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Polarity for SMT Devices



Polarity for SMT Devices | 28 May, 2002


I know I will probably get a dozen "check the archives" answers, but what the heck. What is YOUR typical polarity mark for SMT devices? Do you use silk screen or cut a corner off a pad? How do you identify pin one on ICs?

Let me know and thanks.

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Polarity for SMT Devices | 28 May, 2002

Silkscreening is falling out of favor, as it should. SMT IC devices can have their "pin one" solder lands indicated by a round mark, like a fiducial or a "1" mark, both made from either the copper clad PWB material or silkscreened. I have seen some with elongated solder lands indicating pin 1. Silkscreening boxes around the insides of the solder lands of IC devices to indicate orientation causes problems with low-profile devices whereas the silkscreen deposition is too high and the IC leads are not sitting flat on the solder lands. For caps and diodes, again a silkscreen or etched line is common, on the outside of the solder land. I have seen the solder land shape modified to indicate polarity for caps and diodes, well.

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Polarity for SMT Devices | 28 May, 2002

That is the rub here, isn�t it? For those posting, it�s easier [but much slower] to start a new thread than to search the archives. [And posters could view their issue so unique and novel that it could never have been discussed several years earlier, as implied when contemplating 'search'.] And on the other hand, it�s easier for responders to send someone to search the archives than to respond to a posting.

In keeping with that precept, I�m tempered to suggest that you �searching the fine SMTnet Archives�. Unfortunately, I can�t recall a thread that responds to your question, should I send you there.

Several points on polarity marks are: * Any polarity mark is a good polarity mark. We prefer silk screen over cut corners of pads, because silk screen remains easily observable after mass soldering. * Generally, polarity marks on the bare board should directly correlate with polarity marks on the component. * Recently, we see less silk screen on boards than in the past. * Basically, 2221 - Generic Standard On Printed Board Design says that if you change the pad shape to indicate pin 1, you should be careful not to mess-up.

Our typical mark varies with the component * Diodes and caps: �+� mark, line under the part closer to one end than the other, �U� with chamfered corners * IC with leads on two sides: Dot or triangle in the center of the part at the top/bottom closest to pin 1. * IC with leads on four sides: Chamfer or dot on one corner if pin 1 is at that corner. Dot or triangle in the center / above the row of leads, if pin 1 in the center of a row. * Connector with multiple row of leads: �U� with chamfered at the corner for pin 1.

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