Flexible circuit boards: enabling cheaper substrates

Indeed, it is so durable and insensitive to temperature that PI is routinely used in laboratory ovens, cryostats, and (when laminated with aluminium) wrapped around satellites.

Downsides of polyimide

These capabilities make it sound like the dream material for flexible electronic applications, but there are some downsides. Firstly, it is expensive at around 100x the price of PET (polyethylene terephthalate), a common transparent plastic used in drinks bottles. This does not matter if the plastic film is going in a laboratory or around a satellite, but as a circuit substrate for disposable smart packaging or other low-cost applications, it is a major downside.

Secondly, at the risk of stating the obvious, it is orange. This prevents use in optical applications, such as sheets covered with LEDs. Thirdly, the counterpart to its durability is that it does not stretch, so it is challenging to use in wearable skin patches or e-textiles.

All this means that moving to cheaper but more thermally fragile transparent substrates such as PET, PEN (polyethylene naphtholate) and stretchable TPU (thermoplastic polyurethane) is a major trend in the flexible electronics industry. However, it does necessitate a significant technological shift in how electronic components are attached to the substrate. This attachment of conventional components to flexible substrates with printed (rather than etched) interconnects is termed flexible hybrid electronics (FHE).

The new report from IDTechEx, ‘Flexible Hybrid Electronics 2020-2030: Applications, Challenges, Innovations and Forecasts’, discusses the technologies and innovations needed to enable electronic manufacturing on low-cost substrates. It is based on extensive primary research, including interviews and visits with companies from around the world, and is drawn on in the rest of this article.

Flexible hybrid electronics needs low cost substrates

Flexible hybrid electronic (FHE) promises to combine the functionality of conventional rigid electronics with the flexibility of printed electronics. The diagram shows a prototypical FHE circuit, in which an integrated circuit is combined with multiple other elements, including (for example) printed sensors, an antenna, a thin film battery and even thin film PV.

Not all components need to be included, or even printed, for a circuit to count as FHE, but it must include as a minimum both printed and placed functionality. The opportunities and challenges of FHE are comprehensively reviewed in the new IDTechEx report, ‘Flexible Hybrid Electronics 2020-2030: Applications, Challenges, Innovations and Forecasts’.

With a conventional rigid PCB, or indeed an FPCB, attaching components is done with solder, a crucial technology that is largely taken for granted. Solder has multiple benefits: the surface tension helps to pull placed components into the correct alignment thus reducing the requirements for precision placement, errors can be rectified by hand, and most importantly it provides excellent electrical and mechanical connections.

Unfortunately for manufacturers aspiring to use alternative cheaper thermally fragile substrates, conventional SnAgCu (SAC) solder requires reflow temperatures around 250 C, which is no problem for PI but causes melting and degradation in most other substrate plastics.

Anisotropic conductive adhesives

To date, this conundrum has been resolved by using anisotropic conductive adhesives (ACAs), which can be thermally cured at suitable temperatures. However, IDTechEx believe that they represent an interim solution for many applications, particularly those that require high durability - they are around 3-10x the price of solder, and most notably do not offer any benefits.

As such, much effort has been devoted to developing low-temperature solder, or on photonic sintering that can cause solder to reflow without damaging the substrate. Which of these approaches is ultimately most widely adopted is still an open question, but the development of PET compatible component attachment methods is undoubtedly a crucial step in furthering the adoption of low-cost flexible hybrid electronics.

These component attachment materials, along with case studies of innovative approaches including nanoparticles containing supercooled solder and field-aligned ACAs, are evaluated in detail in the new IDTechEx report, ‘Flexible Hybrid Electronics 2020-2030: Applications, Challenges, Innovations and Forecasts’.

A roadmap for substrate adoption

Given the requirements in terms of both technological development and manufacturing methods for low-cost substrates, IDTechEx envisage that they are most likely to be adopted in high volume, cost-sensitive applications. IDTechEx thus suggest that paper substrates will gradually become dominant for RFID tags and smart packaging. There will also shift away from PI towards cheaper, more thermally fragile substrates such as PET, led by applications that have lower durability requirements.

A comprehensive overview of FHE

The new report from IDTechEx, ‘Flexible Hybrid Electronics 2020-2030: Applications, Challenges, Innovations and Forecasts’, offers a comprehensive analysis of this emerging technology frontier. IDTechEx have been assessing and following the printed electronic technologies and markets for well over a decade. This report is based on fresh primary research including interviews and company visits with all the key players worldwide. It identifies and examines all the key innovation trends across the material and production systems, covering attachment, substrate, and metallization materials well as production techniques including high-throughput pick-and-place and various S2S and R2R printing techniques.

This report builds an application roadmap, showing how FHE will grow in complexity from simple RFID tags of today to complex flexible hybrid electronics of the future, enabling applications in smart packaging, industrial monitoring and wearable devices and more. It offers insightful analysis into application timeline, challenges, and innovation opportunities. Furthermore, it forecasts the growth of this enabling technology, showing that the market can reach X by Y with the inflection point at which rapid uptake occurs likely arriving in 2025.