On 02:18 PM 25/06/2001 -0700, Abd ul-Rahman Lomax said:
>At 09:36 AM 6/25/01 +0100, Andy Gulliver wrote:
>>My experience of metric-specified parts on an imperial grid has led me to
>>create the PCB symbols on a 1-mil grid, rounding the *absolute* co-ordinates
>>of each pad to the nearest mil. (The use of absolute co-ordinates is
>>critical to avoiding cumulative rounding errors, which can be significant on
>>even medium-sized parts). The positional variation from this method is
>>small, and the time taken to create the symbol is more than saved when it
>>comes to tidying up the autorouter's efforts!
>
>I'm not sure I understand why the use of absolute coordinates improves the
>situation. Perhaps I should simply say I don't understand. I'm not even
>sure what Mr. Gulliver means by "absolute coordinates" as it relates to
>footprints.
I read Andy's comments as meaning - work out each correct pad location as
accurately as possible and then round (each individually) to the nearest
mil. Do not simply work out nearest whole mil pad spacing and place all
your pads on this spacing. The errors will rapidly accumulate such that at
the end of the row pads and leads do not correspond.
Ian Wilson
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