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February 2014 • SMT Magazine 49 In addition to this, the pixel-to-mi- cron resolution of the vision system af- fects the system's inherent ability to ad- equately locate a fiducial feature similar to the motion system encoders. The vi- sion system algorithm's ability to consis- tently locate the target position within the captured image then affects the tar- get positioning for where to dispense the fluid. Substrate and fiducial imaging also depends upon having adequate lighting to create sufficient contrast in a captured image for the software to correctly iden- tify the fiducial position. Thus, even with perfect motion system accuracy, if the vi- sion system does not properly target the correct position, the total system accu- racy can suffer. Interpreting accuracy and repeatability Specification In evaluating performance specifica- tions, it is important to have clear informa- tion on how claims to dispensing system specifications are obtained. Comparing data obtained under different conditions can make specification comparison difficult and can also lead to incorrect conclusions. Artificially high positioning accuracy and repeatability could be achieved when the equipment is operated below normal speed, for example. Therefore, the evaluator must under- stand actual conditions for the stated specifica- tion: • Does the specification represent a best-case scenario or a normal operating condition? • What is the data spread of the claimed specification? • Is the specification achieved at the positioning sensors or at the fluid dispense location? Rather than theoretical capability at the positioning sensors, true system positioning specification is the measured capability at the fluid dispense location. True system positioning specification considers factors beyond X-Y posi- tioning accuracy and repeatability. It also takes into consideration the effects of calibration, vi- sion (camera) system resolution and the offset between vision system and actual dispensing position. Figure 8 illustrates total system capa- bility at the fluid dispense location in which the final system accuracy reflects both the ac- curacy (based on the sample set mean) and re- peatability to achieve results around that mean position. Conclusion Accuracy and repeatability are important factors in determining fluid dispense capabil- ity. They are often the key attributes to evalu- ate whether equipment can meet target process parameters. However, claimed specification numbers are highly dependent on underlining conditions. Therefore, when comparing specifi- cations between two systems, it is crucial to un- derstand the complete story behind the specifi- cation. It is always recommended to check the conditions under which the claimed accuracy and repeatability are specified to determine true system specification and to be able to compare two systems equally. SMT Figure 8: Total system accuracy. FEATUrE MaKING SeNSe OF aCCuraCy, rePeaTabILITy aND SPeCIFICaTION continues