The first talk will focus on solder reaction, spalling of intermetallic compound in thin under-bump metallization (UBM), and Kirkendall void formation in thick UBM.

Then, the unique failure mode of electromigration in flip chip solder joints will be presented and explained on the basis of current crowding. Joule heating that leads to melting of flip chip solder joints will be discussed. The interaction between electrical force and chemical force will be analyzed.

In the last part of the seminar, the mechanism and prevention of spontaneous Sn whisker growth will be presented. Spontaneous Sn whisker growth is an irreversible process, in which there are two atomic fluxes accelerated by two driving forces. The Cu atoms which diffuse from the leadframe into the solder finish are driven by chemical potential gradient to form the intermetallic compound of Cu₆Sn₅ in the grain boundaries of the finish. The growth of this compound at room temperature generates a compressive stress in the finish. To relieve the stress, Sn atoms which are driven by the stress gradient will diffuse away to grow a stress-free Sn whisker. The typical industry solution is to insert a diffusion barrier of Ni between the finish and the Cu to prevent the diffusion of the latter into the former. It is insufficient, however, because we have to uncouple the irreversible processes and stop both fluxes of Cu and Sn. By understanding the interaction between chemical force and mechanical force, we propose a better solution to prevent the spontaneous growth of Sn whiskers.

Speaker Biography
Professor K.N. Tu holds a B.S., National Taiwan University; M.S., Brown University; Ph.D. in Applied Physics, Harvard University(1968). He worked at IBM's T.J. Watson Research Center for 25 years, managing their Thin Film Science department and as Senior Manager of the Materials Science Department. For the past 12 years he has been on the faculty of the Materials Science & Engineering department at UCLA, including 5 years as chair of the department.

Dr. Tu has been a Science Research Council Senior Research Fellow and The Royal Society Guest Research Fellow at Cavendish Laboratory, UK; Fellow of American Physical Society; Fellow of the Metallurgical Society; Overseas Fellow of Churchill College; Application to Practice Award of the Metallurgical Society; Alexander von Humboldt Research Award for senior US scientists; President of the Materials Research Society in 1981. His research interest is in kinetic processes in thin films, metal-Si interfaces, electromigration, Pb-free solder metallurgy, low dielectric constant thin films, and phase changes driven by high current density and high electric and magnetic fields. In addition to over 300 articles, he has published a book entitled Electronic Thin Film Science.