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92 PCB007 MAGAZINE I OCTOBER 2024 mechanical CAD soware. When used in the design and manufacturing engineering pro- cess, there are four levels of measures applied to a product: • Metrics: Both the product and the process are measured by physical data using SPC and TQM techniques (Predictive engi- neering process) • Figure of merit: Both the product and the process are scored by linear equations developed by consensus expert opinion (Expert opinion process) • Opinion: Opinion, albeit from an expert, is applied aer or concurrent with design (Manufacturing engineering inspection process) • No Opinions: No attempt to inspect or improve the design is done during the specification, partitioning, or design stage (Over the wall process) e critical parameter in the calculation of FPY (Figure 2) was the metric called com- plexity index. is is a dimensionless parame- ter that I call a pseudo-independent variable: It replaces a number of important independent variables that would be complicated to take individually. is variable is invented, but it follows several pseudo-independent variables that are essential for modern engineering: the dimensionless numbers. For example, you might remember the Reynolds Number 2 , a measure of inertia force divided by viscous force (Figure 3) used in fluid flow calculations. e Reynolds Num- ber was first described quantitatively in 1883. Reynolds found that fluid velocity (l/t), fluid density (m/l), fluid viscosity (ml/t), and pipe diameter (l) determined the nature of pipe flow. e four variables are combined into a single dimensionless parameter. In fact, there are 154 dimensionless parameters Figure 2: First pass yield (FPY) of PCB fabrication as a function of PCB design complexity.