In my experience the metric units are the more exact dimensions,  with the inch
units being only a close approximation.

Found this when using a Zilog VQFP once.  Board came back,  and part
would only line up on pads about 3/4 way down one side of the package.
Looking closer at the mechanical data it said "controlling dimensions MM".
After converting inches to mm,  and looking at the various dimensions
very closely for awhile,  I began to see why this was....the inch
dimensions had been rounded up or down to just 3 decimal places.  (ie. .020
or something,  where the actual number was like .0196...)  It was
close enough for a few pins,  but when multiplied by 25 pins on a
side,  It was enough to cause pad to pin misalignment




---Phil

DH> It appears to me that no one is very consistent
DH> about the use of metric/"english" measurements in their package definitions 
DH> (specifically with reference to pin pitch).  For example, on QFPs the same 
DH> manufacturer will refer to a part as having a 25 "mill" (0.025 inch) pin 
DH> pitch and then provide a package drawing in metric showing a pitch of 0.65 
DH> millimeters "Typical"



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