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48 SMT Magazine • February 2014 pletely died out or the system is dynamically moving (dispensing on-the-fly), there is some induced error in the read position of the sys- tem at the encoder and the end effector's actual position. This error will then translate to some level of degradation of the accuracy of the ac- tual dispensing of the equipment. end effector and Fluid The end effector and fluid type can further affect the accuracy and repeatability specifica- tions of a given dispensing system. Valve type can affect the operation of the dispenser as a whole and thus impact the dynamic versus static conditions. The mass of the valve and the fluid reservoir will affect the inertia of the system. Additionally, for valves or applications that require a much tighter dispense gap between the substrate surface and the dispensing tip, a system must move in the X-Y po- sition then move down in Z position. This pause to allow motion in the Z-axis acts as an artificial settling time and can improve the actual dispense accuracy. Similarly, the gap between the dispense tip/orifice and the substrate can create some minor offset in the fluid position if the fluid does not eject vertically down from the dispense tip to the substrate. In such a case, having a larger dispense gap introduces a larger X-Y error. Vision System Most automated fluid dispensers today incor- porate some means of a vision system to identify target fiducials on a sub- strate part to locate where to dispense. As a result, the ability of a dispenser to ac- curately find and locate a fiducial factors into the total system accuracy. Vision system accu- racy itself has many factors affecting proper fi- ducial capture and finding, some of which are tied to the motion system. The frame-capture time and the correlation of a frame capture to a specific position in the dispenser's X-Y mo- tion area creates similar issues to those previ- ously mentioned in the motion system section. In most cases, the camera and end effector dis- pense point are mechanically offset from one another. In such cases, the system software must calibrate the distance between these two positions and can create an offset error if the two positions vary relative to each other or the calibration is too coarse (Figure 7). Figure 7: Camera to dispense valve offset. FEATUrE MaKING SeNSe OF aCCuraCy, rePeaTabILITy aND SPeCIFICaTION continues

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