9.1 Introduction

Adhesives play an important role in the manufacturing of high-density surface mount assemblies. Adhesives are used to secure components during processing (chip bonders or surface mount adhesives), form an electrical or thermal interconnection (conductive), increase reliability (flip-chip and CSP underfill), and even protect the entire device once assembly is completed (conformal coating). This chapter will discuss some of the important physical properties related to these polymeric materials such as rheology, glass transition temperature ( T _g), coefficient of thermal expansion (CTE), and methods of curing. With respect to the various applications, a majority of the discussion will focus on the surface mount adhesives, but other applications will also be discussed.

When choosing an adhesive system to use within an application, several factors need to be considered. First are the properties of the uncured material, such as shelf life, pot life, and rheology. Second, the method by which the adhesive is processed, dispensed, and applied relates back to the rheology, and the method of curing must also be considered (e.g., heat or ultraviolet radiation). Finally, the properties of the final cured material are significant. Typically, one starts with the final item and works backward, but all three factors play an important role in selecting the correct material for an application. This chapter will first discuss these properties in detail to provide a better understanding of adhesives. Then, different applications using adhesive materials will be reviewed. Knowledge...