Лако-красочные материалы — производство

Both thermoplastic and thermosetting adhesives are used to formulate anisotropically conductive adhesives. During assembly, thermoplastics must be heated above their glass transition temperature (Tg) to achieve good adhesion and electrical interconnection. The Tg value must be sufficiently high to avoid polymer flow during use but sufficiently low to prevent thermal degradation of the substrate and device. The ease with which joints can be assembled and repaired are the primary advantages of thermoplastic matrices. Typically, the electrical interconnections are characterized by moderate reliability and their use is restricted to consumer products.

Thermosetting matrices, such as epoxies and thermosetting hot-melt adhesives, are used where increased reliability is required. Repair of anisotropically conductive intercon­nections assembled with thermoset adhesives is problematic, however, as the adhesive matrix must be removed completely from the substrate and device prior to reassembly.

An additional consideration for the selection of an adhesive system is that robust bonds must be formed to all surfaces involved in the interconnection. Materials commonly found include metallizations on the substrate and components (e. g., gold, solder, copper, aluminum, and indium tin oxide), polymer substrates and coatings (e. g., polyimide, polye­ster, epoxy, and acrylic adhesives), and chip passivation layers (e. g., SiO2 and SI3N4). Adhesion promoters may be required.

The thickness of the adhesive applied to the substrate should be considered in the early stages of formulation. The required thickness is a function of the geometry of the substrate and device to be interconnected. If a film adhesive is used, the film thickness must be sufficient to fill the gap between substrate and device to prevent void formation, as illustrated in Fig. 6a. The film thickness cannot be arbitrarily large, however, as the bonding temperatures, pressure, and time must be sufficient to displace the excess

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Figure 6 Effect of adhesive thickness on anisotropically conductive adhesive interconnections.

adhesive. Loss of electrical contact can result when the final bond line is greater than the diameter of the fillter particles, as shown in Fig. 6b.

Paste adhesives are less sensitive to height variations due to their lower viscosities. Particles in a low-viscosity matrix, however, are more susceptible to settling and agglom­eration. This may lead to loss of isolation if the paste thickness applied is too great.