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June 2003

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Subject:
From:
"Brooks,Bill" <[log in to unmask]>
Reply To:
TechNet E-Mail Forum.
Date:
Mon, 23 Jun 2003 10:44:33 -0700
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Hi Steve,
I agree with your assessment that the 'K' or Cathode side of the diode is
typically the indicator for the polarization of the part. There are no
references that I have seen in my 30+ years of PCB design that indicated
that it should be otherwise. I did find the industry change on radial
electrolytic CAPs to be a bit disturbing back in the 1980's they started
marking the NEG side of the capacitor with a stripe that indicated the minus
symbol. Before that, we always indicated the Positive terminal of the Cap on
our silk screen legends. The connection between pin one and the polarization
of the part is traditional, and has been an industry wide practice since the
1960's.. so I am told. I was in grade school back then so I guess I will
have to take their word for it. We often indicated the polarity or pin one
by using a square or rectangular pad shape in that component lead instead of
a cut pad or oval pad. Our indications for polarity came from observing the
manufacturer's standard practice. Transistors typically indicated the lead
that was connected to the emitter junction as opposed to indicated the base
or collector leads. We followed their choice for indication on the legend.
IC's are also customarily oriented by the location of pin one with a DOT or
an indentation on the correct end of the molded plastic package...

In Darryl Lindsey's book "The Design and Drafting of Printed Circuits"
published by Bishop Graphics back in 1979, Chapter 3 refers to discrete
component polarization and he says that Diodes MUST be identified by the
cathode end.  I started laying out boards in 1970 and it has always been the
'standard' practice to identify the cathode, not the anode, of discrete
diodes.

All Diode manufacturers indicate the cathode on the actual physical part
with some sort of band or marking close to the lead or surface mount
termination. They do not indicate the anode side of the diode.

Therefore I would suggest that perhaps your customer is trying to set a new
precedent in the component marking standards, hoping that the rest of the
world will follow their lead.... (don't count on it) or, they just made a
mistake in creating the symbol and should correct their database.

I hope that helps a little.

Bill Brooks, CID

-----Original Message-----
From: Steve Vargas [mailto:[log in to unmask]]
Sent: Friday, June 20, 2003 7:18 AM
To: [log in to unmask]
Subject: [TN] Diode Pin Assignment

All:

    As a SMT subcontractor, we see CAD files from many different sources.
An item of frustration for us is how some customers assign a Pin 1
designator to diodes. Primarily, we see Pin 1 being assigned to Cathode and
Pin 2 to Anode. For those customers who use Pin 1 to anode, we manually
rotate the part within our CAM system to stay consistent with what our
operators typically see on their visual aids. This is both cumbersome and
inefficient.
   I don't believe (or should I?) that there is an industry standard that
addresses this, but is there some type of defacto standard that exists
which I can use as evidence to some of our customers that they should
consider using the 'Pin 1 to cathode concept'? Thanks.

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