Printed Wiring Boards High density

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. 

Printed wiring boards (PWBs) with microvia hole structures are called different names, such as HDI, SBU (sequential build-up), and BUM (build-up multilayer). However, HDI covers a broader range of high-density wiring boards such as extremely high-layer-count multilayer boards (MLBs) without microvia holes. MLBs with microvia holes are not necessarily built sequentially, nor do they necessarily have build-up structures.These definitions are not appropriate for the discussions in this chapter, and therefore we shall address MLBs with microvia holes simply as microvia hole boards (all microvia hole boards are essentially multilayer boards). 

Sakamoto, Kazunori, Yoshida, Shingo, Fukuoka, Kazuyoshi, and Ando, Daizo, The Evolution and Continuing Development of ALIVH High-Density Printed Wiring Board, presented at IPC Expo 2000 featured in Circuit Tree, May 2000, pp 34-37. 

Fukuoka, Yoshitaka, New High Density Printed Wiring Board Technology Named B2it, JIPC Proceeding, Vol. 11, No. 7, November 1996, pp. 475-478. 

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

G. T. Paul, Cracked Innerlayer Foil in High Density Multilayer Printed Wiring Boards, Proceedings of Printed Circuit Fabrication West Coast Technical Seminar, San Jose, Aug. 29-31,1983. 

However, conductive elastomers have only ca <10 of the conductivity of soHd metals. Also, the contact resistance of elastomers changes with time when they are compressed. Therefore, elastomers are not used where significant currents must be carried or when low or stable resistance is required. Typical apphcations, which require a high density of contacts and easy disassembly for servicing, include connection between Hquid crystal display panels (see Liquid crystals) and between printed circuit boards in watches. Another type of elastomeric contact has a nonconducting silicone mbber core around which is wrapped metalized contacts that are separated from each other by insulating areas (25). A newer material has closely spaced strings of small spherical metal particles in contact, or fine soHd wires, which are oriented in the elastomer so that electrical conduction occurs only in the Z direction (26). 

To facilitate the many choices of laminates and their associated properties, industry standards groups such as the IPC have defined minimum performance specifications and have issued several specifications to inform the selection process. Some of the most commonly used material specifications are those that deal with laminate,prepreg, and copper foil. IPC lOl, Specification for Base Materials for Rigid and Multilayer Printed Boards, and IPC-4652, Metal Foil for Printed Wiring Applications, are the primary specifications for clad laminates, prepregs, and foils. Another specification, IPC- 4104, Specification for High Density Interconnect (HDI) and Microvia Materials, deals with many of the new materials for HDI, such as epoxy-coated microfoils, as discussed in this chapter. 

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