Contact-type mechanical interfaces for high speed digital interconnects

Abstract

The objective of this work was to determine the feasibility of Ball Grid Array (BGA) planarization and to investigate the effects of mechanical contacts on signal integrity for high­speed digital signals. As devices become smaller and clock frequencies increase, new technologies for contacting devices are being developed, especially in the field of semiconductor test. These contactors will be incorporated into a socket or an interconnect used in testing devices or printed circuit boards. The heights of the solder ball leads on BGA packages have a high degree of co-planarity. As BGA pitches become tighter, the requirement for improved co-planarity persists. This thesis shows that through diamond turning techniques, the co-planarity of the solder ball leads can be improved. Planarization of solder balls enables the use and development of rigid interconnects. Interconnections generally have a built in compliance to them to accommodate for co-planarity. Reducing co-planarity eliminates the need for compliant interconnects. Contact mechanics identifies the conical shaped tip as the optimal tip shape, providing the best mechanical connection. The intent of the tip shape investigation is to correlate indentation theory with signal integrity. The electrical performance of the contactor tip is evaluated using the industry standard pogo pin. Mechanical parameters are varied to observe the effect on the signal integrity. At high frequencies, the interconnect provides a vulnerable location for signal losses to occur. A new interconnect design to address the electrical performance of existing designs is introduced. The cantilevered interconnect consists of a coplanar waveguide cross-section. The interconnect is both mechanically and electrically sound. This thesis presents aspects of mechanical contact interfaces for electrical signal testing purposes. It focuses on device interfacing, contactor tip shape, signal integrity and new interconnect designs.