Chapter 5 Device-level Packaging
5.2 Metal Packaging
areas, such as military, space, and underwater applications, sealing is still necessary.
In 1947, the first semiconductor transistor was invented. That’s the beginning of electronic packaging history.
In the 1950s, TO-type shell packages were developed with three electric pins and metal-glass sealing. During the same period, ceramic casting was invented, which provided a process lead for multilayer ceramics. In 1958, the first integrated circuit came out. Though metal glass packaging still took the leading role, the development of the multiwire package began to attract much more attention. With the development of the integrated circuit from small-scale to middle-scale then to large-scale, the scale of integration was getting higher and higher and more and more I/O pins were required, thus promoting the development of multilayer ceramics.
In the 1960s, the double in-line package (DIP) with a ceramic shell was born, that is, ce-ramic double in-line packaging (CDIP). With excellent electrical and thermal performance and high reliability, CDIP was highly favored by integrated circuit manufacturers and de-veloped quickly. It soon became a leading product in the 1970s. Later, plastic DIP was developed. It costs little to mass produce. Now it is still used widely in the low-end product market, with the pin number less than 84 and pitch usually 2.54 mm.
In the 1970s, with the development of SMT, a series of electronic package products for the SMT process were developed, such as leadless ceramic carrier, plastic leaded chip carrier, and quad flat package (QFP), and were put into commercial production in the 1980s. For its high density, small pitch, low cost, and easy surface mounting, plastic QFP became a leading product in the 1980s.
In the 1990s, more and more I/O pins were required for packaging ultra-large-scale in-tegrated circuit and pitch was getting smaller and smaller. Since then, electronic packages developed include quad flat package, such as QFP with 356 pins and 0.4 mm pitch, area array package, such as pin grid array (PGA) with 750 pins and 1.27 mm pitch, and ball grid array (BGA), over 3000 pins and 0.8 mm pitch. Now the development of advanced packaging for high density I/O is still booming.
Since the beginning of the 21st century, different CSPs of smaller size packages have become the emphasis of research and development. Packaging, at the same time, is moving in the direction of high density and high performance, such as 3D stack package, SIP, or SOP.
Various packaging methods are designed for different chips, which should satisfy special requirements of assembling and next level package or mounting. Depending on the package materials used, packaging can be divided into metal packaging, plastic packages, and ceramic packages, including metal ceramic package. These three packages will be discussed below.
5.2 Metal Packaging 5.2.1 Concept of Metal Packaging
Metal packaging uses metal as a shell or base to seal and protect a chip or chips. The I/O leads for transmitting electric signal, power, and ground in metal package is mostly sealed with glass metal or metal ceramic material.
Because of its good performance in thermal dissipation and electromagnetic shielding, metal packaging is usually used in highly reliable or customized hermetic packages. The main application of packaging modules, circuits, and devices includes packaging microwave multichip modules and hybrid circuits, power devices, ASIC, photo-electronic devices, and other specific devices.
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5.2.2 Advantages of Metal Packaging
Metal packages feature high precision, strictly controlled size, low cost, batch production capability, excellent performance, wide applicability, high reliability, and large packaged free space.
With various methods and flexible processing technologies, metal packages have good compatibility with some components, such as hybrid integrated A/D or D/A modules, ap-plicable for packaging of low I/O single chips or multichips. In addition, it is apap-plicable for packaging of MEMS, RF, microwave, optoelectronic, surface acoustic wave, and large power devices. It can meet the requirements of packages in small batch production with high reliability. Moreover, in order to provide good thermal dissipation, the metal structure also serves as a heat sink in most metal packages.
5.2.3 Process of Metal Packaging
A typical process of metal packaging is shown in Figure 5.3. The first step starts by making the metal lid and shell of metal package. Then electrodes on the shell are made to provide input and output ports for power supply and electrical signal transmission. The electrodes are sealed and insulated with the metal/glass/metal sealing method. After the thinning and dicing processes, the chip will be mounted on the shell with the attaching and wire bonding steps. The last step is capping.
Making metal lid Making metal shell Making pins Glass insulator
Making mold Assembly pins and
insulator Assembly to shell
High temperature sintering
Capping Attaching and
wire bonding Thinning and dicing
Figure 5.3 A typical process of metal packaging
Metal parts should be baked before the chip assembly to remove the gases or moisture out of the surface of the metal to reduce device defects caused by corrosion. During assemblly, high temperatures cannot be kept for a long period of time. Instead, a temperature curve should be followed to reduce the thermal influence of the postprocesses on the preprocess etch steps.
Capping is a special technique used in metal packages. The usual capping techniques include resistance welding seam welding, pulse heat fusing, laser welding, and solder sealing or any other microjoining method. During the capping process, there should be no gap between the sealing surface of the package cover and shell, and the sealing parts should be precisely aligned, since sealing defects of the devices Otherwise there may be. Capping is usually done in a dry protective environment, such as nitrogen, to reduce moisture and other harmful gases.
