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76 The PCB Magazine • March 2015 by Steve Williams STEVE wILLIaMS COnSuLTInG LLC PoiNt of View Best Practices 101, Part 5: Process Capability ColuMn Early in my career, a wise old mentor told me, "Steve, never argue about what can be mea- sured." As an engineer by trade and German by lineage, he knew a little about precision crafts- manship. This advice has stuck with me, and in the quest for continuous improvement it has translated into "How can we get better if we don't know where we are now?" followed by "How can we know where we are now without metrics?" Process Capability I will try to follow my KISS philosophy and stay away from all the scary math as much as possible, so let's begin by reviewing the funda- mentals of statistical process control (SPC). It is important to note at this point that not every process is a good candidate for statistical con- trol, and that in these instances alternate pro- cess control methods may be required. The laws of physics dictate that although every single process has variation, once a process is stable, that variation follows a repeatable pattern that is called a normal distribution. That means that only some of the product (any process output) will be exactly the same as the process average (mean). It also means that the rest of the prod- uct will either be less or greater than the average, and will occur in decreasing frequency the fur- ther away from the mean the data stray. If you were to draw this product data set in graphical form, it would take the shape of a bell, which is why a normal distribution is also called a bell- shaped curve. Another thing that is known about a normal distribution is that the relationship of the product that falls on either side of the mean is predictable. In other words, the data can be divided into groups based on the distance (de- viation) from the mean. The term standard de- viation is used to describe these groups. Every product has an optimum value, and because every process has variation, it also has a tolerance. This is defined as specification limits, with both an upper and lower spec limit (USL, LSL) surrounding the optimum value. Simply stated, when a product or process is outside of either of these spec limits, bad product is pro- duced. How well the process variation is cen- tered and contained within these spec limits is called process capability. The relationship of this

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