Emerging Interconnect and Pb-free Materials for Advanced Packaging Technology: Electromigration, Thermomigration and Electrochemical Behaviors
Sponsored by: TMS Functional Materials Division, TMS: Electronic Packaging and Interconnection Materials Committee
Program Organizers: Fan-Yi Ouyang, National Tsing Hua University; C. Robert Kao, National Taiwan University; Albert T Wu, National Central University; Fay Hua, Intel Corporation; Yan Li, Intel Corporation; Babak Arfaei, Binghamton University; Kazuhiro Nogita, The University of Queensland

Tuesday 8:30 AM
February 28, 2017
Room: 30E
Location: San Diego Convention Ctr

Session Chair: John W Elmer, Lawrence Livermore National Laboratory; Yan Li, Intel Corporation

8:30 AM  Invited
The Grain Refinement of Metal Alloy by Electromigration: PinChu Liang1; Kwang-Lung Lin1; 1National Cheng Kung University
    Previous studies reported that the peak intensity of in-situ synchrotron XRD spectra of solders diminished during electric current stressing. The decline behavior was ascribed to the formation of high dislocation density induced by electromigration force. Grain refining was achieved for pure Sn with appropriate control of the recrystallization behavior triggered by the high density dislocations. The investigation of grain refining by electric current stressing was extended to CuZn brass. The grain refinement can be optimized by monitoring current stressing and Joule heat dissipation. The appropriate control of the current stress-quench cycle parameter reduced the average grain size of the CuZn brass from 15 μm of the annealed as-prepared brass foil to around 4 μm as evidenced by EBSD investigation. The EBSD results show the increase in the fractions of misorientation angles and twin structure, which evidenced the recrystallization of the brass foil, upon the current stress-quench cycle operation.

8:50 AM  
In Situ Characterization of Electromigration Damage in Single Crystal and Bi-crystal Pure Tin Solder Joints: Marion Branch Kelly1; Antony Kirubanandham1; Nikhilesh Chawla1; 1Arizona State University
    Reduction in solder joint size and the number of grains per joint increases the severity of electromigration damage due the anisotropic nature of tin. Since electromigration is a diffusion-dependent phenomenon it is important to understand the effects of both through-grain and grain boundary diffusion on electromigration. Single crystal and coarse-grained samples were fabricated from larger sandwich-type solder joints and used to study electromigration in single crystals and bi-crystals. In situ scanning electron microscopy (SEM) and electron backscatter diffraction (EBSD) characterization were used to quantify damage evolution over time. The effects of grain orientation and grain boundary effects were also investigated and will be discussed.

9:10 AM  
DZ* Value of the Sn Diffuser in Cu6Sn5 under Various Current Densities: Cheng-Hsien Yang1; Pei-Tzu Lee1; Han-Lin Chung1; Cheng-En Ho1; 1Yuan Ze Univeristy
    The DZ* value of Sn in the Cu6Sn5 phase under various current densities (j = 0–104 A/cm2) at 150 C was acquired via Cu/Sn/Ni diffusion couples. During current stressing, an electric current was imposed from the Ni side to the Cu side of the couples, and the reaction kinetics of the Cu6Sn5 phase at the Cu/Sn interface was analyzed, to characterize the Sn electromigration behavior in growth of Cu6Sn5. A linear increase of the Cu6Sn5 (or Sn) quantity was obtained with increasing j, resulting in a unique growth mode of Cu6Sn5 in the Cu/Sn reaction. A calculation based on the fundamental electromigration theory in combination with experimental measurements indicated that the DZ* value of Sn in Cu6Sn5 fell within the range of 2×10-14 m2/s – 5×10-14 m2/s for j = 0–104 A/cm2.

9:30 AM  
Study of Electromigration Mechanism in Pb-free Tricrystals Ball Grid Array Solder Joints: Yu Tian1; Jing Han1; Fu Guo1; 1Beijing University of Technology
    The EM mechanism of SAC305 solder joints was closely related to Sn-grain orientation, especially in tricrystals BGA solder joints. A BGA specimen of Sn-3.0Ag-0.5Cu (SAC305) solder with cross-sectioned edge row was subjected to 2.1103A/cm2 at room temperature for 600 hours. The results showed that the growth mechanism of Cu6Sn5 IMCs had a close relationship with the c axis three-dimensional direction of Sn grain. When the direction of c axis in Sn grain was nearly parallel to the direction of current, the Cu6Sn5 IMCs grew rapidly and stacked at the tricrystals grain boundary. That was because the diffusion rate of Cu atoms was fast along the direction of c axis, and the high interfacial energy at the grain boundary. Futhermore, based on lower resistivity of c axis, the Cu6Sn5 IMCs preferred to internal growth when the c axis of Sn grain along the cross section inwards.

