Has the time come to say goodbye to printed circuit boards? Maybe not today, but soon, say two researchers from UCLA. Puneet Gupta and Subramanian Iyer make the case for what they call ‘silicon-interconnect fabric’ – SiIF for short – in which bare chips are connected directly to wiring on a separate piece of silicon.

“A group of us at the University of California, Los Angeles, and the University of Illinois at Urbana-Champaign architected such a wafer-scale system comprising 40 GPUs. In simulations, it sped calculations more than fivefold and cut energy consumption by 80 percent when compared with an equivalently sized 40-GPU system built using state-of-the-art multichip packages and printed circuit boards,” they write in a fascinating report (Goodbye, Motherboard. Hello, Silicon-Interconnect Fabric) posted on IEEE Spectrum yesterday.

Not that it was easy.

“Power turned out to be a major constraint. At a chip’s standard 1-volt supply, the wafer’s narrow wiring would consume a full 2 kilowatts. Instead, we chose to up the supply voltage to 12 V, reducing the amount of current needed and therefore the power consumed. That solution required spreading voltage regulators and signal-conditioning capacitors all around the wafer, taking up space that might have gone to more GPU modules. Encouraged by the early results, we are now building a prototype wafer-scale computing system, which we hope to complete by the end of 2020.”

In fact there are a variety of challenges, however the advantages outweigh them argue Gupta and Iyer. The researchers suggest printed circuit board could be replaced with silicon and that doing so 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.”

Unlike connections on a printed circuit board, “the wiring between chips on our fabric is just as small as wiring within a chip. Many more chip-to-chip connections are thus possible, and those connections are able to transmit data faster while using less energy,” write Gupta and Iyer.

Here’s an excerpt from their IEEE Spectrum article:

“We believe that a better solution is to get rid of packages and PCBs altogether and instead bond the chips onto a relatively thick (500-µm to 1-mm) silicon wafer. Processors, memory dies, analog and RF chiplets, voltage-regulator modules, and even passive components such as inductors and capacitors can be bonded directly to the silicon. Compared with the usual PCB material—a fiberglass and epoxy composite called FR-4—a silicon wafer is rigid and can be polished to near perfect flatness, so warping is no longer an issue. What’s more, because the chips and the silicon substrate expand and contract at the same rate as they heat and cool, you no longer need a large, flexible link like a solder bump between the chip and the substrate.”

The researchers acknowledge many of the challenges involved with switching to SiIF and note, for example, the cost of crystalline silicon is higher than that of FR-4. “However, our analysis indicates that when you remove the cost of packaging and complex circuit-board construction and factor in the space savings of Si-IF, the difference in cost is negligible, and in many cases Si-IF comes out ahead.” Their essay is a good read.

Link to the IEEE Spectrum article (Goodbye, Motherboard. Hello, Silicon-Interconnect Fabric): https://spectrum.ieee.org/computing/hardware/goodbye-motherboard-hello-siliconinterconnect-fabric