Development of Biocompatible Coatings on Flexible Electronics

Development of Biocompatible Coatings on Flexible Electronics Rabindra N. Das, Frank D. Egitto, Mark Poliks Endicott Interconnect Technologies, Inc., 1093 Clark Street, Endicott, New York, 13760. Telephone No: 607-755-1389, E-mail: rabindra.das@eitny.com Abstract This paper discusses silicone-coated flexible substrates to provide biocompatibility for implantable devices. In particular, we highlight recent developments on silicone coatings on high density, miniaturized polyimide-based flexible electronics. A variety of high density circuits ranging from 11 microns lines/space to 25 microns lines/spaces were processed on polyimide flex substrates and subsequently coated with biocompatible silicone coatings. The electrical performance of silicone coated batteries was characterized by voltage measurements. The final structure enhances the stretching capability. Biocompatible coatings were characterized by optical microscope to ascertain coating thickness and surface characteristics. An Impedance Analyzer was used for electrical characterization. Silicone coated batteryies are stable after water, acid and base treatments. Voltage change was insignificant even after dipping 75% of the coated battery (i.e., excluding the exposed terminals) in strong KOH (ph:13) solutions for over 60 days. The paper also describes a novel approach for the fabrication of silicone coated, flexible wearable miniaturized electronics for possible clothing applications. 1. Introduction: There is a strong desire to develop biocompatible electronic substrates that can meet the growing demand for miniaturization, high-speed performance, and flexibility for medical devices. To accomplish this, new packaging structures need to be able to integrate more dies with greater function, higher I/O counts, smaller pitches, and high reliability, while being pushed into smaller and smaller footprints [1,2]. As a result, the microelectronics industry is moving toward alternative, innovative approaches as solutions for squeezing more function into smaller packages [3]. Flexible packages using polyimides as base materials, with fine-line circuit features having traces narrower than 12m, have been developed for a variety of high-performance medical applications [4]. Flexible polyimide with one or two metal layers provides the smallest possible roll diameter for systems such as catheters. Although polyimide is extensively used for making flexible electronics, it is not biocompatible. On the other hand, silicones are used as flexible, stretchable, biocompatible materials. However, it is difficult to fabricate high-density electronic circuitry on silicone substrates. Silicones are also known for contaminating plating baths. In this paper, the authors report a novel approach to coating of biocompatible materials onto flexible electronics surfaces. The silicone-coated flexible electronics were designed for use in biomedical applications, especially for implantable devices [5]. Biocompatibility of silicone-coated polyimide material represents a key advantage for flexible electronics systems in that they enable high-density fine-line structures not achievable using pure biocompatible materials such as silicones. Silicone coatings are applied by a lamination process. A variety of silicone coating thickness was achieved using this process. The biocompatible coatings were used to encapsulate circuitized polyimide substrates having one or two metal layers. Biocompatible coated flexible electronics offer many advantages over normal flexible substrates. They convert flexible substrates to biocompatible flexible substrates. They provide controlled coating over a wide range of thickness. Thick silicone coating imparts additional stretchability. The concept is extended to develop silicone-coated flexible electronics that are colored for wearable electronic applications. 2.1 Microflex and Ultra Fine Pitch Flip Chip Assembly Flexible technologies are important at all levels of microelectronics due to their desired properties, bend radius, and ability to fit within the devices. The classification of flex is varying with its rapidly growing technological significance in electronics. Continuous technological growth and application of new complex structures are forcing changes in the limits placed on the term of classical flex. Now flex defines the technology relating to any bendable or stretchable structures or devices. The performance of flex is primarily determined by its bend radius. The final structure bend radius solely depends on dielectric layer (composition, thickness),circuit design, and number of layers. Most of the miniaturized SWaP (size, weight and power) reduction applications use high density flex