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Dual-function Underfills Offer Versatility

BY BRUCE CHAN, ROBERT CHU, and BRIAN TOLENO, Ph.D., The Electronics Group of Henkel

Underfills enable numerous package designs and provide the required support and reliability needed for highly miniaturized and lead-free devices. Without these essential materials, many of today’s advances would not be possible. Developments in underfill technology such as enhancements in filler technology, better control of flow rates, new cure mechanisms, improved modulus properties, and alternative application techniques 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, 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 no-flow) underfills, cornerbond, and edgebond systems. Each has relevance for certain applications, but newer devices — and even some older-generation packages — may benefit from a breakthrough underfill material technology in the reflow-cured encapsulant class. This epoxy flux system enables many applications in both semiconductor packaging and PCB assembly, as well as some of the emerging device configurations such as package-on-package (PoP).

Figure 1. Reflow curable underfills offer throughput advantages.
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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, eliminating the need for a dedicated dispensing system as well as the time required to dispense and cure (Figure 1). These 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


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