Pb-Free Solders and Next Generation Interconnects : Thermal Cycling, Sn Whiskers, and Mechanical Properties
Program Organizers: Thomas Bieler, Michigan State University; Babak Arfaei, Binghamton University; Chris Kinney, University of California, Berkeley
Monday 2:00 PM
October 8, 2012
Room: Room 325
Location: David L. Lawrence Convention Ctr
Session Chair: Fu Guo, Beijing University of Technology; Chris Kinney, University of California, Berkeley
2:00 PM
The Effects of Thermal Cycling on Electromigration Behaviors in Lead-Free Solder Joints: Fu Guo1; Limin Ma; Yong Zuo1; Sihan Liu1; Xitao Wang2; 1Beijing University of Technology; 2University of Science and Technology Beijing
The failure mechanism of solder joints used in microelectronic packaging has been widely investigated under the mono conditions of thermal cycling or electromigration (EM). However, the coupling effects of thermal cycling and EM on the solder alloy was not obtained enough attentions which was much more close to the operation conditions of electronic products and helpfully when mentioned to evaluate the real-life time of the solder joint. In this study, the effects of thermal cycling on EM behaviors and the evolutions of grain orientation, microstructure, and resistivity of the solder joint were observed. The primary results indicated that the changes of resistivity of the solder alloy can be attributed to the microstructure evolution and the modification of the grain orientation. Hence, the relationship between resistivity and microstructure was established in this study to illustrate the EM damages in the solder joint under the electric current stressing and thermal cycling conditions.
2:20 PM
Fabrication and Optimization of Low Temperature Sintering Die Attach Preforms Via the Tape Casting Method: Jared McCoppin1; 1Wright State University
Die-attach solutions are needed for high temperature electronics applications. In this work, tape casting is utilized to fabricate preforms of micro/nano silver material. The preforms use a polypropylene carbonate/propylene carbonate binder/plasticizer system. A bimodal powder distribution of silver powder having ranges of diameters from 1.3-3.2um, along with silver nanopowder having a range of diameters from 30 to 50nm allows effective packing of the particles, decreases shrinkage, and increased grain size. The tape cast preform is devoid of solvent reducing the organics content. This combination of reduced shrinkage and less vapor evolution during burnout may allow tape-cast nanosilver preforms to be utilized on larger dies than are currently feasible by other approaches. Microstructure, thermal, and mechanical characterization is performed on the tapes using SEM, TGA/DSC and tension testing.
2:40 PM
Controlled Growth of Spontaneous Tin Whisker: Albert T. Wu1; Chien-Hao Su; 1National Central University
Broken spots on surface tin oxide layer are crucial for spontaneous tin whisker growth. Whiskers grow from these positions when atoms diffuse to the root of the whiskers under stress gradient generated by intermetallic compound formation. In this study, weak spots were deliberately created on tin oxide by lithography; the growth of each whisker could be monitored and yield quantitative measurement of the dimensions. Corresponding to the growth, the evolutional stresses of the films were accurately measured by synchrotron radiation X-ray diffraction. The results showed that the stress in the film was not a constant at the initial stage but evolved with total volume of the extruded whiskers. The stress approaches yielding strength of tin after steady-state. The growth kinetics and the comparison with mathematical model will be discussed in this paper.
3:00 PM
Propensity for Tin Whisker Formation Due to Thermal Fluctuations and Intermetallic Compound Growth: Benjamin Anglin1; Wei-Hsun Chen2; Pylin Sarobol2; Ricardo Lebensohn3; John Blendell2; Carol Handwerker2; Anthony Rollett1; 1Carnegie Mellon University; 2Purdue University; 3Los Alamos National Laboratory
Stress relief has long been theorized to be the driving force of metallic whiskers, particularly those that form on tin finishes. However, the source of the stress is still subject to debate. In this work it is postulated that thermal excursions from room temperature induce sufficient stress to cause whisker formation. Starting from a measured two-dimensional tin whisker microstructure, a three-dimensional physical model, including an intermetallic compound between the tin and copper layers, is built. The resulting stress state from thermal excursions is simulated using a thermoelastic spectral method, which accounts for local changes in crystal orientation and elastic properties. A measured microstructure provides more realistic grain shapes and orientations compared to those constructed synthetically.
3:20 PM Break
3:40 PM
Evaluation of Anisotropic Effect on Defect Formation in Tin Film under Compressive Environment and Thermal Hold Condition: Wei-Hsun Chen1; Pylin Sarobol1; John Koppes1; Peng Su2; John Blendell1; Carol Handwerker1; 1Purdue University; 2Component Quality and Technology Cisco Systems, Inc.
