Formulation preform adhesives

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. 

Figure 3.23 Manufacturing steps in formulating fikn/preform adhesives.

Figure 3.22. Manufacturing steps in formulating film/preform adhesives.

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). 

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. 

This has become the most important type of isocyanate-based adhesive system. In this method a preformed, fully reacted, high molecular weight polymer is employed as a vehicle in the adhesive formulation. The strength of the vehicle holds adherend members in exact position after assembly... 

Dielectric heating can be used to preheat powder, pellets, or preforms to remove water before processing the material. Dielectric heating can also be used to generate the heat necessary for curing polar, thermosetting adhesives, and it can be used to quickly evaporate water from a water-based adhesive formulation. 

Bondline thickness. The control of bondline thickness is essential for high-power devices. A thin bondline is required since thickness is directly proportional to the jimction-to-case thermal resistance, djc (see Ch. 2). Specially formulated solvent-based hybrid adhesives can produce imiform bondline thicknesses of one mil or less. Small hard spacers, acting as shims, added to paste adhesive formulations, have also been used to control thickness. The use of preforms instead of paste adhesives also assures... 

Butyl rubber is used in preformed, hot melt adhesive rods which can be fed through various types of automatic dispensing equipment. Butyl provides both strength and flexibility to the adhesive rope before application, flow control during application, and improves the properties of the adhesive bond. A formulation for a typical rope or coil form of hot melt adhesive is shown in Table 8. 

Method D appears to be possibly the most important type of isocyanate-based adhesive system. It is similar to Method B in that a preformed, fully reacted, high molecular weight polymer is employed as a vehicle in the adhesive formulation. The strength of the vehicle holds adherend members in exact position after assembly until the full bond has formed. Method D differs from Method B in that its vehicle polymer is a polyurethane. A further difference is that the inherent adhesive character and strength of the polyurethane vehicle frequently enables its use without added di- or poiyisocyanate. This strength may be realized in essentially amorphous compositions such as the thermoplastic polyurethane elastomers or millable gums. Or it may be achieved with crystallizing urethane adhesive polymers. 

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