(2009.03.12) scott.schue...@gmail.com:
> Hey guys,
> i have some questions about teh considerations when making a package in
> eagle.  I'm working on the TPS2490.
> First:
> the datasheet shows the dimension tolerances for this part in a max/min
> format, for instance: width of the chip without pins is shown as 3.1/2.9,
> and width with pins is 5.05/4.75.  Tim, I believe in the tutorial, you
> managed this issue for the chip width (w/o pins) by simply making the line
> with which you drew the footprint have a width = to the tolerance.  Thus,
> for this chip I would make the line 0.1 mm wide to compensate for the +/-
> 0.2mm overall tolerance.  My question is how do we handle the situation with
> pin length tolerance?

Do the chip outline as a wire on layer tPlace, use a 0.2 mm line width

  ( 3.1 - 2.9 = 0.2 )mm

For the pins use rectangles on layer tDocu. The best length to use is
5.05 mm. By using the longest length (and widest width) the pin outline
will represent the worst case placement of the pin. (This is called the
"Maximum Material Condition" in mechanical engineering speak.)

> At this point, to calculate pin length I did the following:
> ((Min. width w/ pins) - (max width w/o pins))/2 = 0.825mm
> As i think about it though, I'm not sure this is the right thing to do as I
> am unsure as to what would should constitute the worst case scenario as far
> as dimensions are concerned.
> Also, once the appropriate pin length is calculated, where should it be
> placed with respect to chip width?  in other words, should the edge of each
> pin be placed at the min (2.9mm/2) from center, the median (3mm/2) from
> center, or max (3.1mm/2) from center?

There is more than one way to do it but it seems to me the most useful
thing to do is to put the outer end of the pin at the maximum position
and the inner part of the pin at the _nominal_ position, in this case
that would be 3 mm.
> Second:
> the datasheet also includes dimensions for gauge plane and seating plane.
> Do these need to be taken into account, and if so, how do I do that in
> eagle?

Nope, for our purposes only 2-D stuff needs to be entered into Eagle.

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