3

Firstly, by low-scale I mean the sense of very-large-scale-integration, so the question is: Why are there so few and such thick wires on a typical PC printed circuit board, compared to the (orders of magnitude) smaller and more numerous connections in a typical CPU IC?

My guess would be that there would be a big benefit from making these PCB wires thinner so that they can create many more connections between the CPU and other devices, in order to increase bandwidth significantly. It is kind of surprising to me that there are only about 600 pins connecting the CPU to the other components, whereas there are orders of magnitude more connections between the components inside a CPU. I would guess that this would be a bottleneck, with a big benefit to adding more and shorter connections.

Presumably they might also place the components closer to each other and hence decrease latency, and possibly use the extra space for new components.

Is there a technical reason why PCB wires have to be so much thicker/bigger and less numerous than wires inside an IC chip?

user56834
  • 177
  • 5
  • 1
    Look up interposers, which are silicon circuit boards that can have tens of thousands of wires between components but are more expensive. Technology exists to do very fine features, but the cost is often not justified. – user1850479 Oct 22 '21 at 06:16
  • You might want to look at ceramic hybrid circuits for an intermediate style of manufacture. –  Oct 22 '21 at 12:45
  • NRE for PCB is $500, NRE for IC is $500,000, they serve a different purpose and scale in integration. – crasic Oct 22 '21 at 15:08
  • I'll add my own separate thoughts. When designing an IC, the designer has only rather vague ideas about what the pins will drive (capacitance load, dissipation requirements, etc.) Consequentially, they must design their IC pad drivers to be "huge." If, on the other hand, the ICs were designed with an exact, precisely known load those sections on the IC could be tailor-made for that purpose. And this would allow not only far better use of the IC die space but also, likely, a different interconnect design. This is done on Intel CPUs, by the way, where several dies are wire-bonded in the package. – jonk Oct 22 '21 at 19:12

3 Answers3

4

It’s the limits of PCB etch and drill process that require such large feature sizes compared to what’s possible on an IC. On the other hand, PCB processes are much cheaper than ICs in an area-for-area basis.

That said, there are much finer PCB geometries possible than the typical 5/5 PCB, but they cost more money.

hacktastical
  • 49,832
  • 2
  • 47
  • 138
  • To tool up to make an ic is a very significant amount of money vs pcbs that are relatively cheap in comparison. – Kartman Oct 22 '21 at 05:53
1

Is there a technical reason why PCB wires have to be so much thicker/bigger and less numerous than wires inside an IC chip?

The are practical reasons why track & gaps cannot be etched smaller than the copper thickness. 1/2 oz copper is 18 um 2.8 mil thick.

Surface etching of copper is far more difficult to control below 3 mil tracks with 3 mil gaps supported by the best quality fab sources.

This l/w ratio of tracks also increases impedance of tracks which may affect some signal's integrity.

Tony Stewart EE75
  • 1
  • 3
  • 54
  • 182
0

Considering the IC manufacturing processes such as photolithography and wire bonding, PCBs are way easier and cheaper to manufacture compared to ICs.

Of course, it may be possible to reach the finest trace widths on ordinary PCB materials with copper, but it'll be costly and time-consuming.

Rohat Kılıç
  • 26,954
  • 3
  • 25
  • 67