# Decoupling capacitor placement

I have often seen decoupling capacitors "hugging" the IC that they're protecting - but that's on '80s era through-hole boards, and things have changed.

It also doesn't help that the design of ICs, for some reason, puts Vcc and GND on diagonally opposite pins!

I know that decoupling capacitors should be "as close as possible" to their ICs, but I assume that the Vcc pin is more important? With (two layer) circuit board designs often having large ground areas, it implies that any ground is a good ground, and you can get better track layouts if you mount the cap perpendicular to the power pin, with the ground side "hanging in the breeze" on a convenient ground pad. I assume that running a specific ground rail to the IC's ground pin(s) is not necessary?

### EDIT

OK, there seems to be some confusion (perhaps on my side!). Assume a one layer board, with an IC and capacitor with the following pinout:

14 13 12 11 10  9  8
V+       O Decoupling
V-                         O capacitor
1  2  3  4  5  6  7


I can either do this (option 1):

14 13 12 11 10  9  8
+--O
+--------------------O
1  2  3  4  5  6  7


or this (option 2):

   14 13 12 11 10  9  8
O--------------------+
O--+
1  2  3  4  5  6  7


or this (option 3):

14 12 12 11 10  9  8
+--O
O----(some adjacent ground path)
1  2  3  4  5  6  7


I am inferring that #3 is a bad idea?

### EDIT #2

Or maybe, since the decoupling is so important, I could do this (for a huge DIP40 chip)?

+-- (some convenient Vcc path)
|
|   40 39 38 37 36 ... 22 21
O                          +--O
O---+                         O----(some convenient ground path)
1   2   3   4  ... 19 20

• The diagonal power pins are a hold over from 2-layer through-hole boards laid out in a grid. Chips without power pins at the corners (eg SN5490) were actually an annoyance. Oct 1, 2016 at 12:43
• @SpehroPefhany I can understand that chips with power pins in the middle of a side were a problem - but why didn't they put both the supply pins on the same side of the chip? On a DIP14, why use pins 1&8/7&14, rather than 1&14 or 7&8? Oct 1, 2016 at 12:58
• I think the routing channels would be more constrained if they were both on the same side so the board would end up bigger but it's been a long time since I dealt with anything resembling the old grid layouts. Note the disk caps that fit nicely across the ends without obscuring any pins when the power pins are on opposite sides. Not like today with SMD parts with many power rails. Oct 1, 2016 at 13:18
• I've edited the question to assume a one-layer board Oct 1, 2016 at 13:30
• @SpehroPefhany That picture is... exactly what I am familiar with! Ah, the memories - and the realisation that the 80s were a long time ago! Oct 1, 2016 at 14:08