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Design007-Nov2018

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78 DESIGN007 MAGAZINE I NOVEMBER 2018 the proper hole size. Component holes should be sized correctly to allow 12–16 mils in diam- eter larger than the component pin at maxi- mum material condition (MMC). MMC is the condition where the hole is drilled at the low end of the tolerance range and the pin mea- sures at the high end of the tolerance range. Pin locations should be placed at nominal locations or the basic dimensions shown on the datasheet. 3. Datasheets Can Disagree with CAD Software Third-party CAD libraries can contain mil- lions of different parts, so it's no surprise that a few bad apples lurk among them. When the datasheet and library part don't match up, your project is dead before you make the first connection. Always check any library part for accuracy before you use it the first time. The datasheet for a part usually tells the real story—a long story, but one worth read- ing. Some datasheets can run upwards of 200 pages, and it's just a few lines that provide you with the information you need to make crucial decisions about sizing. It's important to read and comprehend the datasheets, so problems in CAD don't lead to the wrong sizing and spacing on the PCB. 4. Pay Attention to Pinouts When Using Alternative Vendor Parts Even if you've paid attention to pin and through-hole size and made sure your solder joints are good, a part can still not work as expected. Similar parts with the same footprint might look like they should act identically, but they won't always have the same pinout. Sure, each transistor has a gate, drain, and source, but different manufacturers can vary in what goes where. A Motorola part can differ from a Texas Instruments (TI) part, and if you're buy- ing generics, all bets are off. The same basic component will come in multiple packages. Sometimes the variations are tossed into the back of a datasheet as an afterthought, but these can be critical. Similarly-named pack- ages can even come in different widths. 5. Be Aware of Mechanical Fit It's not just the footprint and through-holes that you need to pay attention to. The physical size of your component body can keep parts from fitting into designated spaces. Again, pay close attention to the MMC size and the tol- erance range—both are critical. As parts get larger, or you start sourcing your parts from multiple vendors, your footprint size may need to expand considerably to accommodate all of the dimension and tolerance variables. When combining multiple part body dimensions, always take the largest dimension, or you could end up trying to violate the Pauli exclu- sion principle, which states that two identical fermions (particles with half-integer spin) can- not occupy the same quantum state simultane- ously. In short, you are going to smash some- thing. 6. Datasheets Are Helpful but Do Not Replace Experience When it comes down to it, the datasheet is considered king, but that doesn't mean you shouldn't question it when necessary. Many problems with fit can be counterintuitive or difficult to diagnose, so understanding size requirements will prevent a lot of headaches. Every dimension needs to be looked at, from pin and through-hole size to capacitor height. Frequently, these issues will crop up when replacement components are used. Watch those tolerance ranges carefully. When you evaluate replacements, make sure your replace- ment parts fall between the minimum and maximum measurements stated in your origi- nal part datasheet. You could get away with parts that are smaller in some regards, but this should be carefully evaluated. And don't for- get to pay attention to pinouts and alternative packages. DESIGN007 Bob Tise and Dave Baker are engineers at Sunstone Circuits. To read past columns or contact Tise and Baker, click here. Bob Tise Dave Baker

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