Datum and insertion question

[Jackie Oh!]

Hi again,

As you know, the Orcad Layout Library Manager always puts pin one at 0,0 and then you can put the insertion origin wherever you want.

I am working with a relatively new library that has about 250 components in it. They all (including the SM parts) have the datum at pin 1 and then the insertion origin in the centre of the part.

I want to create new SM parts with the datum as well as the insertion origin in the center of the part. I won't change the datum of the already existing parts.

Does anyone know of any reason why I shouldn't build my parts with datum in the center of the part instead of at pin one?

Thanks in advance,

Jackie Oh!

[TonyQ]

The only reason I can think of for keeping the datum on pin 1 has to do whether you work on a grid-based design. Let's say you create a part with "modified" SO8 footprint with a lead pitch that matches a 0.1mm grid. Assuming your work environment is locked to a 0.1mm grid, placing and moving the part will always guarantee that the pins fall on the grid. Placing the datum at the center of the part may not guarantee this.