Cpk is sometimes referred to as the voice of the process because it is measuring how tight your process is in relation to the specification limits. A Cpk of 1 means that you are running a 3 sigma process (plus or minus 3 sigma). This means that your calculated control limit equals your specification limit, and that 99.73% of your product will fall inside the +-3sigma control limit (meaning a defect ppm of 2700ppm). Usually, a minimum of 1.33Cpk is required to get a process validated. A Cpk of 2 means that your calulated control limits are 1/2 of your specification limits, getting you to that wonderful place known as 6 sigma (or in Motorola terms, a defect of 3.4ppm) How I got there with component placement... Using a coordinate measurement system, I measured different components in relation to the x-y center from the CAD data. I made the specification limits the IPC placement limits (calculated a high and low value) based on the 50% requirement. You have to be careful to check the component size vs. the pad size or your will get an erroneous reading. I found the averages and used the formulas: Cpk=(USL-Xavg)/3sigma or (Xavg-LSL)/3sigma The Cpk is the smaller of the two numbers.
Most of the placement equipment can get to high Cpk's, but you have to make sure you do everything you can to redcue any possible variation. This includes, but is not limited to: machine preventative maintenance, good part shape definitions, feeder setups, feeder maintenance, and various and sundry other factors. Before you try calculating Cpk's, define all the factors in the process and insure that you have minimized the variation in each one. The process must be in statistical control prior to calculating Cpk.
If you want more information on this, please feel free to e-mail me. There are several sources of training I can recommend.
Brian W.
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