circuits due to their flexibility, reduced weight, and space savings. For example, flexible solar cells, displays, sensors typically use high density flex substrates. Multi-layer rigid structures can be converted into single or double layer high density flex structures and open up new directions for SWaP reduction in next generation technology applications. Our current flex technology is capable of fabricating a variety of flex devices by utilizing thin or ultra-thin polyimide flex material and the fine line circuitization required for SWaP reduction. Circuitized solder dams help to prevent solder bridging during flip chip assembly, and is key for use of bare die for fine pitch applications. And finally, thin or ultra-thin flex substrates must be maintained flat and taut during die placement and assembly processing at elevated temperatures. Fine line circuitization was achieved using a semi-additive, or pattern plating, process. Figure 1 illustrates a SEM micrograph and a photograph of a cross section of a double-sided high density flexible substrate. Minimum 11 µm line width and spacing between lines are defined using a semi-additive plating

process. In this case, minimum 11 µm line width and spacing between lines are achieved using 2-10 µm thick metal layers. Vias having diameters of 25 to 50 µm are drilled through the polyimide film using a frequency-tripled Nd-YAG laser, and subsequently plated using the semi-additive plating process. The circuitized flex substrate was coated with flexible soldermask, 6-10 µm thick, on both sides of the substrate, prior to placement of die and/or other surface mounted components on the flex. research groups have been searching for new ways to fabricate biocompatible, miniaturized flexible substrates for implantable devices. Silicon, silicon dioxide, silicon nitride, silicon carbide, metallic gold, SU-8 photoresist, parylene, and silicones are used for biocompatibility applications [6-10]. In the present study, silicone is chosen for its biocompatibility, stechibility and commercial availability. In the present paper, a novel biocompatible coating approach that have the potential to surpass conventional coating to produce thin, stretchable, and applicable over large flexible polyimide substrate is reported. Specifically, we are focusing on new composite biocompatible silicone materials that can be deposited into substrates or used in a roll-to-roll manufacturing processes. In the present process, silicone increases overall strechability, whereas the polyimide provides better processability, flexibility and mechanical robustness. The effects of coating thickness on the observed flex performance and the stability of coated substrates are presented. A variety of silicones and their deposition into polyimide were investigated in order to achieve thin uniform coatings. In a typical procedure, silicone was prepared by mixing appropriate amounts of the individual components. A thin coating of this silicone was then deposited on a circuitzed substrate. A circuitized polyimide substrate was sandwitched between two thin silicone coatings and then laminated together. A silicone-coated battery was prepared to measure voltage under extreme environmental conditions. A Figure 1. Top: SEM microghaph of circuit traces and plated vias on a flexible poyimide substrate. Bottom: Optical photo in cross section of a double-sided flex substrate with 25 µm plated vias and 11 µm plated metal traces. 2.2 Biocompatible Coatings: US implantable medical devices are forecast to exceed $50 billion in 2015. Pacemakers, implantable cardioverter defibrillators ICDs, neurostimulators etc are among the most common implantable device available in the market. Today s implantable devices demand increased reliability, extended operational life, as well as significant SWaP reduction. Advanced miniaturized flex solutions are achieving significant reduction in physical size for existing printed wiring board assemblies. Primary reductions in size and weight are due to polyimide film and high density circuits. Although polyimide is an excellent materials for high density flexible circuits, it is not biocompatible. Over the years several B

