Defective parts per million

The process capability index, Cpk, is related to the so-called process sigma such that a 6a process corresponds to a Cpk of exactly 2.00, or 2.0 defective parts per billion (PPB), assuming N (0, a) quality variance distribution (can alternative calculation for process sigma estimates 3.4 defective parts per million for a 6a process). Examples of the correspondence between Cpk, process sigma, and defect rate for N(0, a) distributions are shown in Figure 2. The process capability (based on observed yield) of pharmaceutical manufacturers has been cited by some benchmark studies to be roughly 0.7 (2.1a) [28]. 

Nevertheless, it should be understood that, in their safety achievements, a few companies have done better than Six Sigma. Assume that an OSHA recordable incident is the defect to be measured. How does 3.4 defective parts per million relate to an OSHA recordable incident rate OSHA rates are computed from a base of 200,000 hours worked. To be at an OSHA incident recordable rate of 3.4 incidents per million hours, the computed rate using a 200,000 hour base would 0.68. That rate has been bettered by a few of the best performing companies, but only a few. Thus, on occasion, humans are capable of doing better than Six Sigma. 

The statistics for quality have been around for a long time. Common performance measures include Pp, Pp, Cp, Cp first-time capability (FTC), line speed, and defective parts per million (ppm). This section provides an overview of common process capability terms and addresses their importance in supply chain management. 

Quality levels are the result of a ratio of parts defective to parts produced. The current trend is to use parts per million (ppm) but this is not always practical for some processes. Painting processes for instance cannot achieve blemish-free surfaces in the order of one blemish per million parts painted ... 

The inherent limitations of attribute data prevent their use for preliminary statistical studies since specification values are not measured. Attribute data have only two values (conforming/nonconforming, pass/fail, go/no-go, present/absent) but they can be counted, analyzed, and the results plotted to show variation. Measurement can be based on the fraction defective, such as parts per million (PPM). While variables data follows a distribution curve, attribute data varies in steps since you can t count a fraction. There will either be zero errors or a finite number of errors. 

The requirements placed on the performance and reliability of CVD coatings are continuously upgraded. For one thing, this means the need for an ever increasing degree of purity of the precursor materials since impurities are the maj or source of defects in the deposit. The purity of a gas is expressed in terms of nines, for instance, six nines, meaning a gas that is 99.9999% pure, which is now a common requirement. It is also expressed in ppm (parts per million) or ppb (parts per billion) of impurity content. 

The quantum defects of the n = 10 states of He are given in Table 18.2. The quantum defects for the s and p states are derived from optical measurements, specifically from the term energies given by Martin.26 The quantum defects of the i = 8 and 9 (10 and 10m) states are calculated using Eq. (18.2), and the quantum defects of the 2 < i 7 states are obtained from the calculated i = 8 quantum defect and the measured intervals reported by Hessels et a/.15 The quantum defects given by Table 18.2 are by no means indicative of the possible precision of the measurements. For example, Hessels etal. have reported measurements of n = 10 intervals which have precisions of parts per million.15... 

The reliability parameters, such as the mean time to failure, have to be determined in experiments under well defined conditions. Failure rates of microsystems for automotive applications are typically in the range of a few ppm (parts per million). This may sound negligible, but due to the large number of sensors sold every year and their increasing numbers in each car, even this failure rate must be decreased further. However, the engineer who tries to investigate failure mechanisms is confronted with the problem of lack of failures in the sense that he finds too few defective samples for a thorough failure analysis. Thus, due to the lack of a statistical basis, the quality of lifetime predictions under normal in-use conditions would be poor. 

Volume defects include voids and local regions of different phases, such as a precipitate or an amorphous phase. In Si, oxygen precipitation is the most important volume defect. Silicon crystals are grown by either the Czochraski technique or by the float-zone technique. The typical concentration of oxygen in Czochraski Si crystals is about 10-20 ppm (parts per million) or 5 x 1017-1 x 1018 cm-3. The float-zone technique introduces less oxygen in Si than does the Czochraski technique. Most of the oxygen in the as-grown crystal is atomically... 

Three of the old measures used to evaluate suppliers were cost, delivery, and quality. Qrrality is measured as parts per million (PPM) defective and is reported to the supplier on a regtrlar basis. Delivery is based on materials being received on time. The number of shipments is used for the measurement in this area. Price is what was agreed to in the contract. In most cases this is the cheapest one. These were good measures, but as competition gets stronger we will need to get better and our measures will have to change. 

Caution must be exercised in interpretation of the physical data for the tetracyanoplatinate complexes (as well as all other one-dimensional systems) because purity and morphology are extremely critical for one-dimensional systems. For example, a 1.00 x 0.01 x 0.01 mm perfect needle crystal of K2Pt(CN)4Xo.3 would contain — lx 10 parallel strands each of 3.5 x 10 collinear platinum atoms. Thus, purity (foreign impurities, end groups, and/or crystalline defects) levels of one part per million indicate that each strand averages more than three defects, which may drastically alter some (and in particular transport) measurements. Besides the intrinsic purity problem of one-dimensional systems, the physical properties of K2Pt(CN)4-Xo.3(H20)a are a strong function of hydration. Dehydration alters the crystal structure and thus properties of the complexes (78). Care must be maintained to ensure that dehydration is not caused by the measurement technique. For... 

What is Six Sigma It presents an outstanding quality assurance standard. Envision a normal distribution chart, a bell curve. Variability from the mean, the center point of the distribution, is measured in units called sigma, which is defined as the standard deviation. At plus or minus three sigma, three standard deviations, 99.7% of a population would be included. In a manufacturing process using a three-sigma standard, approximately 2700 parts per million could be defective. 

If a quality performance standard of Six Sigma is achieved, at plus or minus six standard deviations from the mean, it can be expected that no more than 3.4 parts per million will be defective. Four-sigma capability is ten times better than three-sigma capability five sigma is 30 times better than four sigma and six sigma is seventy times better than five sigma. One can appreciate the stretch in the quality standard adopted. 

A Six Sigma capability means no more than 3-4 parts per million defects. Recently Six Sigma programs have become more general in their approach, reflecting overall efforts to make improvement as well as error-free production. 

Parts per million (PPM) The number of failures in one million parts. A statistical estimation of the number of defective devices, usuaUy calculated at a 90% confidence level. 

Process capabihty Cp is dehned as the specification width divided by the process width. The main assumption for all of this is that many physical processes have sample distributions that are described by the Gaussian or normal distribution. A second assumption is that the 3process normal distribution describe the process width. If the process width coincides with the specification limits, then the Cp = l.O. This is somewhat arbitrary but has been the standard usage for many years and continues to be accepted. If Cp = 1, the defect rate is approximately 2700 parts per million (ppm), a generally unacceptable defect rate for most products in today s competitive marketplace. If the center of the process is not coincident with the center of the specification width, the process capability is reduced and must be calcnlated by Cpic = Cp l — K) where K is the difference between the target value and the process mean divided by one-half the specification width. 

Defects are defined as those artifacts that are outside the window of acceptable attributes for the finished circuit board assembly. Thus, defects are not limited to the solder joints, specifically, but can also include damage to the circuit board material as well as degradation to component structures (e.g.,molding compound, leads or terminations, etc.). Defect types and their allowable frequencies (often expressed in parts-per-million solder joints or product units) vary with the different assembly processes and applications. Therefore, product drawings, in conjunction with industry standards (e.g., IPC-610), are used to establish accountable defect types for printed circuit boards. 

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