IC pins

[meowth08]

I was making our project when I observed the designation of pins to the ICs specifically dual in line packages. I am wondering why for an 8 pin IC, pin 4 is gnd and pin 8 is VCC. For 16 pins, pin 8 is gnd and pin 16 is VCC. One thing more, pin 3 is the output as of an ne555 or the beginning of the output as in 4017. Is there any convention in designation of IC pins? Your answers will ease my curiosity.

 

[RCinFLA]

Most packaging has a pedestal to mount the semiconductor die. This pedestal becomes the substrate of the IC when it is mounted with conductive epoxy resulting it being negative most voltage for the I.C. So it is brought out as the ground pin. The standard packages has pin 4 for 8pin package, pin 7 for 14 pin package, and pin 8 for 16 pin packages.

 

[meowth08]

Most packaging has a pedestal to mount the semiconductor die. This pedestal becomes the substrate of the IC when it is mounted with conductive epoxy resulting it being negative most voltage for the I.C. So it is brought out as the ground pin. The standard packages has pin 4 for 8pin package, pin 7 for 14 pin package, and pin 8 for 16 pin packages.

I think I'd have to search IC fabrication to understand it better.

 

[unclejed613]

because it makes sense, when you have hundreds of ICs on a board to have the power and ground at opposite corners of each chip. power and ground are either the first or the last things done in a board layout, and it's convenient to know without looking up each chip that the power and ground are on opposite corners.

 

[colin55]

Putting the power pins on the diagonally opposite corners was the most stupid idea of all. Remember the EPROM chips that were damaged when inserted around the wrong way?
In addition, some chips have power pins other than opposite corners.
You never know where the power pins are.
3 terminal regulators are especially different to prevent a negative regulator being inserted in place of a positive regulator.

 

[Reloadron]

Colin55

In addition, some chips have power pins other than opposite corners.

The LM324 comes to mind. Always use the correct data sheet and make sure for every part. Bad things happen when you don't!

Ron

 

[KeepItSimpleStupid]

OK, while we are at it, because I think the discussion is related. A 74HCT154 is available in the following packages: 24-SOIC, 24-TSSOP, 24-VFQN. 24-DHFQN and a 24-PDIP.

If you would rank them in order of preference (meaning: the least likely to become unobtanium) how would you rank them? Why?

If you were doing an SMT design, which package would you pick and why?

There also appears to be some synonyms floating around like 16-QFN and say a 16-Lead Lead Frame Chip Scale Package [LFCSP_WQ]. So what's going on?

PS:
BGA packages are just plain nutty.

 

[KeepItSimpleStupid]

In terms of other signals, the wire bonder has to attach to the IC at specific points without crossing the uninsulated, wires.

 

[RCinFLA]

OK, while we are at it, because I think the discussion is related. A 74HCT154 is available in the following packages: 24-SOIC, 24-TSSOP, 24-VFQN. 24-DHFQN and a 24-PDIP.

If you would rank them in order of preference (meaning: the least likely to become unobtanium) how would you rank them? Why?

If you were doing an SMT design, which package would you pick and why?

There also appears to be some synonyms floating around like 16-QFN and say a 16-Lead Lead Frame Chip Scale Package [LFCSP_WQ]. So what's going on?

PS:
BGA packages are just plain nutty.

Depends on your particular PCB and fabrication expertise. For an average homebrew builder, dealing with pad pitch smaller then 50 mil can be quite challenging. Problem is most commercial users (where the volume sales are to the IC suppliers) want smallest possible packaging. 12.5 mil pitch is pretty much beyond 'build it is your kitchen sink' capability. Many IC's are now only available in these fine pitch packages.

If you are just starting out stick with 50 mil pitch. You can hand solder these with a soldering pencil. When you get real good you can tackle some 25 mil pitch. 12.5 mil pitch pretty much requires solder pasting screen printing with careful placement and reflow. This can be quite expensive for a homebrew project.

My personal preference is Master Ultratorch or Weller Portasol PSI-100k for SMD repair work. These are butane torches with hot gas tip. They produce a low flow volume, very hot gas which I prefer over hot air repair machines that tend to blow parts around due to their strong hot air flow rate. The hot gas contains less oxygen then a hot air machine so flux stays viable longer with less oxidation to exposed leads. I have repaired 400 pin BGA's with the butane touches without damaging PCB which is nearly impossible with a hot air machine.

Get a good plunger solder sucker and learn how to properly use solder wick. (don't use one of the solding iron/sucker that heat and suck off solder, all you will do is damage pads) If you don't succeed on first attempt getting a clean solder removal, reapply solder to get fresh flux and make another plunger sucker attempt. A final heating with an Exacto knife to pry the final lead tacking to the pad will remove the part without damaging the pad. Practice on some scrap boards to develop your technique.

Athough not the greatest, the only readily available board cleaner is 90% isopropyl alcohol. I have my own private stash of Tri-Chloroethane solvent I bought many years ago which does a fabulous job of cleaning flux off SMD board but it is no longer readily available to purchase because of its vapor toxicity. Freon is also good but it is now outlawed for cleaning.

 

[KeepItSimpleStupid]

Hey thanks.

I basically did one SMT part long ago and that was a microprocessor in a car radio. It should not be your first, but it was. I'm like that. I was 6 years old and my first paint my number was like 3 ea (12" x 36") and they still look good.

I did use the "cut the pins method" and I soldered it upside down.

I have a "Circuit Specialists" hot air, solder sucker and iron, but I have only used the iron ONCE.

I have a Master Ultra Torch and thanks for the tip. It's autoignition. Your right, I could use the air attachment. I tend to like that torch. I think it's better than a 100 W soldering iron.

Eventually, I'm going to start playing with SMT and it doesn't look like a lot of fun or it means an investment in new technology.

I can see where the BGA package can be very reliable, but the board expense seems way too high. It would seem that you may need an 8 layer board to use a 16 pin BGA with 0.6mm pitch.

 

[OlPhart]

Us geezers 'mem a fair number of logic chips having power & ground in midchip: 3 for + & 11 for gnd (or maybe reverse that..). I HATED that BS... the simple presumption of power/ground on same corners allows me (and, I'd think others) to pay attention to more important aspects.

It sucks enough that uC chips and/or families (let alone diff mfgs) have different power/gnd pins.

It IS evolving to better than it's ever been, but will never be as good as it could be... <<<)))