Printed Circuit Design & Fab - March 2009 - (Page 21) co-desIGn throughout the 1990s up until now has improved the situation ‘373 (octal transparent latch) and ‘374 (octal D-type flip-flop) but it’s a double-edged sword: respectively, that took care of the address de-multiplexing. A schematic snippet of the address latch from a typical design is ■ We do not deal with multiplexed buses as often, but instead shown in fIGure 1. we have numerous extra connections coming out of the cenThe ‘373/4 was a classic case of a horrible pinout. The ter of the package (i.e., BGAs). schematic looks great, but when you bring it over to the PCB, ■ The more complex packaging requires extra layers from the you have a lot of crossovers and a tough time routing it as seen PCB for fan-out, finer tolerances for manufacturing, extendin fIGure 2. A common trick to mitigate this was to change ed layout and routing times and challenges with yield. These factors result in higher product costs to the consumer. the order of the flip-flops on the schematic by manual pin To mitigate current and future tradeoff problems, there are a few swapping, creating a library of alternatives. In the 1980s and approaches that can be taken. 1990s, PCB design tools were very limited, and none had pinIf you have the luxury of developing the PCB in concert swapping automation. This problem often resulted in confusion or error, and in the worst case, the program memory was not read in the correct sequence, resulting in a re-spin. So as the 74xx series evolved, easier devices were released, largely owing to silicon processes allowing an extra layer of die connections. Then the pinout improved to favor the PCB. fIGure 3 illustrates the newer ‘574 device (IC6). Note the pin numbering sequence as compared to Figure 1. Naturally, it made for a much faster job routing the PCB and resulted in a neater design with lower via count required. Hence, overall product cost was reduced. The updated PCB is shown in fIGure 4. IC6 is shown semi-transparent in 3D to reveal improved routing. These innovations – including surface mount – significantly improved designs where component density was paramount. But to obtain the density required for certain applications, it was necessary to eliminate the package altogether. Making use of chip-on-board and wire bonding presented a whole new chalProtoMat® S-Series lenge. Other hybrid technologies, such as PCB Milling Machines thick film printed components, were also Electrical engineers agree: with a Protomat S-Series a challenge. Higher levels of integration prototyping machine at your side, you’ll arrive at the in silicon proved to make these methods best solutions, fast. These highly accurate benchtop short-lived with hybrid circuit assemblies PCB milling machines eliminate bread-boarding and being popular through the mid 1980s, yet allow you to create real, repeatable test circuits— they are making a comeback in medical including plated vias—in minutes, not days. and aerospace today. Co-Design Today ECAD tools need to be flexible in order to cater to such specialized applications. Most tools allow customization via scripting to add functionality that is peculiar to their product or industry, but tools that have a more abstract and flexible approach at viewing and editing design data make it easier and can remove any need for customization. Development of denser packaging MARCH 2009 • Declare your independence from board houses • Affordable, entry-level price tag • The best milling speed, resolution, and accuracy in the industry • Single-sided, double-sided, and multilayered machining without hazardous chemicals • Optional vacuum table and autosearch camera for layer alignment For complete details visit: www.lpkfusa.com 1-800-345-LPKF or call: PRINTED CIRCUIT DESIGN & FAB 21 http://www.lpkfusa.com http://www.lpkfusa.com
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