Electronic – the relevance of “typical values” in datasheets

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Of course you have to pay attention to min and max values! This should be self-evident. Every manufacturing process has variations. There is no such thing as a fixed exact dimension. With the min/max values, the manufacturer is telling you what range the parts you will receive will fall within. You could get a whole batch right at one end or the other of that range. If you want your board to be reliably produced, it has to work properly with parts anywhere within that range. This should be obvious.

For example, if some surface mount chip is specified with a nominal .3" distance between the ends of the pins with a 10 mil tolerance, what exactly does the .3" spec mean? Nothing. It's only there to give you a quick conceptual idea of the size, but otherwise they are saying the size falls from .290 to .310 inch. Let's say you center the chip at the origin when defining the package in your CAD system. That means the absolute coordinate of the ends of the pins will be half the width. The ends could extend as far as .310"/2 = .155" from center. Of course there will always be some placement error and you want the pad to extend a little past the end of the pin for a good solder miniscus and possibly to ease manual soldering in case you ever need to. My general rule of thumb is to add 20 mils for all the above. That's a general rule. There are reasons you might want a bit more or less depending on how the chip will be placed and soldered and some details of the process. In this example, we'll go with the general 20 mil rule. That means the ends of the SMD pads need to be at the absolute coordinate .175".

The same logic applies to the inner edge of the pad. Again you look in the datasheet and find the minimum distance from the center of the chip where the pin can come down and touch the board. On the inside, you don't need 20 mil of additional length. I usually add about 10 mil, although particular processes may dictate something different.

Using this logic, you end up with a footprint that will nicely accept any chip the manufacturer sends you. If it's a standard footprint you may only want to enter it into your CAD system once (at least for the general case). There may be slight spec differences between manufacturers. It is a good idea to check datasheets from a few different manufacturers for the same supposedly standard footprint. This gives you a better idea of the true variation out there. Usually they are close, but there can be small differences.