Flexible electronics have been around for years. Decades ago, car manufacturers started using flexible printed-circuit boards (PCBs) behind vehicle dash panel instrumentation clusters, but they were large and clumsy. For today’s design engineers, the Holy Grail of electronic component flexibility would be a chip that can bend and stretch like human skin.
Most electronic systems and components are still rigid affairs, though, except for those that have mechanically enabled flexibility. The question is if there are enough applications that need a flexible chip. The answer is yes, there are plenty.
Implanted health monitors, for example, could wirelessly transmit patient data to a health centre. Smart textiles could be fashioned into intelligent clothing. And, mobile communication systems could be integrated into clothing.
These applications now look achievable. Imec’s associated lab at the University of Ghent has integrated an ultra-thin, flexible chip with bendable and stretchable interconnects into a package that adapts itself to curving and bending surfaces. The circuitry can be embedded in medical and lifestyle applications such as wearable health monitors or intelligent fashion where user comfort and unobtrusiveness are key design parameters.
So what’s at the heart of this flexible technology? Researchers thinned a standard commercially available microcontroller to 30 µm while maintaining its electrical performance and functionality. This die was then embedded in a slim polyimide package (40 to 50 µm thick).
Next, this very thin chip was integrated with stretchable electrical wiring, which was created by patterning polyimide-supported horseshoe-shaped wires developed at the Ghent laboratory. The package then was embedded in an elastomeric substrate called polydimethylsiloxane (PDMS). The conductors in PDMS behave like two-dimensional springs, enabling greater flexibility while preserving conductivity (see the figure).
“Future electronic circuitry will stretch and bend like rubber or skin while preserving its conductivity,” explained Jan Vanfleteren, who was responsible for the research on flexible and stretchable electronics at the lab.
An internationally recognised expert in this field, Vanfleteren has published work on the 3D stacking of ultra-thin chips and chip packages. His research has investigated flexible and stretchable electronics, especially wafer and chip thinning and the packaging and embedding of ultra-thin chips.
“This breakthrough achievement demonstrates that flexible ultra-thin chip packages (UTCPs) can be integrated with stretchable wiring, paving the way toward fully flexible applications. We anticipate the first appliances will be used in intelligent clothing, with medical applications following later. Once commercial products are introduced, I expect to see clothing with signalisation by using LEDs and embedded sensors to track movements,” Vanfleteren said.
Within the auspices of an European Commission funded project known as SHIFT (Smart High Integration Flex Technologies), technology has been developed to embed a 25-μm thin chip in an organic carrier consisting of cured polyimide layers.
Chip contacts are created using laser drilling or reactive ion etching through the polyimide to the chip pads and thin-film interconnection metallisation. The result is a bendable package that’s 60 μm thick, including the chip.
This type of package can be embedded in the inner layers of a PCB or a flex interconnection substrate. Packaged as a UTCP, the chip is embedded in the flex substrate during the flex production process. Passive components are assembled below and above the embedded UTCP.
For Vanfleteren, an important goal is to develop technology for an ultra-compact package of a number of chips, based on the stacking of ultra-thin chips.
Currently, chips are stacked using die-to-die and die-to-wafer bonding, followed by wire bonding. But according to Vanfleteren, the stacking of a multitude of dies combined with an increase in the number of interconnections results in severe yield and reliability problems.
UTCP stacking is expected to eventually combine the advantages of both package-on-package and through-silicon-vias technologies. It’s also expected to be cost-effective and extremely compact.
So intelligent clothing now looks possible. Maybe we aren’t far way from boldly wearing a Captain Kirk tunic that lets us telecommunicate by merely tapping and speaking into an insignia on our chests.