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Title: A study of micro and nano-particle doped lead-free composite solders for advanced electronic packaging
Other Titles: Ying yong yu gao ji dian zi feng zhuang zhi wei na ke li shan za wu qian fu he han liao zhi yan jiu
Authors: Gain, Asit Kumar
Department: Department of Electronic Engineering
Degree: Doctor of Philosophy
Issue Date: 2011
Publisher: City University of Hong Kong
Subjects: Electronic packaging.
Lead-free electronics manufacturing processes.
Solder and soldering.
Notes: CityU Call Number: TK7870.15 .G34 2011
xxix, 214 leaves : ill. 30 cm.
Thesis (Ph.D.)--City University of Hong Kong, 2011.
Includes bibliographical references (leaves 192-207)
Type: thesis
Abstract: The rapid advances in the miniaturization of chip-scale packaging technologies have prompted a rapid increase in the density of solder joints in microelectronic devices. Among the most advanced packages, ball grid array (BGA) technology is expected to have increasing applications because of its higher input-output connection density achieved through area array solder joints. However, if anyone of these solder joints fail, the whole microelectronic device malfunctions. Thus, it is crucial to evaluate the integrity and reliability of solder joints which is in turn related to the interfacial reaction phenomena between the molten solder and substrate during soldering as well as the solid state aging. With the advancement of micro-/nano-system technologies through the years, microelectronic components have evolved to become smaller, lighter and more functional. Therefore, conventional solder technology can no longer guarantee the solder joint reliability of electronic components due to the high diffusivity and the softening nature of the solder. To overcome this problem, a potentially viable and economically affordable innovative approach to improve the mechanical properties of a conventional solder is to add micro-/nano-sized metallic or ceramic particle to a solder matrix so as to form a composite. The scope of this dissertation is to understand the reaction phenomena between micro/Inano-sized metallic or ceramic particle doped lead-free composite solders and flexible substrates. At an initial stage, lead-free composite solders were prepared by mechanically dispersing different weight percentages (1, 3, 5 and 7) of micrometer sized Sn-Ag-Cu powder into Sn-9Zn and Sn-8Zn-3Bi solder pastes. In plain solder joints, scallop-shaped AuZn3 intermetallic compound (IMC) layers were found at the interfaces and in the solder ball regions, needle-shaped Zn-rich phase was observed in the β-Sn matrix. After Sn-Ag-Cu additions, an additional AgZn3 IMC layer was adhered to the top surface of the AuZn3 layer at the interface and finespherical-shaped AgZn3 IMC particles were detected in the solder ball regions together with a Zn-rich phase. After the addition of Sn-Ag-Cu, the shear strength of solder joints increased due to the formation of the fine AgZn3 IMC particles. The effects of reflow on the interfacial reactions of nano-metallic (Ag, Ni and AI) particle doped lead-free composite solders and Au/Ni metallized Cu pads on BGA substrates were investigated. After the addition of nano-metallic particles to Sn-Zn lead-free solder alloys, layer type IMCs have been found at interfaces. On the other hand, a layer-type spal1ing of the interfacial IMCs was observed very early in the liquid state reaction for plain Sn-Zn based solder alloys with Au/Ni metallized Cu pads. The active nature of Zn confirmed an instant reaction zone at the interface to maintain the bonding between the solder and the substrate. Also, no spalling of the IMCs was observed in the liquid state reaction of the Sn-Zn solders with the addition of nano-sized Ag and Ni particles. In Sn-Ag-Cu solder joints and solder joints containing Al nano-particles, scallop-shaped Sn-Ni-Cu IMC layers were clearly observed at the interfaces after multiple reflows and the IMC layer thickness was substantially increased with an increase in the number of reflow cycles. However, after the addition of Al nano-particles, additional Sn-AI-Ag IMC particles were