Лако-красочные материалы — производство

1. Epoxy

Some of the first commercial conductive epoxy adhesives were simply based on silver powder dispersed in a liquid epoxy resin [e. g., diglycidyl ether of bisphenol A (DGEBA)] with an aliphatic amine [e. g., triethylene tetramine (TETA)] as a curing agent. Although capable of room-temperature cure, commercialization of this type of system was hampered by severe mix ratio disparity (typically, 50:1 by weight), problematic mixing due to visc­osity differences, short pot life, and safety concerns. See Table 3 for typical properties of this type of formulation.

Solvents were often used to help disperse the silver particles, to lower viscosity, or to enhance conductivity. The presence of solvent can accomplish several objectives: first, by lowering the resin viscosity, additional silver can be added to the mix, increasing the likelihood of exceeding the percolation limit; and second, certain solvents can dissolve or displace lubricants on the surface of the silver flake (necessary for manufacturing the fine particles) that may interfere with the electrical contact between particles. The selection of a solvent involves consideration of its effect on both the epoxy cure reaction and the long-term performance of the adhesive. Most solvents reduce the cure reactivity of epoxy adhesives and some can prevent the development of a complete cure (lower alcohols are known to act in this fashion). The suitability of a particular solvent will also depend on its ability to leave or remain in the cured adhesive as intended. Low-boiling solvents may evaporate from a curing adhesive without void formation if the cure temperature is mild and the bond area (die) is small. Conversely, a high-boiling solvent may be retained in the cured adhesive to act as a plasticizer.

With improved formulations, organic-based conductive adhesives became feasible replacements for eutectic solders. Table 4 shows typical one — and two-part conductive adhesives from the 1970s. The one-part adhesive system employed a latent catalyst for long pot life but required a high-temperature cure cycle. The two-part system, formulated

Table 4 Die Attach Conductive Adhesives

Parts by weight

Two-part adhesive

for a 1:1 by weight mix ratio could be cured at a lower temperature but had a shorter pot life. Formulations similar to the two-part system are still commercial today and are used where the low-temperature cure is critical. Some suppliers now offer this type of conductive adhesive is premixed, frozen syringes for added convenience.

The one-part formulation of Table 4 was improved over the years with respect to the curing temperature by the development of cure accelerators for DICY. However, concerns over residual outgassing of potentially corrosive cure by-products (e. g., ammonia) have resulted in the substitution of newer latent curing agents in modern formulations. An example of this type of curing agent is 1-cyanoethyl-2-ethyl-4-methylimidazole (sold as 2E4MZ-CN by Shikoku Chemicals Corporation), used at a level of 4 to 8 parts by weight of resin (phr), where the resin could be a liquid epoxy novolac [46].

As the die attach business became more competitive, adhesive suppliers refined their formulations to take advantage of small improvements in rheology, high-temperature performance, electrical conductivity, cure cycle, and ionic purity to satisfy customers’ real or perceived needs. As a result, the current generation of conductive adhesive for­mulations may include new resins with lower viscosity, lower levels of ionic contamina­tion, higher functionality, particulates with specific shapes, or combinations of shapes (e. g., flake and spheres); additives to control flow properties for better application tech­niques; conductivity enhancers to allow better particle-to-particle contact or higher con­centrations of particles for improved conductivity; and adhesion promoters. Flow control additives are used in very low concentration and can be based on silicone oils, fluorocar­bon or hydrocarbon surfactants, low-molecular-weight acrylic polymers, and so on. Some materials that provide conductivity enhancement include low-molecular-weight polypro­pylene glycols, select solvents, and some reactive diluents (low-molecular-weight mono — and difunctional epoxy resins). Organosilanes are the most commonly used adhesion promoters, but organotitanates can also be used. The use of additives with low boiling points that cannot be incorporated into the polymer structure should be minimized, however, to reduce void formation.

Residual stress in the die after assembly remains a concern when using epoxy die attach adhesives, especially with a large die. Various formulation techniques to reduce stress have been reported that include lowering the Tg of the matrix [47,48], formulating the matrix to exhibit two Tg values [49], reducing the modulus of the adhesive by mini­mizing silver content, and lowering cure temperature [47].