PCB007 Magazine

PCB-Nov2014

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52 The PCB Magazine • November 2014 yield somewhere between 93 and 99%—the old three sigma complacency. Statistics are a won- derful tool, but like most things in life, you will only get out of them what you put in. It all boils down to what the organizational objective is; superficial window dressing or honest-to-good- ness improvement. Inflating process yields by excluding things like rework, customer waivers, or returns does nothing but mask problems and will not result in true improvement. If window dressing is indeed your goal, then I would sug- gest tossing this article and immediately pick- ing up a copy of Extinction for Dummies, by Peter T. Platypus. Why 99% is Not Good Enough I remember the not-too-distant past when a 99% yield rate would earn bragging rights (myself included). Looking at what that real- ly means by today's metrics shows that a 1% scrap rate converts to 10,000 defective parts per million (DPPM). As a customer, imagine a supplier striving to give you only 1% defective parts! World-class six sigma levels allow only 3.4 DPPM. If you are still reading at this point, I would hope that you agree with me that 10,000 DPPM is totally unacceptable and are prepared to do something about it. (Note: It must be mentioned that the 3.4 DPPM attributed to six sigma levels was devel- oped by Motorola, and based on the assump- tion that over time, a process is likely to have a shift in the mean of up to +/- 1.5 sigma. This potential shift is factored into the 3.4 DPPM. Statistical purists would argue that a six sigma level is actually .002 DPPM, but since the Mo- torola interpretation is universally accepted, I use 3.4 DPPM to represent a six sigma level.) When companies like Motorola and Gener- al Electric began communicating six sigma ex- pectations to their suppliers in the early 1980s, what began as a ripple quickly developed into a shockwave throughout the supply chain. To say that this concept was met with some resistance is a monumental understatement. Companies had absolutely no idea how they were going to effect a change of such magnitude that their process defect rate would drop from 10,000 to 3.4 DPPM. Through a slow and painful process, companies began to understand that the way to achieve these quantum paradigm changes was through Lean best practices. The interesting BEST PRACTICES 101, PART 4 continues figure 1: life at 99% Good. (source: elusive lean)

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