Acta Metall Sin  2018, Vol. 54 Issue (7): 1077-1086    DOI: 10.11900/0412.1961.2017.00426
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Interaction Between β-Sn Grain Orientation and Electromigration Behavior in Flip-Chip Lead-Free Solder Bumps
Mingliang HUANG(), Hongyu SUN
Key Laboratory of Liaoning Advanced Welding and Joining Technology, School of Materials Science and Engineering, Dalian University of Technology, Dalian 116024, China
Abstract

With the increasing demands for miniaturization, the electromigration (EM)-induced failure by diffusion anisotropy in β-Sn is expected to be more serious than that induced by local current crowding effect, especially with the downsizing of solder bumps. In this work, the effects of Sn grain orientation on intermetallic compound (IMC) precipitation, dissolution of Ni under bump metallurgy (UBM) at the cathode, EM failure mechanism as well as the EM-induced β-Sn grain rotation in Ni/Sn-3.0Ag-0.5Cu/Ni-P flip-chip interconnects undergoing solid-solid EM under a current density of 1.0×104 A/cm2 at 150 ℃ were in situ studied. (Ni, Cu)3Sn4-type IMCs precipitated in these β-Sn grains with a small angle θ (between the c-axis of Sn grain and electron flow direction), i.e., along the c-axis of β-Sn grains. Stress relaxation, squeezing β-Sn whiskers near the anode, was also observed during EM. A mathematical model on the relationship between the dissolution of Ni UBM and β-Sn grain orientation was proposed: when the c-axis of β-Sn grain is parallel to the electron flow direction, excessive dissolution of the cathode Ni UBM occurred due to the large diffusivity of Ni along the c-axis; when the c-axis of β-Sn grain is perpendicular to the electron flow direction, no evident dissolution of cathode Ni UBM occurred. The proposed model agreed well with the experimental results. EM-induced β-Sn grain rotation was attributed to the different vacancy fluxes caused by EM between adjacent grains of various grain orientation, when vacancies reached supersaturation and undersaturation at the interfaces of the anode and the cathode, respectively. Vacancy fluxes went through free surface along the interface, resulting in a normal vacancy concentration gradient. Accordingly, stress gradient produces a torque to rotate the β-Sn grain.

 ZTFLH: TN405
Fund: Supported by National Natural Science Foundation of China (Nos.51475072, 51511140289 and 51671046) and Fundamental Research Funds for the Central Universities (No.DUT17ZD202)
 Fig.1  Schematic of the Ni/Sn-3.0Ag-0.5Cu/ENEPIG lead-free solder joint (ENEPIG—electroless nickel electroless palladium immersing gold, BT—bismaleimide triazine, PCB—printed circuit board, UBM—under bump metallurgy) Fig.2  Cross-sectional micromorphologies of as-soldered flip chip lead-free solder joint(a) macrograph (b) interface at the chip side (c) interface at the PCB side Fig.3  Cross-sectional micromorphologies of No.1 flip chip lead-free solder joint after electromigration at 150 ℃ under the current density of 1.0×104 A/cm2 for 0 h (a), 150 h (b), 250 h (c), 350 h (d), 500 h (e) and the corresponding EBSD map in RD direction (f) (RD—rolling direction) Fig.4  Cross-sectional micromorphologies of No.2 flip chip lead-free solder joint after electromigration at 150 ℃ under the current density of 1.0×104 A/cm2 for 0 h (a), 150 h (b), 250 h (c), 350 h (d), 500 h (e) and the corresponding EBSD map in RD (f) Fig.5  Cross-sectional micromorphologies of No.3 flip chip lead-free solder joint after electromigration at 150 ℃ under the current density of 1.0×104 A/cm2 for 0 h (a), 150 h (b), 250 h (c), 350 h (d), 500 h (e) and the corresponding EBSD map in RD (f) Fig.6  Cross-sectional micromorphologies of No.4 flip chip lead-free solder joint after electromigration at 150 ℃ under the current density of 1.0×104 A/cm2 for 0 h (a), 150 h (b), 250 h (c), 350 h (d), 500 h (e) and the corresponding EBSD map in RD (f) Fig.7  Cross-sectional micromorphologies of No.5 flip chip lead-free solder joint after electromigration at 150 ℃ under the current density of 1.0×104 A/cm2 for 0 h (a), 150 h (b), 250 h (c), 350 h (d), 500 h (e) and the corresponding EBSD map in RD (f) Fig.8  Cross-sectional micromorphologies of No.6 flip chip lead-free solder joint after electromigration at 150 ℃ under the current density of 1.0×104 A/cm2 for 0 h (a), 150 h (b), 250 h (c), 350 h (d), 500 h (e) and the corresponding EBSD map in RD (f) Table 1  Anisotropic Properties of β-Sn[8,9,10] Table 2  Parameters of the material used in the calculation Fig.9  The consumption of cathode Ni UBM as a function of angle q Fig.10  Schematic of grain rotation caused by electromigration in a Sn-3.0Ag-0.5Cu flip chip lead-free solder joint($Jvc$ and $Jva$ are the self-diffusion vacancy fluxes of Sn atoms along the c-axis and a-axis of Sn grain, respectively; v is vacancy)