Seam welding is one of the highly reliable capping technologies.[1] Materials like cover plates or lids may greatly affect the hermetic performance and the leaking rate of hermetic end products. High-quality cover plates for seam welding should have the following features:
5.2 Metal Packaging 91 (1) The thermal expansion coefficient should be similar to that of shell and solder rings and close to that of the ceramic substrate. (2) The melting point of bonding parts should be as low as possible. (3) They should have excellent corrosion resistance. (4) They should have a small tolerance in size. (5) They should have a uniform and smooth surface without any burr or contamination.
The base materials mostly used now are alumina ceramics and Kovar alloy. The metal solder ring that can match ceramic’s coefficient of thermal expansion (CTE) is Kovar alloy or 4J42 ferro-nickel alloy.
Kovar alloy’s melting point is 1460◦C. In order to reduce the melting point of the lid, nickel-phosphorus alloy is plated onto the bonding surface, and a low bonding temperature of 880◦C is thus achieved.
Tin-lead solders are widely used for bonding; alloying additions such as indium and silver are sometimes added to improve the strength or fatigue resistance. Use of bismuth-tin alloys for sealing with a lower melting point than eutectic tin-lead solder has also been suggested. And in addition, Au-Sn is a common bonding material, especially for solder bonding between two materials with similar CTEs. Eutectic (80:20) Au-Sn alloy soldering is also called brazing. In furnace sealing, the typical reflow time is 2–4 minutes above the eutectic temperature of 280◦C, with a peak temperature of about 350◦C. When bonding two parts with mismatched CTEs, fatigue failure may appear after thermal cycling tests. In addition, Au-Sn is also a fragile material and can only stand low stress.
5.2.4 Traditional Metal Packaging Materials
To provide chips with mechanical support, electric interconnection, thermal dissipation, and environmental protection, metal package materials should meet the following require-ments:
(1) Low CTEs similar to that of chips or ceramic substrates to reduce or avoid thermal stress.
(2) High thermal conductivity to provide thermal dissipation.
(3) Good electrical conductivity to reduce transmission delay.
(4) Good EMI/RFI shielding performance.
(5) Low density, high strength and rigidity, and good processing or molding characteristics.
(6) Good characteristics for plating, welding and corrosion resistance, easy to attach chip onto substrates reliably and seal lid with shell hermetically.
(7) Low cost.
Selection of metal materials may directly affect the quality and reliability of metal pack-ages. The common-used materials are Al, Cu, Mo, W, steel, Kovar alloy, CuW (10/90 or 15/85), SilvarTM(Ni-Fe alloy), and CuMo (15/85). All of these materials have good ther-mal conductivity with higher CTEs than that of silicon. The density, CTE, and therther-mal conductivity of some materials are listed in Table 5.1.
Table 5.1 Main performance of commony used package materials
Materials Density (g·cm−3) CTE (×10−6K−1) Thermal conductivity [W/(m·K)]
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5.2.5 Novel Metal Packaging Materials
Traditional metal packaging materials are single metals or alloys except Cu/W and Cu/Mo.
These materials all have their own limitations and are not able to meet the requirements of the modern package technology development. In recent years, a lot of metal matrix composites (MMCs) have been developed. They are composites with Mg, Al, Cu, and Ti or metal alloys such as the matrix, and they have granules, whiskers, short fiber, or continuous fiber as reinforcement. Compared with traditional materials for metal packaging, MMCs have the following advantages:
(1) It is possible to change the thermal physical properties of materials by adjusting varieties of reinforcement parts, volume fraction, fiber orientations, or matrix alloy to meet the requirements of package thermal dissipation and even simplify the package design.
(2) Flexible material manufacturing, especially direct molding, can prevent expensive processing costs and material waste caused in processing.
(3) The specially developed low-density and high-performance MMCs are quite likely to be used for aviation and avionics applications.
The main composite materials used for microsystem packages are composites of Cu-matrix and Al-matrix thermally matching with Si material. Their properties are shown in Table 5.2.
Table 5.2 Properties of Cu-matrix and Al-matrix composites Metal-matrix Reinforcement Thermal conductivity [W(m−1K−1)]
CTE (×10−6K−1) Density (g·cm−3)
Along with the development of electronics packages for high performance, low cost, low density, and system integration, requirements of metal packaging materials are getting higher and higher. MMC will play a more and more important role. Therefore, the research and application of MMCs will become a hot topic in the future.
5.2.6 A Case Study of Metal Packaging[2]
Most MEMS devices need vacuum packaging to ensure a good working environment for their movable parts. Metal packages with excellent sealing features are the first choice for high-performance MEMS vacuum packages.
Figure 5.4 is the schematic diagram of the MEMS vacuum package. The cap and house of the package are made of Kovar material. Then electrodes are formed by sintering the glass tubing with Kovar wire throughout the house. A house is fabricated, where low-temperature bonding material is used to form a hermetic seal between the cap and house.
The process flow of temperature sealing is as follows: first fix MEMS chip and low-temperature getter inside the house and cap, respectively, and introduce low-low-temperature solder inside the slot of the house. Then electrically interconnect the chip with feedthrough electrodes of the house by the wire bonding process. Second, assemble the house and cap into their positioning frames and move them into the vacuum chamber. Third, after pumping the vacuum chamber to a set pressure, heat up the cap at 400–500◦C for several minutes based on the activation specification of a low-temperature getter to activate the getter and
5.3 Plastic Packaging 93