BOSTON – Flexible hybrid electronics is expected to be an approximately $3 billion market by 2030, according to IDTechEx. Flexible hybrid electronics is an emerging technology that removes a previous tradeoff, specifically between the flexibility of printed electronics and functionality of conventional PCBs, and is thus rapidly gaining traction, the firm added.

“There is rapidly increasing interest in flexible hybrid electronics. This is an approach that resolves the downsides of fully printed electronics and will ultimately be employed across many sectors," said IDTechEx CEO Raghu Das.

An FHE circuit uses printed conductive interconnects, antennas, and possibly sensors, while mounting complex components such as ICs. FHE has applications across multiple sectors, ranging from wearable technology to smart packaging.

One of the challenges is developing the ability to attach mounted components to flexible substrates, given the difference in mechanical and thermal properties, says the firm.  Furthermore, component attachment materials must have curing/reflow temperatures that are compatible with thermally fragile, low-cost flexible substrates such as PET and PEN. Conventional solder with a reflow temperature of 250°C can only be used with polyimide, leading to the orange color that characterizes the current generation of flexible PCBs.

A range of component attachment materials is being developed, including low-temperature solders with reflow temperatures compatible with low-cost polymeric substrates and field-aligned anisotropic conductive adhesives that enable components with closely spaced contacts to be mounted more rapidly.

Perhaps the most straightforward solution to high reflow temperatures is to produce solder with a lower temperature by changing the alloy composition, says IDTechEx. Solder companies are developing both low temperature (reflow < 170°C) and ultra-low temperature solder (reflow < 150°C), with the latter targeted at attaching components to transparent PET substrates, and thus for flexible hybrid electronics. Low-temperature solder alloys often include a substantial proportion of bismuth, which can make connections mechanically brittle.

An arguably more innovative approach to low-temperature solder has been developed by an early stage US company that has developed nanoparticles that contain supercooled liquid solder. This enables the solder flow to be achieved without heat, with either mechanical force or chemicals breaking the outer shell and enabling the previously encapsulated liquid solder to flow.

An alternative to solder that does not require reflow is conductive adhesive, which can be either isotropic or anisotropic. Isotropic conductive adhesives conduct in all directions and are suitable for LEDs, resistors, capacitors. The adhesive needs curing, usually thermally (although UV is also possible), although the temperature requirements are much lower than conventional solder. An example application is bonding RFID ICs to antennas.

Anisotropic conducting adhesives are more complex materials that conduct only in the z-axis. They are applicable for IC attachment, where applying individual dots of an ICA to I/O pads with a fine pitch may be challenging, risking a short circuit. In contrast, ACAs can be deposited in a homogeneous layer because conductivity is only in the z-axis and is therefore often available as films rather than pastes. ACAs also need heat and pressure to form a vertical electrical pathway by trapping the conductive particles embedded in the epoxy between the contacts. There is currently substantial innovation in ACAs, with self-assembly under magnetic or electric fields improving conductivity and reducing the temperature and pressure requirements, says IDTechEx. Expect to see increased adoption of ACAs as ICs with more I/O ports are incorporated within FHE circuits.

The emerging component attachment materials discussed above are at different levels of technological and commercial readiness. Field-aligned ACAs are expected to be used in commercial production sometime in 2021.

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