Innovation in materials and substrate technology

Today’s flexible circuits are bendable (e.g. flex flat wires with rigid component parts) but not stretchable. The STELLA project will develop a stretchable electronic circuit technology that can be integrated in stretchable applications like bandages, floor carpets, elastomeric parts in mechatronics… The stretchability poses particular challenges with regard to the desirable stretchability of the final product and the need to achieve continuous operation through the complete range of strain.

Stretchable thermometer demonstrator (IMEC)

Specific challenges are in the development of stretchable materials (substrates and electric conductors) that have the right properties to allow fabrication in an interconnection pattern. Stretchable conductive materials are a new range of materials to be used in electronics. Apart from using new materials as stretchable conductors, also metallic conductors can be used that are formed in an appropriate shape so that they can be stretched together with the substrate. The material research, the process technology but especially also the characterization of the materials is a completely new area in electronic interconnection.

Innovation in interconnection and packaging technology

The interconnection technology for electronic components on stretchable substrates is highly innovative in that it combines components with highly different materials properties: solid vs. stretchable. Interconnection technologies will be employed that are available in either of the contributing technology fields, i.e. on one side soldering, flip chip technology, and under-bump metallization technologies will be used to produce mechanical anchoring devices, on the other hand dispensing, hot pressing (shaping?) and moulding technologies as used in elastomer manufacturing. The technologies have to be adapted to each other to form novel process technologies, that are also used for the substrate technology with stretchable conductors..

FEM simulation of interconnection area (IMEC)

Since assembly and interconnection up to now has exclusively been constrained to rigid or at maximum flexible (and/or stiffened) substrates, the full integration into the flexible matrix gives rise to new applications and markets.

Furthermore, with the continuing trend toward smaller packaging and to System in a Package (SiP), the integration of individual ultra-thin chips embedded at the same level as the interconnection appears as a must. However, up to now, this is mostly done on substrate hosts, which has a limited accuracy - and therefore limited integration density – and cannot be extremely thin. Further miniaturisation is obtained when thinned chips can be embedded into multilayer thin films. A technology for interconnection and assembly on a carrier substrate from which a PolyImide (PI) structure is released by dissolution of a sacrificial layer will be further developed in the STELLA project, with stretchable elastomer materials instead of PI, resulting in an innovative very high density electronic circuit.

Innovation concerning modelling, characterization and reliability measurement

Stretchable electronics with a sufficient mechanical reliability under complex mixed loading is fully new. When this consortium can provide reliable technology in combination with intelligent mechanical design, it will open a whole new market of possible applications. New mechanical and thermo-mechanical models for these stretchable electronic assemblies will provide the optimal design for the stretchable conductors in a polymer matrix, the flexible areas and the interconnection between stiff/stretchable parts. In that sense, the “parameterised modelling approach has unique capabilities to avoid the iterative approach of building prototypes.

Horse shoe shape conductor bundle (IMEC)

Characterization methods for stretchable electronics is new and will be developed inside the STELLA project. Dedicated equipment for cycled testing under complex loading conditions will be used here.

The modelling and characterization activities, together with technology development, will lead to new design rules, now non-existing for stretchable electronic assemblies. Reliability measurements and the modelling work will provide necessary inputs for a complete design rule set.

New insights will also be gained in investigation of the failure mechanisms of the stretchable assemblies, which will certainly be totally different from failures in regular (rigid and flex) electronic circuits.

Innovation concerning power management in stretchable electronics

New developments in the STELLA project will be done in the field of very thin batteries and their associated power management. Especially challenging will be the high temperatures (up to 170°C or even 250°C) that are required for the subsequent processes that will be necessary to incorporate some of the stretchable electronic parts into larger systems (floor coverings, elastomer parts…).

Power management will also foresee the possibility of connecting energy scavenging systems to the electronic circuit and its energy storage system. This will be conceived as a generic solution for autonomous portable or large area systems.

Innovative character of system design with stretchable electronics

The low power requirement of the STELLA systems will have a significant impact on the embedded software design. This will require special power management functionality to use the capacity of the battery or energy scavenging sources as little as possible.

The combination of flexible and rigid components & sensors in a stretchable design has a highly innovative character. Especially the system partitioning has to count with the possibilities and limitations of stretchable design, stretchable conductors, maximum size of rigid parts, etc.

The conductivity limitations of conductors in certain low-cost technology solutions will be another challenge to cope with in system design.

The low power requirement of the autonomous systems will have a significant impact on the embedded software design.

A very demanding and innovative part will be the design of suitable antennas integrated in the stretchable substrate : the antenna has varying dimensions and thus changing characteristics. This will cause particular problems in the design of the wireless system.

Innovation in manufacturing technology

A material that has no fixed surface structure is difficult to handle in conventional production equipment. For instance, printing processes are widely used in the electronics industry but will possibly be very limited in their use for stretchable substrates due to misalignment. Furthermore, a stretchable substrate must be able to support multi-layer interconnections for some of the electronics we wish to implement. Lastly, the aspect of wireless communication function brings the requirement for micro-electronic components that are used as standards in the industry. It is unclear if wirebonding is at all possible and the flip-chip process has never been done yet either on such new substrates, especially not in a production environment.