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IPC-7351 gives a pin out and a 0 degree rotation for SOT23-3 packages; but, IC vendors use many different pin numberings.

When I make my library component should I use the IPC pin numbering for all of my parts or use the datasheet pin numbering for each part?

I can put functional labels and put the part body outline in the schematic symbol to reduce confusion there.

I am most concerned about confusion at the board assembly house. It seems to me that they would prefer consistent IPC numbering and rotation. But they might look at the datasheet or some other info from the manufacturer resulting in confusion.

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  • \$\begingroup\$ As long as the correct wires get connected to the correct pins in your layout, it doesn't matter. The assembly house isn't going to get confused about how to rotate a SOT23-3 because it doesn't have the same number of pins on the two sides. \$\endgroup\$
    – The Photon
    Dec 22, 2016 at 21:54
  • \$\begingroup\$ Of course they will eventually get it oriented correctly; but, I think that there is an advantage to having the placement file reflect a consistent rotation. The IPC defines 0 rotation as pin 1 in the upper left quadrant - what happens if you have a different pin 1? \$\endgroup\$
    – PaulB
    Dec 22, 2016 at 22:01
  • \$\begingroup\$ Ask the board assembly house. There may be odd numberings for SOT-23's, but normally pin one and two are on one side, and pin three is on the other side. When pin three is facing to the right, viewed from the top, pin one is upper left, and pin two is lower left. \$\endgroup\$
    – user57037
    Dec 23, 2016 at 6:56
  • \$\begingroup\$ Post a link to this part, let's check out the pin arrangement for you. \$\endgroup\$
    – Neil_UK
    Dec 23, 2016 at 7:35

2 Answers 2

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Pin numbering is for you and will show in the schematic. It is irrelevant to a assembly house in this case since the package is not symmetric.

In cases where a package is symmetric, like a SOT23-6 for example, you have to show where pin 1 is supposed to go, and the package will have a marking showing pin 1 also. With such packages, there is no ambiguity how the pins are numbered. They start at 1 at some place that is specially marked, then go around from there counter-clockwise when viewed from top. This standard has been around since tubes, which were numbered going around clockwise when looking from bottom up, starting at some specially marked place, a gap, or something.

You are over-thinking this.

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    \$\begingroup\$ I may be "over thinking" but I am stickler for manufacturing data files being correct. What about the rotation angle in the placement file? IPC generally specifies 0 rotation as pin 1 in the upper left quadrant. Should I use the datasheet's pin 1 or the IPC's pin 1 to determine 0 rotation. \$\endgroup\$
    – PaulB
    Dec 22, 2016 at 22:08
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You ALWAYS use the manufacturer's data sheet. If it differs from the industry standard 'pin 1 in the top left corner, next to the index mark, increasing anti-clockwise round the package', then put a warning note on the schematic in text.

But first, does the part really have a non-standard pinning? Have you read the data sheet correctly? Have you confused a pins-up with a pins-down drawing? Has the data sheet been published with an error (it can happen). Check the pinning of some pin-compatible parts from other manufacturers, or contact the manufacturer. Once you are sure what the data sheet says, use that. I have had 'function' compatible parts that would work on the same footprint, with different pin numbers from different manufacturers, that required a table on the schematic to avoid confusing myself as well as others.

As in everything in life, the specific (data sheet) always supercedes the general (the standard).

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