature lead free operations present an additional challenge. Because of the higher temperatures required, these processes have even smaller operating windows. Visual inspection of lead-free solder joints also presents new difficulties, and since most hand soldering rework occurs ‘on-the-fly,’ actual defect rates are difficult to measure. There are other process solutions available which involve very little capital expenditure, but can significantly increase operator output. These solutions are effective in eliminating many of the process defects associated with wire solder. They will usually result in a faster and more controllable process that reduces scrap and improves overall productquality. Alternative soldering methods
Solder paste and paste flux are relatively simple improvements to implement, as they can be applied through a variety of deposition methods. For planar surfaces, it can be printed in place through stencils, and for nonplanar surfaces and/or singular deposition points it can be deposited via syringe through a wide variety of needles and other apertures. Many machines are available to automate deposition. These systems range from bench-top operator fed devices to inline, conveyor fed robotics. Some systems are even scalable to allow for a gradual process evolution from bench-top to automated assembly line. Solder paste deposition is usually a more controllable process than wire. The actual amount of solder applied can be tuned to a fine degree. Add to this the increased flux levels found in solder paste as compared to wire, and a better solder joint is often achieved. Many find that they use far less solder per joint when moving to paste processes, while producing stronger, higher quality bonds. Process flexibility also increases, due to the incredibly wide variety of paste formulations. Flux type, paste viscosity, rheology, metal content and alloy are all customizable to a wide degree, allowing for non-standard processes, parts and materials to be soldered. Comparing solder paste to traditional wire
In addition to the consistency and quality comparison, solder paste differs physically from traditional wire. These physical differences change the options available to the assembler. Solder paste is a homogenous mix. Each alloy sphere is coated with flux, typically 85 percent metal content and 15 percent flux (figure 6). Traditional wire contains approximately 2 to 3 percent flux and commonly experiences voids in the flux core that result in soldering defects, such as excessive solder, solder spikes and icicles. Paste flux is also more tolerant of heating that liquid flux. It stays in place and survives a longer heating process before running outof activity. 
The additional flux in solder paste provides a better wetting surface when creating a joint. The flux cleans and prepares the surface to be soldered better than core wire, improving alloy spread and accelerating intermetallic growth. In comparison, wire solder must be fed into the joint until the desired wetting is achieved. With solder paste’s improved flux performance, less alloy is needed to produce the same solder joint. Solder paste’s binding system prevents separation, and the tacky consistency allows for better part placement without the need to fixture the components to be soldered. Deposited paste stays where it is put and holds components in place for the reflow step (figure 7). 
Solder paste can be deposited in hard to reach locations (figure 8), allowing for easy assembly whereas positioning a wire and iron in place at the same time is a difficult and demanding exercise. By using a solder paste thatis specially formulated for use with the rapid and high temperature heating of a soldering iron, greater care can bepaid separately to placing the correct amount of solder andensuring the best heating is achieved. 
Solder paste is easy to transition to semi- and fullyautomated assembly with many process options available; whereas wire is either manually or gripper roller fed. Conclusion
The ideal solution to the problems associated with hand soldering would be independent of employee skill level so that any employee would be able to consistently produce high-quality solder joints. To avoid problems associated with soldering tip maintenance, it would also utilize a noncontact heating method. The heating method would provide reliable, even heating of the joint without any danger of overheating. Solder joint volume would be uniform and the location of the solder joint would be consistent from part to part. The solution would include the ability to use a variety of solder alloys and fluxes, and would be easily transitioned between leaded and lead free operations. And of course, the ideal solution would have a minimal investment cost. Such solutions exist and do have a low investment cost. Solder pastes exist that serve a nearly infinite variety of heating and wetting needs. It is not necessary to tolerate the quality problems and expense associated with hand solderingin order to keep manufacturing costs down. --------------------------------------------------------------
Common soldering defects Manual soldering operations result in many types of defects. These defects are well documented in IPC A-610D, Acceptability of Electronic Assemblies. Within IPC A-610D, standards for acceptability were established based on the type of product being manufactured. Here are the six most common defects: • Too much solder: Excessive solder can be caused by operator error and flux core solder wire quality variation. Even the best operator cannot get solder to wet when they reach a section of wire where the flux core is reduced or missing entirely. An excessive solder joint causes poor contact angles, stress concentration points, and can even result in bridging between components (figure 1). 
