Gary,
Re Component Orientation, or Zero Rotation - I fought Dieter and Tom on this
one long and hard, in IPC-7351, but ultimately they gave in and went with
the rule of:
"Pin 1 Upper Left in all instances, for any IC's (excepting Legacy PLCC
Packages (which per JEDEC are not to be used for any new IC design) which
have Pin 1 in Top Center)."
All 2 terminal devices will be located with Pin 1 to the Left (with Pins in
a Horizontal Plane), and all Polarized Capacitors (or other 2 Pin Polarized
devices) shall have Positive Polarity (+) to the Left, and all Diodes shall
have the Cathode to the left. Period - No Exceptions.
This means that Positive Polarity (+) of a Capacitor should be Pin 1, and
that the Cathode of a Diode should also be Pin 1.
Please note that there will always be people and software that for "Legacy",
or "personal preference", or "other" reasons, will just not want to comply
with this.
Unfortunately, many of the Libraries of today's various CAD Systems have
just gone with the Libraries inherited from previous Versions or Systems, or
simply "drawn" new Components/Devices as they most conveniently fit the
viewing area of the computer screen, which usually meant that the longest
dimension of the Component/Device went in the horizontal plane, for none
other than reasons of "drafting convenience" and the maximum utilization of
available viewing space.
The problem is that people have used any and every possible orientation for
any possible component, in the past, in what we might call "Legacy Designs"
and "Legacy Component Libraries".
This is well and good, but the problem lies in how do we standardize
"PWB/PCB Design for Automation", and in Particular for "Automated Component
Placement", or what is commonly called "Pick and Place", during Manufacture,
for the Twenty First Century.
For a while people just went for a copy of the orientation of the Device as
it was shown in the Databook/Datasheet, but then we got conflicting
Datasheets from different Manufacturers of the same Device, etc., etc.,
etc..
Even within the JEDEC Standards (which was the initial orientation used for
Components in IPC-7351) there is confusion with the orientation of similar
components.
Ultimately, everyone agreed that we needed to establish a "Standard
Component Orientation - Zero Rotation" for Components/Devices as used in
"New Design" for PWB's/PCB's. This orientation would apply to
Components/Devices as related to the Orientation of the PWB/PCB as shown in
the PWB/PWA Drawing / Fab/Assy Drawing, which establishes "Standard
Orientation - Zero Rotation" for the PWB/PCB itself.
This now allows Components/Devices to be rotated within the Design, and yet
have a Standard Reference by which to reference the amount of rotation of
that given Component/Device, for the sake of Automated Assembly.
Many of today's Components/Devices are supplied in either "Tape and Reel"
with "Orientation/Rotation" as governed by ANSI/EIA-481, or in "Trays" as
governed by various JEDEC Standards, and there are yet still those who do
not comply to any of the above.
We, in IPC-7351, are not in a position to dictate any "Orientation/Rotation"
to the be used in any of these other "Standards", throughout industry,
either here in the US, or internationally.
We are, however, in a position to resolve and standardize the problem of
"Orientation/Rotation", as respect to the "New Design" of Printed
Wiring/Circuit Boards and the "Orientation/Rotation" of the
Components/Devices used in their Assemblies.
This allows, among other things, for anyone who is responsible for the
installation of any Components/ Devices in a PWB/PCB which conforms to
IPC-7351, to know what compensation, if any, needs to be made to the
"Orientation/Rotation" of any Components/Devices which are going to be
installed by Automatic Equipment, or other means, including Manual Assembly.
So this still may answer all questions, excepting one: How did we get to the
lowly Cathode being Pin 1 and to the Left? Simple really - the Cathode, the
Band on the Diode, has, through out all industry, always been the
"Indicator" of "Orientation/Rotation" for the Diode. Pin numbers and names
may come and go, but the Cathode will forever remain the Cathode and the
"Band". However you define it in your system (ie: Cathode, Pin 1, or simply
the letter "C"), I would recommend that it be as simple and failsafe as
possible. For many, that would be to have a visible Pin Number 1 (to the
Left of course), with the hidden Pin Name of "C" (or if you are really a
dinosaur, the throwback to "K" in MIL-STD-15).
