Circuits Assembly - April 2008 - (Page 19)

directly for further information. Briefly, the difference between “true Cpk” and “estimated Cpk” is true Cpk uses population parameters µ and , (µ refers to the mean of the population and refers to the standard deviation of the population), and estimated Cpk uses and s (where X refers to sample average and s refers to sample standard deviation). We rarely, if ever, have the population parameters (e.g., µ and ) to obtain true Cpk; instead, we typically have the estimates of those population parameters: the sample average, X , and the sample standard deviation, s. With that in mind, we can use the following formulas to estimate Cp and Cpk. Cp USL LSL , and 6 X 3 LSL USL , 3 X result tells us Printer A, with its accuracy of +10 µm, is not very capable. This effect is further demonstrated in Figure 1. Printer A, even though printing within the pad, may cross over at any time. This constitutes a poor process. Still not convinced? Consider Figure 2 (I get an unanimous agreement when I present this to my SPC class). Which pilot would you rather fly your plane? I rest my case! ■ Screen Printing Cpk = min where LSL = lower specification limit USL = upper specification limit. Cp is known as the “inherent process capability” index and Cpk is known as the “actual process capability” index. The reason for Cpk being referred to as the actual process capability index is that it, unlike Cp, bases its calculations on two parameters: the average of the process, X , and the variability in the process, s. On the other hand, Cp does not use information about the centering of the process. Therefore, to get a true understanding of whether the process is actually producing parts outside of specification limits, Cpk should be used. A Cpk of 2 is associated with a process capable of performing at a 6-Sigma level. Getting back to our example of printers A and B, consider the hypothetical data generated from the two printers with respect to Y-axis offset. The data set and process capability indices are shown in Table 1. Although all the data points for printer A are within +10 µm, it has a Cpk value of 0.45 compared to printer B, which has a Cpk of 3.45 This Rework & Repair One product line to exceed your expectations and take care of all your lead-free rework and repair needs. All Chemtronics lead-free products are: Engineered for high temperature lead-free applications Specifically designed for all lead-free solder RoHS compliant CALL NOW! Contact ITW Chemtronics today for more information about the new Lead-Free product line. Call our technical hotline at 770-832-4401, or email us at askchemtronics@chemtronics.com. Nothing Cleans Like Chemtronics Soder-Wick®, Flux-Off®, Chemask®, CircuitWorks® and Chemtronics® are registered trademarks of ITW Chemtronics. ©2005 All rights reserved. 800-645-5244 / 770-424-4888 • www.chemtronics.com circuitsassembly.com Circuits Assembly APRIL 2008 19 http://www.chemtronics.com http://www.chemtronics.com http://circuitsassembly.com

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Circuits Assembly - April 2008 Contents Caveat Lector Industry News Market Watch Talking Heads Screen Printing Better Manufacturing Design and Modeling of High-Speed, High-Density 3-D CSPs and Memory Modules The ‘Big Brush Off’ Revisited Impact of Soldering Atmosphere on Solder Joint Formation Beyond Moore’s Law ESD Control For Class 0 ESDS Devices Growing Your Brand This Year’s Model Tech Tips Reflow Soldering Process Doctor Pb-Free Lessons Learned Getting Lean Equipment Advances Apex Product Preview Ad Index Assembly Insider Technical Abstracts

Circuits Assembly - April 2008

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