THE CHEMISTRY & PHYSICS OF UNDERFILL
THE CHEMISTRY & PHYSICS OF UNDERFILL
Dr. Ken Gilleo
ET-Trends
gilleo@ieee.org
Abstract
Underfill is today’s polymer magic that enables the increasingly popular
2nd generation Flip Chip – lower cost assembly on organic substrate. This paper will provide a basic understanding of the chemistry of underfills and the physical properties that are important to their successful use. Underfills are a carefully formulated composite of organic polymers and inorganic fillers. Fillers are the most important single ingredient in the modern underfill. We must therefore examine the affects of filler characteristics on underfill pre-cured and post-cured properties in some detail. Finally, we will investigate the total rheology of underfill in order to understand the material-machine interface and better define the underfill process.
Introduction
Underfill was originally tested as a sealant to protect Flip Chip (FC) joints from corrosion on IBM mainframe computer modules. This early work led to the unexpected discovery that thermomechanical fatigue could be reduced. The improved thermocycle performance, while of small value for already robust ceramic FC systems, is absolutely essential for reliable FC-organic systems with their significant thermomechanical mismatch.
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