Since their inception over 30 years ago, underfills have enabled numerous new packages and have provided the required support and reliability needed for highly miniaturized and lead-free devices. It is safe to say that without these essential materials, many of today's advances would not be possible. Continued developments in underfill technology such as enhancements in filler technology, better control of flow rates, new cure mechanisms, improved modulus properties and alternative application

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... echniques have brought enhanced performance capabilities to the market. But, as the industry continues its march forward toward more efficient, flexible and miniaturized devices and component configurations, even more underfill system capabilities will be required. To date, the four most commonly used types of underfills are capillary flow materials, fluxing (often referred to as noflow) underfills, cornerbond and edgebond systems. Each of these have relevance for certain applications but some of the newer devices -and even some older generation packagesmay benefit from a breakthrough underfill material technology in the reflow cured encapsulant class. The new material system -called epoxy flux -is enabling many applications in both semiconductor packaging and printed circuit board (PCB) assembly, as well as some of the emerging device configurations such as package-on-package (PoP). Epoxy Flux Underfill Technology Designed to offer process efficiency, epoxy flux underfills deliver a fluxing component that facilitates solder joint formation as well as an epoxy system that offers added device protection by encapsulating individual bumps. Because epoxy fluxes are cured during the reflow process, they offer an in-line alternative to other underfill mechanisms and eliminate the need for a dedicated dispensing system and the time required to dispense and cure. (Figure 1 ) These new underfill systems also provide deposition flexibility and, depending on the application and process, can be screen printed, dipped, jetted or dispensed as required. While there are certainly other fluxing -or no-flow -underfill materials that offer in-line processing, none deliver the processability of epoxy fluxes. No-flow underfill encapsulants, for example, have been used in both semiconductor packaging and PCB assembly and, although process efficient, there can be challenges with performance and reliability. Using the noflow technique, material is applied to the substrate prior to component or die placement and then is cured during reflow. However, since moisture outgassing from the substrates and packages into the no-flow material causes voids, many packaging and assembly specialists have migrated toward reflow-cured cornerbond or edgebond materials that do not fully underfill the device or traditional capillary flow materials. Epoxy fluxes, on the other hand, only encapsulate individual spheres or bumps and, therefore, leave channels underneath the device that allow any volatile gasses from the substrate to escape, while still providing solder joint protection. And, as mentioned previously, these versatile materials can be used for a variety of applications. Figure 1: Reflow curable underfills offer throughput advantages Ball Attach From water washable to no clean, there are countless tacky flux formulations used for solder ball attach, each with unique features and benefits. Epoxy flux, however, may prove to be the most effective attachment method from a reliability standpoint. Recently, a study was conducted to test the shear strength of four flux types to evaluate the most robust solder sphere attachment mechanism. In the experiment, three solder sphere alloys (all SAC variants) were used: SAC-1, SAC-2, and SAC-3. The shear strength of each solder sphere alloy was tested against four different flux types: two water washable fluxes (Flux A and Flux B), a noclean flux (Flux C) and an epoxy flux (Flux D). The flux was dispensed as single drops on the copper coupon and the balls were deposited individually by a ball dispenser, which picks up the ball by suction and places it onto the dispensed flux. Using the single ball shear test at a shear height of 30 um and a shear speed of 0.5mm per second, each material combination was evaluated. With each of the three alloys, it was proven that the epoxy flux material delivered the strongest solder joint as compared