Interconnect substrates

Ceramic boards are currently widely used in high-performance electronic modules as interconnection substrates. They are processed from conventional ceramic precursors and refractory metal precursors and are subsequently fired to the final shape. This is largely an art a much better fundamental understanding of the materials and chemical processes will be required if low-cost, high-yield production is to be realized (see Chapter 5). A good example of ceramic interconnection boards are the multilayer ceramic (MLC) stractures used in large IBM computers (Figure 4.11). These boards measure up to 100 cm in area and contain up to 33 layers. They can interconnect as many as 133 chips. Their fabrication involves hundreds of complex chemical processes that must be precisely controlled. 

Extensive work has been reported on the deposition of individual cell layers and of full anode-electrolyte-cathode fuel cells on metallic interconnect substrates, much of it by VPS, with no sintering or other post-deposition heat treatments required [112]. However, so far relatively thick YSZ electrolytes, approximately 25 to 35 pm, have been needed to provide sufficient gas tightness [108, 114], so further optimization of the process is required to produce thinner, gas-tight electrolytes. Peak power densities of 300 mW/cm2 have been reported at 750°C for APS single cells [114], with four-cell stacks exhibiting power densities of approximately 200 mW/cm2 at 800°C [55],... 

In the multichip package, a variety of pretested chips (e.g., bipolar, MOS, and GaAs) and discrete components (decoupling capacitors and termination resistors) may be mounted on the high-density interconnection substrate. This approach is sometimes termed hybrid-wafer-scale integra-... 

For chips mounted face up, heat is transferred to the substrate by conduction through the interconnection layers, and because the polymer dielectric has poor thermal conductivity, heat conduction is often promoted by an array of metallized vias through the interconnection layers (Figure 16) (100). For face-down-mounted chips, the heat may be removed from the back side by using pistons (as in the TCM) or conductive fluids, or heat may be conducted through the solder bonds to the interconnection substrate (98). 

Multi-chip modules, in which chips are attached with TAB directly to an interconnection substrate, typically silicon or aluminum nitride or other ceramics, represent an emerging technology. The module interconnect circuits are generated with IC fabrication technology, but at a relatively modest level of resolution. Typically, as shown in Figure 6, the substrates can include power and ground planes plus one or more signal planes, and low dielectric constant interlayer materials such as polyimides. These modules, which have feature sizes ca. 5 to 50 pm, and which are mounted directly on the PC board or serve as the board itself,... 

Ordered polymer films made from poly benzthiazole (PBZT) and poly benzoxazole (PBO) can be used as substrates for multilayer printed circuit boards and advanced interconnects to fill the current need for high speed, high density packaging. Foster-Miller, Inc. has made thin substrates (0.002 in.) using biaxially oriented liquid crystal polymer films processed from nematic solutions. PBZT films were processed and laminated to make a substrate with dielectric constant of 2.8 at 1 MHz, and a controllable CTE of 3 to 7 ppm/°C. The films were evaluated for use in multilayer boards (MLBs) which require thin interconnect substrates with uniform controllable coefficient of thermal expansion (CTE), excellent dielectric properties, low moisture absorption, high temperature capability, and simple reliable processing methods. We found that ordered polymer films surpass the limitations of fiber reinforced resins and meet the requirements of future chip-to-chip interconnection. 

Figure 2). Orientation of the films is used to tailor and control the CTE, and the low dielectric thin film layers provide more controlled impedance and reduced crosstalk than other substrate materials. In addition to MLBs, ordered polymer films can be used to advantage in multichip molecules where silicon chips are directly bonded to the interconnect substrate. The very low permeability of PBO and PBZT films will protect the chips from moisture. Flexible circuitry is another promising application area for these films. 

In assembling hybrid microcircuits or multichip modules, ceramic interconnect substrates fabricated using thin-film or thick-film processes are attached to the inside base of a ceramic or metal package. Generally, film adhesives that have been cut to size are used to attach large substrates (greater than 1-inch square) while either paste or film adhesives may be used for smaller substrates. Substrates may be alumina, beryllia, aluminum nitride, or silicon. 

Flip-chip devices have solder bumps, other metal bumps, or even conductive adhesive bumps on the face of the device for I/O connections. During assembly, the devices are flipped face down, then mated and bonded to corresponding solder or metal pads on the package or interconnect substrate. In the quest to eliminate tin-lead solders, electrically conductive epoxy adhesives are beginning to be used for the bumps. 

The die may be attached and wire bonded or flip chip bonded to a small rigid interconnect substrate such as a ceramic or a plastic laminate, for example, bisma-leimide triazine (BT). Conventional wire bonding and overmolding processes, as in leadframe CSPs, are also used. 

Multichip packaging involves the attachment and interconnection of a variety of chip devices on an interconnect substrate that may be single layer or multilayer. There are two basic types of multichip packaging hybrid microcircuits and multichip modules. 

MCM-C interconnect substrates are produced from either low-temperature cofired ceramic (LTCC) or high-temperature cofired ceramic (HTCC). Either process can produce multilayer substrates having high numbers of conductor layers (up to 100), although for most applications 2-20 layers are sufficient. The fabrication of MCM-C involves thick-film processes that have wider lines and spacings (5-20 mils) than MCM-D, but are lower in cost. 

MCM-Ls are multichip modules whose interconnect substrates are produced from plastic laminates similar to PWBs, but having much finer dimensions. 

Chip-on-flex is similar to chip-on-board except that the die, chips, or CSPs are wire bonded, flip chip attached, or epoxy connected to a flexible circuit instead of to a rigid interconnect substrate such as a PWB. 

The prime function of adhesives is to mechanically attach or bond devices, components, heat sinks, wire, connectors, and other parts onto a circuit board or an interconnect substrate. Adhesives are also used as pastes or films to attach lids in sealing cavity packages and as dielectric films in fabricating multilayer interconnect substrates. The most important consideration in obtaining a reliable adhesive bond is the ability of the adhesive to flow and wet the surfaces. For a reliable bond, strong adhesion to both surfaces and strong cohesion within the adhesive are necessary. 

Besides this prime role of attachment, electrically conductive adhesives are widely used to form electrical contacts between components and the printed-wiring board or other interconnect substrate, such as thin-film or thick-film ceramic substrates or flexible cable. For this function, adhesives serve as low-cost alternates to wire, solder, and other metallurgical connections. 

The flow of heat from the junction of a semiconductor device through the device material and on through several layers of an interconnect substrate may be treated as a series of temperature drops or thermal resistances at each interface. The junction-to-case thermal resistance jc expressed in °CAV is widely used in thermal analysis. The junction-to-case thermal resistance is proportional to the drop in temperature from the semiconductor junction to the case and inversely proportional to the power dissipation, thus ... 

Nanoparticles of silver and carbon are also being studied as replacements for copper or gold plating or for copper filling of microvias in interconnect substrates. ... 

Table 4.4 lists commercial cleaning solutions currently used in the electronics industry for cleaning printed-circuit boards, assemblies, ceramic printed circuits, thick-film interconnect substrates, electronic components, wiring harnesses, and stencils and screens (used in applying adhesives and solder pastes). 

After the adhesive has been dispensed onto the PCBs or other interconnect substrate, bare die and other electronic components must be precisely placed. Of course, pick and place can be done manually using vacuum pick-up tools or tweezers, but this approach is useful only for small quantities, prototypes, or for rework. The risk of... 

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