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May 2015 • SMT Magazine 69 SELECTING aN aUTOmaTIC PICk-aND-PLaCE maCHINE, PaRT 3 continues sMt quiCk tips make corrections to the position programmed, if needed, to within a few microns of the ac- tual X-Y location for the component placement (typically 12,800 increments, or steps, for each inch of travel). The best machines in this cat- egory use servo motors. Pros: Very high accuracy, to within +/- .0005"; very repeatable. Cons: More costly, but necessary for high- value production. NOTE: The quality of the encoder (the posi- tion feedback sensor), is an important element in the whole system and does affect accuracy. machine Construction When selecting a pick-and-place machine, be aware that its construction will dictate its ef- fective CPH range and footprint, including con- siderations for the number of component feed- ers it can accommodate. 1. All-welded steel: The most accurate ma- chine will have a frame that is constructed of a solid welded structural steel tube. This provides significant stability necessary for accurate posi- tioning and high-speed movement of X-Y axes. This construction method is recommended for any production environments, and it will remain stable without requiring ongoing calibration. 2. Bolt-together frame: Extruded alu- minum or formed sheet metal frame will come with a lower initial accuracy than a welded frame and will need to run more slowly because it can't handle the rapid inertia shifts of X-Y axes movement. Further, it will likely go out of calibration frequently, which will adversely impact labor time, downtime and yield. (Lower cost usually reflects a weaker construction.) Solder Paste/Fluid Dispensing Any pick-and-place machine should be ca- pable of offering fluid dispensing systems. Most common liquids include solder pastes, adhe- sives, lubricants, epoxies, fluxes, glue, sealants, and more. This is a valuable option when build- ing prototypes or one-off PCB assemblies that do not warrant the cost of a dedicated printer stencil or foil. Component Feeders If the machine's production will be dedicat- ed to jobs with a small number of components it's very easy to identify the number and type of feeders. However, that is not usually the case with contract assembly shops, since they don't know what type of board and how many differ- ent components the next job will require. Some OEMs also need flexibility for a wide range of board configurations, especially if they intend to use the same machine for prototypes and several different production boards. So it is useful in those cases to consider a machine with the greatest number of feeder po- sitions and options that can accommodate the footprint your space can handle. figure 3: fluid/solder paste dispenser, typically mounted to the head.