Printed Wiring Boards Additive

Furthermore, the human health section of the EU Risk Assessment has concluded that the fire retardant additive TBPBA carries no risks, and the EU Scientific Committee on Health and Environmental Risks (SCHER) has confirmed the EU (Risk Assessment) conclusions that TBPBA presents no human health risk. TBPBA is a brominated flame retardant (used in electrical equipment including computers, televisions, and in printed wiring boards (PWB), and so on [21],... 

Polymer adhesives have found their place in numerous electronics applications. Major uses include eommercial/consumer products computers and military, space, automotive, medical, and wireless communications. Some adhesives may be used aeross several applications while others have been formulated to meet applieation-specific requirements. For example, reworkability is not a consideration for high-production, low-cost consumer products such as cell phones or calculators, but is important for high-value, high-density printed-wiring boards (PWBs) used in military and spaee electronics. Further, thermal stability at high temperatures is required for near-engine electronics in automobiles, aircraft, and for deep-well sensors, but not for office computers. The major applications for polymer adhesives are to attach and electrically insulate or to electrically connect components, devices, connectors, cables, and heat sinks to printed-circuit boards or to thin- or thick-film hybrid microcircuits. In addition, over the last several decades, new uses for adhesives have emerged for optoelectronic (OE) assemblies, microelectromechanical systems (MEMS), and flat-panel displays. 

Saytex CP-2000 is widely used for printed wiring boards laminates made from FR4 epoxies. It is reacted into the epoxy, so that there is no potential for leaching out of the resin. The FR meets the German Dioxin Rule. It is environmentally acceptable, thermally stable at high temperatures and does not disrupt resin properties when soldered. It can be used as an additive in ABS and also as a raw material in brominated epoxy oligomers in ABS and high impact polystyrene (HIPS). 

In addition to bare die and surface mount techniques, there are still the older device packages for through-hole applications, where the lead of the package goes through the printed wiring board (see Fig. 8.128 and Fig. 8.129). Modules are also available that are assemblies of either packaged parts or die. 

These trends have led to the larger use of nonorganic base substrates, such as aluminum and soft iron. In addition, alternate ways to create boards have been developed. These will be discussed in this chapter, along with the traditional board structures and processes. The terms printed wiring board, PWB, and board will be used synonymously. Also, the words laminate, substrate, and panel will be used interchangeably. 

Conventional multilayer printed wiring boards must be subjected to drilling and through-hole plating to create interconnections. These holes represent inefficient use of PWB area. In addition, to connect the printed wiring with part connection lands, some through holes must be provided in areas other than where the lands are located. 

In addition to providing circuit interconnection, a multilayer printed wiring board (ML-PWB) provides the electrical and mechanical platform for the system. This means that the electrical and thermal properties of the ML-PWB material are very important for the proper functioning of the system. Among the properties of importance are dielectric constant, Du (also known as Er) dielectric loss, Df (or tan 8) glass transition temperature, Tg time to delamination,Txxx thermal decomposition temperature.Ta coefficient of thermal expansion, CTE and moisture absorption. The following sections discuss the importance of these properties to an ML-PWB snbstrate. 

N. Ohtake, New Printed Wiring Boards by a Partly Additive Process, PC World Conference IV, Tokyo, Paper No. 44, June 1987. 

S. Imabayashi, et al., Partly-Additive Process for Manufacturing High-Density Printed Wiring Boards, IEEE Trans. 0569-5503/92/0000-1053,1992. 

H. Akaboshi, A New Fully Additive Fabrication Process for Printed Wiring Boards, IEEE Trans, on Comp., Hybrids and Mfg. Tech., Vol. CHMT-9, No. 2, June 1986. 

R. Enomoto, et al., Advanced Full-Additive Printed Wiring Boards Using Heat Resistant Adhesive, PC World ConfV, Paper No. B6-1, Glasgow, Scotland, June 1990. 

Now that Europe s RoHS legislation restricts Pb to a level of less than or equal to 1,000 parts per miUion by weight in electrical and electronic assemblies, there is much more focus on tin whiskers. Traditionally, many electrical components had tin-lead surface finishes, as did printed wiring boards (HASL). In cases where pure tin finishes were used on components, small additions of lead were added. The resulting lead-doped tin lattice imparted some compliance to the structure and rednced the risk of tin whisker formation. For the vast majority of electrical and electronic prodncts, that is no longer an option for many classes of components. 

Electrical and electronic devices are made utilizing several various types of plastic materials, thus when discarded their waste is difficult to recycle. The plastics employed in housing and other appliances are more or less homogeneous materials (among others PP, PVC, PS, HIPS, ABS, SAN, Nylon 6,6, the pyrolysis liquids of which have been discussed above). However, metals are embedded in printed circuit boards, switches, junctions and insulated wires, moreover these parts contain fire retardants in addition to support and filler materials. Pyrolysis is a suitable way to remove plastics smoothly from embedded metals in electrical and electronic waste (EEW), in addition the thermal decomposition products of the plastics may serve as feedstock or fuel. PVC, PBT, Nylon 6,6, polycarbonate (PC), polyphenylene ether (PPO), epoxy and phenolic resins occur in these metal-containing parts of EEW. 

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