Performance Analysis of Physically Flexible Commercial Off-The-Shelf Microcontroller with Soft Polymer Encapsulation

Flexible electronics have a large application spectrum including wearables, healthcare, human-machine interface and robotic skins. For such a system to be effective, it is crucial to render most of its components fully flexible while maintaining high performance. Traditional packaged complementary-metal-oxide-semiconductor (CMOS) chips offer high performance. However, they lack the physical flexibility highly coveted in flexible electronics. On the other hand, systems based on organic materials possess excellent physical conformity. However, many efforts are still being made to improve their performance, reduce their drive voltage and miniaturize them.

Thus, in this study, a commercial off-the-shelf (COTS) bare-die system-on-chip (SoC) microcontroller was integrated into a polymeric interposer, rendered flexible and encapsulated with flexible polymer. A thin metal stack was deposited and patterned on a flexible polyimide interoser using electron-bream evaporation and photolithography. The COTS bare-die SoC microcontroller was flip-chip-bonded onto the interposer. Photoresist spacer was formed to prevent lateral silicon etching during the silicon thinning step based on Bosch process. Finally, the device was encaupsulated with an SU-8 layer for mechanical support.

To quantify the microcontroller’s performance, sub-main clock frequency and power consumption were measured with an oscilloscope and a source meter, respectively, since they are critical parameters in digital logic and standalone embedded systems. Under zero mechanical stress, the flexible microcontroller showed no deterioration in clock frequency and power consumption. Furthermore, to investigate the flexibility and mechanical reliability of the system, its electrical performance was measured under mechanical stress, such as bending at bending radii of 50 mm, 20 mm, 10 mm and 5 mm and after 500, 1000, 5000, 10000 and 15000 bending cycles at a bending radius of 5 mm. The device did not exhibit degradation in its sub-main clock frequency and power consumption. This study confirms the potential of flexible bare-die integration for flexible transformational electronic systems, demonstrating its capacity for high data processing throughput, physical conformity and minimal footprint through heterogeneous integration.