Лако-красочные материалы — производство

1. Adhesives

The most commonly used adhesives in the electronics industry are thermosetting one-component epoxy resins and UV-irradiation cross-linking acrylates [7].

Thermosetting epoxy resins. Generally, thermosetting epoxy resins are mixtures of the following:

Resins 50-80 wt %

Hardeners 20-50 wt %

Accelerators 0-3 wt %

Fillers, dyes, modifiers 0-20 wt %

At this time, thermosetting epoxy resins are the most important adhesive systems used for mounting [2]. One reason for their successful use for a long period is due to the great variability of their properties, which can be adjusted to a lot of requirements. Other advantages are simple processibility and good thermal stability. Furthermore, using modern efficient accelerator systems, the curing temperature of epoxies could be decreased to temperatures lower than 100°C and times shorter than 15 min. In this way, the thermal stressing of temperature-sensitive components can be minimized.

UV-cross-linking acrylate resins. Generally, UV-curing acrylate resins contain [8]:

Reactive oligomers/prepolymers 50-80 wt %

Monomers/reactive thinners 10-40 wt %

Photoinitiators/stabilizers/accelerators 1-5 wt %

Inhibitors/dyes 1-5 wt %

The advantages of UV-curing acrylates are curing times under UV irradiation of only a few seconds and the low energy consumption of UV lamps compared to the thermal energy needed for thermosetting systems. On the other hand, these adhesives cannot be cured completely by UV irradiation. Therefore, postcuring is necessary. Commercially available adhesives (e. g., epoxy acrylates, urethane acrylates, polyester acrylates, silicon acrylates, and methacrylates) contain functional groups, which allow complete hardening. It must be noted that due to the lower glass transition temperature of these polymers than of thermosetting epoxy resins, their maximum operating temperature is limited.

Electrical-and/or thermal-conducting adhesives [1,9]. The epoxies and acrylates described above are filled with metal powders to get electrical-conducting adhesives. For special applications polyimide and silicone adhesives are used also. Since the metallic particles must touch each other inside the resins to reach a sufficient level of conductivity, a metal content of 70 to 80 wt % is necessary. Silver is the metal generally used, since specific resistances of the filled adhesives down to about 10“4 ^ cm can be achieved (metallic silver has a specific resistance of 1.6 x 10“6 ^ cm). Using other metals, such as copper or nickel, the accessible electrical conductivity is too small. On the other side, copper-filled resins show good thermal conductivity and are therefore used for such
applications where heat dissipation is of importance. To reach high heat dissipation levels, ceramic fillers such as aluminum oxide or boron nitride in a quantity of about 60 to 75 wt % are used. The resulting adhesives reach thermal resistances of 5 to 7 K/W. The main advantages of electrical conducting adhesives are (1) better resistance against mechanical stresses, resulting from large temperature variations compared with solder connections, and (2) electrical connections obtained at low temperatures since soldering is not required.