Emerging Interconnect and Pb-free Materials for Advanced Packaging Technology: Tin Whisker and Wettability
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

Wednesday 2:00 PM
March 1, 2017
Room: 30E
Location: San Diego Convention Ctr

Session Chair: Babak Arfaei, Binghamton University ; Eric Cotts, Binghamton University

2:00 PM  
Influences of Wettability and Volume of Sn-based Solder Alloys on Self-Alignment Accuracy: Hwan-Pil Park1; Gwancheol Seo1; Young-Ho Kim1; 1Hanyang University
    Solder self-alignment is a typical and important phenomenon enabling cost-effective device packaging requiring accurate positioning. In this study, we discussed the wettability of Sn-based solder alloys on different substrate and their critical solder volume affecting self-alignment accuracy. To investigate the influence of solder wetting and volume on self-alignment accuracy, the contact angle of SAC305 and Sn3.5Ag solder on two different commercial surface finish substrates (ENEPIG and OSP) was measured and solder volume was controlled by varying their bonding height from 10 to 100 μm, respectively. By using a transparency glass chip, the measurement of exact location was analyzed by the high resolution optical microscopy and scanning electron microscopy. The alignment accuracy depends on solder wettability and volume. Controlling solder wettability and volume, alignment of less than 1 μm can be obtained. This study will contribute to reduce the die position error in flip-chip technologies and fan-out package manufacturing approaches.

2:20 PM  
Role of Indium Doping on Whisker Mitigation in Electroplated Sn: Bhaskar Majumdar1; Sherin Bhassyvasantha1; 1New Mexico Tech
    In recent work, dramatic reduction in whisker growth in Indium doped electroplated Sn has been reported. Here we present results of our studies aimed at identifying the primary mechanisms responsible for near mitigation of whiskers. Auger studies reveal the presence of mixed In2O3 and SnO/SnO2 at the surface. In addition, the surface layer shows an excess of In2O3 in the oxide scale compared to the base alloy. The large negative enthalpy of indium oxide versus tin oxide is likely a driver for the surface indium excess. FIB sections show similar reaction zone and columnar grain structure for undoped and indium doped material. It is suggested that the indium serves to compromise the tenacious oxide layer and indirect evidence is provided through polarization studies. Results from XPS measurements as well as stress measurements using XRD techniques will be presented. We gratefully acknowledge support through NSF CMMI-1335199/1335491 grant.

2:40 PM  
Impact of In Addition to Electroplated Sn in Mitigating Whisker Growth: Susmriti Das Mahapatra1; Bhaskar Majumdar2; Indranath Dutta1; 1Washington State University; 2New Mexico Tech
    The effect of In-doping of electroplated Sn on whisker growth from Sn films is reported, and it is demonstrated that 5-10% In throughout the thickness of the Sn films, whisker growth is completely eliminated. By comparing whisker growth in Sn-In coatings with monolithic Sn coating and Sn-Sn-Sn tri-layer, the reasons behind this mitigation are explored, using FIB, AES, XPS and XRD. FIB is used to assess the role of internal interfaces, and XRD is used to elucidate the role of Cu-In intermetallic formation. FEA is used to investigate the role of In-incorporation in the surface-oxide, as revealed by XPS and AES. Based on the analyses, it is inferred that the observed mitigation due to In addition occurs due to a multitude of factors. Finally, a procedure for co-electrodeposition of Sn-In alloys, and its effect on whisker mitigation are reported. Supported by NSF (CMMI-1335199/1335491)

3:00 PM  
Quantifying the Role of Stress in Whisker Nucleation and Growth: Eric Chason1; Fei Pei1; Justin Vasquez1; Andrew Hitt1; 1Brown University
    Although Sn whiskers have been studied extensively, there is still controversy about what are the driving forces for whisker nucleation and growth. Many studies point to the role of stress, but confirming this requires a quantitative comparison between controlled stress and the resulting whisker evolution. We describe the results of recent studies in which we use applied stress (via thermal cycling or mechanical pressure) to induce whisker nucleation and growth. The measurements are interpreted in terms of phenomenological equations that try to relate the observed behavior to underlying kinetic mechanisms. Studies will be described that determine how stress decreases the barrier for whisker nucleation at different temperatures and strain rates. We will also show results in which mechanical pressure drives the growth of whiskers.

3:20 PM Break

3:40 PM  
Sn Whisker Growth in Air HAST: Chulmin Oh1; Wonsik Hong1; 1KETI
    Sn whisker hasn’t been solved in the electronics industry yet although many researcher have studied the growth mechanism of Sn whisker. The test of Sn whisker has been still proceeded with the JEDEC standard to confirm the mitigation performance of Sn whisker. However, in the high temperature/humidity storage test described in JEDEC standard, the test time (4,000h) is too long to correlate with the development timeline. We here investigate the test method of Air HAST where the partial air is able to insert in the HAST chamber, enable to accelerate the oxidation process at HAST condition. The specimens consisting of QFP, SOP, and MLCC are soldered on FR-4 board with various Br content of Pb-free solder materials. Sn whisker observed by optical microscope and SEM by changing the air pressure and compared the Sn whisker in the high temperature/humidity storage to identify the same growth mechanism of Sn whisker.

4:00 PM  
Sn Film Microstructure on the Kinetics of Spontaneous Whisker Growth: Albert T. Wu1; Hao Chen1; Wen-Chih Lin1; 1National Central University
    Spontaneous Sn whisker growth is harmful to the reliability of electronic devices. In this study, lithography process was used to define the location of the whiskers on Sn films with various thicknesses and grain sizes. The history of the growth for each whisker was recorded to yield quantitative analysis. The results evidently show that both thickness and grain size of the Sn films affect the kinetics of the growth. Since spontaneous whisker growth is accompanied to stress generation and relaxation of the film, synchrotron radiation X-ray was used to measure the residual stress of the films. An incubation time for building up compressive stress and nucleation of whiskers was observed. A model is proposed for steady-state growth kinetics of Sn whiskers on the films with various microstructures.