I just found something that changes what I thought I knew. I have a
PDF of an IPC magazine from 2005 where they are touting a "leap
forward" in land pattern generation. An illustration showing pin 1
in the upper left for SOT components is what I used as my
reference. That and the post in the FreePCB forum of a normally very
reliable source. But I found a copy of IPC-7351 and it clearly says
that for SOT and most other IC parts, the original rotation is with
pin 1 in the LOWER left. That is what FreePCB does in the library
editor by default.
Are you telling me that EIA defined a different original
rotation? It makes no sense to have two competing specs for
something that should be universal. What is the EIA spec
number? The only related EIA spec I can find is for the carrier tape.
Rick
At 05:14 PM 8/15/2010, you wrote:
Rick Collins wrote:
you still have to decide if the bottom side is
viewed from the top or the bottom.
Always take the viewpoint of looking down at the board
as it physically would be, never as a see through image
is what they want.
When I have asked assembly houses about what they assume as
convention, I never get an answer. They just tell me that they
need the X and Y data along with the side. They basically figure
out or at least verify the rotation data for themselves.
Is that what you find?
Yes, they don't get all the info from the design drawings.
They're used to having to figure out some of it from a single hand
assembled prototype, or notes, or photos of a prototype.
It just seems very odd that there is no accepted
and widely used convention for rotations. I found info from IPC
that says pin 1 in upper left corner is 0 degrees for ICs. But
I've seen nothing that addresses how to spec the bottom side components.
They are assuming you mean "look at the bottom side of the board
and pin 1 in upper left corner is unrotated."
They do NOT want directions explained as if you
"See through board with X-ray vision".
Besides IPC there's EIA and they're different on package origin points.
Fabs verify these things by looking at them.
John
--
Ecosensory Austin TX
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