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Re: gEDA-user: Thermals on Pads

On Sun, 2011-01-30 at 16:37 -0500, rickman wrote:
> What geometry problems do you have?  There are plenty of references
> in 
> regard to thermals.  I don't recall seeing any other than bridges
> that 
> span a uniform gap around the pad.  The only variation I can recall
> is 
> the number and rotation angle of the pattern.   But most, if not all 
> that I have seen use four bars either along the x and y axes or at 45 
> degree angles.  I think there are even some built in commands for
> this 
> in the RS-274X Gerber file spec.

Firstly, the general shape is not so much the problem, but dimensions of
the shape.

Second - PCB calculates what it thinks the thermal finger width should
be, but actually calculates the geometry wrong - so the thermal you see
is not what the calculation PCB does suggests it should be

This is evident at certain clearance sizes, where the thermal cutouts
start to overlap and cause polygon problems.

At some geometries, the general shape is wrong - we need to restrict
some of the dimensioning to keep the thermal shape valid, or add
routines for a different thermal shape in these cases.

Peter Clifton

Electrical Engineering Division,
Engineering Department,
University of Cambridge,
9, JJ Thomson Avenue,

Tel: +44 (0)7729 980173 - (No signal in the lab!)
Tel: +44 (0)1223 748328 - (Shared lab phone, ask for me)

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