PRINTED WIRING BOARD TECHNOLOGIES

Japan is enhancing printed wiring board (PWB) technology to meet market demands for low-cost, thin, and lightweight consumer products. PWB enhancements fall into several categories:


Figure 4.21. Low-cost fine line thin film process.


Figure 4.22. Shape and accuracy of conductor pattern
by additive process (Ibiden).

Japan has invested in a variety of PWB materials that include FR-4, polyimide-glass, maleimide styryl, BT resin, and a new aramid-based laminate consisting of aramid-based paper as a reinforcement in a matrix of a new epoxy resin by Teijin Limited. The superior properties of this aramid-based board for potential MCM applications include low TCE (6-16 PPM/deg. C), very high electrical resistance, very low impurities in the aramid fiber, and processability with fine via holes.

The shape and accuracy of conductor patterns, as well as the mounting reliability of solder bridging, as illustrated in Figure 4.22, compares with the standard subtractive process. Table 4.11 indicates the dielectric and metal ground rules, as well as the drilled and photolith dimensions resulting in fine-line, thin-film structures on both sides of a PWB. The structure of the additive process as practiced by IBM (Japan) and Ibiden is illustrated in Figure 4.23, using two layers on each side of the PWB.

Table 4.11
Characteristics Of Additive-Plated PWB


Figure 4.23. Additive process enhancement (IBM Japan, Ibiden).

Conductive Adhesives

Conductive adhesives are alternatives to solder and braze connections in chip and packaging assemblies. The concept shown in Figure 4.24 allows electrical connection to be made as a result of the z-axis alignment of particles. Because of low cost and process simplicity, the technology is currently used for LCD connections. Conductive adhesives are being explored and applied by almost all major Japanese consumer product companies. The materials typically consist of thermoplastic and thermoset materials that provide adhesion to surrounding metallic particles, typically Ag solder; these are then applied as paste.


Figure 4.24. Anisotropic conductive conductor system.

The conductive adhesive technology that includes a specialty technology referred to as anisotropic adhesive connection (ACA) is ideally suited for low-cost, lightweight, and low-profile applications. Examples of the use of this technology include (1) the connection between an LCD driver IC and the display panel, as in Fujitsu's notebook computer, and (2) the connection between a bumped chip and a PWB used by Fujitsu (see Fig. 4.19). A variation of the conductive adhesive used by Sharp in display modules uses spherical polymeric materials coated with nickel and gold, presumably by electroless plating. Such a system provides stress relief in the conductive adhesive joint due to the low modulus of core organic particles, yet it provides the required electrical conduction.

Flex Circuits

Japan places very high priority on flex film technology, which it has used in cameras, connectors, and video games. It has also used this technology to connect ICs to LCD displays. Its advantages include 3-dimensional conformal nature, light weight, and thin film. There are several examples of direct chip on flex, as used in the wrist pager, making packages even more compact. Sharp is currently developing multilayer flex film for future consumer product applications.


Published: February 1995; WTEC Hyper-Librarian