Why do they put 'NC' pins on chips? - Page 1
Forum section: electronics / projects

author: Fungus
author_rank: Super Contributor
timestamp: July 23, 2015, 04:25:07 pm
content: I get that if you're making an 8 pin chip and you only need 7 pins that you'd leave one of the pins unconnected.But ... I'm just looking at this chip:http://datasheets.maximintegrated.com/en/ds/DS3231.pdfIt's a 16-pin package and8of the pins are 'NC' - half of them!Why don't they put it in an 8-pin package? Surely that would be cheaper/easier for everybody involved.Is it because their production facilities don't do 8-pin chips or is there a more sinister reason for doing that?(nb. In the datasheet it says "Must be connected to ground" so they're not really NC).

author: Richard Crowley
author_rank: Super Contributor
timestamp: July 23, 2015, 04:33:10 pm
content: Typically, that many "ground" pins is actually a "heat-sink" connection to better conduct heat out of the chip and onto the board.OTOH, that chip does feature a temperature sense feature, so perhaps the many ground pins better allows the chip to sense the ambient (or at least PC board) temperature.

author: trophosphere
author_rank: Frequent Contributor
timestamp: July 23, 2015, 04:33:21 pm
content: The die/internal components (oscillator and capacitors) may be larger than what can be accommodated in an 8-pin package.

author: retrolefty
author_rank: Super Contributor
timestamp: July 23, 2015, 05:05:51 pm
content: Well the datasheet you posted does state:QuoteSo it certainly not a "no connection" in the most liberal meaning.

author: SeanB
author_rank: Super Contributor
timestamp: July 23, 2015, 05:11:01 pm
content: You find a lot of thermal sensors with NC pins so you can get better thermal conduction to the die inside, often all the pins on the one side. As well you get power devices like the LM380 where you have 6 pins dedicated to cooling the die, while still being a DIP device. As well, as mentioned above, you also get some chips that require only a few pins ( serial input and output) but where the die itself is too large to fit into the available space in the small package, so they use a larger leadframe and case that will fit the die, with the extra pins all being unused.Last use in in single chip switching power supplies, where you have pins that are either omitted or are designated as no connection to provide creepage and clearance for the high voltage internal switch device. Typically a DIP8 with 7 pins, the HV connector having no pin next to it do it will have 1kv creepage clearance. As the device typically will dissipate 150mW a DIP8 package is fine and cheap to fit it in, other packages are a lot more expensive or require special processing in production, while the DIP8 can be handled in a standard IC production package line with no change required.

author: chicken
author_rank: Frequent Contributor
timestamp: July 23, 2015, 05:12:20 pm
content: Sometimes it's also done to keep the pinout compatible with ICs that offer more functionality.E.g. the NC pins of the Silabs Si4362 radio receiver are used for TX in the Si446x transceivers. One PCB can be designed to be used in receivers as well as transceivers. In addition, the transceiver IC can be used as (more expensive) replacement part for the receiver IC when in a pinch .Another example is MIC5504 in SOT23-5, which has a NC where other LDOs either have a bypass cap or adjust pin.

author: Kalvin
author_rank: Super Contributor
timestamp: July 23, 2015, 05:14:44 pm
content: And sometimes the unused pins are used for factory testing and/or trimming.

author: Pjotr
author_rank: Frequent Contributor
timestamp: July 23, 2015, 05:40:32 pm
content: Quote from: Fungus on July 23, 2015, 04:25:07 pmSimply because the embedded tuning fork crystal doesn't fit into an 8-pins SOIC. That crystal is housed in a separate evacuated vacuum cavity.The newer "M" version uses a MEMS resonator (which is a lot smaller) anddoescome in an 8-pins package as well:http://www.maximintegrated.com/en/products/digital/real-time-clocks/DS3231M.html

author: DanielS
author_rank: Frequent Contributor
timestamp: July 24, 2015, 02:49:19 am
content: On some devices like monolithic isolated DC-DC converters and photocouplers, the NC pins are simply for mechanical support since all the active pins are on the same side of the DIP package.

author: hendorog
author_rank: Super Contributor
timestamp: July 24, 2015, 03:19:49 am
content: Looks like it is done to improve isolation from external signals and improve isolation between pins. The datasheet even says not to run signals underneath the chip:"Avoid running signal traces under the package, unlessa ground plane is placed between the package and thesignal line. All N.C. (no connect) pins must be connectedto ground."

author: bitwelder
author_rank: Super Contributor
timestamp: July 24, 2015, 05:36:28 am
content: If those pins *must* be connected to ground, isn't there a better name in use in the industry than 'N.C.' ?

author: zapta
author_rank: Super Contributor
timestamp: July 24, 2015, 05:45:23 am
content: That can be an interesting topic for a video blogger with a good microscope. What's going inside that package?

author: Pjotr
author_rank: Frequent Contributor
timestamp: July 24, 2015, 07:47:51 am
content: Quote from: hendorog on July 24, 2015, 03:19:49 amYup. Note this is a highly accurate RTC that runs from just one uA and as such is a high impedance circuit. Any induced noise/hum/leakage will impair its accuracy. Although the NC pins of the lead frame aren't connected to anything, they help shielding. Here is an app note from Intersil about this issues from a comparable highly accurate RTC:http://www.intersil.com/content/dam/Intersil/documents/tb48/tb484.pdf

author: coppice
author_rank: Super Contributor
timestamp: July 24, 2015, 08:08:47 am
content: If there truly is a crystal, rather than a MEMS resonator, in that package, they probably just needed space for it.

author: daqq
author_rank: Super Contributor
timestamp: July 24, 2015, 08:46:57 am
content: Thermal, mechanical reasons most of the time, some times it's compatibility, testing.Thermal - more pins = more thermal connection.Mechanical - more pins soldered = more physical stability, or bigger package = more stuff inside. The link you provided shows a device with a built in crystal - that is most likely taking up most of the space inside the package.Compatibility - I know of a DAC family that can have 2 or 4 channels. The digital interface is exactly the same, the package is the same, the pin locations are the same, but the 2 channel one has two NC pins in place of channel 2 and 3 ( see DAC124S085 and DAC122S085)Testing - some pins DO have functions, but are used only in the factory for calibration, testing, etc. Most of the time a datasheet will explicitly say what you should do with them

author: george graves
author_rank: Super Contributor
timestamp: July 24, 2015, 09:03:28 am
content: They do it just to mess with your head. I'm 99% sure that's how all chip designers work."Oh, lets design this 7 segment LED driver in the worst possible pin out, so the silly little engineers will have to make 100 vias! Bahahahah! That will be funny!"

author: Fungus
author_rank: Super Contributor
timestamp: July 24, 2015, 04:11:29 pm
content: Quote from: george graves on July 24, 2015, 09:03:28 amThis seems the most likely explanation.Thanks to everyone who answered!

author: Stray Electron
author_rank: Super Contributor
timestamp: July 24, 2015, 05:36:41 pm
content: Quote from: Richard Crowley on July 23, 2015, 04:33:10 pmThat and for mechanical strength/stability.