Is This The World’s Smallest Computer?

How small could you make a computer? In a way, that’s a question that requires that a computer be defined, because you could measure the smallest computer simply in terms of the smallest area of silicon required to create a microprocessor. So perhaps it’s better to talk about a smallest working computer. Recent entries in the race for the smallest machine have defined a computer as a complete computer system which holds onto its program and data upon power-down, but this remains one that is hotly debated. You might for instance debate as to whether that definition would exclude machines such as the crop of 1980s home computers that didn’t store their programs and data, was your Sinclair Spectrum not a computer?

At the University of Michigan they have opted for the simpler definition with their latest entry in the race to be the tiniest. Their latest machine packs an ARM Cortex M0 into a 0.3mm cube, along with photoreceptors and LEDs for programming, data throughput, and power. It is designed to be a temperature sensor and logger for medical implantation, but it stands more as a demonstration of technological prowess than as a usable product.

Pictures of a tiny computer “dwarfed by a grain of rice” make for good mass media consumption but where’s the relevance for us? The interesting part comes from the tantalizing glimpse of its construction: this is a hybrid device upon which we can see the optoelectronic components have been wire-bonded. Unfortunately the paper, catchily titled “A 0.04mm3 16nW Wireless and Batteryless Sensor System with Integrated Cortex-M0+ Processor and Optical Communication for Cellular Temperature Measurement” does not appear to be free-to-view online, so we don’t have any more information. We wish that such feats were possible within our community, but suspect those days are still pretty far away.

The Mystery Behind The Globs Of Epoxy

When Sparkfun visited the factory that makes their multimeters and photographed a mysterious industrial process.

We all know that the little black globs on electronics has a semiconductor of some sort hiding beneath, but the process is one that’s not really explored much in the home shop.  The basic story being that, for various reasons , there is no cheaper way to get a chip on a board than to use the aptly named chip-on-board or COB process. Without the expense of encapsulating  the raw chunk of etched and plated silicon, the semiconductor retailer can sell the chip for pennies. It’s also a great way to accept delivery of custom silicon or place a grouping of chips closely together while maintaining a cheap, reliable, and low-profile package.

As SparkFun reveals, the story begins with a tray of silicon wafers. A person epoxies the wafer with some conductive glue to its place on the board. Surprisingly, alignment isn’t critical. The epoxy dries and then the circuit board is taken to a, “semi-automatic thermosonic wire bonding machine,” and slotted into a fixture at its base. The awesomely named machine needs the operator to find the center of the first two pads to be bonded with wire. Using this information it quickly bonds the pads on the silicon wafer to the  board — a process you’ll find satisfying in the clip below.

The final step is to place the familiar black blob of epoxy over the assembly and bake the board at the temperature the recipe in the datasheet demands. It’s a common manufacturing process that saves more money than coloring a multimeter anything other than yellow.

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Thermosonic Wedge Bonding

hot

In the past, if we’ve been doing smd soldering, we’ve used pretty basic hot plates. This project takes that idea a bit further.  Since [kc6qhp] will be using parts that aren’t conducive to soldering, he has to use wire bonding. After locating a fairly cheap wire bonding machine and microscope, he built the heated stage to fit perfectly with his other tools.  You’ll notice that he has machined a lip around the heat plate for small custom C-clamps as well as made it adjustable height. Very nice work [kc6qhp].