By definition, 1 inch = 25.4mm exactly. This was standardised
some decades ago.
It's when rounded numbers are used that problems occur.
Cheers,
John Haddy
> -----Original Message-----
> From: TSListServer [mailto:[EMAIL PROTECTED]]On
> Behalf Of [EMAIL PROTECTED]
> Sent: Tuesday, 6 March 2001 10:44 AM
> To: Multiple recipients of list proteledausers
> Subject: Re: [PROTEL EDA USERS]: Confused Newbie on Footprints
>
>
>
>
> 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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