war between China and United States. The latter has put Huawei on its entity list or ban list. The hardware in 5G consists of satellites, servers, optical fiber lines, base stations, cell phones, and terminal sensors, and the software consists of various mobile internets, such as Android of Google. Huawei is especially good at making base stations.
Data and signals can be transmitted via satellites in sky, but satellites can be shot down easily in the beginning of a war. It can be transmitted via optical fibers underground and across oceans. The making of optical fiber requires the doping of rare‐earth elements, so the strategic importance of rare‐earth elements can be appreciated. Also it can be transmitted via base stations on land surface, so the construction of a large number of base stations is of national security concern. It is worth mentioning that Taiwan has successfully used its network of base stations to detect and control the motion and contact of people with Covid virus.
In the late nineties, dot‐com was developed but soon burst because cell phones were not available. After cell phone becomes popular, Apple, Microsoft, Amazon, etc. are now the biggest companies in the world, no more GE, IBM, and Exxon, because of the wide applications of mobile technology.
About the global standard of requirements in 5G technology, see Table 1.1, the first is point‐to‐point latency of signals, which is only a few milli second. Latency means the total time spent to send out a signal and to receive it back. In a chain of moving cars, if the first car stops suddenly, the second car must stop within the time of latency, otherwise an accident would occur. If we consider a human‐less vehicle, the LiDAR (not radar) on top of the car should be able to detect a sudden appearance of a pedestrian or a car, so that it can stop to avoid an accident.
Moreover, LiDAR and radar are line‐of‐sight techniques. Yet, we need to have a network of vehicle‐to‐everything in order to have non‐line‐of‐sight awareness to know what is behind a stationary or a moving object. Also it should have an ultrafast rate of transport of data in order to show clearly the change of images of the surrounding of a car moving at high speed. Besides, it can download or upload instantly news or weather reports. To download a movie will now take only three seconds in 5G technology. For the success of 5G, it must have a very large number of base stations, so that information can be received and transmitted continuously from place to place. It is no need to explain other standard requirements in Table 1.1, they should be clear. For example, any device being used under the hood of a car should have a high reliability because of heat. Then, low cost is important for the use of internet of everything in our home and office.
No doubt, 5G technology will enhance AI applications. At the end of this book, in Chapter 14, we shall discuss the need of using AI to accelerate the study of reliability, so that we may change it from a time‐dependent event to a time‐independent event. Another area for the use of AI will be the biomedical and health applications. For example, Chinese medicine has been based on big data for many years, and the technique of acupuncture could be improved with modern microelectronic devices. The link between microelectronics and biomedical applications will be the most important advanced technology in the future.
1.4 3D IC Packaging Technology
As the trend of miniaturization in Si technology slows down, microelectronics industry has been looking for ways to keep the downsizing momentum going, meaning to go to more‐than‐Moore! [1–3] The critical feature size in Si devices has already reached nanoscale, below 10 nm. Hence, it is harder and harder to make transistor circuits on a Si chip smaller and smaller without a large cost increase. At present, the most promising way to extend Moore’s law is to go from 2D IC to 3D IC. Actually, the paradigm change has occurred more than 10 years ago, but 3D IC is not in mass production, because of cost and reliability.
In semiconductor manufacturing, because the product quantity is extremely large, so high yield and high reliability are critically important. Low yield will increase cost, and poor reliability will lead to recall; one example is the battery failure of cell phones. For any consumer electronic product in mass production, the concern of reliability is critical, especially the electronic packaging in 3D IC for advanced consumer electronic products, which are widely used now for distance teaching and home office.
In this introductory chapter, we explain what is electronic packaging? Also, what are the science and engineering in it, especially those relate to reliability? If we want to add more functions to hand‐held devices, the operations of memory, logic, and special functions must be increased. At the same time, power as well as battery capacity must be increased too. A larger size battery will squeeze the volume of the rest of the device, which makes the heating problem worse. To remove heat, we must have a temperature gradient. If we consider a temperature difference of 1 °C across a microstructure of 10 μm in diameter, the temperature gradient is 1000 °C/cm, which will induce thermomigration. In turn, Joule heating will enhance electromigration, and thermo‐stress will induce stress‐migration. While these are time‐dependent events, they are of major reliability concern.
Figure 1.4 is a scanning electron microscopy (SEM) image of the cross‐section of a 2.5D IC test device. It has only two pieces of Si chips stacking on a polymer board. Electrically, they are interconnected by three sets of solder joints. At the bottom or on the outside of the polymer board is the set of the largest solder balls of diameter up to 760 μm, which is called the ball‐grid‐array (BGA). These balls allow the test device to be connected to the circuits on a printed circuit board. Within the polymer board, there are Cu wirings, as well as Cu plated‐through‐holes, which are not shown in the image. On top of the polymer board, there is the second set of flip chip solder balls of diameter about 100 μm, the so‐called C‐4 (controlled collapse chip connection) solder balls, connecting the board to the first Si chip, which is the “interposer.” In this test device, there is no transistor on the interposer, which is passive and serves only as a substrate without introducing thermal stress to the active Si chip on the top. Often this test device is called 2.5D IC due to the fact that the interposer has no transistors. If the interposer has transistors, it becomes 3D IC.
Figure 1.4 Scanning electron microscopy (SEM) image of the cross‐section of a 3D IC test device. It has only two pieces of Si chips stacking on a polymer board.
In the interposer, there are arrays of vertical through‐Si‐vias (TSV) plated with Cu, making connections to the third arrays of solder joints of diameter about 10–20 μm, the so‐called micro‐bumps or μ‐bumps, which join the interposer to the top Si chip. The top Si chip is an active device chip, so it has transistors. The thickness of the device in Figure 1.4 is about that of a US penny. The thinness of the device is a critical requirement due to the limit of form factor of mobile consumer electronic products. Consequently, the thickness of Si chips is thin too. The thickness of the Si interposer is about 50 μm, which is much thinner than that of a convention Si chip of 200 μm in thickness. The thin interposer has caused the warpage problem, as well as the heat conduction issue, to be discussed in the later chapters. The diameter of the TSV in the interposer is about 5 μm, so the aspect ratio of the TSV is 10.
In the above example, besides the active Si chip, the rest, which includes the interposer, can be regarded as electronic packaging. The packaging enables the Si chip to function, as well as to allow us, to interact with the outside world. In the packaging, it is worth mentioning that between two sets of solder joints of different sizes, there should be a redistribution layer (RDL) structure for circuit fan‐out. It increases the number