1
Overview of Flexible Electronic Encapsulating Technology

Zhenguo Liu and Yongji Chen

Northwestern Polytechnical University Ningbo Research Institute, Qingyi Road 218, 315821, Ningbo, China

1.1 Flexible Electronics Overview

Flexible electronics, with its unique flexibility, ductility and high efficiency and low cost manufacturing process, have wide application prospects in information, energy, medical, national defense and other fields. As with traditional integrated circuit (IC) technology, manufacturing processes and equipment are also the main drivers for the development of flexible electronics technology. Flexible electronics manufacturing technology level indicators include chip feature size and substrate area size; the key is how to create a smaller feature size of flexible electronic devices on a larger format substrate at a lower cost.

Compared to traditional electronics, flexible electronics have greater flexibility and can adapt to a certain extent to different working environments to meet the requirements of the device's deformation. Flexible electronics covers organic electronics, plastic electronics, bioelectronics, nanoelectronics, and printed electronics, including radio frequency identification (RFID), flexible display, organic electroluminescent (Organic Light-Emitting Diode, OLED) display and lighting, chemical and biological sensors, flexible photovoltaics (PVs), flexible memory or storage, flexible batteries, wearable devices, and many other applications. With its rapid development, the involved fields have been further expanded, and now it has become one of the research hotspots in cross-disciplinary research (Figure 1.1) [1].

In recent years, with the further improvement of flexible electronic technology, we have seen some unimaginable products. For example, the current attention is on folding-screen (Figure 1.2) and wrap-around-screen cell phones. In fact, whether it is a folding screen or a wrap-around screen, the essence of the use of flexible screen technology is that it is a form of flexible electronics technology. The flexible screen, flexible chip, and flexible electrode are only the tip of the iceberg of flexible electronics technology. In fact, information technology involves a variety of sensing, information transmission, information processing, energy storage, and other links that are expected to achieve flexibility [2].

Figure 1.1 The fields of flexible electronics.

So, how exactly is flexible electronics achieved?

First, let us understand the materials used in flexible electronics. Common materials for flexible electronics include flexible substrates, metallic materials, organic materials, inorganic semiconductor materials, and carbon materials represented by graphene (Figure 1.3).

After the raw materials are available, let us look at how flexible electronic devices are manufactured. There are three common flexible electronics fabrication methods: transfer printing, inkjet printing, and fiber structure formation. Among them, transfer printing is a series of functional arrangement techniques used to deterministically assemble micromaterials and nanomaterials into spatially organized structures with two- and three-dimensional layouts [4]. Inkjet printing, on the other hand, as the name implies, allows the direct deposition of functional materials to form patterns on substrates [5]. Flexible electronics fabrication methods based on fiber structures are well suited for wearable electronics that are lightweight, durable, flexible, and comfortable [6].

Since, as mentioned above, flexible electronics have so many advantages and broad application prospects, why has their development been slow to open up?

Figure 1.2 Folding-screen phone.

Figure 1.3 Thermally reduced graphite oxide (TRG).

Source: Kim et.al. [3]; © 2010, Reproduced with permission from American Chemical Society.

Two major obstacles impede the development of flexible electronics: mechanics and encapsulation. Shen Yang, vice president of the School of Materials at Tsinghua University, has said that the first challenge in the development of flexible electronics is the mechanics of the problem: flexible electronic components in repeated folding and bending will be constantly subjected to alternating stress over time, making them easy to crack. This problem can be overcome mainly through structural design. The second challenge is the problem of electronic encapsulating, which is to integrate the components on the flexible substrate tightly encapsulated together and achieve the desired function.

Figure 1.4 Flexible sensors.

Source: Hammock et al. [7]; 2013, Reproduced with permission from John Wiley and Sons.

Furthermore, the slow progress in the development of flexible electronics can be attributed to the absence of a significant "viral effect" in terms of application scenes. In other words, the folding-screen cell phone is not an industry pain point. However, another application of flexible electronics - flexible sensors - may be the real revolutionary change in the industry application scene.

Using flexible sensors and conductors, scientists can convert the external force or heat into electrical signals, which are transmitted to the robot's computer for signal processing, so that it can be made transparent, flexible, extensible, freely bendable, foldable, and wearable electronic skin in order to monitor the human body indicators in real time and accurately [7], as shown in Figure 1.4.

Recently, the Institute of Mechanics of the Chinese Academy of Sciences, in cooperation with researchers from Dalian University of Technology and Beijing University of Aeronautics and Astronautics, has developed a thin-film patch-type flexible curvature sensor for wearable devices from the mechanical structure design (see Figure 1.5). This sensor can accurately measure the dynamic bending curvature and bending angle of the measured surface, and its bending measurement results are not affected by tensile deformation. So, in practical application, it does not require the sensor to be perfectly bonded to the measured surface but simply fit, so it is no problem at all even with gloves or tights on. Also, this sensor is very suitable for integration with wearable apparel and can be applied to flexible smart wearable devices such as joint flexion monitoring, gesture recognition, and sitting posture monitoring [8].

Currently, there are two main approaches to the selection of flexible electronic materials internationally. One approach is to shift from traditional inorganic materials to organic materials, such as polymer materials and organic semiconductors, for flexible electronic applications. Another approach involves the combination of organic and inorganic materials, utilizing composite materials to develop flexible electronic devices.

Figure 1.5 Curvature sensors for joint flexion deformation, gesture recognition, and sitting posture monitoring. (a) Strain sensor. (b) Curvature sensor.

Source: Liu et al. [8]; © 2018, Reproduced with permission from John Wiley and Sons.

Since the discovery of graphene, two-dimensional materials consisting of single layers of atoms, such as boron nitride, molybdenum disulfide, and black phosphorus, have garnered attention from the semiconductor industry. Research related to these two-dimensional materials holds promise for the advancement of flexible electronics.

The successful application of flexible displays, flexible sensors, and other flexible electronic components signifies the transition of flexible electronics from theory to practice. This advancement may herald a new era of electronic device revolution, bridging the gap between humans and machines and fostering closer interactions.

1.2 Development of Flexible Electronic Encapsulating Technology

One generation of encapsulating, one generation of products. One generation of encapsulating, one generation of products. After the flexible electronic device is manufactured, before it is brought to market as a product, it needs to be packaged to isolate water vapor to ensure its stable operation and complete function. The encapsulating of flexible electronic devices is the same as the encapsulating of traditional electronic devices and is a branch of the encapsulating of electronic devices. When it comes to the word encapsulating, it first originated from the encapsulating of ICs, and the development of IC encapsulating was developed along with the development of IC chips. The history of the development of encapsulating is also the history of the continuous improvement of chip performance and the continuous miniaturization of the system. As the size of IC devices shrinks and the operating speed increases, new and higher encapsulating requirements are placed on ICs.

Figure 1.6 TO-type encapsulation (a) and double inline encapsulation (b).

Source: Reproduced with permission from huangye88.com / http://yiqiyibiao.huangye88.com/xinxi/5529un9e0ef8b2.html / last accessed 02 August 2023.

Therefore, before introducing flexible electronic device encapsulating, we review the development history of the IC encapsulating industry,...