SMTA International Conference Proceedings


Authors: Paul Morganelli, Ph.D.
Company: Emerson & Cuming
Date Published: 9/26/2004   Conference: SMTA International

Abstract: Various underfill chemistries have been developed over the past decade in response to assembly requirements. Olefinic (free-radical cured), propargyl ether, and bismaleimide are examples of emerging chemistries, but the vast majority of commercial underfills are based on epoxy and cyanate ester chemistry. Of these epoxy-based materials are by far the most common, but cyanate esters are well known for their moisture resistance.

The fact that so many different materials have been studied as underfills is indicative of the difficulty that has been encountered meeting a wide range of requirements. To be sure, there is no single underfill that performs optimally against all the requirements of any given application. In product development, as in underfill selection, one must compromise certain needs in order to achieve a balance of properties that best meets the process and reliability of the assembly of interest.

This paper will compare and contrast the performance of epoxy and cyanate ester underfills with emphasis on FCOB requirements. We will look at both processing characteristics as well as reliability, leading to an overall picture of the advantages and disadvantages of each type of material. Process steps, such as dispensing, flow, and cure characteristics of commercial epoxy and cyanate ester will be compared and contrasted, followed by a discussion of flux compatibility and its relationship to defect generation.

Physical property characterization and its correlation to reliability against moisture and mechanical stresses such as thermal shock will also be discussed. Future directions in underfill development for FC assembly will be presented.

Key Words: Underfill, Flip Chip, Cyanate Ester, Reliability.

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