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Please use this identifier to cite or link to this item: http://hdl.handle.net/2031/6232

Title: Electromigration and thermomigration studies of lead-free solder joints
Other Titles: Wu qian han liao han dian de dian qian yi he re qian yi yan jiu
無鉛焊料焊點的電遷移和熱遷移研究
Authors: Gu, Xin (谷新)
Department: Department of Electronic Engineering
Degree: Doctor of Philosophy
Issue Date: 2010
Publisher: City University of Hong Kong
Subjects: Electronic packaging.
Solder and soldering.
Lead-free electronics manufacturing processes.
Notes: CityU Call Number: TK7870.15 .G8 2010
xxiii, 167 leaves : ill. 30 cm.
Thesis (Ph.D.)--City University of Hong Kong, 2010.
Includes bibliographical references (leaves 155-165)
Type: thesis
Abstract: Electromigration (EM) of solder joints in microelectronic devices has become a critical reliability issue due to continuous miniaturization and requirements for enhanced performance of devices. Also, thermomigration (TM) has been regarded as another reliability concern in recent years as it has been found to accompany EM in flip chip (FC) solder joints. Lead (Pb)-free solders are being widely used in the electronic industry to replace Pb-containing solders in recent years. The coming of the Pb-free solder era is driving researchers to investigate the reliability of the available Pb-free solders for practical applications. This dissertation is aimed at investigating the EM and TM behavior of Pb-free solder joints. An atomic flux model is proposed and applied to explain the combined effects of EM and TM on Pb-free solder joints. The current-crowding effect was reduced in Cu/Sn58Bi/Cu solder joints by using a line-type structure. The EM behavior of Sn58Bi solder in the solid state and effects of EM on the evolution of Cu-Sn IMC were investigated. Driven by the DC current, the Bi atoms were found to migrate towards the anode side and formed a Bi-rich layer there, where the growth of the Bi-rich layer followed a linear dependence on the current stressing time. The growth of Cu-Sn IMC layers both at the anode and the cathode sides was enhanced by the electric current. Moreover, the IMC layer at the cathode side was found to grow faster than that at the anode side, and the growth of IMC layers at the cathode followed a parabolic law. The intrinsic diffusivity (D0) and the activation energy of the growth of the total Cu-Sn IMC layer at the cathode were thus calculated to be 9.91×10-5 m2/s and 89.2kJ/mol (0.92 eV), respectively. When Sn58Bi solder was in a molten state or partially molten state during the EM test, it was found that the Cu/Sn58Bi/Cu solder showed a different EM behavior as compared with that was in solid state, two separate Bi-rich layers formed at the anode side and a great many Cu6Sn5 IMC precipitates formed between the two Bi-rich layers. The combined effects of TM and EM in Sn58Bi solder were detected in Cu/Sn58Bi/Ni solder joints supplied with DC currents in opposite directions. Under the thermal gradient, Bi atoms were found to move towards the lower temperature side. A thermal gradient of 527°C/cm was verified to exist in a solder joint stressed with a 5×103 A/cm2 DC current at 50°C. Depending on the direction of the current, TM was found to assist or counteract EM on the diffusion of Bi atoms. The atomic fluxes of Bi induced by EM and TM were estimated separately. EM-enhanced cross interaction between Cu and Ni across the solder joints was also detected, which led to the IMC layer at the Ni side or the Cu side of the joint with Cu as the cathode was thinner than that of the corresponding IMC layer in the joint stressed with a current in the opposite direction. For a joint as-reflowed, the IMC at the Ni side was (Cu,Ni)6Sn5 instead of Ni3Sn4 and the IMC at the Cu side was Cu6Sn5 without containing Ni. For the joint with Cu as the cathode, the IMC at the Ni side remained as (Cu,Ni)6Sn5 even after the joint was stressed for 384 h. However, for the joint with Cu as the anode, (Cu,Ni)6Sn5 at the Ni side turned into (Ni,Cu)3Sn4 after the joint was stressed for 384 h. Regardless of the direction of the electric current the IMC formed at the Cu side was Cu6Sn5. For the joint with Cu as the anode, the growth of Ni3Sn4 at the cathode side