Flexible hybrid electronics (FHE), where an integrated circuit is combined with several other elements such as printed sensors and a thin-film battery, are very promising because they combine the functionality of rigid electronics with the flexibility of printed ones.
Now, in a report published on May 22nd, the market research firm IDTechEx has shared its thoughts on the evolution of FHE and what it could mean for the industry in the future.
The report offers a comprehensive analysis of printed electronic technologies and markets, an area the company has been assessing and following for well over a decade.
Fundamental to FHE is the mounting of an integrated circuit (IC) fabricated via photolithography onto a flexible substrate and linking it to printed interconnects. This is not an easy process and presents a whole range of challenges.
First, silicon dies are rigid and have a different form factor and thermal expansion coefficient to the flexible substrate. Second, silicon semiconductor alternatives such as organic ones have lower mobility and are harder to produce with high enough yields.
In IDTechEx’s report, these technical challenges are identified and evaluated along with potential solutions.
One solution, which IDTechEx admits is a temporary “interim strategy” is to solder a conventional packaged rigid IC to a flexible substrate. These circuits are known as flexible printed circuits (FPCBs) despite having etched interconnects and their flexibility impeded by mounting a packaged IC.
A solution to this solution, says IDTechEx, is to use bare dies which have a very small IC attached at two points using a conductive adhesive to an antenna.
This application voids two of the difficulties associated with adding larger and more powerful ICs because, firstly, they are so small, and secondly, large attachment pads are viable since only two electronic connections are required. As the ICs become more capable and larger (incorporating Bluetooth and memory) their lack of flexibility becomes a problem for FHE applications since they cannot conform to the underlying substrate.
An example of a simple flexible printed circuit. Image credited to Billpnh, CC BY-SA 4.0
Flexible ICs from Thinned Silicon
Because of this, flexible ICs are now starting to emerge, says the report. The approach to flexible ICs is taking two distinct paths:
The first is to thin existing silicon chips by grinding them to just above the first junction. The fragile thin die ends up encapsulated in a thin polyimide layer which adds fracture protection, making handling easier.
The second solution is to use natively flexible ICs made from metal oxide. Applying the photolithographic techniques to deposit material, insulator, and metal oxide films on a polyimide substrate have enabled the creation of cheap, flexible ICs ideal for applications such as smart packaging where low cost is an ideal characteristic and lack of complexity is not a limiting factor. At the moment, natively flexible metal oxide ICs are primarily used in RFID tags.
Alternatives to Rigid Packaged ICs Are Needed
In the report’s view, FHE inherently needs alternatives to rigid packaged ICs that are used in conventional PCBs and FPCBs.
Thinned silicon dies made more robust via thinning may find use in applications with more complex processing requirements whereas natively flexible metal-oxide ICs, while currently limited to “simpler” applications due to their limited capabilities, may find more advanced applications as the technology develops.
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