• Not enough solder: An insufficient solder joint is at risk due to poor mechanical strength. In surface mount applications an insufficient joint can be difficult to see because so much of the solder joint is under the component lead. In through-hole applications, there can be poor hole-fill that is not visible (figure 2). 
• Overheating: Leaving the iron in contact for too much time or having too high an iron temperature can damage components and circuit boards. The burns are the easy to see defect. Less easy to detect are cracked components that can easily make it to the field before making their presence known. Component cracking is caused by thermally inducedexpansion mismatch (figure 3). 
• Contamination: High iron temperatures and inadequate iron maintenance can leave behind burned flux and oxidized solder. Conversion to lead free solders typically requires higher iron temperatures that aggravate flux burning. Burned flux residue can contaminate joints, causing voids and interfere with wetting by covering surfaces that should be soldered (figure 3). • Cold joints: Insufficient heating and insufficient flux content in wire can both result in joints where the solder does not wet well. There is a minimum activation temperature below which the flux will not properly remove metal oxides. All the surfaces to be soldered must reach a minimum temperature for intermetallics to form. Putting a drop of molten solder on a piece of copper will not result in a joint unless there is enough heat in the molten solder to heat the copper to that temperature. Also, solder alloys wet and spread better the higher the soldering temperature. This improved wetting is caused by changes in surface tension and intermetallic growth rate. Poor wetting results in lower jointstrength, voiding, and in some cases no joint at all (figure 4). 
• Solder spikes and icicles: Solder spikes have been reported as the most common soldering defect. They are the result of several sources. Poor soldering iron technique can draw spikes of solder by allowing the joint to partially solidify before the tip is removed. Insufficient heating also causes solder spikes. Insufficient heating can be difficult for even a skilled operator to overcome. Many of the more inexpensive irons available have poor iron temperature control. Smaller joints often require a smaller tip that conducts less heat. The issue of flux consumption also produces solder spikes. Solder re-oxidizes when heated and insufficient fluxing action results in an oxide shell that increases the probability of a solder spike. Solder spikes are vulnerable to touching other electrical components and breaking loose to fall onto theproduct (figure 5). 
All of these defects have a common root cause. They are the result of a manual operation, and manual operations by definition are more likely to cause variation. Handmade items have value when we are looking for unique products, but ‘one-of-a-kind’ is not a selling point for solder joints. Employee training is essential to making good solder joints, but training courses cost money and take away from valuable production time. Even highly trained and skilled employees must deal with the effects of fatigue. High rates of employee turnover result in even more training expense, and smaller operations with fewer trained employees mustfind ways to deal with absenteeism and vacation time. Evaluating true cost of quality Many manufacturers do not have processes in place to calculate the total cost of quality. They have a good understanding of how to trim the cost of the bill of material and make their assembly operation more productive but not how to manage the soft costs associated with inspection, rework, training, and complaints and returns. Table 1 illustrates therelative quality impact and cost of each of these variables. 
• Solder material: Solder wire is sold as a commodity. The percentage of a product’s cost from solder is usually quite low and may not even be on the bill of materials. The difference in performance between solders can result in yield loss change of 50 percent or more. Compare the cost of scrap to the cost of solder and you will usually find the solder cost is insignificant. • Inspection costs: These are the costs associated with lost productivity from time spent inspecting solder joints for defects. The operator may perform this inspection as the joints are manufactured, or it may involve secondary oreven tertiary inspection operations. Inspection costs increase when there are customer complaints and returns that requirecorrective action. • Rework costs: This is the cost of time spent remanufacturing product and the materials consumed in repeating the soldering operation a second time. This rework can take place immediately as the joint is made, or can result from post production inspection and rejection. • Scrap costs: Perhaps the most devastating cost is when a defective solder joint cannot be reworked, or results in damage to the product. This cost includes the cost of the raw materials and all of the value-added time invested. • Training costs: The manual production of solder joints requires skilled employees, and skilled employees require training. In addition to the actual hours spent in training and certifying employees, there are the costs of training materials and instructors. There is also the cost of ‘on-the-job’ training. Skill derives not just from training, but also from experience. Training continues on the production floor, and experience is making a mistake, correcting for it and learning from it. All this happens at the expense of effective production. • Complaints and returns: In addition to the direct costs of dealing with customer complaints and returns, there is the intangible cost of future business being lost because of poor quality. In some cases a customer will only require replacement which more than doubles your cost of manufacture for that sale. In others they require the implementation of corrective action which does much morethan double costs in the short term. -------------------------------------------------------------- | 
