At 12:21 PM 12/13/01 -0800, Samuel Cox \"Sam\" wrote:
>Most of the time the component is and I believe should be built with the 
>origin (Ref) at the center of PIN 1
>so that when placement is performed you have the best chance to align all 
>or the most number of pins to a
>testable /routing grid..  Since Protel does such a great job of 
>calculating the centroid for the mid of the component
>anyway.  Secretly the object of the game is to get as many pins as 
>possible on an even increment of the routing grid.
>Which should be an even increment of the placement and tester grid..

Centroid is better, in my opinion. Yes, the routing grid could possibly be 
improved by having a guaranteed pad on grid, but much more often, now, pads 
spacings are not a grid multiple so this is useless. In fact, it is worse 
than useless: try lining up a series of 0603 resistors, for example, that 
are to be oriented some with pin 1 up and some with pin 1 down. If centroid 
were used, no problem, they naturally align.

If pads are on grid, centroid will be on grid or half-grid at worst. For 
example, an SOIC will have the pads on 25 mil grid (in the direction of 
tracks that must pass between pads, which is where grid is important.

Rotating parts with centroid reference is a bit easier, they stay put. 
Discover that a diode is wired backwards in the schematic after the PCB 
design is done? With centroid reference, one rotation and you are done. 
With pin 1, you must rotate and translate. If you have a row of 100 of 
these, you can rotate them with a global edit, you will be very glad you 
have centroid reference.

Protel is to be commended for providing alternatives....

Abdulrahman Lomax
Easthampton, Massachusetts USA

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