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When I attended a viva about the IC Applications, I was asked What is the use of NC pin in 741 opamp? so I answered as it just indicates no need of any connection to that pin in return I was asked then why this pin is placed? Unfortunately I didn't answer.

I researched about it and found that it is filler space from this article.

Is there anything else I didn't reached in this case? and What is Filler space?

3 Answers3

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The reason for no connect pins is that the manufacturers use standard ic packages.

The package for a given device must have (clearly) at least the number of pins required to bring out all functional pins; in the case of the 741 (and countless other devices) the number of pins required for functionality is less than the number of pins on the package.

Those pins that are not required for functionality are still required for the standard package and are simply not connected to anything.

On complex devices with hundreds (or more) pins, it is not uncommon to see numerous NC pins.

Updated for a comment by Spehro: I should have noted this, especially as I have recently been using a device that has just such an arrangement:

The LT3752/LT3752-1 are available in a 38-lead plastic TSSOP package with missing pins for high voltage spacings.

Peter Smith
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  • Yes!, You're right, moreover *DIP chips are not made with an odd number of pins.* – Prakash Darji Apr 02 '16 at 15:09
  • There are many packages (not DIPs perhaps) that *do* have an odd number of pins (particularly those with an exposed pad on the underside). – Peter Smith Apr 02 '16 at 15:23
  • Yep, I have never seen a DIP package with a thermal pad or an odd number of pins. If you can extend the set to SIL packages you can get odd pins though. – Vladimir Cravero Apr 02 '16 at 15:32
  • @VladimirCravero or just a metal can uA741 – Asmyldof Apr 02 '16 at 15:33
  • @Asmyldof yep, well I think that the fancy round thingies are even more difficult to be added in the DI{P|L} family – Vladimir Cravero Apr 02 '16 at 15:35
  • @PeterSmith I think he means an "odd" number in the sense of an unusual, nonstandard number; not in the sense of a number that isn't divisible by 2. –  Apr 02 '16 at 18:41
  • There are DIP packages with [*missing*](http://www.sharpsma.com/download/pr39mf2-epdf) pins (typically for reasons of clearance/creepage) and likely some with pins tied together externally as one (but that fan out into individual through-hole (typically for reasons of power dissipation or current carrying), but convention is to number as if the pins were standard. – Spehro Pefhany Apr 02 '16 at 19:50
  • @PeterSmith, they do make DIP ICs with odd numbers of pins these days but not in the time the 741 was designed. The DIP packages back then were very few and very simple. The 741 was originally available in a 'top-hat' metal can and that IFIRC lacked the n.c. pin. – TonyM Aug 01 '16 at 19:33
  • Sometimes NC pins have actual thermal significance; some stepper motor chips have pins that are strictly "radiator pins" for binding them thermally with radiators. Though often these are GND instead of NC. – SF. Dec 22 '16 at 13:57
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I would like to add to the previous answers that there are actually two distinct cases.

1) Extra pin in package not physically connected to the die. It is okay to connect other things to this pin (I have done this on a super tight board because it helped the routing).

2) The pin is used during the manufacturing process to test/trim something. This is much less common. In this case the pin must float as directed by the datasheet for proper operation.

Houston Fortney
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  • Amazing.. NC pin has helped you in *routing* , can i know some more details about this... –  Apr 03 '16 at 09:30
  • The board was 1 by 5 cm. the ic was up against the edge of the board so the traces had to leave along the inside of the chip. There was a via I couldn't move blocking one of the pins on the inside. So I jogged through the neighboring NC pin and then out. Pretty specific scenario but it did the trick. – Houston Fortney Apr 03 '16 at 10:12
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There is another reason for NC pins. Sometimes those pins are used by the manufacturer to put the IC in test mode during manufacturing validation tests.

For example, See Altera's "MAX® V Device Family Pin Connection Guidelines"PDF. In that document it says of NC pins:

Do not connect these pins to any signal. These pins must be left unconnected.

which would not matter if the pins were truely unwired. Usually the manufacturers, for fear of reverse-engineering, don't disclose the functionality of those pins.

Brock Adams
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Claudio Avi Chami
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  • Can you give a specific example? – Tim Spriggs Apr 02 '16 at 16:56
  • Sure. For example, you can take a look at the datasheet of TI DSP TMS320C6415. You will see that there are several pins marked RSV (reserved). Some of those pins MUST be connected to Vdd for proper operation, some others MUST be left unconnected. The reason for that is that some of them are for test, and if you don't put the values they ask for, your device can enter test mode and behave funny. Usually the manufacturers, for fear of reverse-engineering, don't disclose the functionality of those pins. – Claudio Avi Chami Apr 02 '16 at 17:03
  • @BrockAdams then maybe you want to take a look at this datasheet of the Max V from Altera. https://www.altera.com/content/dam/altera-www/global/en_US/pdfs/literature/dp/max-v/pcg-01012.pdf - Regarding the NC pins it says: Do not connect these pins to any signal. These pins must be left unconnected. – Claudio Avi Chami Apr 02 '16 at 20:50
  • The problem, Brock, is that manufacturers don't like to talk about this and in many cases you can't really know if an NC pin is really not connected to the die, or is a test pin, or a disabled feature, or a future expansion. You can only suspect, specially if you are ordered to leave them unconnected or to connect them to a specific gnd or voltage. – Claudio Avi Chami Apr 02 '16 at 21:24