I think this discussion illustrates why standardization is so important.
For a moment, put yourself in the shoes of the assembly house... If each one
of you designers were sending your boards to the same assembly house with
your 'standard pin one assignments'... The assemblers could be very confused
trying to interpret the differing methods used in your data.
Not only that, but I imagine each of you as experienced designers has a
different method of graphically indicating the polarity of the parts on your
boards. Some use a dot, or a fat line, a plus or minus symbol, a schematic
symbol drawn in the legend, the numeral one, a small triangle or square, a
pad shaped differently than the others in the part, etc.
While this may seem trivial to some from the designer's perspective, it can
cause a great deal of trouble for the folks who are using your database to
build from. Imagine trying to keep track of and interpret each person's
methods... One customer does it their way, another does it a different way
and still another has some other way of doing it.
In my case, for example, the Altium Designer program I use gives the
designer the option of not calling the pin out with a 'number' but allows
using alphanumeric representation for the pins... so I can call the Anode
'ANODE' and the Cathode 'CATHODE'... there are no numbers needed in the
database or netlist for that particular footprint. This works just as well.
Obviously, not all CAD programs let you do that...
In the past when I have used programs that restrict you to the use of a
number, the Cathode has always been the 'polarized' end with the polarized
mark on the body. The 'standard' I have always used is to put pin one on the
polarized end of the part. In the case of the diode, that would be the
cathode end. That is true of capacitors, as well. The polarized pad would
typically be called 'pin 1'.
This practice goes way back to the early 1970's when I first started doing
board design. The Bishop graphics book, published back in the early 1980's
emphasized the importance of indicating the polarity of components and when
we moved to CAD in the early 1980's the pin one for the cad footprint and
schematic symbol was always on the polarized end of the part.
Good discussion... :)
Best regards,
Bill Brooks
PCB Design Engineer, C.I.D.+
Tel: (760)597-1500 Fax: (760)597-1510
Datron World Communications, Inc.
Vista, California
-----Original Message-----
From: Kevin L. Seaman [mailto:[log in to unmask]]
Sent: Tuesday, March 20, 2007 6:14 AM
To: [log in to unmask]
Subject: Re: [DC] Footprint standards for Polarized components (Diodes)
Gary,
We use CAD tools that allow alphabetic pin "numbers".
So, we use A and C as pin "numbers" for both the schematic symbol
and the PCB footprint of all diodes.
For SOT23 diodes we use AA, AC, CC, and NC as required.
For transistors, we use E, B, C, G, S, and D.
This has eliminated all polarity problems with these parts.
Thank you,
Kevin L. Seaman
OrCAD CIS & Allegro PCB Library Liaison
Sr. Staff PCB Layout Engineer
Broadcom Corp. Irvine, CA
(949) 926-5656
========================= Original Message ==========================
From: DesignerCouncil [mailto:[log in to unmask]] On Behalf Of Gary
Bremer
Sent: Monday, March 19, 2007 6:25 PM
To: [log in to unmask]
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_hmemailtaglinemar
ch07
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