C bonding and good quality coatings. Figure 2 shows a series of filled silicone coated thin and thick films. Thicker silicone coating facilitates stretching as can be seen in Figure 2(D) and Figure 2(E). The color of the coated polyimide film can be seen through the somewhat transparent thin silicone ocatings. Film transparency reduces with increasing thickness and eventually attains the color of the filled silicones. Figure 2(A) represents a thin transparent coating using unfilled silicones. It is interesting to note that present technology can produce transparent, stretchable, thin silicone-coated flexible electronics (see Figure 2A). D E Figure 2: Various silicone-coated substrates. (A) Transparent silicone coated electronics, (B)-(C) Thin colored silicone coated flexible substrates, and (D)-(E) Silicone coated flex before and after stretching, respectively. A real challenge in the development of large area thin silicone coatings is the incompatibility that exists between the polyimide and silicone matrix. As a result inferior coatings with poor performance are obtained. Proper design and surface treatment have been identified that result in excellent Figure 3: Photographs of silicone coated flexible electronics shown in cross section. (A)-(E): Lower to higher

magnification photographs of silicone-coated flexible electronics containing multiple silicon dies. Figure 6: Voltage change with time. Figure 4: Photographs of silicone-coated flexible electronics shown in cross section. (A)-(C): Lower to higher magnification photographs of silicone-coated flexible electronics containing circuit traces. Figure 3 shows cross sectional photographs of siliconecoated flexible electronics. Specifically, Figure 3 shows the die area in cross section. Flex modules containing multiple dies and circuit traces are coated with silicone. There was no defect observed in the dies, indicating that the coating process was not affecting the electronics. Figure 3C-E shows high magnification photographs. It is clear from the cross sections that the silicone material flows and fills the gap between the dies. Figure 4 represents silicone-coated flex substrates containing circuit traces only. Lamination of silicone fills the spaces between circuit traces uniformly. Figure 5: Silicone coated battery in KOH solution (ph: 13). Figure 5 shows silicone battery immersed (75% of the coated battery) in KOH solution. Silicone coating was used in such a way that only the battery terminals (+ve & -Ve) are exposed for testing. In general body fluid solution ph is around 7.4. The battery was immersed in acid (ph: 2.5) and base (ph: 13) solutions. Acid or base solution will destroy a battery with inferior coating. It is possible to make a wide variety of silicone-coated batteries with different coating thickness. The electrical properties of coated batteries fabricated from silicone coating showed stable voltage even after immersion at 13 ph for about 2 months. Initial voltage (prior to coating) of the battery was 4.4V. Over the last two months, voltage changed from 4.4 V to 4.3V. The small change in voltage is due to manual measurement which damages the electrode. On the other hand, for a battery immersed in based solution for 10 minutes, voltage measured after drying the battery was 3.7 volt, and thereafter continuously degraded. So, acid or base solutions were not able to penetrate the silicone coating barrier. Figure 6 shows the room temperature voltage profile of coated battery immersed in acid and base solutions. The voltage profile is fairly constant in acid and base solutions. This indicates the silicone coating is stable enough for the battery or electronics to serve as an insulating material and will have no effect under body fluid solution (ph 7.4). 2.3 Wearable electronics: Wearable electronics are important for commercial and military applications. The market is forecast to exceed $1.5 billion in 2014. According to Google and Apple, the future is in wearable computing devices. Recently, Google s smart glass, Apple s smart watch, and Machina s MIDI controller jacket [10] are getting more and more attention for commercial applications. On the other hand, wearable electronics is essential to get real time information on the battle field. Military combat clothing are passive and need technology to embed electronics into the clothing to achieve better performance on the battle field. This paper presents a novel high-density, circuit-based, wearable electronics approach for extending performance beyond the limits imposed by traditional approaches. Specially, we are focusing to apply SWAP reductions to develop miniaturized