found on the top surface of the Sn-Ni-Cu IMC layer. The nano-metallic particle containing composite solder joints consistently displayed higher shear strengths than that of the plain solder joints due to a second phase dispersion strengthening mechanism by the formation of fine IMC particles as well as a controlled fine microstructure. Nano-sized, nonreacting, noncoarsening ceramic (Al203, Ti02) particles have been incorporated into lead-free solders to investigate the microstructure, hardness and shear strength on different surface finished flexible BGA substrates. In the plain Sn-Zn solder joints and solder joints containing Al203 nano-particles, scallop-shaped AuZn3 IMC layers were found at the interfaces. In the solder joints containing Al203 nano-particles, a fine acicular-shaped Zn-rich phase and Al203 nano-particles were found to be homogeneously distributed in the β-Sn matrix. On the other hand, at their interfaces, different types of scallop-shaped IMC layers such as CU6Sn5 for a Ag metallized Cu pad and Sn-Cu-Ni for Au/Ni and Ni metallized Cu pads, were found in plain Sn-Ag-Cu solder joints and solder joints containing 1 wt% Ti02 nano-particles. In addition, the IMC layer thicknesses increased substantially with the number of reflow cycles. In the solder ball region, Ag3Sn, CU6Sn5 and AuSn4 IMC particles were found to be uniformly distributed in the β-Sn matrix. However, after the addition of Ti02 nano-particles, Ag3Sn, AuSn4 and CU6Sn5 IMC particles appeared with a fine microstructure and retarded the growth rate of IMC layers at their interfaces. The shear strengths and hardnesses of solder joints containing ceramic nano-particles exhibited consistently higher values than those of plain solder joints due to the control of the fine microstructure as well as the homogeneous distribution of ceramic nano-particles giving as a second phase dispersion strengthening mechanism. During the reflow process, the formation of IMCs is one of the mechanisms for establishing a strong joint between liquid solder and substrate. However, due to the rapid formation of brittle IMC layers growth affects the mechanical reliability of the solder joints. Therefore, to develop reliable lead-free composite solder joints, it is desirable to better understand the kinetics governing the growth of the interfacial IMC. The growth kinetics of interfacial IMCs of lead-free composite solders doped with different metallic and ceramic nano-particles and OSP-Cu or Cu substrates were investigated. From kinetic analyses, the activation energies for the growth of the Cu-Zn-Ag IMC layer for Sn-Zn-Bi-0.5Ag/Cu and the Cu-Zn-Ni IMC layer for Sn-Zn-Bi-0.5Ni/Cu systems were about 41.1 and 46.5kJ/mol, respectively. In Sn-Ag-Cu/OSP-Cu system, the calculated activation energies for the total (Cu6Sn5+Cu3Sn) IMC layers for Sn-Ag-Cu solder joints, Sn-Ag-Cu-l wt% ZrO2 and Sn-Ag-Cu-0.5AI composite solder joints were about 53.2kJ/mol, 59.5kJ/mol and 55.lkJ/mol, respectively. On the other hand, the calculated activation energy for the Sn-Ni-Cu IMC layer for Sn-Ag-Cu-0.5Ni composite solder joints on OSP-Cu pads was about 49.3kJ/mol. The reliability of nano-metallic and ceramic particle doped lead-free composite solders with different surface finished substrate solder joints were evaluated in terms of under-bump-metallization (UBM) dissolution, IMC formation in the solder joint in conjunction with revolutionized concepts of high temperature aging, high temperature/humidity (85°C/85%) testing and thermal shock testing. After the addition of ceramic nano-particles, the IMC layer thicknesses at their interfaces and IMC particle sizes in solder ball regions were substantially decreased due to the second phase reinforcement ceramic nano-particles which promote a high nucleation density in the eutectic colonies during solidification and significantly improved the mechanical reliability of solder joints. In high temperature/mechanical damping tests nano-particle doped composite solders exhibited a low damping capacity and a high creep rapture life time found in creep tests as compared with plain Sn-Ag-Cu solders.
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