Microelectronics Packaging:Packaging Substrates

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Packaging Substrates

An IC package falls into three basic categories: single-layer molded IC packages, single-layer ceramic packages, and multilayer packages.

In a single-layer molded IC package, the IC chip is first mechanically bonded to a lead frame and then electrically interconnected with fine wires from the chip bond pads to the corresponding lead-frame fingers. The lead-frame subassembly is imbedded in plastic after molding. In a single-layer ceramic package, the ceramic chip carrier is fabricated using either ceramic green-sheets or dry-pressing processes. For the multilayer type packages, the IC chip is assembled into a prefabricated multilayer substrate made of plastic or ceramic.

Single-Layer Molded IC Packaging

The molding compound used is man-made organic polymer (plastic) which is relatively porous and absorbs or transports water molecules and ions easily. Plastic packages are not very reliable because the aluminum metallization is susceptible to rapid corrosion in the presence of moisture, contaminants, and electric fields. Besides, impurities from the plastic or other materials in the construction of the package can cause threshold shifts or act as catalysts in metal corrosion. Fillers can also affect reliability and thermal performance of the plastic package.

Single-Layer Ceramic Packaging

Pressed ceramic technology packages are used mainly for economically encapsulating ICs and semiconductor devices requiring hermetic seals. Any contaminant present before sealing must be removed to an acceptable level before or during the sealing process. The hermetic package must pass both gross and fine leak tests and also exclude environmental contaminants and moisture for a long period of time.

Multilayer Packaging

The fabrication of a multilayer plastic substrate begins with impregnation of glass cloth with a thermo- setting resin solution to form a stable material termed “prepreg.” Several plies of prepreg are sandwiched between sheets of treated copper foil and laminated to form a copper-clad, fully cured epoxy–glass composite core. The cores are then circuitized using photolithographic process. The composite circuit board is then fabricated by interleaving the cores with additional sheets of prepreg and copper foil. Lamination, hole drilling, photolithography, and plating processes are repeated to construct a multilayer printed circuit board.

Multilayered ceramic substrates consist of a cofired stack of ceramic green sheets on which metal wiring is printed and vias are punched for interlayer connections. Ceramic powders of the desired composition are mixed and ground with organic binders, solvents, and plasticizers to form a slurry which is then cast with a doctor’s blade to form green sheet. Metallization of the green sheet and viafill are accomplished by thick film screen printing or extrusion filler. A schematic [3] of the whole process of making cofired multilayered substrates is illustrated in Figure 8.4. The tapes shrink during sintering and it is important to obtain uniform and repeatable sintering shrinkage throughout each part. The firing temperature dictates the metal wiring used for the circuitry. The sintering temperature of the powder has to be less than that of the melting temperature of the metal used for conductors. Pure alumina sinters at above 1500°C and hence requires refractory metals such as W or Mo for the wiring. These are generally referred to as high-temperature cofired ceramics (HTCC) and are losing popularity in the industry because of their high-temperature processing. In addition, alumina has a high dielectric constant and high CTE, further limiting its applications.

Glass ceramics can be sintered at relatively lower temperatures (<1000°C) and hence, more con- ductive metals such as Ag–Pd and Cu are used for the interlayer circuitry. These materials are referred

Microelectronics Packaging-0120

to as low-temperature cofired ceramics (LTCC). Glass ceramics typically consist of glass-forming compounds, mixed with alumina or silica. The glassy phases melt at low temperatures, completely wet the alumina, and aid in sintering. Compositions of glass ceramics can be adjusted to obtain the desired properties such as a thermal expansion coefficient close to that of Si and a low permittivity. The major drawback to glass ceramics in many applications is their very low thermal conductivity. Miniaturization and improved device performance can be achieved by integrating passive compo- nents such as resistors, capacitors, and inductors into LTCC substrates. Components made from LTCC are receiving wide attention for high-frequency/RF applications in the telecommunications industry [8].

An ideal substrate material should combine all of the following general characteristics:

• Low dielectric constant and loss factor

• Thermal expansion coefficient close to that of silicon

• High thermal conductivity

• High mechanical strength

• High dielectric strength and resistivity

• Good thermal shock resistance

• Chemical and thermal stability under conditions of processing and use

• Nontoxicity

• Cofirable with Au, Ag, or Cu

• Low cost.

Table 8.6 summarizes the thermal and electrical properties of some materials used in packaging [1,3,30–32].

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