21.0 INTRODUCTION

The semiconductor die or chip typically is bonded to a substrate for several critical reasons. First, an unattached die is subject to much greater mechanical abuse during handling of the package and assembly during manufacturing. The final end product can expect considerable abuse during its lifetime, mostly as mechanical shock. Portable products must be especially robust because users have come to expect a system that can be mistreated, including dropping. Attaching the die to a lead frame, chip carrier, or circuit board makes the system very much more robust and immune to most mechanical breakage of both the die and the interconnection structure, which can be very fragile. The attached die is also better able to dissipate heat to the substrate, particularly when a metal lead frame or thermally conductive metal or ceramic package is used. The die-attach material also can be designed with electrical conductivity in order to ground the back of the die.

There are three specific categories of die-attach materials: metallurgic, inorganic (ceramic), and polymeric. The earlier materials were either pure metals or alloys that provided an excellent thermal path. Metallic die-attach systems are still used but represent only a small niche today. Ceramic adhesives, most notably silver-glass, also have been used for some time. However, polymer adhesives are the dominant materials for today and are used widely for area array packaging. More than three decades ago, polymer-based adhesives began to displace metals as the preferred attachment materials. The polymer adhesives...