Stress relaxation in Sn films can occur through the formation of hillocks and whiskers. The stress state around a defect, however, is hard to determine experimentally. In this work, finite element analysis is performed to simulate Sn films under a compressive stress and thermal hold condition. Computer-generated microstructures with different textures are used to determine the effect of crystallographic anisotropy on the local stresses. In addition, microstructures using orientation information based on EBSD and synchrotron diffraction measurement have been generated. The elastic stress, strain and strain energy distributions have been determined using OOF2 and related to the location of defect and defect density. These simulation results are also compared with experimental observations of strain from the synchrotron measurements. The resolved shear stress in each grain is also calculated to investigate the relationship between plastic deformation and defect formation.
4:00 PM
Correlation between Interfacial Reaction and High Speed Impact Reliability in Sn-Ag-Cu/Au/Ni Solder Joints: Chi-Yang Yu1; Shao-Wei Fu1; Tae-Kyu Lee2; Jenq-Gong Duh1; 1National Tsing Hua University; 2Cisco Systems Inc.
The interfacial reactions and the high speed impact reliability of Sn-Ag-Cu/Au/Ni solder joints before and after aging are investigated. During aging, (Cu,Ni)6Sn5 intermetallic compounds (IMCs) with high and low Ni contents formed at the Sn-Ag-Cu/Ni interface, represented as H-(Cu,Ni)6Sn5 and L-(Cu,Ni)6Sn5 respectively. This study proposes detailed formation mechanisms of H-(Cu,Ni)6Sn5 and L-(Cu,Ni)6Sn5. By employing high-speed shear testing, the impact toughness and peak force of Sn-Ag-Cu/Au/Ni solder joints degraded after aging. In addition, the impact fracture modes could be related to the interfacial microstructure. The nano-indenter was used to measure the hardness and toughness of H-(Cu,Ni)6Sn5 and L-(Cu,Ni)6Sn5 IMCs. Correlating the impact fracture behavior and mechanical properties of H-(Cu,Ni)6Sn5 and L-(Cu,Ni)6Sn5, it is demonstrated that the location of the crack propagation in the Sn-Ag-Cu/Au/Ni solder joints can be revealed under high speed impact testing.
4:20 PM
Effect of Zn Addition on the Deformation Behavior of Sn–3.5Ag Lead-Free Solder Alloy
: Sam Mahin Shirazi1; Reza Mahmudi2; 1Binghamton University; 2University of Tehran
The deformation behaviors of Sn–3.5Ag and Sn–3.5Ag–1.5Zn alloys were investigated at temperatures ranging from 298 to 400 K, and strain rates ranging from 5 × 10–4 to 1 × 10–2 s–1. After melting and casting, the samples were rolled to sheets, from which tensile specimens were punched and pulled to fracture in uniaxial tension tests. Scanning electron microscopy (SEM) was used to study the microstructure and fracture surface of the samples. Addition of 1.5% Zn into the binary alloy resulted in an increase in ultimate tensile strength (UTS) and a decrease in ductility. The increase in strength and decrease in ductility were attributed to the formation of the second phase AgZn intermetallic compound in the Sn matrix. The strain rate sensitivity (SRS) indices (m) of 0.12 to 0.17 and 0.11 to 0.19 were found for the Sn–3.5Ag and Sn–3.5Ag–1.5Zn alloys, respectively.
4:40 PM
Sn Whisker Growth Behavior in Thermal Cycling: Ying Wang1; Pylin Sarobol1; Peng Su2; John Blendell1; Carol Handwerker1; 1Purdue University; 2Cisco Systems,Inc
The local microstructural response of Sn films on 300nm Cu-coated Si substrates to stresses induced by thermal cycling (-40°C/85°C in air) was monitored by Scanning electron microscopy as a function of the number of thermal cycles. By following the morphology of individual whiskers and their surroundings,four distinct features were observed: deep grooves at the whisker root, a decrease in the diameter of whiskers as they grew, whisker pinch-off, and the formation of new whiskers with thermal cycling. Whisker density increased initially but then decreased due to continuing whisker pinch-off and a decreasing rate of new whisker formation with thermal cycling. In comparison, ambient storage produced longer whiskers, with a lower density and no apparent change in whisker diameter with growth. The difference between microstructure evolution during thermal cycling and during ambient storage will be compared, and the mechanisms responsible for the characteristic features of thermal-cycled whiskers will be discussed.