A Parylene-Based Ultra-Thin Printed Circuit Board As a New Platform for Flexible Sensors and Wearables
Authors: Selbmann Franz , Roscher Frank , Gueth Frederic , Wiemer Maik , Kuhn Harald , Joseph Yvonne 
Published: November 2022

Flexible electronics and sensors are a key enabling element for the realization of wearables and geometry adaptive devices needed to follow current trends such as the Internet of things or Industry 4.0. Within this paper, we present a new and flexible packaging platform by the fabrication of an ultra-thin and highly flexible printed circuit board (PCB).
The thermoplastic polymer Parylene, which combines a variety of unique properties such as optical transparency, biostability and biocompatibility according to ISO 10993, thermal stability as well as low permeability for gases and water, was used as the base for the new PCB.
Particularly, its mechanical properties, especially the absence of intrinsic stresses, as well as its low Young’s modulus and good bendability are advantageous when using Parylene films as a free-standing substrate for flexible applications. The chemical inertness of Parylene against all common acids, bases and solvents ensures its compatibility with established standard microsystem technologies.
For the realization of ultra-thin flexible PCBs, Parylene was used as a substrate as well as for the encapsulation and protection layer, respectively. Furthermore, Parylene was the dielectric between the metallic conductive layers. These redistribution layers (RDL) were deposited and patterned by standard microsystem technologies, such as sputtering, lithography and wet chemical etching. Hence, structure sizes of down to 10 µm were successfully realized. Different metals, such as gold, copper, and aluminum were tested for the RDL. Alternatively, printing technologies such as screen printing and aerosol jet printing were successfully demonstrated for the if conductive silver layerse. The electrical connection between the various RDL was of special interest throughout the study and was realized by etching vertical vias through the Parylene using oxygen plasma. Due to the low total thickness of the flexible Parylene PCB of 20 µm or even less, the obtained vias through the Parylene feature an advantageous low aspect ratio. For the contact formation through the vias, different methods were investigated, e. g. sputtering, printing and electrochemical deposition.
Finally, the fabricated flexible Parylene PCB were characterized with respect to their bendability and electrical properties. Doing so, the resistivity and capacitance were measured as well as the frequency response between 20 kHz and 500 MHz. Particularly, for frequencies below 1 MHz the realized flexible Parylene PCBs show a good electrical performance.
A third function of Parylene with respect to the realization of an ultra-thin and flexible PCB is its usage as a barrier or capping layer. Doing so, new approaches such as embedding electronic or sensor components in the flexible PCB can be enabled. Other integration technologies such as wire bonding, printing or gluing were investigated with respect to their usability for the new flexible PCB as well. Besides, also the direct fabrication of sensors on the Parylene flexible PCB was successfully demonstrated by the realization of a flexible potentiometric pH sensor.
The presented new type of ultra-thin PCB is a versatile platform for the realization of flexible electronics, sensors and devices. Due to its biocompatible properties, the flexible Parylene PCB can be used for medical applications but also for the integration of MEMS, chemical sensors and optical components, respectively. Doing so, it enables the realization of smart systems for wearable applications and their integration into light weight constructions. Within this paper, the concept of this new approach, the respected fabrication and integration technologies as well as possible applications are presented in detail.

Figure 1