What are the increasing number of CPU pins used for?

RalphTheCow

Senior member
Sep 14, 2000
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I know I could look this up, but it seems like an interesting question for this forum. What are they used for? Does the decreasing size of the traces in the chip (ie 90 nm) make it possible? Why can't they design a socket that will have a life of more than a year or two?

Also, has flip-chip become standard so that they just don't say it anymore, as in my P3 is also called FC-PGA but I don't see any new CPUs called that? Or have they turned chips back upside down?

On this subject, if I go AMD, it seems like I should try to go with 939 over 754, but there are two theories if I tend to upgrade only on a pretty long 4-5 year cycle.

1. Buy cheap, it will be obsolete in 4-5 years anyway. Plus it's proven.

2. Spend a little more for 939 in the hopes that it will still be upgradable in 4-5 years.

Is there a reason to wait for nForce 4? I really need to build now, so I really can't play this waiting game, even though I would like PCIe. I also noticed on the nVidia site that it goes up to 3 GB/s on SATA, but do any drives match that? Only WD Raptors?

OK, the following snip from Wikipedia's Athlon 64 article gives me a clue:

_________________________________________________________________________
# 754: The Athlon 64 value/budget line, 64-bit memory interface
# 939: Athlon 64 performance line and newer Athlon 64 FXs, 128-bit memory interface
_________________________________________________________________________

So could 128 of the pins be for memory? Are these really pins? They must be exceedingly small and fragile if so.
 

Mark R

Diamond Member
Oct 9, 1999
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Well, the pins on a chip are used to get data in and out, and supply power and control signals to the chip.

For example on an Athlon 64 (S939) - the chip has 939 pins. They are distributed roughly as follows: 258 pins for DIMM connections, 511 for power and power monitoring, 80 for the hypertransport links to the motherboard chipset, 68 pins aren't used, and 22 for various control functions - e.g. clock input, thermal diode, etc.

The main thing that has made this possible has been 'flip-chip' mounting. The original mounting method involved welding tiny wires to pads on the edge of the core which then went to the pins. How do you put 939 connections round the edge of a core? You can't. Not enough space. Flip chip basically has tiny pads in a grid over the surface of the core, and the core is basically soldered to a special PCB which forms the substrate of the chip, with tiny PCB traces connectig the core to the pins. Imagine the core connecting to the main body of the chip, in the same way as the chip connects to the mobo - just a lot smaller.

Why so many pins for power? - well modern CPUs take a lot of power, and specifically they take it at a very high current at a very low voltage - this means that the connection needs to be outstandingly good. Also, the pins are very small and delicate - so you need a lot of them.

Well, the manufacturers do try to keep the same socket as long as possible - it costs a lot of money to redesign the socket. However, things change, faster CPUs need more power than older sockets can provide - or new types of FSB need more pins. Sometimes there were problems, the original Opteron Socket 940 didn't work as well as was hoped - it was expensive for mobo manufacturers (basically pins were in awkward positions, so connecting the DIMMs was very difficult). Socket 939 has the pins in a better position, and it's much easier to make mobos for it.

It's very unlikely any socket you use now will give optimal performance in 2 or 3 years time. E.g. with AMD, the sockets will need to change to accommodate the extra connections needed for faster DDR2 memory next year.

If you are worried about getting obsolete hardware - well that's what computers are all about. Just buy what will do what you want now. It doesn't amtter what you get, it will be obsolete in a years time.
 

RalphTheCow

Senior member
Sep 14, 2000
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Thank you, Mark R. That was great! Just one thing - those high-density pins must be incredibly fragile, aren't they?
 

sswingle

Diamond Member
Mar 2, 2000
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Just be careful when handling your CPU. I have never lost any pins. Had a few bent, which I very carefully bent back though.

I hear it is easy to lost pins when you take a cpu OUT of the motherboard. The suction between the heatsink and the cpu sometimes pulls the cpu out of its socket. with the ZIF arm down, its possible to lose pins. Main thing to remember here is to wiggle the heatsink back and forth to loosen it up before you pull it out, then you don't get that problem. On my very first A64 build I managed to pull the cpu out twice this way, but fortunately did not cause damage.