23.0 INTRODUCTION

Electronic packaging must provide physical protection during component storage, handling, and assembly to the circuit board and long-term operation within the finished product. Environmental protection is also required, especially resistance to moisture and ionic materials. The end user typically will expose the package to extremes of temperature, mechanical shock, and harsh environments. The finished package must have proper dimensions for machine handling while also allowing identification nomenclature to be applied. All these requirements can be achieved using polymers in the transfer-molding process. Plastic packages make up a large segment of area array packages. Both larger plastic ball grid arrays (PBGAs) and smaller chip scale packages (CSPs) use molding processes. Transfer molding is by far the predominant process employed because it has a long and successful history in the production of all types of packages. Transfer-molding processes and materials will be described in detail with this chapter. For now, it will suffice to describe the method that uses solid preforms of polymer precursors that are melted, injected into molds, and permanently hardened by thermally induced polymerization.

Encapsulants were described in Chap. 11, so the discussion here will only be a summary. The most common materials are epoxy mold compounds (EMCs). The mixture contains reactive epoxy resins, a hardener, at least one reaction accelerator, flame retardants, a filler, a lubricant, a coupling agent, a flow promoter, a stress modifier, an ion getter (chemical scavenger), and a colorant. The basic epoxy-hardener...