Design007 Magazine

Design007-Jan2021

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82 DESIGN007 MAGAZINE I JANUARY 2021 If hole sizes are too tight, pins may not fit into the holes, pins bend during insertion, or they go into the holes but may not solder well. Proper through-hole soldering requires sol- der to flow up through the gap between the pin and the hole barrel. If there is not enough space to allow enough solder mass to flow up through the hole, a cold solder joint can result and premature failure of your circuit is likely to occur. To avoid this issue, make sure that when designing, you know the pin size and tolerance range for your components. Component holes should be sized correctly to allow between 12 and 16 mils diameter larger than the com- ponent pin at maximum material condition (MMC) when possible. MMC is the condition in which the hole is drilled at the low end of the tolerance range, and the pin measures at the high end of the tolerance range. Pin locations should be placed at nominal location or the basic dimension shown on the datasheet. 3. Data sheets can disagree with CAD software. Third-party CAD libraries can contain mil- lions of different parts, so it should not come as a surprise that a few bad apples lurk among them. When the data sheet and the 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 data sheet for a part usually tells the real story, and it's usually just a few lines that provide you with the information you need to make the crucial decisions about sizing. It's important to read and comprehend the data sheets so problems in CAD don't lead to the wrong sizing and spacing on the PCB. 4. Pay attention to pinouts when using alternate vendor parts. Even if you've paid attention to pin size and through-hole size, and you've 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 iden- tically, but they won't always have the same pinout. Sure, each transistor has a gate, drain and source, but different manufacturers can differ in what goes where. A Motorola part can differ from a Texas Instruments (TI) part, and if you're just buying generics, all bets are off. The same basic com- ponent will come in multiple packages. Some- times the variations are tossed into the back of a data sheet as an afterthought, but these can be critical. Similarly named packages 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, MMC is the rule, and with maximum compo- nent body size, paying close attention to the tolerance range is critical. As parts get larger, or you start sourcing your parts from multiple vendors, your footprint size may need to expand considerably to accommo- date all the dimension and tolerance variables. When combining multiple part body dimen- sions, always take the largest dimension, or you could end up trying to violate the Pauli exclusion principle, which states that two identical fer-

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