Лако-красочные материалы — производство

A relatively new application for conductive adhesives, and a very important one in terms of technological advance and future manufacturing techniques, is the assembly of liquid crystal displays (LCDs). Interconnecting flexible circuit connectors, tape automated bonding packages, and bare IC chips to glass panels have been the principal technology drivers for the development of anisotropically conductive adhesives. A brief history of LCD interconnection technology [7] as well as an analysis of current manufacturing techniques for display-based consumer electronics equipment [8] have been reported that illuminate material requirements for this rapidly evolving type of product. The principal reasons for developing anisotropic adhesive systems for interconnections to glass panel displays are:

1. Low assembly temperatures to prevent glass fracture

2. Very fine pitch (repeat distance between adjacent conductors) of 2 to 20 mils

3. Low cost

Neither isotropic conductive adhesives nor conventional solder interconnections, can easily satisfy these requirements.

Early LCDs contained the electronic drivers on a rigid circuit board, whereas the glass panel contained only the active display area. The two substrates were connected by an adhesively bonded flex circuit or heat seal connector (HSC). HSCs are the most primitive type of anisotropically conductive adhesive material. They are in wide use in Japan and are commercially available in United States.[17] HSCs are manufactured by screen printing conductive traces onto a flexible substrate (usually polyester) using either a graphite, silver, or silver-graphite ink. Alternatively, the conductor patterns may be formed by etching copper foils laminated to flex substrates. The conductive traces may be coated with a second layer of ink containing metallized spheres. A hot-melt adhesive, such as neoprene/ethylene-vinyl acetate [9], is applied over the entire top surface. The HSC is tacked into position at low temperatures onto one substrate, tested for accurate alignment, then fully cured at elevated temperatures (thermosets only). This process is repeated to join the second substrate. Thermoset hot melts offer improved reliability over thermoplastic adhesives [9,10]; however, the thermoplastics offer processing advantages, as they can easily be removed and repositioned if the HSC is not properly aligned to either the glass panel or rigid PWB. Interconnection pitch as fine as 11 mils can be achieved.

Most current LCDs are assembled using tape automated bonding (TAB) to package the driver chips. The TAB packages can also act as connectors between the PWB and the glass display. Connections are made to the glass panel by use of anisotropically conducting polymer films and to the PWB by solder reflow [9,11]. Anisotropically conductive adhesives for attaching TAB packages to low-cost polymer thick film (PTF) flex circuits were first used for the assembly of low-cost calculators [12,13]. Principles of anisotropi — cally conductive adhesive formulation are described in (Section VB, pages 867-870).

Flip-chip interconnection of silicon ICs to glass displays, as well as to multichip module substrates, is the focus of current research efforts. Although most of the flip — chip effort is directed toward solder bumping for multichip modules [14], adhesive inter­connections dominate the emerging display assembly technologies. Technologies that employ anisotropically conductive films [15,16], isotropically conductive adhesive bumps [17,18], and nonconductive adhesives [19] have been reported. An especially novel system that ensures isolaton in the X-Y plane, even at 2-mil pitch, has also been reported [7]. An ultraviolet (UV)-curable adhesive is applied to a large-scale integration (LSI) wafer and exposed through a mask such that only the material above the metallic contact pads on the die remains tacky. The surface is then coated with metallized polymer spheres, 10 pm in diameter; the spheres adhere only to the tacky regions above the 1-mil2 pads. The chips are diced, assembled to glass substrates, and cured under pressure with an additional adhesive. Deformation of the elastomeric spheres, and flow of the adhesive during cure, ensures good electrical contact.

The future use of lead in the manufacture of electronic products is problematic. Lead is toxic, and although current worker exposure is low (and consumer exposure is negligible), regulatory and/or consumer forces may make the cost of using lead in electronics manu­facturing prohibitive. A thorough assessment of lead use, as well as an evaluation of alternative joining technologies, has been published [20]. In addition, editorials have endorsed the elimination of lead from electronic products as a means of attracting consumers and increasing global market share in the next decades [21].

Conductive adhesives are one of the feasible alternatives to lead for electronics assembly. Isotropically conductive adhesives are suitable for standard pitch (50- to 100-mil) surface-mounted components and numerous commercial materials are available (see commercial supplier listing, Section VI. E). Anisotropically conductive adhesives are more suited to flex to rigid connections, fine pitch components (15- to 20-mil pitch), and flip-chip assembly (4- to 12-mil pitch) [22]. Adhesives are not ready to replace solder throughout the electronics industry, however, due to questions that remain concerning the reliability of electrical interconnections. Their implementation is currently limited to low-cost applications using polyester substrates and specialty applications where solder cannot be used. Additionally, the lack of equipment for large-volume assembly with anisotropically conductive adhesives, which require the simultaneous application of heat and pressure, impedes the acceptance of these promising materials.