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76 SMT Magazine • April 2016 amount of flux per area, preheat temperature, solder temperature, and contact time. Machine and process Capability A machine capability analysis may be ap- plied to a selective soldering machine to ascer- tain its suitability to flux, preheat and solder a circuit assembly. It is a short-term study with the sole aim of discovering the machine-specific effects on the soldering process. Process capability is more of a long-term study. In addition to variations arising from the machine, all other external factors that influ- ence the soldering process over a longer operat- ing period must be taken into account. Process capability can only be performed di- rectly on the production line. Parameters tested may include topside board temperature and sol- der contact time. Parameters that influence the process and are related to the materials that are used include the printed circuit board itself. Selective soldering is a method often used in the automotive industry. Machine capability is part of the quality program that ISO-certificat- ed companies ascribe to. The ISO auditor may want to see how the company assures the as- sembly process. Quality tools including process FMEA, machine capability analysis, process ca- pability and statistical process control are part of their daily business. A machine capability analysis (MCA) is typically done before the acceptance of the machine, when the machine is installed in the production line and/or when the machine is moved to another place in the manufactur- ing facility to ensure that the performance of the machine is not affected. Some automotive customers require a MCA before a new produc- tion line starts up that is dedicated to a specific (new) product/model. Fluxing process The majority of flux application systems in selective soldering are dropjet devices. In the flux process there are some very important parame- ters that may affect reliability. The most critical is when flux penetrates into a SMD or other area where it is not activated, due to lower process temperatures. Non-activated flux in combina- tion with humidity can cause electro-migration when the product is used in the field. Therefore, the spreading of the flux is the most critical pa- rameter in this process, although it may have no impact on the final soldering result. This is a problem because engineers focus on soldering results and not on flux residues that are very hard to identify. To avoid reliability risk, it is the best to choose a selective soldering flux that is inert even when it isn't activated [2] . Today many engineers are struggling to con- trol the applied flux amounts. For good solder- ing, more flux is sometimes needed. Applying more flux creates, however, the potential risk that flux may penetrate into SMD areas. To avoid this, it is important to understand properties of flux and the printed circuit board. The solder mask on the PCB is an impor- tant component that significantly impacts the spreading of the flux. The flux spread depends on several factors, including the following: • Flux surface tension (flux type) • Surface energy of the board (solder mask) • Temperature of the flux • Temperature of the board • Flux amount As one may notice, none of these are ma- chine parameters expect for the flux amount that can be controlled by adjusting flux ma- chine parameters. The situation is even more complex. Flux spreading is critical for reliability, but the amount of dry solids of flux per square inch affects soldering quality. Thus, a trend in new fluxes for selective soldering is to increase flux solid content, which allows minor flux (less spreading) and yet retains a high concentration of solids in the soldering area. A typical soldering process requires 500– 2000 μg/in² solids. Figure 1 shows the impact of the flux sur- face tension and the surface energy of the solder mask on flux spreading. Printed circuit boards made for the wave soldering process may fail in selective soldering due to the higher surface energy of the solder mask that makes the flux spread away from the soldering area. All of these different parameters affecting the spreading (and thus also the amount of flux per consisTEnT conTrol ovEr ThE sElEcTivE soldErinG ProducTion ProcEss

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