9:50 AM  
Intermetallic Compound Movement Behavior of Cu Reinforced Composite Solder under Current Stressing: Fu Guo1; Yan Wang1; Jing Han1; 1Beijing University of Technology
    Sn grain orientations in solder matrix dominate the principal failure mechanisms in solder joints under electric current stressing. Due to Sn anisotropy, Cu diffusivity in Sn along c-axis is about 500 times faster than that along a or b axis. In the current research, 2 vol.% micron-seized Cu particles were doped in Sn-3.5Ag solder to form Cu reinforced composite solder. One-dimensional solder joints, designed to prevent the current crowding effect, were stressed under a constant current stress density of 104A/cm2. The effect of various Sn grain orientation on the movement behavior of the interfacial IMC and Cu6Sn5 reinforcement in solder matrix during electromigration was preliminary investigation. Under electromigration, Cu6Sn5 migrated along c-axis of Sn grain through solder joint to form specifically “Cu6Sn5 area”, while Cu6Sn5 moved away in the “non-Cu6Sn5 area” and only Sn-based solder was left. Beside, the interfacial IMC in the cathode was consumed rapidly at “non-Cu6Sn5 area”.

10:10 AM Break

10:30 AM  
Effective Suppression of Thermomigration-induced Cu Dissolution in Micro-scale Pb-free Interconnects by Ag3Sn interlayer: Gong-Lin Hong1; Yu-Fang Lin1; Fan-Yi Ouyang1; 1Dept. of Engineering and System Science, National Tsing Hua University
    3D IC technology is the major trend of electronic packaging in microelectronic industry. Dissolution of Cu under bumping metallization (UBM) and decomposition of intermetallic compounds (IMC) induced by a temperature gradient have been reported to be serious reliability issues in 3D IC package. This study proposed to use Ag3Sn interlayer as a diffusion barrier to inhibit the thermomigration (TM) of Cu in micro-scale interconnects. To understand the effect of Ag3Sn layer on TM of Cu, two kinds of samples were introduced and compared, Cu/Ag3Sn/Sn3.5Ag/Cu and Cu/Sn3.5Ag/Cu. The results show that the dissolution of IMCs at hot end in Cu/Ag3Sn/Sn3.5Ag/Cu was slower than that in Cu/Sn3.5Ag/Cu, and the calculated JCu,TM for samples with Ag3Sn was about one-third times smaller than that in the specimen without Ag3Sn. In addition, the orientation of Sn grain was found to be strongly affected by Ag3Sn interlayer. The corresponding mechanism would be discussed and compared.

10:50 AM  
Corrosion Resistance for High Reliability Devices: Tsan-Hsien Tseng1; Albert T. Wu1; 1National Central University
    High sulfur (S) and moisture environment causes corrosion in electronic components. It is crucial to develop a new coating of surface finishes on the printed circuit boards (PCBs). This paper proposes cobalt (Co) as a barrier layer to prevent corrosion in harsh environment. PCBs for automobile devices were coated with cobalt layers. Cross cut adhesion test showed that zero percent of the films were removed. Surface roughness was analyzed by atomic force microscope (AFM). It suggests that the EC process can modify the substrate surface morphology. Corrosion tests were performed in a chamber with high humidity. The relative humility was set at the range from 75% to 95% at a temperature between 80 and 150 oC. Dramatic surface corrosion can be observed without the coatings of Co. After the deposition of Co, the corrosion was inhibited and shows good corrosion resistance.

11:10 AM  
Failure Mechanism and Reliability of Ag-4Pd Alloy Wire Bonded on Al-Si Metallization under High Temperature Storage and Thermal Cycle Tests in Corrosive Environments: Yan Wen Tsau1; Jui-Nung Wang1; Fan-Yi Ouyang1; 1National Tsing Hua University
     Due to the rising gold price, alternative materials, such as Ag, with profound electrical and mechanical properties, become potential candidates to replace Au in wire bonding technology. In this study, we investigated the failure mechanism of Ag-4Pd alloy wire bonded on Al metallization under accelerated and corrosive environments. The samples were first conducted with high temperature storage test (HTST) at 150C for 500 hours and thermal cycle test (TCT) in the range of -55C to 125C for 250 cycles as acceleration tests. The samples were further subject into salt spray chamber for chlorine corrosion test. The as-bonded samples encountered the fastest failure during salt spray test, caused by galvanic corrosion on Al metallization. For samples under HTST, failure caused by cracks propagation through the IMC layer. The samples undergoing TCT exhibited best corrosion resistance because thin IMC layer acts as a barrier to prevent galvanic corrosion.