Currently, the circuit boards that connect components found in most of today's electronics are made using automated machines. The machines pick up tiny parts, place them onto the boards and solder them into place. It’s a process that requires a costly, high-precision robotic arm, as well as an expensive motorized stage. As the size of electronics continues to shrink, the precision requirements increase – and cost follows suit.
But a new technique that uses light, not robots, to assemble electronic components offers the promise of a cost-effective platform for the mass production of electronic components.
As reported in the journal Optics Express, the method is based on optical traps, which use light to hold and move small objects in liquid. The use of optical traps as a manufacturing tool has previously been hampered by the fact that the liquid must be removed – displacing the pattern or structure that has been formed. However, an innovative freeze-drying method developed by Shuailong Zhang, a member of Dr. Steven Neale’s Micromanipulation Research Group at the University of Glasgow, Scotland, changes that dynamic.
Zhang’s method involves freezing the liquid, then reducing the surrounding pressure to turn it directly from a solid to a gas – enabling it to be removed without disturbing the assembled components. When combined with an advanced optical trapping approach known as optoelectronic tweezers, electrical contacts can be assembled through a process that Neale compares to magic. "The forces formed by these optoelectronic tweezers have been compared to Star Trek-like tractor beams,” he says. “(They) can move objects through a medium with nothing touching them… discrete components assemble themselves almost magically, as they are guided by the patterns of light."
The researchers demonstrated the technique by using an optoelectronic trap to assemble a pattern of tiny solder beads. After removing the liquid and applying heat to fuse the beads together, electrical connections were formed.
The tweezers themselves are formed with a layer of silicon that changes electrical conductivity when exposed to light; the electric field that forms in the areas exposed to points of light allows particles to be precisely moved. Creating patterns of light points allows simultaneous movement of multiple particles.
The tweezers can be used to form thousands of traps at once -- offering the potential of massive parallel assembly. "In principle, we can move 10,000 beads at the same time,” said Zhang, who is now at the University of Toronto. “Combining this with our freeze-drying approach creates a very inexpensive platform that is suitable for use in mass production."
In addition, the freeze-drying method can be used with any type of electrical trap, or even traps formed with acoustic waves. When combined with the tweezers as an overall optical micromanipulation approach, a broad range of objects can be assembled -- semiconductor nanowires, carbon nanotubes, microlasers and microLEDs. "Eventually, we want to use this tool to assemble electronic components such as capacitors and resistors as well as photonic devices, such as lasers and LEDs, together in a device or system," Zhang adds.
The approach may also help to make it less expensive to connect everyday objects, from clothing to household appliances, to the Internet – the basic concept behind the internet of things (IoT).
The researchers are currently working on combining the tweezer and freeze-drying process in a single unit, along with developing a software interface to control the generation of a light pattern. Neale’s group has recently received funding to use the new optical micromanipulation approach to create high-energy density capacitors as a safer and faster-charging replacement for mobile device batteries.