Multilayer circuit boards

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. 

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

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. 

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]. 

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 dressing of the through-hole and Cu pad is the next step. An excess of solder remaining in the hole will hinder lead insertion for the new component. It is difficult to extract physically all solder residue form inside a hole, even at modest aspect ratios (board thickness-to-hole diameter). This difficulty is further compounded with thicker,multilayer circuit boards because the soldering iron tip cannot compensate for the large thermal mass of the laminate or the heat-sink effects caused by the internal Cu layers. Finally, it is often necessary to contact the Cu pads of the circuit board briefly, with the braid between the pad and the tip, to remove any excessive solder residue. 

An adhesive film is laminated on a rigid board with a release sheet first. The boards are then routed by punching or by NC router. It is a very simple process if the adhesive material is pressure-sensitive adhesives (PSAs). Each piece of stiffener is placed on the flex circuits with appropriate pressure, mostly by hand.The process is more compUcated when a thermo-set type adhesive material is required. A temperature higher than 160°C with a pressure higher than 20 kg/cm is required for more than 30 min. A similar heat press used for multilayer circuit boards or film coverlay is necessary. Dummy boards should be prepared to make the pressure uniform. 

In many cases phenolic resists have poly(p-vinyl phenol) as the matrix. They are easily prepared from p-vinyl phenol and can be polymerized under free radical conditions and crosslinked with HMTA or epoxide resins. They can also be used in combination with epoxide resins in the production of multilayer circuit boards. 

While double sided circuit boards usually have the circuit traces running horizontally on one side and vertically on the other, even they quickly became limiting. With the ever-increasing need for higher densities, multilayer circuit boards came into prominence. Typically print and etch inner layers are prepared, placed in a sandwich with interposed epoxy sheets and placed in a press to make a single structure. The outer layers are then processed in a DSPTH process. The number of layers refers to the layers of circuitry. Very complex boards may have as many as 40-60 layers. 

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