Multilayer circuits

Silver Thick Films. About half of the silver consumed in the United States for its electrical properties is used by the electronics industry. Of this amount some 40% is used for the preparation of thick-film pastes in circuit paths and capacitors. These are silk-screened onto ceramic or plastic circuit boards for multilayer circuit sandwich components. 

Polymer films that are sensitive to light, x-rays, or electrons— known as photoresists—are nsed extensively to transfer the pattern of an electronic circuit onto a semiconductor surface. Such films must adhere to the semiconductor surface, cross-link or decompose on exposure to radiation, and nndergo development in a solvent to achieve pattern definition. Virtually all aspects of photoresist processing involve surface and interfacial phenomena, and there are many outstanding problems where these phenomena mnst be controlled. For example, the fabrication of multilayer circuits requires that photoresist films of about 1-pm thickness be laid down over a semiconductor surface that has already been patterned in preceding steps. 

More than 80% of all sodium chlorite produced is used for the generation of chlorine dioxide. Sodium chlorite is also used in disinfectant formulations and sterilization. Like chlorine dioxide, it must be registered with EPA under FIFRA for each specific application use as a disinfection. Sodium chlorite is used in other industrial settings in NO and SO combustion flue gas scrubber systems in the treatment and removal of toxic and odorous gases such as hydrogen sulfide and mercaptans and as a solution formulation to oxidize copper surfaces in multilayer circuit boards (Kaczur and Cawlfield 1993). 

Consider an I8 cm X 18-cm multilayer circuit board dissipating 27 W of heat. The board consists of four layers of 0.2-mm-lhick copper (k — 386 W/m °C) and three layers of 1.5-mm-thick epoxy glas.s (k = 0.26 W/m "C). sandwiched together, as shown in the figure. The circuit board is attached to a heal sink om both ends, and the temperature of the board at those ends is 35°C. Heat is considered to be uniformly generated in the epoxy layers of the board at a rate of 0.5 W per 1-cm X... 

Martin et al. (1989b) focus on cure of epoxies and cyanate ester resins for lamination of prepregs into multilayered circuit boards. The following dual Arrhenius engineering model was fitted to viscosity profiles ... 

Figure 2. Multilayer circuit board with surface mounted components.

Rigid-rod polymer films made from PBO and PBZT are being developed because they provide important performance improvements over competing materials such as fiber reinforced composites and metals. Table I summarizes some of the highest payoff applications. Thin PBZT films combined with low dielectric constant resins are being developed for multilayer circuit boards and multichip modules to increase interconnection density and speed with matched circuit impedance and reduced crosstalk. 

PBO and PBZT films can be made very thin (less than 0.002 in.) and can be impregnated with secondary resins, resulting in a substrate with a dielectric constant less than 2.8, an isotropic planar CTE of 7 ppm per °C or less, and temperature resistance over 250°C. Copper circuits and ground planes can be added by a variety of additive or substrative means, and multilayer circuit boards can be fabricated using plated through holes. Further development of these thin film dielectric substrates should result in interconnection density over 100 times greater than is currently possible with fiber reinforced epoxy multilayer boards. 

A cost reduction area that could have significant opportunities for chemicals companies is the interest in cheaper track generation systems such as screen printed conductive inks and the conductive pastes that are being used to replace plated through holes (PTH). Copper-based inks are already being used to form crossovers and simple circuits, and the use of thick film techniques of multiple layer printing is being evaluated to replace some multilayer circuits. 

Plated through hole multilayer circuit boards, MIL P 55640. Also see Printed wiring for elec tronic equipment, MIL STD 1495. 

Solid adhesives may also be hot melt thermoplastic films or pellets that liquify on heating and resolidify on cooling and are useful in the fabrication of high-density multilayer circuits. 

Surface-mount technology replaces previous methods of inserting component leads into plated through-holes of PWBs, wave soldering from the back side to flow solder into the holes, and simultaneously forming both mechanical and electrical connections. SMT is highly automated and currently the most widely used production process for the assembly of single-layer, double-sided, and multilayer circuit boards. 

Polyimides for die and substrate attachment and for multilayer circuit insulation where resistance to high temperatures and electrical stability are required. 

One concern expressed about LCP films in multilayer circuit boards is the relatively high CTE in the thickness direction. Values of 100 to 140 ppm/°C have been reported [21,23] however, the stiffness in the thickness direction is... 

PBTs have received attention in recent years as a result of their unique physical properties. PBTs are among the most thermally stable polymers reported so far. The high degree of molecular rigidity in their backbone produces high-modulus polymers, which are applied as fibers and multilayer circuit boards [50,71,72]. 

Stetson of RCA, "Method of Making Multilayer Circuits," U.S. Patent 3,189,978. Teaches the use of co-sintering metallized and laminated tapes into a monolithic structure. 

Dielectric constant, loss tangent, and thickness are some of the parameters (NASA Tech Briefs, 2008) to be considered for the nonconductive fabric layer. The circuit design of the conductive fabric layer is secured onto a nonconductive fabric layer using sewing, embroidery, adhesive means, and/or 3D weaving (Courtesy of Bally Ribbon Mills). Multilayer circuits can be built using conductive threads to make electrical connections between layers. 

A process has been described for laminating polyimide films to each other and to conductive metal foils in the production of flexible electrical circuitry. This will find use in aircraft, spacecraft, and missiles as well as industrial applications. It is expected that the market for such constructions will grow as the market share of multilayered circuit boards increases. 

The most common definition of a hybrid circuit is a circuit that contains two or more components, one of which must be active, which are mounted on a substrate that has been metallized by one of the film technologies. This definition is intended to eliminate single-chip packages, which contain only one active component, and also to eliminate such structures as resistor networks. A hybrid circuit may, therefore, be as simple as a diode-resistor network or as compKcated as a multilayer circuit containing in excess of 100 integrated circuits. 

It has not been unusual for LTCC substrate manufacturers to be presented with designs in which the overall part size cannot be changed. The cost of LTCC multilayer circuits is very dependent on overall part dimensions and the relationship to the foundry panel size, panel tooling, and the usable area within the panel. It is important to maximize the total number of parts within the usable area to achieve the lowest possible unit cost. A small dimensional increase could eliminate a row or column of parts or produce excess waste... 

A typical design cycle for multilayer circuits with embedded passives. 

Inspection is an integral part of the screen-printing process, particularly where complex multilayer circuits are involved. A single void, anywhere in any trace or in any layer, can result in the entire structure being scrapped. 

PEEK film finds uses as a substrate for flexible printed circuit boards. Mitsubishi has developed a blend of PEEK and polyetherimide (PEI) in which the slow crystallisation characteristics can be utilised to fuse layers of circuit without the need for adhesives. This is a potentially low-cost route to multilayer circuits [14]. Film can also provide a useful moisture barrier in applications such as membrane switches. There are also a number of niche applications such as environmentally resistant RFID tags. 

Materials that are not fuUy cured can cause problems in circuit manufacturing processes and with finished circuit reliability. For example, an undercured multilayer circuit at the drUhng process can result in excessive resin smear across the internal circuit connections in... 

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