3 ms·

You can fit FAR more signals on the bottom of a chip and in a multi-layer PCB than you can on any socket you might think of. There is a practical limit to how

You can fit FAR more signals on the bottom of a chip and in a multi-layer PCB than you can on any socket you might think of. There is a practical limit to how small the pitch of the pins on a socket can be if it's going to be field-replaceable; at high pin counts, the length of the socket itself becomes a driving factor. As an example, even moving away from edge-connectors (which are just a non-starter for 800+ pins) Samtec's highest-density high-speed connector, the NovaRay line [0] advertises 224 signals per square inch, so an 800 pin module would require 4 square inches of connector space alone, not to mention making the system PCB assembly thicker and compromising on speed, power and thermals. [0] https://www.samtec.com/connectors/high-speed-board-to-board/high-density-arrays/novaray https://www.samtec.com/connectors/high-speed-board-to-board/...

kaibee 5y ago
So design it like a CPU socket? Laptops usually don't have socketed CPUs because they also need to mount a heatsink, but RAM doesn't have that issue.
- InitialLastName 5y ago
  So an FM2 socket (to pick one of the more reliable ones), which has ~900 pins has a 4.5in^2 area and adds ~.25" in thickness, while reducing the maximum speed you can get through the chip at the very low voltages necessary to run LPDDR, which is what I meant by compromising speed and power. Not to mention that socketting parts reduces your production yield over SMT, increases your assembly cost, and cuts reliability (it's fine on a desktop because you almost never move your desktop, and there's usually a big thermal assembly bolted on top).
  - ant6n 5y ago
    Pentium m used to be socketed, pretty reliable, and available for thin laptops. I'm not trying to dismiss the difficulty, just saying they could perhaps be overcome
    - InitialLastName 5y ago
      Hardware engineering is ever thus: Difficulties can always be overcome, but every cost adds 5x that cost to the MSRP.

kaibee 5y ago

So design it like a CPU socket? Laptops usually don't have socketed CPUs because they also need to mount a heatsink, but RAM doesn't have that issue.

InitialLastName 5y ago
So an FM2 socket (to pick one of the more reliable ones), which has ~900 pins has a 4.5in^2 area and adds ~.25" in thickness, while reducing the maximum speed you can get through the chip at the very low voltages necessary to run LPDDR, which is what I meant by compromising speed and power. Not to mention that socketting parts reduces your production yield over SMT, increases your assembly cost, and cuts reliability (it's fine on a desktop because you almost never move your desktop, and there's usually a big thermal assembly bolted on top).
- ant6n 5y ago
  Pentium m used to be socketed, pretty reliable, and available for thin laptops. I'm not trying to dismiss the difficulty, just saying they could perhaps be overcome
  - InitialLastName 5y ago
    Hardware engineering is ever thus: Difficulties can always be overcome, but every cost adds 5x that cost to the MSRP.