was enhanced by EM and TM, while the growth of Cu6Sn5 at the anode side was retarded. TM and EM in Sn58Bi ball grid array (BGA) joints were investigated using a specifically designed daisy-chain supplied with a 2.5 A DC current at 110°C. The individual effect of TM and EM, and the combined effects of TM and EM were detected. The effects of electron charge swirling were also detected when the electric current passed by the top of the solder bump instead of entering into it, where the electron charge swirling and TM played combined effects on the migration of Bi atoms. For the joint without current passing by or passing through, only TM induced the migration of the Bi atoms. The combined effects of TM and EM were investigated in Cu/Sn3.0Ag0.5Cu/Ni solder joints by applying a 5×103A/cm2 DC current to the joints. With a thermal gradient, the Cu atoms and Ni atoms were found to migrate towards the lower temperature side while the Sn atoms were found to migrate towards the higher temperature side. For the solder joint stressed with a 5×103A/cm2 DC current, TM was found to play a dominant effect on the migration of Cu atoms in the solder because the Cu atomic flux in the solder which was induced by EM was smaller than that was induced by TM. The Ni atomic flux in the solder that was induced by TM was smaller than that induced by EM, but it could not be ignored because the values were the same order of magnitude. The Sn atomic flux induced by TM could be ignored due to its smaller magnitude than that induced by EM. The Cu3Sn IMC layer and Cu6Sn5 IMC layer at the Cu/solder interface in the joint with Cu as the anode were thicker than corresponding layers at the Cu/solder interface in the joint with Cu as the cathode. The IMC at the Ni/solder interface of joint with Cu as the anode was the Ni3Sn4 phase while that in the solder joint with Cu as the anode was the (Cu,Ni)6Sn5, and the former was thicker than the latter. EM and TM induced dissolution of the Ni3Sn4 IMC into the solder was found to be the main cause of the formation of voids at the Ni3Sn4/solder interface from a joint that used Cu as the anode. For the solder joint that used Cu as the anode, the Ni3Sn4 IMC layer at the Ni/solder interface was as thick as the Cu-Sn IMC layer at the Cu/solder interface. By contrast, for the solder joint that used Cu as the cathode, the IMC layer at the Ni/solder interface was thinner than that at the Cu/solder interface. Microstructural evolution of Cu/Sn8Zn3Bi/Cu joints under DC current stressing was investigated. Only Cu5Zn8 IMC could be observed in the solder joints at the initial stage of the test since the formation of the Cu5Zn8 IMC is easily achieved compared to that of Cu6Sn5 IMC. With an increase of the stressing time, Cu6Sn5 IMC was detected at both interfaces. The polarity effect of EM on the growth of the Cu5Zn8 and Cu6Sn5 IMC layers in solder joints was also detected, where the Cu5Zn8 IMC formed at the cathode side was thicker than that at the anode side, whereas the Cu6Sn5 IMC layer at the anode side was thicker than that at the cathode side. Microstructural evolution of Cu/Sn8Zn3Bi/Ni joints under a 5×103 A/cm2 DC current stressing at 110°C was also investigated. It was found that the growth of the Ni-Zn IMC at the Ni side is rather slow when a solder joint was aged at 110°C without current stressing. If the solder joints were supplied with a DC current, all the IMC layers at the interfaces were thickened. For the joints with Cu as the anode, after 384 h of DC current stressing, all the Zn phases in the solder turned into IMC phases, while a large number of rod-shaped Zn-rich grains could be observed for a solder joint with Cu as the cathode stressed under the same experimental conditions. The Ni5Zn8 IMC layer at the Ni side of a solder joint with Cu as the anode was thicker than that at a solder joint with Cu as the cathode. With an increase of the stressing time, voids were found to develop at the Cu6Sn5/solder interface from a solder joint with Cu as the cathode, while both of the interfaces remained intact at a solder joint with Cu as the anode.
Online Catalog Link: http://lib.cityu.edu.hk/record=b3947843
Appears in Collections:EE - Doctor of Philosophy

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