Column: Research notes


Diversification of the 3D electronics market


By Dr Matthew Dyson, Technology Analyst, IDTechEx M


ention an electronic circuit and you are likely to picture a printed circuit board (PCB) – a rigid rectangle in


a characteristic green colour with copper lines and a bewildering array of components soldered onto it. But does adding electronic functionality mean using a PCB and thus requiring shoehorning a rigid rectangle into the product? Te emerging approach of 3D electronics


suggests not. Instead of making circuits on a separate rigid board, 3D electronics instead involves integrating electronic functionality within or onto the surface of objects. Whilst antennas and simple conductive interconnects have long been added to the surface of injection- moulded plastic objects, 3D electronics is undergoing extensive innovation with new materials, metallisation methods and manufacturing methodologies. Whilst aerosol and material jetting enable


conductive interconnects to be applied to surfaces, in-mould and 3D-printed electronics enable complete circuits to be integrated within an object. Between them these approaches offer multiple benefits that include simplified manufacturing, reduced weight and novel forms.


Electronics on the surface Te best-established approach to adding electrical functionality onto the surface of 3D objects is laser direct structuring (LDS), in which an additive in the injection-moulded plastic is selectively activated by a laser. Tis forms a pattern that is subsequently metallised using electroless plating. LDS saw tremendous growth around a


decade ago, and is now used to manufacture hundreds of millions of devices each year, about 75% of which are antennas. However, despite its high patterning speed and widespread


adoption, LDS has some weaknesses that leave space for alternative approaches to surface metallisation. First, it is a two-step process that can require sending parts elsewhere for plating, thus risking intellectual property (IP) exposure. It has a minimum resolution in mass production of around 75µm, thus limiting the line density, and can only be employed on moulded plastic. Most importantly, LDS only enables a single layer of metallisation, thus precluding crossovers and, hence, substantially restricting circuit complexity. Given these limitations, other approaches


to applying conductive traces to the surfaces of 3D objects are gaining ground. Extruding conductive paste, a viscous suspension containing multiple conductive flakes, is already used for some antennas, and is the approach of choice for systems that deposit entire circuits on 3D surfaces. Aerosol jetting is another metallisation


approach, in which a liquid of relatively low viscosity, usually conductive ink, is atomised. Tis spray is then combined with an inert carrier gas and ejected from a nozzle.


In-mould electronics In-mould electronics (IME) offers a commercially compelling method of integrating electronics into injection- moulded parts, reducing manufacturing complexity, lowering weight and enabling new forms, since rigid PCBs are no longer required. Furthermore, it relies on existing manufacturing techniques such as in- mould decoration and thermoforming, reducing the barriers to adoption. Te basic principle is that a circuit is printed onto a thermoformable substrate, and SMD components mounted using conductive adhesives. Te substrate is then thermoformed to the desired shape and infilled with injection-moulded plastic.


10 November/December 2020 www.electronicsworld.co.uk


IME is especially well suited to human-machine interfaces (HMIs) in both automotive interiors and the control panels of white goods, since decorative films can be used on the outer surface above capacitive touch sensors. Whilst IME is likely to dominate HMIs in


the future, it does bring technical challenges. Chief among these is developing conductive and dielectric materials that can withstand the temperature of the thermoforming process, along with the heat and pressure of injection moulding. As a result, material suppliers are developing portfolios of materials aimed at IME, with conductive inks that can be deformed without cracking.


Fully 3D-printed electronics Least-developed technology is fully 3D-printed electronics, in which dielectric materials (usually thermoplastics) and conductive materials are sequentially deposited. Combined with placed SMD components, this results in a circuit with a, potentially, complex multilayer structure embedded in a 3D plastic object. Te benefit is that embedded circuit can be manufactured to a different design without the expense of manufacturing masks and moulds each time. Fully 3D-printed electronics are well suited


to short-notice manufacturing. Indeed, the US Army is currently trialling a ruggedised 3D printer to make replacement components in forward operating bases. Te technology is also promising for applications where a customised shape and functionality are important, for example, medical devices such as hearing aids and prosthetics. Te ability of 3D-printed electronics to


manufacture different components using the same equipment, and the associated decoupling of unit cost and volume, could also enable a transition to on-demand manufacturing. Te challenges are that manufacturing is fundamentally a much slower process than making parts via injection-moulding, since each layer needs to be deposited sequentially. Whilst the printing process can be accelerated using multiple nozzles, it is best targeted at applications where the customisability offers a tangible advantage. Ensuring reliability is also a challenge, since


with embedded electronics post-hoc repairs are impossible; one strategy is using image analysis to check each layer and perform repairs before the next layer is deposited.


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