Electronic Packaging Science and Technology. King-Ning Tu
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Название: Electronic Packaging Science and Technology

Автор: King-Ning Tu

Издательство: John Wiley & Sons Limited

Жанр: Химия

Серия:

isbn: 9781119418337

isbn:

СКАЧАТЬ technology. The first is the need of denser and denser I/O, which means the diameter of micro‐bump and the pitch between them has to be reduced. As to be shown in Chapter 3, hybrid‐bonds consist of Cu‐to‐Cu bonds together with dielectric‐to‐dielectric bonds are being developed. The second is Joule heating and heat dissipation, which will be discussed in Chapter 9.

      About the increase of I/O, from BGA to C‐4 joints, there is a RDL of Cu wires in the upper part of the polymer board. From C‐4 joints to μ‐bumps, there is an RDL of Cu wires at the lower part of the interposer chip. This second RDL is invisible in the figure, but it is new in 3D IC because it does not exist in 2D IC devices, where typically there are only two levels of solder joints. The failure of the new RDL is of concern.

Schematic illustration of (a) synchrotron radiation tomographic images of a similar device as shown in Figure 1.1. (b) Synchrotron radiation tomographic image of a 2.5D IC device, having a length about 4 mm, and a thickness and a height of about 0.5 mm. Schematic illustration of the cross-section of a typical 3D IC device.

      In comparing the structure of 3D IC to that of 2D IC, the difference is the stacking of multilayer of chips and the interconnects using TSV and μ‐bumps. On processing TSV, the thinner the chip, the easier the drilling of vias. On making μ‐bumps, its melting point should be lower than that of C‐4 joints, so that the latter will not melt upon the melting of the former. Thus, the basic challenges are that the wafer is thinner and the processing temperature is lower.

      From the viewpoint of packaging technology, we may say that the essence or the major challenge in 3D IC is to scale down the dimension of packaging structures so that it can match those in the chip technology. There is no Moore’s law in packaging technology, so it has room to shrink.

      What are the key functions of electronic packaging? The cell phone held in our hands is a movable electronic packaging product or a mobile computer, which enables us to compute and to communicate with the world around us. The set of chips in the cell phone can be arranged horizontally, side by side, but it takes space. Or they can be arranged vertically, one on top of the other, this is called 3D IC, and it reduces the form factor and takes less space. However, heat dissipation in 3D IC is harder because the packing is denser. When over‐heat occurs, it induces reliability problems. Over all, the product should be electrically, mechanically, chemically, and thermally stable.

      where T is the temperature, V is the volume of sample, dS/dt is the entropy production rate, and j2ρ is the Joule heating per unit volume per unit time. Typically, the power from Joule heating is written as P = I2R = j2ρV, where I is the applied current and R is the resistance of the sample. Thus, j2ρ is power density or Joule heating per unit volume per unit time of the sample, in units of Watt/cm3, and I2R is Joule heating per unit time for the entire sample, in units of Watt. Clearly, this is the reason why we need low‐power devices or low entropy production devices.

      While the cost of production of 3D IC can be reduced when it is in mass production, the problem of reliability due to over‐heating has to be solved fundamentally by a smart system design or by design‐for‐reliability (DfR) and by a critical selection in materials integration. To put it simply, we need to design low‐power devices, and also we need to understand heat production (Joule heating) in irreversible processes and heat dissipation in the device structure. [6] Hence, the science and engineering of electronic packaging come into focus.

      Entropy production is the most relevant understanding of failure induced by electromigration, thermomigration, and stress‐migration in irreversible processes. [7] Statistical analysis of failure requires the knowing of mean‐time‐to‐failure (MTTF). An example is Black’s equation of MTTF for elctromigration. In Chapter 13, we shall present a unified model of MTTF for electromigration, thermomigration, and stress‐migration on the basis of entropy production.