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16 The PCB Magazine • November 2017 erage leads per component on a log-log graph, the PWB wiring density in inches per square inch (or centimeters per square centimeters) and assembly complexity (in leads per square inch or leads per square centimeter) can be cal- culated. The assembly density is just the X-ax- is times the Y-axis. The PWB density was de- rived by assuming an average of three electri- cal nodes per net and that the component lead was a node of a net. The result was an equation that says the PWB density is β times the square root of the parts per square inch times the aver- age leads per part. β is 2.5 for the high analog/ discreet region, 3.0 for the analog/digital region and 3.5 for the digital/ASIC region: PWB Density = β √[parts per sq. in.] x (avg. leads per part) In Figure 3, the assembly complexity lines cross the wiring density lines. At high discreet levels, less wiring is required for the amount of assembly density. At high ASIC (and low dis- creet) levels, much more wiring is required to connect the components. This makes assembly metrics like leads per square inch a good indica- tor, but not adequate to substitute for the PWB wiring density. Metrics are an important part of solutions creation. If we can't measure something, it's very difficult to improve it. EDA Tools Need Additional Functionality Current design tools do not provide the analysis and advice on the challenges of fine pitch and increasing density. This forms a real obstacle for implementation of high density in- terconnects. One reason may be the absence of density metrics. Performance measures are need- ed for: the difficulty to assemble surface mount- ed components with fine-pitch; the amount of density required on the printed circuit to mount all these fine-pitch devices in the area provided; and the degree of sophistication of new compo- nents with faster rise-times. Maybe the problem is that I am approach- ing my 50 th year in printed circuits. It's been fun and I look forward to what each day may bring. For the last 35 years I have been involved in the PCB design process. I have used CAD systems to design boards, provided advice for designers on their boards and worked on CAD tool de- velopment. Currently, I am involved in the de- velopment of prediction tools to allow the ac- curate estimation of design rules, board struc- tures and costs from schematic and part infor- mation before the board is physically designed. This, along with new design metrics, allows for the optimization of the board. This optimization follows a major theme for many electronics companies—that of Total Quality Management (TQM). TQM can be ap- plied to the PCB design process by acknowledg- ing that the next step in the process after design is PCB fabrication and assembly. These, then, are the customers of the design process. And a major measure of performance by those cus- tomers is quality. Quality of design is interpret- ed by manufacturing as producibility. Unfortu- nately, for CAD tools, producibility is not one of their features! When I once asked a CAD com- pany executive about his definition of "quality of design," he talked of the number of software bugs per 1,000 lines of code. He refuted the idea that CAD tools had any responsibility for the TQM process. In the last 35 years, my opinion of the PCB design process is, never has so much been done by so few, to build so many electronics products, to such little recognition! It's like looking at a chasm between two thriving plateaus. On one side are the areas of engineering excellence, concurrent engineering, CAE and state-of-the-art compo- nents. On the other side of the gulf is progres- sive manufacturing (PCB fabrication, assembly and test). Bridging them takes a leap of faith by PCB design. Falling short means plummet- ing into the abyss. We need to build sturdy, reliable, predictable "bridges" over the chasm. But I don't see any- one doing it. Having faster, cheaper CAD tools with more features is a necessary piece of the bridge, but in itself will not create those bridg- es. In fact, if they continue to increase in com- plexity, they may become a part of the prob- lem. The last papers I read on predicting wiring were written in 1990. I have not come across any more current work. I have read papers on the CAD tools, on their autorouters and on the software architecture, but nothing on the pro- cess and planning of PCB design. 35 YEARS OF HDI FABRICATION PROCESSES AND OBSTACLES FOR IMPLEMENTATION