kicad-developers team mailing list archive

[Lorenzo Marcantonio <l.marcantonio@xxxxxxxxxxxx>]

On Tue, Feb 11, 2014 at 11:40:29PM +0100, Nick Østergaard wrote:
> In addition, I think that the default shape of the pads should be
> rectangular, that is "*oval": "False". I have never seen a datasheet
> suggesting oval pads for a QFP. I did not add that to the patch since that
> was not the problem I wanted to fix and it can be selected by the user
> anyway.

There is a (somewhat involved) IPC rule defining if the pad should be
rectangular or oval: in short if the pad have to be trimmed to keep side
clearance or underfill it has to be rectangular, otherwise it should be
oval (rationale: round corners have better mechanical properties, but if
you trim you want to keep all the attachment surface available).

So, more or less, if pitch is < 1 mm *or* standoff height (the A1 JEDEC
dimension) is < 0,15 mm pads should be rectangular. In short every
modern component has low standoff so it need to trim for underfill…

However when/if IPC7351C become standard and widely applied there will
be a resurgence of oval pads :D

Even shorter explanation: if you don't do high reliability board, just
use rectangular pads; otherwise that would be the SMALLER of your problems:D


-- 
Lorenzo Marcantonio
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