John,

Assume that you don't have any info from the mfg other than the package dimensions. How do you then come up with a footprint? Specifically, I find that the dimensions requested by Protel are typically not the dimensions provided by the mfg datasheet, so it can get confusing to figure out where the pads need to be.

As an example, I've got a 8x8 QFP (specifically an FTDI FT232BM USB chip) that apparently isn't a standard package. In going through the Protel process of creating a custom footprint, the dimensions given in the datasheet tell the size of the plastic package and the extent of the pins, but Protel wants the distance between the center of the pads from one side to the other and the vertical distance between the the centerline of the horizontal pins and the nearest vertical pin. These calculations, at least for me, seem to take several tries before I get something that looks right.

Moreover, selecting a pad size seems arbitrary at first glance, but I'm sure there are good conventions for chosing the pad size. If there are rules of thumb for determining pad size, I'm interested in knowing them, so if you know of any, lead on.

Bryn Wolfe

John Haddy wrote:

Try:

http://tsc.jeita.or.jp/eds/DATA/PACKAGE/ED731120.PDF

John Haddy



-----Original Message-----
From: Rene Tschaggelar [mailto:[EMAIL PROTECTED] Sent: Tuesday, 3 June 2003 6:51 PM
To: Protel EDA Forum
Subject: Re: [PEDA] metric footprint



Is there no footprint given in a manufacturers datasheet ? It just a minute or two to make this footprint, much faster than a websearch.

Rene






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