Printed Wiring Boards Types

This chapter outlines the basic considerations, the main choices, and the potential tradeoffs that must be accounted for in the selection of the interconnection methods for electronic systems. Its main emphasis is on the analysis of potential effects that the selection of various printed wiring board types and design alternatives could have on the cost and performance of the complete electronic product. 

Silicon—Ca.rbon Thermoset. The Sycar resins of Hercules are sihcon—carbon thermosets cured through the hydrosilation of sihcon hydride and sihcon vinyl groups with a trace amount of platinum catalyst. The material is a fast-cure system (<15 min at 180°C) and shows low moisture absorption that outperforms conventional thermosets such as polyimides and epoxies. Furthermore, the Sycar material provides excellent mechanical and physical properties used in printed wiring board (PWB) laminates and encapsulants such as flow coatable or glob-top coating of chip-on-board type apphcations. 

Scanning acoustic microscopy (SAM) is an ideal nondestructive method for revealing internal flaws within materials or between material interfaces. SAM is extensively used in detecting voids, delamination, and other separations that can occur in adhesive-attached parts, especially after thermal cycling. SAM is particularly useful in the analysis or evaluation of many types of electronic parts, including ceramic and plastic-encapsulated ICs, plastic-encapsulated microcircuits (PEMs), hybrid micro-circuits, CSPs, PBGAs, and printed-wiring boards. 

Electrical Laminates. Printed wiring boards (PWB) or printed circuit boards (PCB) are used in all types of electronic equipment. In noncritical applications such as inexpensive consnmer electronics, these components are made from paper-reinforced phenolic, melamine, or polyester resins. For more critical applications such as high end consnmer electronics, computers, complex telecommunication equipment, etc, higher performance materials are required and epoxy resin based glass fiber laminates fnlfill the requirements at reasonable costs. This application constitntes the single largest volume of epoxies used in structural composites. In 2000, an estimated 200,000 MT of epoxy resins were used globally to manufacture PCB laminates. 

Similarities to epoxy resin include no volatile polymerization byproducts stable B-stage resins possible low shrinkage, high adhesion and void free structures. High purity makes cyanate esters very attractive for use in electronic applications. The performance of bisphenol A dicyanate resins in printed wiring board laminates was described by Weirauch et al(4). Mobay Chemical (and Bayer) introduced products of this type for PWB applications in the United States in 1976-78. These "Triazine A resins were not commercially successful and were withdrawn from the market about 1979. 

Since the invention of printed wiring technology by Dr. Paul Eisner in 1936, several methods and processes have been developed for manufacturing printed wiring board (PWBs) of various types. Most of these have not changed significantly over the years however, some specific trends continue to exert major influences on the types of PWBs required and the processes that create them ... 

Due partly to the rather low photosensitivity and resolution capabilities (>2mm) of negative photoresists based on condensation types of photoreactions, they are no longer in use for microelectronic applications. However, currently they do find some use in the fabrication of printed wiring boards and in photolithographic printing plates. 

The types of end products covered are primary plastic materials (mainly pellets, i.e. granules not to be confused with transportation pallets), loose-fill packaging material (packaging chips), films, bags, mulch films, printed wiring boards (for electronics), thickener for lacquer, two different panels for passenger cars, and transport pallets. These products are compared with equivalent products made from petrochemical polymers - in many cases polyethylene, polypropylene or polystyrene. 

The European electronics industry is lobbying for an extension of exempted lead applications listed in the Annex. The new applications would include (a) lead contained in high-melting-temperature-type solder used within an electronic component for internal connection purpose between functional element and wires, terminals, heat sinks, etc. and (b) lead contained in high-melting-temperature-type solder used for mounting electronic components onto printed wiring boards. 

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). 

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