Depends on the format of the original. 0.1inch is far more accurate than 2.54mm
because 0.1 = 0.100000..............
If you go with the sequence 2.54, 1.27, 0.635 or 0.1, 0.05, 0.025 the imperial
system
is the more accurate as it has more decimal places. For high pin count devices
it
could become critical.
0.0196 inch would be a rounded 0.5 mm? but if you get your calculator out (and
you
use 1 inch = 25.4mm) it actually becomes 0.49784mm. Best bet is to toggle
protel
between imperial or metric [q] and design using the format in the datasheet. If
you
did convert it from metric, your 0.5 mm becomes 0.019685 inches and that is not
0.02
inches
The other problem is the Protel wizard requires the dimensions of the centres of
the
pads in a different style than shown on the datasheets. You have to figure out
the
extra clearance and it always pays to do a 1:1 print and drop your part on the
paper
to see if it fits ok.
_______________________________________________________________
Clive Broome
IDT Sydney Design Centre Ph: +61 2 9763 3513
8 Bayswater Dr, Homebush Fax: +61 2 9763 3409
Sydney, NSW, 2127 Email:[EMAIL PROTECTED]
Australia
Australia's Leading Semiconductor Designers
---------------------------------------------------------------
Phillip Stevens <[EMAIL PROTECTED]> on 03/06/2001 03:15:35 AM
Please respond to [EMAIL PROTECTED]
To: Multiple recipients of list proteledausers
<[EMAIL PROTECTED]>
cc: (bcc: Clive Broome/sdc)
Subject: Re: [PROTEL EDA USERS]: Confused Newbie on Footprints
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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