Basically, IPC-7351 has simply sought to "Standardize" the way that almost
all Engineers and Designers have always "looked at and thought of"
Component/Device "Orientation/Rotation", for the last forty some years, from
the "databook/datasheet" and "Schematic" perspective, such that we can
"Standardize" Design from the PWB/PCB perspective, as we try to fully
automate as well as fully integrate our corner of the industry, with the
rest of the industry and the rest of the world.
Is everybody going to be happy with this?
Nope!
Is everybody and his brother and even his mother going to have a better way
to do it?
Yep!
But somebody somewhere has to pick the "one out of one hundred" possible
"Orientations/Rotations" and reasons, and make that the "Standard
Orientation/Rotation", such that we can move forward, and with everyone
participating, make the system work.
If you and your Company are in the middle of a bunch of boards, it obviously
may not be the best time for you to start changing over to IPC-7351, but on
the other hand, If you are beginning a new Project, now might be the time to
switch over to IPC-7351, including "Orientation/Rotation".
You gotta start somewhere. Why not make it with this Diode you are asking
about?
Legacy Components/Devices are not necessarily a problem, in that if you want
to use them in conjunction with new IPC-7351 Component Footprints, you can
simply copy them into a new IPC-7351 (compatible) Library, and you can
rotate the Component to correspond to IPC-7351 "Standard
Orientation/Rotation" at the same time you that you copy it in and verify
that other things also comply with IPC-7351, such as Pad Size and Pad
Location (especially with respect to the center of the Component, such that
the center of the Component is the Location "Origin" (as opposed to Pin 1 as
found in some Systems and Libraries)).
You can probably come up with several of your own reasons not to comply with
IPC-7351 with respect to Component/Device "Orientation/Rotation", but then
again, why not just go with the "Standard" and help us all move forward with
respect Industry Automation.
JaMi Smith
----- Original Message -----
From: "Gary Bremer" <[log in to unmask]>
To: <[log in to unmask]>
Sent: Monday, March 19, 2007 5:25 PM
Subject: [DC] Footprint standards for Polarized components (Diodes)
> Hi,
> I have been asked if there is a standard for creation of footprints
> espicially for diodes. One of the designers reversed the cathode and anode
> causing all the diodes to be placed backwards. The rational was the pin 1
> was the anode for this device for other diodes pin 1 is the cathode.
>
>
> Gary Bremer CID
> Manufacturing Engineer
>
> _________________________________________________________________
> Exercise your brain! Try Flexicon.
>
http://games.msn.com/en/flexicon/default.htm?icid=flexicon_hmemailtaglinemarch07
>
> --------------------------------------------------------------------------
-------
> DesignerCouncil Mail List provided as a free service by IPC using LISTSERV
1.8d
> To unsubscribe, send a message to [log in to unmask] with following text in
> the BODY (NOT the subject field): SIGNOFF DesignerCouncil.
> To temporarily stop/(restart) delivery of DesignerCouncil send: SET
DesignerCouncil NOMAIL/(MAIL)
> Search previous postings at: www.ipc.org > On-Line Resources & Databases >
E-mail Archives
> Please visit IPC web site http://www.ipc.org/contentpage.asp?Pageid=4.3.16
for additional information, or contact Keach Sasamori at [log in to unmask] or
847-615-7100 ext.2815
> --------------------------------------------------------------------------
-------
---------------------------------------------------------------------------------
DesignerCouncil Mail List provided as a free service by IPC using LISTSERV 1.8d
To unsubscribe, send a message to [log in to unmask] with following text in
the BODY (NOT the subject field): SIGNOFF DesignerCouncil.
To temporarily stop/(restart) delivery of DesignerCouncil send: SET DesignerCouncil NOMAIL/(MAIL)
Search previous postings at: www.ipc.org > On-Line Resources & Databases > E-mail Archives
Please visit IPC web site http://www.ipc.org/contentpage.asp?Pageid=4.3.16 for additional information, or contact Keach Sasamori at [log in to unmask] or 847-615-7100 ext.2815
---------------------------------------------------------------------------------
|
