TABLE OF CONTENTS Applications for organic polymer coatings have grown over the years, paralleling advances made in microelectronics. From the traditional conformal coatings used to protect printed wiring boards and insulate wire, coatings are now used as high-performance dielectrics as an integral step in the processing of semiconductor chips, multichip module interconnect substrates, flip-chip devices, and ball-grid-array packages. Polymer coatings, especially the polyimides and benzocyclobutenes (BCB), now play a major role in the electronic functioning of assemblies such as RF, microwave, and high-speed circuits. In addition, with the increased use of sensors in automobiles and in implanted medical devices, coatings have had to play a new role in meeting diverse requirements. Examples are protecting electronic circuits in the hot and dirty near-engine environment of automobiles and from the biofluids of the human body. It is also anticipated that dielectric coatings will be specifically engineered for emerging optoelectric and MEMS (Micro-electromechanical systems) applications.
Besides the permanent protective coatings and dielectrics, this chapter addresses coatings that are temporary, yet essential, in the processing of microelectronic circuits. Among these temporary or sacrificial coatings are the photoresists and maskants used during etching, plating, and soldering operations.
Organic polymer coatings have been used for over forty years as moisture protective conformal coatings for printed circuit boards (PCB) and for printed wiring assemblies (PWA) used in high-reliability military and space applications and, to a lesser extent, in commercial electronic hardware.[1][2] The main functions of organic conformal coatings for PWAs include:
