Underfill adhesives commercially available

In the application of capillary-flow underfills, there are so many material, process, and equipment variables that empirical methods are widely used to establish the optimum processing conditions. The viscosity and flow properties of underfill adhesives are among the most important variables in rapidly filling different gap sizes and devices. A list of some commercially available underfill adhesives and their flow properties, as related to filler size and gap height, is given in Table 5.3. 

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). 

---