Large-area electronics
It is believed that in the future, many electronic assemblies on rigid substrates (e.g. FR4 printed circuit board (PCB) will be replaced by mechanically flexible or even stretchable alternatives. This is a consequence of the ambient intelligence vision where the citizen carries along more and more electronics systems, near the body, on or even inside the body.
These systems must be light weight, must preferably take the shape of the object in which they are integrated, and must even follow all complex movements of these objects, hence the need for stretchability. Typical examples are implants, intelligent textile, portable electronic equipment (e.g. mobile phones), robotics, car electronics, etc.
Since more than 2 years, the TFCG Microsystems Group of IMEC is developing technologies for elastic electronic circuits and systems in the frame of a number of collaborative projects RP020, RP167, P15277. Currently, 4 technologies are under investigation having the following common characteristics:
(1) the components have standard rigid packages and are connected with elastic electrical interconnections;
(2) electrical interconnections consist of meander-shaped metal tracks, which under deformation act as 2-dimensional springs;
(3) the circuits are embedded in an elastic material like PDMS (silicone) or poly-urethane (PU).
Depending on the technology, the metal meanders are formed using either electro-plating, photolithography and wet etching, or laser patterning. Electronic components are mounted using conductive adhesives or conventional lead-free SnAgCu soldering.
In 3 of the 4 technologies, interconnection patterning and component assembly are done on a rigid or flexible sacrificial or carrier substrate and elasticity of the circuit is introduced only during the moulding at the end of the process, when the sacrificial substrate is removed.
In this way, the processes are close to conventional printed circuit board (PCB) technologies, which is favorable for later industrialization of the elastic circuit fabrication technologies. Stretchability of up to 100% of these circuits has been demonstrated. The technologies are being developed for applications in intelligent textiles, robotics, implantable devices, health & wellness, etc.
For intelligent textile applications, washability of the circuits is also under investigation and in preliminary tests has been demonstrated successfully on a 20 light-emitting diode (LED) circuit, submitted to 2 consecutive conventional domestic washing cycles, using a normal household washing machine, a commercial detergent, 900rpm tumbling and 40degC washing and rinsing.
Figure 1: Functional LED circuit integrated on textile substrate after 2 domestic washing cycles.
Several functional demonstrators have been realized to demonstrate the feasibility of the technologies: a stretchable thermometer including a temperature sensor, a stretchable wristwatch with embedded liquid-crystal display (LCD), battery, push buttons and microcontroller chip as well as an inductive link for wireless transmission of energy and information for which a 2-month continuous underwater operation has been demonstrated.
Figure 2: Inductive link operating while immersed in water (together with KULeuven/ESAT/MICAS).
Currently, more complex circuits are under study, for which careful system partitioning into component islands, combined with elastic interconnections, is required. Also, high-density interconnect technologies like the ultra-thin chip package (UTCP) packaging technology will be combined with stretchable circuit technology in order to obtain a maximum comfort and unobtrusiveness.
Figure 3: Ultra-thin UTCP with 20µm thick TI MSP microcontroller die inside.