Pan Pacific Symposium Conference Proceedings


Authors: David Gagnon, Ph.D., Zenner, Zieminski, and Kropp
Company: 3M Adhesives and Tape
Date Published: 2/18/2003   Conference: Pan Pacific Symposium

Abstract: Underfill films are being explored for advanced chip and package underfill applications to increase process efficiency. Process efficiency is enhanced by eliminating time-consuming steps and using less capital-intensive equipment at the assembly line, thus better mimicking standard SMT processes. Underfill materials, either liquid or film, that can be pre-applied to the chip or the substrate are potential enablers for increasing the assembly throughput. Much development work has been done and published on the use of pre-applied fluxing liquid underfills, and indeed a number of products are sold into the industry today. Because these materials are unfilled and were targeted at flip-chip applications, only small chips were previously considered, due to thermal reliability concerns.

Work on film-based, pre-applied underfills is more embryonic. The first efforts in this area have been for highly filled, wafer applied underfill (WAU) films. Recently, advances have been made in the area of unfilled, pre-applied, fluxing underfill films as an alternative to underfill. With the advent of mass produced mobile electronics with high functionality, reinforcement of chip scale packages (CSP) is driving the development of pre-applied films for mechanical reinforcement. There remains the challenge of using a pre-applied film and maintaining high yield in assembly.

One key characteristic, which determines yield, is the rheology of the adhesive during placement and reflow processes. Rheology during lamination, upon chip/package placement and through reflow, is determined by the adhesive formulation and the in-process thermal profile. The rheology of the underfill, as a function of the thermal and mechanical environment during assembly, is critical to the successful implementation of film-based underfills. New underfill film materials will be described and data from a unique measurement test that simulates assembly thermal profiles will be used to correlate assembly yield to the rheological behavior of underfill materials.

Key words: underfill, CSP, BGA, no-flow, fluxing adhesive, rheology, reflow.

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