In IPC-7351, all two terminal components are identified with the Polarity Indicator
at Pin 1 which is always to the left.
The Polarity Indicator on a capacitor is the "plus" or "notch" or "band" which
identifies the Positive Terminal or Polarity (and yes there are exceptions to the
rule such as some radial capacitors which have a stripe which is labeled "-"), and
thus the "rule" for a capacitor is always "Positive Terminal or Polarity" to the
left.
The Polarity Indicator on a Diode is the "band" (or occasionally some other feature)
which identifies the Cathode (by industry convention), and thus the "rule" for a
diode is always "Cathode" to the left.
Yes, the IPC-7351 figures and language need to be cleaned up to avoid any ambiguity.
The problem is that everyone in the industry orients diodes differently, with their
own specific preferences, with some wanting Pin 1 as Anode, some wanting Pin 1 as
Cathode, and some wanting vertical orientation and some wanting horizontal
orientation, some wanting Pin 1 up or to the left, and some wanting Pin 1 down or to
the right.
IPC-7351 has chosen to adopt an orientation as the standard "zero degrees of
rotation" or "zero rotation" in which all two terminal components are oriented
horizontally (which complies with the way that almost all existing libraries orient
the majority of their two terminal components (which itself is primarily due to the
height / width aspect ratio and display area of the standard computer monitor)), and
further to specify that Pin 1 will always be to the left, and further to specify
that the "Polarity Indicator" will always be Pin 1 which again is always to the
left. The "Polarity Indicator" is always the "Positive Terminal" for a Capacitor
(notwithstanding the above mentioned exception) and the "Cathode" for a Diode (due
to the industry wide standard of identifying the Cathode with a "band").
Some will be happy with this orientation, and others will be not.
Once again, the problem is that everybody currently does it differently, which
causes major problems for assembly houses, and results in numerous errors.
IPC-7351 has adopted a "standard orientation" for these components as "zero
rotation" which is designed to eliminate these problems in the design and
manufacture of printed circuit boards.
It is not, and was not, designed from the results of a survey, and certainty not
designed to make everybody happy, which it obviously will not do.
All else said, IPC-7351 does fix the problem, whether or not it you are happy with
the way it does it.
JaMi Smith
----- Original Message -----
From: "Indrek Rebane" <[log in to unmask]>
To: <[log in to unmask]>
Sent: Tuesday, October 26, 2004 9:59 AM
Subject: Re: [DC] Pin Numbering Question
> Hi George,
>
> According to IPC-7351 final draft (final is not released yet as far as I
> know) pin one is always positive and thus an anode for molded
> components. See Figure 16-1. But on same figure you see that for MELF,
> pin 1 is always "Polarity Mark" or cathode. Go figure!?
>
> Indrek
>
> George Patrick wrote:
> > According to Donna Robertson [[log in to unmask]] over on the Cadence List:
> >
> > "If you are following IPC-7351 Pin 1 is the Cathode."
> >
> > DISCLAIMER: I don't have a copy, so I can't verify it :)
>
>
> --
> Indrek Rebane | Borthwick-Pignon
> Electronics Engineer | Tartu Science Park
> Phone: (+372) 7 302 641 | Riia 185, 51014 Tartu
> Fax: (+372) 7 383 041 | Estonia
> [log in to unmask] | www.bps.co.ee
>
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