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Design007-Apr2020

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36 DESIGN007 MAGAZINE I APRIL 2020 Feature by Rick Hartley R HARTLEY ENTERPRISES Editor's note: During a recent conversation with Rick Hartley, he shared one of his favorite PCB design horror stories. This is a cautionary tale about what can happen when design teams place too much faith in app notes and do not follow cost-aware design techniques. Enjoy! When I first went to work at a company in 2003, I was asked to look at a circuit board that was not working properly; the few they did get to work had major EMI problems. I discovered they had placed the parts badly so that routing the memory lines to be even close to a similar length was going to be nearly impossible. The engineer who did the schematic looked at the app note for the particular memory that was used with this processor, which stated, "Route all the memory lines within the same length of one another at ±50 mils." The engi- neer thought, "50 is good, and 25 is even bet- ter," so he put a note on this schematic that the memory lines should all be the same length at ±25 mils. What's really interesting is that en- gineers often don't understand that inner and outer layers propagate at different rates. If you make all the lines the same length, they don't have the same propagation time, and they don't match anyway. Then, they farmed out the layout. And the people who did the layout—because of the poor positioning of components—couldn't get all the lines routed anywhere close to the same length without a ton of tromboning or serpen- tining of the memory lines. They ended up with 14 routing layers, and the only way they could get it to the thickness required was to make it an 18-layer board. They had an 18-lay- er board with only four plane layers and 14 routing layers. Anybody who knows anything about design is already thinking, "How could this possibly work?" It all happened because of this overabun- dance of length-matching. To make matters worse, they had to have 65 ohms impedance on some of the lines because there was a PCI bus on the board. In order to get the dielectric constant—with these very thin dielectrics— low enough to hit the target impedance, they had to go to Rogers 4000 series material (at 6 to 8 times the cost of FR-4). The bare boards were seven by nine inches, and they were go- A Design Economics Horror Story

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