wearable electronics that can be embedded into military clothing. Figure 7 represents silicone-coated wearable electronics that can be used both inside or outside the cloth of military clothing, depending on its applications The substrates are built in parallel, assembled, aligned, and laminated to form a variety of high density flexible structures including embedded battery, system in a package (SiP), or RF substrates. This process offers many advantages compare to the conventional approach. For example, it enables designs having increased wiring density. The parallel processing of substrates leads to reduced fabrication cycle time. Several classes of flexible materials, including polyimides, PTFE, liquid crystal polymer (LCP), have been used to develop highperformance flexible wearable devices. Silicone coating was used to protect the electronics from moisture and other extreme environment. A variety of materials was used to understand durability of clothes. The present process allows fabrication of circuit traces having line widths narrower than 12 m in the flexible substrates. Substrate fabrication, reinforcement, and silicone coating provides a simple solution to build miniaturized flexible electronic circuits for military cloths. Flexible substrates and SWAP technology together have the potential to lighten a soldier s load. Figure 8 shows the TGA analysis of the silicone coated electronics at a heating rate 10 C/minute. TGA analysis of the substrates shows decomposition in the range of 325 C which is good enough for using clothing applications. Figure 9 shows cross sectional photographs of silicone coated flexible electronics. The flex module containing multiple dies and circuit traces are coated with silicone. Figure 9C shows a high magnification photograph. Figure 7: Wearable electronics for possible Military applications. Figure 8: Thermogravimetric analysis (TGA) for the silicone coated wearable substrates. 3. Conclusions A thin biocompatible coating technology based on silicone was developed to produce biocompatible polyimidebased flexible electronics. Lamination was used to fabricate silicone coated flexible electronics. Thicker silicone coatings enable significant stretching of the coated flex substrates.. The technology is able to produce a silicone coated battery. Silicone coated batteries were stable in extreme acid and base environments. Stability of silicone-coated flexible electronics in body fluid solution is under investigation. A variety of biocompatible flex structures have been demonstrated for the manufacture of large-area, flexible/rollable electronics. It is possible to produce silicone coated wearable electronics. These materials enable finefeature definition with excellent control of layer thickness. Collectively, the results suggest that biocompatible, flexible packages are attractive for a range of applications, not only where flexibility is required, but also in large-area microelectronics. The silicone coating offers the necessary isolation and biocompatibility for flexible electronics to be used in body fluid solutions. The observed lifetime of silicone coated device may be sufficient for long-term use as it survives extreme environments. Silicone coating also improves strechability of flex. Thicker coatings increase strechability. Acknowledgments The authors acknowledge the valuable contributions of G. Kohut, F. Marconi,B. Wilson, B. Pennington, and M. Shay. References 1. Rabindra N. Das, Frank D. Egitto, John Lauffer, Tim Antesberger and Voya R. Markovich, Z-axis Interconnections for Next generation Packaging Advancing MicroelectronicsVol.38(5),pp.12-19 (2012) 2. F.D. Egitto, S.R. Krasniak, K.J. Blackwell, and S.G. Rosser, Z-Axis Interconnection for Enhanced Wiring in Organic Laminate Electronic Packages, Proceedings Fifty-Fifth Electronic Components and

Technology Conference, May 31 to June 3, 2005, Lake Buena Vista, FL (IEEE, Piscataway, NJ, USA), p. 1132.] 3. Rabindra N. Das, Steven G. Rosser, Robert Welte, John M. Lauffer, Richard Kelly, Mark D. Poliks and Voya R. Markovich, Fabrication and Electrical Characterization of Embedded Actives and Passives for System Level Analysis: Towards Size, Weight and Power (SWaP) reduction Proceedings sixty first Electronic Components and Technology Conference,,2012,page. 1593 4. Frank D. Egitto, Rabindra N. Das, Glen Thomas, Susan Bagen, Miniaturization of Electronic Substrates for Medical Device Applications 2012, 45th International Symposium On Microelectronics in San Diego, California. 5. National toxicology program Report Number IMM90006, Contact Hypersensitivity studies in Female B6C3F1 mice US department of health and human services. 6. 12] Voskerician G, Shive M S, Shawgo R S, Von Recum H, Anderson J M, Cima M J and Langer R 2003 7. Biocompatibility and biofouling of MEMS drug delivery devices Biomaterials 24 1959 67 8. Liger M, Rodger D C and Tai Y-C 2003 Robust parylene-to-silicon mechanical anchoring IEEE Int. Conf. on Micro Electro Mechanical Systems (MEMS) (Kyoto) pp 602 5 9. Weisenberg B A and Mooradian D L 2002 Hemocompatibility of materials used in microelectromechanical systems: platelet adhesion and morphology in vitro J. Biomed. Mater.Res. 60 283 91 10. http://www.gizmag.com/wearableelectronics/) Figure 9: Photographs of silicone coated wearable electronics shown in cross section. (A)-(C): Lower to higher magnification photographs of silicone coated flexible wearable electronics containing multiple silicon dies.