ORCID Identifier(s)

0000-0001-5298-5065

Graduation Semester and Year

2021

Language

English

Document Type

Dissertation

Degree Name

Doctor of Philosophy in Materials Science and Engineering

Department

Materials Science and Engineering

First Advisor

Suk Kyung Yum

Abstract

The miniaturizing trend for advanced electronic devices has motivated the microelectronic industries to pursue device integration and packaging technology based on the micro-bump solder joint. This type of the joint enables devices with far higher interconnect density and miniaturized form factors which are ideal for mobile applications. The focus of this research is to conduct comprehensive investigation on the electromigration (EM) reliability of micro solder joint. For that purpose, a series of EM tests on micro solder joints under direct current (DC) load was conducted with variation in the contributing factors to EM failure, including solder joint arrangement (change of interfacial layer addition), Cu pillar geometry (elliptical vs. cylindrical), EM test temperature, and current density. For this research, a customized EM test system with an oil-bath was developed to enable effective joule heat dissipation and maintenance of temperature uniformity and stability for the duration of EM testing. Our studies on EM failure kinetics and microstructural mechanism have produced a few important findings: 1) failure rate does not follow a single activation energy model, and it is far more sensitive to test temperature than the predicted by a conventional model. This is determined to be resulted by a limited amount of the solder material that is under two competing processes during EM test, a formation of intermetallic compound (IMC) and EM damage by voiding. The failure mode changes by change of dominant process among these two. Our extensive failure analysis on EM tested samples at different temperatures revealed that EM void damage due to EM of Sn is the dominant factor for the samples tested at low range of EM testing. On the other hand, IMC formation rate is the process that controls the failure kinetic of the joints tested at high temperature range of EM testing. 2) It is found that failure can happen even if the solder joint fully converted to IMC even though IMC joints are immune against EM damage. The results showed that failure location shifted to the interface of Cu interconnect and IMC joint. Unbalanced interdiffusion of Cu and Sn through IMC joint is the source of discontinuity formation, also known as the Kirkendall effect. 3) The investigation of micro solder joints isolated from Cu pillar by a Ni interfacial layer showed that the EM failure can be suppressed by the back-stress effect. Very thin thickness of micro solder joint along with low aspect ratio (thickness to length) makes the back-stress effect to be sufficient to prevent EM damage. The back-stress effect is found to be very sensitive to test temperature, and the back-stress effect diminishes at high temperature as it is outcompeted by EM rate of Sn. 4) Our study on fine pitched solder joint with cylindrical joint geometry leads to a finding of a unique failure mechanism caused by a formation of extended solid solution. The EM testing at extremely high current density causes interdiffusion of Cu and Sn to continue even after full conversion of the solder joint to the IMC. In this case, we observed that grain boundaries in Cu interconnects are the main routes of interdiffusion and responsible for the unique failure mechanism. Finally, our findings suggest the complexity of failure mechanism in this type of solder joint compared to conventional solder joints like BGA (ball grid array) or flip-chip joints. Small amount of solder material along with low aspect ratio of the joint makes the failure kinetics and mechanism to be very sensitive to test temperature and joint geometry. Our findings also suggest that failure mechanism of micro solder joint under EM cannot be understood by considering a single metallurgical process but demands considerations on various contributing factors.

Keywords

Micro solder joint, Electromigration, Failure, Reliability, Electronic packaging, Intermetallic Compound

Disciplines

Engineering | Materials Science and Engineering

Comments

Degree granted by The University of Texas at Arlington

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