GOALI: Creep and Microstructural Coarsening of Lead-Free Solders in Micro-Electronic Packaging Applications

Naval Postgraduate School, Monterey CA

Investigators

Abstract

This project, which is supported by the Division of Materials Research and the Office of Multidisciplinary Activities in the Directorate for Mathematical and Physical Sciences, aims to investigate the life limiting thermal cycling, creep and microstructural instability of lead-free solders for electronic packaging with special reference to flip-chip (FC) and ball-grid array (BGA) packages. The project is a highly leveraged collaborative program of research between Naval Postgraduate School (NPS) and Intel (Chandler, AZ). The objectives of this GOALI proposal are to (1) devise a methodology for rapid creep characterization of FC and BGA solder balls with minimal sample preparation, based on the impression creep approach; (2) develop a unified creep model incorporating the effect of phase coarsening applicable to lead-free solders of 2 representative microstructural types; (3) generate comprehensive creep and coarsening kinetics data for solder joints of 2 selected lead-free alloys belonging to these microstructural types; and (4) provide fundamental mechanistic insight into the roles of microstructural scale and compositional artifacts (associated with attaching tiny volumes of solder to other materials) on the evolution of creep behavior during thermo-mechanical cycling (TMC). A major goal of the project is to directly measure the creep response of individual solder balls joined to a substrate, in lieu of the standard tests on bump arrays and bulk materials. The study will lead to an understanding of the fundamental phenomenological dependence of creep kinetics on the (a) microstructural scale, and (b) process-history dependent compositional variations of the joints. These goals are achieved by a systematic variation of material and process parameters. The work constitutes developing closed-form unified creep laws including microstructural coarsening effects, which may be incorporated into finite element models for solder-joint reliability assessment. The impression creep of single solder bumps is a challenging scientific effort and the successful completion of the project will have direct impact on semiconductor electronic packaging technology in predictive engineering processes. The work is multidisciplinary with impact on experimental aspects in materials sciences and mechanics, as well as predictive modeling efforts involving microstructural as well as finite element aspects. The project takes advantage of the expertise and experimental facilities available at the academic institution (NPS) as well as industrial counterpart (Intel/Chandler). In addition, collaboration with experts at Motorola is pursued. The unique aspect of this GOALI program includes the time spent by the Intel PI at the academic institution. The personnel (PI, post-doctoral fellow and graduate students) from the academic institute (NPS) plan to perform studies at the industrial laboratory (Intel/Chandler) while Intel supports the post-doctoral fellow during that time. The educational and technological impacts of the program are rated superior. The program is a close collaboration between NPS and Intel with secondary interactions with personnel at Motorola with implications to developing a thorough understanding of the life-limiting aspects of lead-free solders in microelectronic packaging. The project has practical importance to the microelectronic industry while addresses basic scientific issues. The study will (a) develop testing methodologies and models for improving current reliability engineering practices, and (b) generate kinetics data for two lead-free solders of strategic importance to the industry. The collaboration of the university personnel with industrial counterparts provides a significant opportunity for students.