Curing underfill adhesives

Underfill adhesive (rapid cure) 30-60% Bisphenol F epoxy reaction product, 15% imidazole catalyst, 30-60% silica filler Loctite 3563 (Henkel)... 

Underfill adhesive (snap cure) Epoxy resins (high purity Bisphenol-A and cycloaliphatic resins) 45-60% Resin modifier 1—5% Curing agents (amine) 3-5% Filler (amorphous silica) 50-80% Loctite 3563, Ablebond 7811... 

Liquid encapsulation and potting compounds, e.g., for automotive parts Thermal and RT-curing adhesives, underfiller materials Composites, fiber-reinforced, e.g., for transportation or in printed wiring boards PCB build-up materials... 

Once cured, the properties of NFUs are similar to those of capillary-flow underfills. One major difference is that the no-flow materials are generally unfilled and, as a result, their expansion coefficients are higher than those of their filled counterparts. However, their lower moduli more than compensate for the mismatches in expansion coefficients. NFU adhesives have shorter shelf lives than capillary-flow types because of the incorporation of the fluxing agent into the adhesive formulation. Table 5.4 is a compilation of underfills and their properties while Table 5.5 lists examples of capillary flow and no-flow underfills and their applications. 

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). 

In a similar process, known as polymer-film interconnect (PFI), an insulative thermoplastic film is laminated over the devices at the wafer stage, and vias are opened over the bonding pads using a laser. At that point, either the normal solder bumps can be formed or a silver-filled conductive adhesive can be stencil printed into the vias to form polymer bumps. After printing, the epoxy is B-staged and the flip-chip devices are diced. In assembly, the devices are heated to a temperature that completes the cure of the B-staged bumps and simultaneously reflows the thermoplastic underfill material. 

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