1

I would like to know what is the purpose of the surface mounted capacitors found directly on the top of a CPU such as the Intel i3 2120 or the one depicted in the photo ? Does this type capacitor directly feed the internal circuitry of the CPU with a charge ? Directly to a specific layer on die of this Intel CPU ? If yes does a SMC and its charge ie voltage somehow follow a trace directly to a location a particular layer on die ?

Intel CPU

JoeT
  • 11
  • 1
  • 5

5 Answers5

4

These are the regular power bypass capacitors. The distance from the die to bottom pads is substantial, so the inductance of nets is substantial too. The CPU has multitude of rails with dynamically changing power consumption. The caps are there to eliminate/reduce current starvation on spikes of consumption. If the caps are placed only on mainboard, plastic substrate lead/plane inductance will not be able to supply changing current, and voltage will drop killing CPU functionality. That's why the caps are placed as close as possible to the die.

Ale..chenski
  • 38,845
  • 3
  • 38
  • 103
  • Ale..chenski@ can you tell me what are rails ? Do they supply voltage/current to the various IC's (such as the CPU Cores, I/O Controller, GPU and so on) on the die ? – JoeT Mar 10 '22 at 22:19
  • @JoeT, yes, these are power rails, to supply power to various internal IP blocks, like interconnect fabric, memory drivers, and to the other logic as you listed. They all are dynamically controlled to save power. – Ale..chenski Mar 11 '22 at 07:23
4

What you call the "CPU" is actually a WLCSP package (the tiny rectangle in the middle, and the actual CPU die) on top of a small PCB carrier. The carrier "converts" the WLCSP package to a PGA, BGA, or LGA package and also provides a small amount of area that can be used for components that wouldn't fit in or be appropriate in WLCSP such as decoupling capacitors and options jumpers.

Ignacio Vazquez-Abrams
  • 48,282
  • 4
  • 73
  • 102
2

These are decoupling capacitors. Their purpose is to "smooth" the supply voltage, which cannot respond immediately to fast current demands.

The aim of each capacitor is not to feed a particular layer on the die, but to feed the whole chip. Each cap is, however, often located as close as possible to a supply pin of the chip. Because if located too far away, they loose their effectiveness due to the resistance of the track from the capacitor to the chip. Which is why they put them directly on the small CPU integrated PCB.

Community
  • 1
dim
  • 15,845
  • 3
  • 39
  • 84
  • dim@ just one question. Where on die and using the photo above is the Memory controller located ? Does it span multiple layers starting at the top assuming the chip is flipped upside down ? – JoeT Mar 10 '22 at 22:22
1

They're probably decoupling capacitors serving the same purpose as decoupling caps placed between power and ground pins near any digital IC (or analog for that matter.)

Larger processors and microcontrollers (ie with more internal circuits) always have multiple power and ground pins simply to decouple all of those circuits.

Supa Nova
  • 1,122
  • 6
  • 9
1

They are bypass capacitors and since the substrate for the cpu has circuit via's (it's like a miniature multi-layer PCB) they are used to reduce power supply impedance to portions of the die. They are typically solid electrolyte or ceramic capacitors. This might help you: https://labs.wsu.edu/advancedmanufacturing/wp-content/uploads/sites/238/2014/10/Embedded-Capacitors-in-the-Next-Generation-Processor.pdf

Jack Creasey
  • 21,428
  • 2
  • 15
  • 29