TECH BRIEF

Motherboardless PCs

University of California at Los Angeles researchers want to do the unthinkable: kill the motherboard. In a recent piece for IEEE Spectrum, the researchers said this act of technological matricide would enable the creation of more powerful systems that aren’t constrained by the printed circuit board (PCB) used today, all thanks to a new silicon-interconnect fabric that can be used in the motherboard’s stead.

The researchers, Puneet Gupta and Subramanian Iyer, said this change would enable the development of all kinds of systems. They contend that relying on PCBs makes it harder for companies to develop smaller devices like smartwatches while also inhibiting the growth of larger devices used in data centers. Their silicon interconnect fabric is supposed to enable smaller and larger devices. They explained:

“Our research shows that the printed circuit board could be replaced with the same material that makes up the chips that are attached to it, namely silicon. Such a move would lead to smaller, lighter-weight systems for wearables and other size-constrained gadgets, and also to incredibly powerful high-performance computers that would pack dozens of servers’ worth of computing capability onto a dinner-plate-size wafer of silicon.”

Gupta and Iyer also said the silicon interconnect fabric would allow chip makers to stop relying on “the (relatively) big, complicated, and difficult-to-manufacture systems-on chips that currently run everything from smartphones to supercomputers.” Instead they would be able to “use a conglomeration of smaller, simpler-to-design, and easier-to-manufacture chiplets tightly interconnected” on their fabric.

They note that relying on chiplets instead of SoCs isn’t a novel idea. Intel, Nvidia and other semiconductor companies have explored the same concept. But the researchers want their silicon-interconnect fabric to go beyond the new packaging those companies are exploring to overcome what they view as fundamental problems with PCBs: their flexibility, their reliance on soldering and their size.

So how would they address those problems? It starts with “a relatively thick (500-µm to 1-mm) silicon wafer” to which “processors, memory dies, analog and RF chiplets, voltage regulator modules, and even passive components such as inductors and capacitors can be directly bonded.” That would also allow “micrometerscale copper pillars built onto the silicon substrate” to replace solder bumps.

Those changes would “produce copper-to-copper bonds that are far more reliable than soldered bonds, with fewer materials involved,” they said. But perhaps more importantly they would mean “the chip’s I/O ports can be spaced as little as 10 µm apart instead of 500 µm” so one could “therefore pack 2,500 times as many I/O ports on the silicon die without needing the package as a space transformer.”

Silicon would also be a better heat conductor than the FR-4 material currently used in PCBs, they said, allowing “up to 70 percent more” heat extraction when two heatsinks are placed on the sides of the silicon-interconnect fabric. Better heat extraction means better performing components that don’t have to be artificially constrained because otherwise they’d get too hot to run safely.

Those are just the benefits afforded to current form factors. The researchers believe silicon-interconnect fabric “should let system designers create computers that would otherwise be impossible, or at least extremely impractical,” too. That’s assuming development on the technology continues, of course. Right now they’re addressing its potential, not promising it’s ready to be used in the real world.

“Our research shows that the printed circuit board could be replaced with the same material that makes up the chips that are attached to it, namely silicon”.