New developments in hollow microspheres that may be added to MCM-L and MCM-C substrates for multichip modules

This paper summarizes the characteristics of hollow microspheres that may be added to thick film dielectrics and printed wiring board laminates to lower the dielectric constant of these substrates. Such printed wiring board laminates and thick film dielectrics may now be considered excellent candidate substrates for multichip modules. One critical parameter for addition of microspheres to these substrates is the particle size distribution. Processes were developed to bring maximum particle sizes down to 15-20 microns. Electrical characteristics are detailed, with data presented to 22 gigahertz. Materials characteristics are detailed, using SEM, XPS, XRD, and electron spectroscopy. The materials characterization is linked to the electrical characterization, leading to the conclusion that a crystalline product forms during the processing of the spheres, and the amount of the crystalline product is a direct indicator of electrical performance.