Film or preform adhesives

Film or preform adhesives are produced by casting the premixed resin and hardener onto release liners. Single or multiple master batches of the resin mixed with the... 

There are several common forms of solid epoxy adhesives. These include film, tape, powder, and preformed shapes. Certain formulations are better suited for specific forms. For example, casting of tape or film adhesive from solvent solutions lends itself to working with multicomponent hybrid systems, where each resin can be solubilized and blended together in a universal solvent. B-staged systems are generally more brittle and better suited for powders or preformed adhesives. 

To assure optimum conductance, the adhesive must be applied as thinly and uniformly as possible. To control the thicknesses bond lines, thermally conductive paste adhesives have been formulated with collapsible spacers. The spacers are reported to control bond line thickness to 30 pm. Bond line thicknesses and uniformity may also be achieved by using film or preform tape adhesives and controlling the applied pressure and heat during cure. 

Preforms, tape, or film adhesives, unlike the paste or liquid adhesives, are already in the solid state. They may be thermoplastic or thermosetting but, in both cases, may be purchased as sheets of various thicknesses and cut to size to accommodate specific device or part sizes. In using preforms, the electronics assembler avoids problems associated with the storage, handUng, and dispensing of liquid adhesives. Parts such as leadframes, lids, and substrates may also be purchased with pre-attached preforms, a convenience to the user, who then avoids having to cut and handle individual preforms. Most parts can also be stored for up to one year in ambient conditions without affecting their properties. 

In this approach, entire wafers or segments of wafers containing ICs are stacked. Interconnections are formed from vias in the silicon. In vertically stacking memory chips or other IC chips, the chips are first thinned to several mils (4 mils or less), then adhesive bonded, and electrically interconnected directly from chip-to-chip, either from the edges or through vias in the silicon. In these approaches, both electrically conductive and electrically insulative adhesives are used. Thermally conductive preform adhesives or thin thermoplastic films are used to bond and isolate the chips within the stack and to dissipate heat. Several processes are available to vertically stack and interconnect chips of the same size and function such as SRAM, flash, and DRAM memory chips. Other processes have been developed to vertically stack chips of different sizes and functions, or to horizontally interconnect different chips in one layer, then vertically stack the layers. ... 

First, in terms of their physical form, adhesives may be either pastes or preforms (also known as films or tapes). Pastes are semisolid materials, easily dispensed through a needle or applied by screen or stencil printing. Film adhesives are solid sheets of thermoplastic or partially cured (B-staged) thermosetting polymers that can be cut to size and generally used to attach large-area components, substrates, and lids. 

One-part epoxy adhesives include solvent-free liquid resins, solutions in solvent, liquid resin pastes, fusible powders, sticks, pellets and paste, supported and unsupported films, and preformed shapes to fit a particular joint. Two-part epoxy adhesives are usually comprised of the resin and the curing agent, which are mixed just prior to use. The components may be liquids, putties, or liquid and hardener powder. They may also contain plasticizers, reactive diluents, fillers, and resinous modifiers. The processing conditions are determined by the curing agent employed. In general, two-part systems are mixed, applied within the recommended pot life (a few minutes to several hours), and cured at room temperature for up to 24 hours, or at elevated temperatures to reduce the cure time. Typical cure conditions range from 3 hours at 60°C to 20 minutes at 100 C. ... 

Coating/dispensing Brush, trowel Spraying, extrusion Roll application (manual or automatic) Hot melt equipment Stencil, dip coating Film adhesives Pressure sensitive adhesives Preformed adhesives and sealants (in solid form)... 

Technical lamination in automotive and furniture industry is used to bond decorative films onto preformed hard substrates, for example, printed PVC films on dashboards or medium density fibre boards. Mainly adhesives based on polyurethane dispersions are used, which provide excellent adhesion and extremely high bond strength. 

Similarly prepared solid epoxy films can be ground into powder. The powder can be pressed into preformed pellets and applied directly to the adhesive joint as annular rings or... 

Solid epoxy adhesives, such as powder, preforms, or film, are the least toxic because they have very low vapor pressures. However, they still can present a hazard when they are heated to a liquid and more volatile state to cure. 

A third approach to underfilling involves applying an insulative thermoplastic preform (film) prior to attaching the device. Under pressure and heat, the preform softens and flows around the solder bumps, then solidifies quickly on cooUng. The devices may then be solder reflowed to make the connections or the preform may be formulated to flow and encapsulate the solder during solder reflow. Table 5.6 lists some commercially available preforms used as underfills. Anisotropic film adhesives have also been used as underfill preforms, in which case z-direction conductive paths are formed beneath the solder bumps at the same time that the rest of the insulating film flows and cures around the bumps. Anisotropic paste adhesives can be similarly used (Figure 5.11). 

Some composite plastics are produced by binding solid polymers together. In backing, preformed solid plastic films are bonded directly to the material to be backed by adhesives or glues. What is known as synthetic leather is a composite produced by the backing process. 

In laminating or coating, a preformed solid film of plastic is bonded with glue or adhesive directly onto the material to be laminated. Composite films of cellulose and poly(ethylene), for example, are produced in this way. In these composite films, the cellulose protects from foreign odors and the poly(ethylene) assures water-tightness. Composite films of poly(isobutylene) and poly(ethylene) are used in the construction of chemical apparatus made of steel. Poly(ethylene) takes care of the protection against corrosion, and poly(isobutylene), with adhesives, the adhesion to steel. Steel sheet laminated with PVC can be processed as normal, but is noncorrosive in the absence of any further treatment. Metal aircraft parts are often coated with films of poly(tetrafluoroethylene). Poly(tetrafluoroethylene) is difficult to wet, and this decreases the tendency to ice over. 

Laminate la-m3- nat (1665) (1, n). A product made by bonding together two or more layers of material or materials. The term most usually applies to preformed layers joined by adhesives or by heat and pressure. However, some authors apply the term to composites of plastic films, with other films, foils, and papers, even though they have been made by spread coating or by extrusion coating. In the reinforced-plastics industry, the term refers mainly to superimposed layers of resin-impregnated or resin-coated fabrics or fibrous reinforcements that have been... 

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