In the last ten years, the technology for manufacturing lightweight, flexible PCBs has made huge progress. Lightweight flex circuits are usually associated with materials like Kapton. The use of those materials is typically limited to high-value applications due to price. Fast forward to 2015, and the landscape has changed dramatically.
Printed electronics makes the news on a regular basis. We hear about breakthroughs in printing semi-conductors, organic photocells, or triboelectric fabric. What often goes unnoticed is that the underlying circuits – manufactured on low-cost flexible substrates with copper traces – have quietly moved from the lab to the production floor. Printed copper flexible circuits are now routinely manufactured by the kilometre in a reel-to-reel process. As production volumes go up, costs come down.Printed copper
Printed flexible circuits are a key element in delivering the ultimate goal of complete printed flexible electronics. Today they are used to create hybrid circuits: flexible substrates carrying conventional SMT devices. Because the tracks are copper we can solder SMT devices using low temperature tin-bismuth solder. Components can be hand-soldered or assembled using pick-and-place machines and soldered in a reflow oven. We see these circuits used for everything from RFID and NFC applications to medical sensors and car seat heaters.
The starting point for this process is inkjet printing onto the substrate. Printing from CAD files means there are no tooling costs, and printing 10 patterns or 10,000 uses the same process and equipment. As the process is reel-to-reel there is no practical limit to the length of the printed circuit.A catalyst for creativity
We use an inkjet printer to print a catalytic ink rather than a conductive ink. The printed material is then passed through an electroless copper plating solution. The catalytic ink promotes a chemical reaction on its surface which causes copper film to grow by autocatalytic deposition. This produces highly conductive solid plated copper tracks.
So what does this process deliver today?
Lightweight, low-cost, flexible, hybrid circuits:Lightweight: A 50 micron PET substrate weighs in at under 80 gsm, like typical photocopier paper. Stiffer 125 micron PET is still lighter than business card material. Compare that to a typical 1.6mm FR4 board which is more like an 80 page document; 50 micron PET offers a 97% weight saving.Low-cost: Prototype quantities cost just £50 per linear metre. For production volumes, costs fall dramatically.Flexible: The printed substrates are flexed, bent, or folded to fit into confined spaces or behind curved surfaces. A printed PET substrate can be folded back on itself.
Even though the circuits are single sided, it is still practical to design quite complex devices. Circuits can use high-powered microcontrollers, compact QFN packages, and RF chips such as Bluetooth to deliver highly functional designs. For example, we have recently built a proof-of-concept sensor circuit with Bluetooth LE communications. It folds into the device and weighs just 0.5g.Lowering the barriers
Perhaps the most important aspect of this shift from lab to volume manufacture is the cost reduction. This opens new possibilities for high volume, low cost, low-end devices, and brings the technology within reach of hobbyists, makers, and innovators.
We believe that the future for flexible PCBs is bright. Autonomous sensors, data loggers, and beacons can be lighter and cheaper. Wearables like wristbands can have components distributed around the whole band. Intelligent packaging can exploit low costs for single use electronics.
So next time you take you creative mojo for a spin, think flex – you could put a new twist on your idea – literally.
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