Well, for instance, if you ar working with the TQFP-packages with a lead
pitch of 0.5mm that equals 0.019685039 inch, and it seems that its rounded
to 0.02 inch which equals to 0.508 mm and having 20 of those pins gives at
least an error of 0.16 mm.

With best regards

Tomas Larsson
Sweden
http://www.naks.mine.nu for downloads etc.
ftp://ktl.mine.nu for uploads. Or use the free www.yousendit.com service.

Verus Amicus Est Tamquam Alter Idem 

> -----Original Message-----
> From: piclist-bounces@mit.edu 
> [mailto:piclist-bounces@mit.edu] On Behalf Of Peter Todd
> Sent: Sunday, February 19, 2006 11:40 PM
> To: Microcontroller discussion list - Public.
> Subject: Re: [EE] Metric units
> 
> On Sun, Feb 19, 2006 at 02:10:21PM -0500, Olin Lathrop wrote:
> > Tomas Larsson wrote:
> > > Imperial is a pain in the arse, when it comes to PCB design.
> > > It seems that most editors has imperial units as base 
> units in the 
> > > database, and that creates round-off problem when working 
> in metric.
> > > Since most Chip manufacturers is moving from imperial to 
> metric it 
> > > creates a problem.
> > 
> > All CAD where I've looked into this use an integer 
> representation that 
> > is a sub-multiple of both mm and mil.
> 
> Using 0.1mm as your base unit would do you just fine as the 
> inch is defined as exactly 25.4mm.
> 
> That said real machining can easilly go down to tolerences as tight as
> +-0.01mm, so you'd want am much smaller base unit than 0.1mm.
> 
> --
> pete@petertodd.ca http://www.petertodd.ca
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> http://www.piclist.com PIC/SX FAQ & list archive View/change 
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