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Highly Reworkable Board-Level Underfill for Large PoP Devices
Keywords: Underfill, Reworkability, Reliability
The primary objective to using board level underfill for CSP and BGA devices has been to increase resistance to drop impact. However the trend toward finer pitch, coupled with new designs such as package-on-package (PoP), has increased the importance of other measures of reliability. Large PoP devices, with dimensions greater than 14 x 14 mm, require the underfill to provide increased thermal cycle reliability and bending resistance. The direction of higher reliability is counter to that of improving reworkability. While, the latter has typically been accomplished through the formulation of lightly crosslinked thermosets that lose strength rapidly at the elevated temperatures, higher thermal cycle reliability requires the use of highly crosslinked, high Tg materials, A new approach is needed to achieve the combination of reworkability with high thermal cycle reliability. This paper will report the development of a new reworkable underfill that provides the high Tg and high adhesion necessary to improve thermal cycle reliability. Thermal cycle, drop impact, and bend test results on 14x14 mm PoP packages will be discussed. In addition, physical property characterization will be reported with emphasis on aspects of high-throughput assembly.
Andrew Collins, Associate Chemist
Emerson & Cuming
Billerica, MA
USA


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