I am with Dale 100% on this one!
We have run into this problem ourselves on the service bureau end. We have
had boards put on hold for this very reason a couple of times. And I would
like to think that the infrequence is due to my diligence in checking
datasheets vs. schematic and PCB symbols. :-) The first time this one bit us
was our ignorance of the fact that this "standard" had changed and was
caught due to a watchful fabricator/assembler. (Thank you, Thank you, Thank
you!) The second time was due to a "bad" datasheet(no notation on the
datasheet itself)....and unfortunately was not caught until the boards had
already been assembled and shipped. Since these incidents I have been
double-checking polarity more intensely (than previously) as the last thing
we (or anyone else) want is to lose/rework expensive boards due to "cheap"
parts! It is unfortunate that we ( at any level of the process) should have
to spend the extra time to double check the "easy" parts when there are so
many newer or more difficult ones to contend with but until this gets
straightened out I don't see any other choice! This will need to be checked
(and rechecked) at all stages until there is a permanent solution (which I
THOUGHT we used to have!)
Thanks,
Dharma
KAW/USA Ltd.
39 Simon Street, #4
Nashua, NH 03060
603-886-8711 x212
603-881-8763 fax
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-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Dale Ritzen
Sent: Tuesday, September 04, 2007 10:29 AM
To: [log in to unmask]
Subject: [TN] SMT Part Polarity Marking Conventions
Good morning all,
Lately we have run into several situations where it appears the old tried
and true polarity marking conventions for capacitors and diodes no longer
apply on SMT parts. The customary silkscreen "+" to denote the anode end of
the device appears to still be in use. However, the actual part marking
(band, or dot, or chamfered edge or other polarity key) is now showing up on
either the anode or the cathode, depending on how the OEM or supplier
decides to mark the device. It has become necessary to question each
polarized placement of a SMT diode or polarized capacitor when setting up
assembly instructions or programming pick & place profiles.
So, what happened to part marking convention in this regard? I don't believe
any marking changes have taken place in the plated through hole world in
regard to diodes and caps. But suppliers of the SMT versions of these
devices are not necessarily following any marking conventions. They appear
to simply mark the device in the manner they find acceptable, then define
the marking (if you're lucky) in their data sheets. Doesn't help much once
the parts are placed incorrectly and the inspection team thinks they are
placed correctly per the visual part markings that have been a standard over
the years.
Is there any movement afoot to get some industry standardization of SMT part
polarity markings in place? Maybe through SMTA? If so, I'd like to join the
effort to make sure some standardization occurs in this area.
Thanks for your feedback in advance!
Dale Ritzen
Quality Manager
Austin Manufacturing Services
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