Every year brings a new crop of digital electronic devices with greater capabilities. There’s a rule-of-thumb called Moore’s Law that says the number of transistors in an integrated circuit will double every 18 months while the cost to produce it remains the same. It wasn’t long ago that a personal digital assistant with an 8 megabyte memory was a hot ticket device, yet today having gigabytes of memory is commonplace.
Miniaturization was crucial during the space race of the mid-twentieth century. The Soviets possessed more powerful rockets. In order to equal their capabilities, the payloads of American rockets had to be smaller and lighter.
As miniaturization increased the density of integrated circuit chips, component density increased on circuit boards. Surface mount components are soldered using automated techniques so there’s no need to maintain generous spacing between them. Technicians have little room for error when performing rework, reflowing solder joints, or replacing components. The space between component leads may be less than 0.010 inches or 0.254 millimetres. The tiny pads on the circuit boards are easily overheated and pulled up even with good soldering technique.
Newer technologies will allow for even greater component density. Right now, most boards have densities in the 10-20% range, but the increasing use of flip chip connections inside integrated circuits will permit density to climb to about 80%. Integrated circuits have tiny internal wires connecting the active devices to larger external leads, but in flip chip technology, the active device is mounted upside down. This allows the use of the bottom of the chip for connections to the circuit board. A modern flash memory chip may have nearly 100 leads, but a similar size flip chip can have over 1000. These connections are called ball grid arrays (BGA) because there are tiny bumps of solder on the underside of the IC. When heated in an oven or under a hot air soldering station, the balls melt to form electrical connections.
The most common transistors found in ICs are MOSFETS which measure from 50-100 nm. Carbon nanotubes, which can be metals or semiconductors depending on their composition, are 2 nm across. CNTFETs could make even greater circuit densities possible, and some of their other properties, like higher currents and lower losses, offer tremendous promise.
Environmental concerns are another key consideration in manufacturing as lead solder is slowly phased out. Conductive adhesives may be a substitute as are lead-free solders, but the long term reliability of these materials is not yet known. For some critical applications in medicine, the military, or aerospace, conventional lead solder may be a better choice for now, but conductive adhesives may be the future when it comes to flip chip technology.
Circuit boards can be manufactured with layers containing passive components like resistors, capacitors, and inductors. Other layers can contain optical circuits for fiber optics, microwave and RF circuits, and even the power supply. Current multi-layer board technology could make this possible very quickly, and is one key aspect of increasing component density. As a result, the differences between components and circuit board may become blurred.
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