David,
currently I am using four SOT23 footprints. These are all reflow
patterns and vary only because of the size and spacing of the physical
device. The footprint reference we use for footprint design lists over 12
SOT23 (I went and checked it is 13) footprints. These cover metric
(typically Japan) verses non-metric parts and all of the variances due to
wide allowances in the Jedec standard.
A caveat which I didn't previously mention is that the reference
footprints we use are designed for high reliability high volume
applications. This means that they have been optimized for optimum reflow
characteristics where a change of a few mils here and there 'can' make a
difference in the reliability of the component soldering, tombstoning and
self-centering during reflow. Some of the variations that are dealt with
vary because of a 12 mil wide lead verses a 17 mil wide lead, a heel to toe
measure of 10mils verses 15mils, etc., etc..
I can picture in my mind (or look at some older designs around here)
and see your footprint, the pads would look huge and when soldered it
probably contains 2 - 3 times the optimum solder volume, but it fits almost
every SOT23 device. If it works for you and your assemblers though, that's
what counts. We do not have an in-house assembly facility so our designs
have to be optimized to a system that should work for any assembly house
anywhere in the world prior to knowing what their rules are. So we rely on
the footprint reference and expertise of it's author who developed this
system through trial, error and experimentation in a large high volume assy
house.
Sincerely,
Brad Velander
Lead PCB Design
Norsat International Inc.
#100 - 4401 Still Creek Dr.,
Burnaby, B.C., Canada.
V5C6G9.
voice: (604) 292-9089 (direct line)
fax: (604) 292-9010
email: [EMAIL PROTECTED]
www: www.norsat.com
-----Original Message-----
From: David Cary [mailto:[EMAIL PROTECTED]]
Sent: Monday, February 26, 2001 11:06 AM
To: Multiple recipients of list proteledausers
Subject: Re: [PROTEL EDA USERS]: My Biggest Mistakes
Dear board designers,
<SNIP>
Brad Velander <[EMAIL PROTECTED]> on 2001-02-26 11:15:56 AM mentioned:
> Would all SOT23 devices fit one standard/global SOT23
> footprint? Not a chance, in the professionally designed footprint standard
> that our company uses there are over one dozen variations of a SOT23
> footprint to cover all manufacturers and devices that they have
encountered
> over the life of their footprint design standard. Need a SOT23 footprint?
> You had better pull out the manufacturers datasheets and ensure that you
> have got exactly the right footprint for that manufacture and that
> manufacturers particular part type (this does not even include the fact
that
> some manufacturers alter the pin numbering scheme around on SOT23s). In
some
> cases, a standard SOT23 footprint is not even a reality across one
> manufacturers part offerings, depends on the packaging facility that
> packaged the dies.
I have only 2 SOT-23 footprints in my footprint library (IR reflow and wave
solder), both with the pins labeled
3
=====
1 2
(top view).
If a manufacturer has some wierd pinout on a SOT-23 device; I just re-alter
it
back to (my) standard. Then I make a new schematic symbol, and fix up pin
1,2,3
on the schematic symbol, *not* the footprint, so I can link that schematic
symbol to my standard SOT-23 footprint.
Am I missing something crucial here ? Have I just been lucky that my one IR
reflow SOT-23 footprint happens to fit every SOT-23 device I've used so far
?
<SNIP>
--
David Cary
* * * * * * * * * * * * * * * * * * * * * * * * * * * * * * *
* This message sent by: PROTEL EDA USERS MAILING LIST
*
* Use the "reply" command in your email program to
* respond to this message.
*
* To unsubscribe from this mailing list use the form at
* the Association web site. You will need to give the same
* email address you originally used to subscribe (do not
* give an alias unless it was used to subscribe).
*
* Visit http://www.techservinc.com/protelusers/subscrib.html
* to unsubscribe or to subscribe a new email address.
* * * * * * * * * * * * * * * * * * * * * * * * * * * * * * *
________________________________________________________
To leave the EDAFORUM discussion list, send a email with
'leave edaforum' in the body to '[EMAIL PROTECTED]'
More Information : http://www.dolist.net
