| Summary: | 碩士 === 國立臺灣大學 === 材料科學與工程學研究所 === 97 === The miniaturization of electronic devices always accompanies the rise of interconnect current density, which makes the subsequent electromigration phenomenon cannot be ignored. Flip chip technology has now been widely used in packaging industry; because of its unique geometry and its involvement in interfacial reactions, electromigration effects in flip chip solder joints are far more complicated than that in Cu and Al interconnects. Therefore, electromigration in flip chip solder joints has been a crucial issue in recent years, and more emphasis will be put on this special topic.
Because extra-high current density in flip chip solder joints always results in immediate failure at Al trace, mostly the applied current density in flip chip solder joints seldom exceeded 104A/cm2 in previous studies. Further, the accelerated electromigration tests in previous studies were usually conducted under high temperature conditions (above 120oC); however, the effects of electromigration on flip chip solder joints involve several diffusion mechanisms, and different diffusion mechanisms demonstrate different dependences on temperature. Subsequently, the accelerated electromigration tests in previous studies under high temperature could not thoroughly demonstrate the reliability of flip chip under normal usage temperature. Therefore, low temperature and extra-high current density are special conditions which were seldom tried in the past, but indeed worth the investigation.
In this research, a cooling system with a PID controller was applied in order to fix the chip temperature at 60oC, and different extra-high current density (5.5×104 A/cm2、5.0×104 A/cm2、4.5×104 A/cm2) was used for accelerated electromigration tests. The objective of the research is to investigate the effects of low temperature and extra-high current density on flip chip solder joints and their interfacial reactions.
This thesis consists of two parts. The first part concerns the literature reviews of interfacial reaction and electromigration physics. First, interfacial reaction and diffusion are thoroughly reviewed from a thermodynamic and kinetic point of view, which include several reaction diffusion theories and kinetic models, their limitations and applications. Next, electromigration driving force and its effects on interfacial reactions are introduced. Finally, two common electromigration-induced failure mechanisms of flip chip solder joints are discussed, which are “void formation-and-propagation” and “UBM consumption”.
The second part concerns experimental details, results, and discussion of this research. The two configurations of flip chip solder joints used in this research are NiV(3.25k)/Cu(8k)/Sn-0.7Cu/Cu and Ni(2μm)/Sn-2.6Ag/Cu structure. And the experimental results showed that low temperature could retard void formation in solder joints (which is a usual phenomenon under high temperature tests), even under extra-high current density tests; Instead, evident IMC dissolution at the cathode and accumulation at the anode was observed. The results implicate that electromigration in flip chip solder joints under different temperature conditions would result in different phenomenon: high temperature electromigration tests would make diffusion with higher activation energy more observable, such as Sn self-diffusion and subsequent void formation in solder; low temperature would make diffusion with lower activation energy more observable, such as Cu diffusion in Sn and subsequent IMC dissolution at the cathode (in this thesis a kinetic model will be suggested). Moreover, especially when IMC was totally consumed by electromigration, interfacial reaction at the cathode could not guarantee the planar interface because Cu was not protected by IMC anymore.
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