On 09:41 AM 15/07/2004, Brian Guralnick said:
Pins 2&3 reversed???

This is my default:

  3
1  2

I think I've seen 1&2 reversed occasionally.

The only time I use something else is with SOT-23 diode & LEDs, where I use A&K & have an appropriate modified footprint.

My rule is use the pin function for SOT-23, not numbers. For this to work I usually set up a specific Sch part and footprint (BC84X for example) that uses the pin functions rather than numbers (in the BC84X case it is E, B and C) - the Sch part matcheds the specific PCB footprint.


I only use a generic SOT-23 footprint on something I don't use commonly. In this case, on the Sch I make sure that the pin designators use the functions, always (EBC, A, K, A1K2 (for series double diodes etc)). The generic SOT-23 footprint (rather than a specific derivative like BC84X) uses default numbering, which for me is the same as Brian. The Sch pins will not match the PCB pad designators and the designer will have to look up the datasheet and manually update the PCB footprint pads to match and then update the PCB again. Forces a check and works for me.

I don't think there is any foolproof method of dealing with this - you have to have systems in place that reduce the chance of error and force checking. When it is a single designer doing Sch and PCB and maintaining the libs there is not a big issue. When you have multiple people doing all the jobs it is a problem. All the small common transistors, diodes etc suffer from it. Not only do some manufacturers number their pins differently, it is possible to buy pin reversed versions of the parts in some cases (BC846R for example).

Ian




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