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Title: Study on reliability of flip chip solder interconnects for high current density packaging
Other Titles: Gao dian liu mi feng zhuang yong dao zhuang xin pian han dian hu lian de ke kao xing yan jiu
Authors: Yang, Dan (楊丹)
Department: Department of Electronic Engineering
Degree: Doctor of Philosophy
Issue Date: 2008
Publisher: City University of Hong Kong
Subjects: Microelectronic packaging -- Reliability.
Solder and soldering.
Notes: CityU Call Number: TK7870.15 .Y36 2008
xvii, 148 leaves : ill. 30 cm.
Thesis (Ph.D.)--City University of Hong Kong, 2008.
Includes bibliographical references (leaves 137-146)
Type: thesis
Abstract: The pursuit of greater performance in microelectronic devices has led to a shrinkage of bump size and a significant increase in current. This has resulted in a high current density and accompanying Joule heating in solder interconnects, which places great challenges on the reliability of electronic packaging. This dissertation focuses on electromigration (EM) and thermomigration (TM) failures due to atomic transport. The current crowding-enhanced EM was detected. Phase separation was evident in Sn37Pb solder joints due to different atomic diffusivities, and Pb atoms were shown to be the faster diffusing species around 100 °C. Void formation was the dominant mechanism in the EM failure of Sn3.5Ag1.0Cu solder joints, and the failure time was identified to be more dependent on void growth than on void nucleation. The growth rate obtained ranged from 0.19 to 0.40 μm/h. The electrical resistance demonstrated a low rate of change over a long period of stressing time until an abrupt increase in final stage. The void propagation resulted in an open circuit because of a complete failure in contact. Also, nano-indentation tests indicated that both hardness and modulus decreased with stressing time due to phase coarsening and atomic bond damage. A combined mechanism concerning the EM of solder and the diffusion of Al is proposed. The accumulated effect of void propagation, chemical dissolution, and the EM of Al induced a melting failure of Sn3.5Ag1.0Cu solder joints. The morphological evolutions in different stages were correlated to the variation of electrical characteristics. A unique time-dependent behavior of melting failure was also clarified. The total incubation time depended on the rates of void growth and Al diffusion. In the last stage, the depletion of Al traces due to EM exhibited a linear relationship with time. The rate of change in resistance was calculated to be 0.9 % h-1, approximately representing the drift of Al atoms. Black’s model was modified to evaluate the reliability of interconnects under EM. A multiplying factor of current density, and an incremental factor of temperature deduced by a temperature coefficient of resistance method were introduced. It was found that the time to failure under EM followed a Weibull distribution. The characteristic lifetime degraded with an increase of current density and temperature, and the shape parameters had similar values as expected. Based on the modified Black’s model, the activation energy and current density exponent for void formation-controlled EM failure were obtained. The individual effect of the TM was investigated. The TM of Pb was directly observed in Sn37Pb solder joints. With a negative heat of transport, Pb atoms migrated from the higher to the lower temperature sides under a thermal gradient. In addition, because of different thermal dissipations, the TM of Pb in the outer layers of solder was significant than that inside the solder center. A thermal gradient of 1100 °C/cm was predicted by finite element simulation. Furthermore, the atomic flux due to TM was estimated to be 1.39×1014 atoms/cm2 s, and the driving force could approach 10-17 Newton, the same order of magnitude as that due to EM. The product of diffusivity and heat of transport was about – 6.56×10-13 m2/s kJ/mole. The TM of Sn3.5Ag1.0Cu solder joints was also explored. Serious voids were seen at the solder/Cu6Sn5 interface at the lower temperature side, which is different from the Kirkendall voids which occur at the Cu3Sn/Cu interface or within the Cu3Sn layer. It is speculated that Sn atoms were driven from the lower to the higher temperature sides, which is opposite to the TM characteristic of Pb atoms. The TM behavior of Sn-based lead-free solder has not been completely clarified.
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