Emerging Interconnect and Pb-free Materials for Advanced Packaging Technology: Mechanical Properties of Pb-free Materials
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

Monday 2:00 PM
February 27, 2017
Room: 30E
Location: San Diego Convention Ctr

Session Chair: Fay Hua, Intel Corporation; Carol Handwerker, Purdue University

2:00 PM  Invited
Impact of Interrupted Thermal Cycling on Sn-Ag-Cu Interconnection Performance: Tae-Kyu Lee1; Zhiqiang Chen1; Greg Baty1; Thomas R. Bieler2; Choong-Un Kim3; 1Portland State University; 2Michigan State University; 3University of Texas, Arlington
    The expected life cycle time for solder interconnects in electronic devices can be evaluated by accelerated thermal cycling test. The characteristic life cycle number or 1% failure rate are used to estimate the expected life cycle time based on the device end-use conditions. But given the fact that the device in real function experience interrupted thermal cycling in service, the impact of the discontinued thermal cycling was investigated. Fine pitch ball grid array (BGA) components with Sn-Ag-Cu interconnects were thermal cycled with interruption cycles in various storage time intervals. The components assembled on 2.4mm thick PCB, were stored at a constant temperature of 50oC in air and in vacuum to observe the environmental affects during thermal cycling interruption. The results show that the life cycle number varies with interval time and storage environment. A potential mechanism of the degradation and improvement of thermal cycling performance will be discussed.

2:20 PM  
Thermal Cycling Performance of Sn-0.5Cu(Pd)-Al(Si)-Ge Solder Joints for Power Control Unit of Automotive: Won Sik Hong1; Chulmin Oh1; 1Korea Electronics Technology Institutue(KETI)
    Sn-Ag-Cu Pb-free solder is limited to apply underhood electronics due to harsh use environment rather than cabin of automotive. Because of ELV (End-of-Life Vehicle) banning, we need to another Pb-free solder alloy for vehicles. In this study, we proposed a new alloy composition which was Sn-0.5Cu(Pd)-Al(Si)-Ge (Tm=230 deg.C). Power control unit (PCU) was soldered with proposed Pb-free solder on FR-4 PCB with OSP finish. To verify the solder joint reliability, we conducted thermal cycling test (10 min dwell) during 1500 cycles from -40 to 125 deg.C, and observed microstructure evolution, interfacial reaction and bonding strength degradation of solder joint. From these result, we have known that the crack propagation was suppressed at the solder joint. Minor Al, Si and Pd addition in Sn-0.5Cu alloy improved solder joint reliability due to microstructure evolution. We have finally confirmed a feasibility of middle temperature Sn-0.5Cu(Pd)-Al(Si)-Ge Pb-free solder for engine room electronics of automotive.

2:40 PM  
Thermocycling Stress Induced Slip Band Sliding in Ultra-thin ENEPIG Joints: Tzu-Ting Chou1; Cheng-Ying Ho1; Wei-Yu Chen1; Jenq-Gong Duh1; 1National Tsing Hua University
    The lead-free solder joints reflowed with ultrathin electroless Ni/electroless Pd/immersion Au (ultrathin ENEPIG) under-bump metallization show distinct failure mode as compared to traditional ENEPIG joint. The ultrathin ENEPIG joint exhibit larger Sn grain size and less grain boundaries that differ from common beach ball structure. Generally, the deterioration occurs along grain boundaries because of mismatch in coefficient of thermal expansion between beta-Sn grains. Due to lack of grain boundaries to release cycling thermal stress, the failure in ultrathin-ENEPIG joints is dominated by slip band sliding of Sn system. The sliding phenomena occurred within the Sn grain was evaluated, showing the trend on the (110) plane along [001] direction. This study demonstrates that how the cycling thermal stress affects the slip band degradation in ultrathin ENEPIG solder joints.

3:00 PM  
The Variation of Grain Structure and the Enhancement of Shear Strength in SAC305-0.1Ni/Cu Solder Joint before and after Aging: Collin Fleshman1; 1National Tsing Hua University
    The correlations between grain structures and shear strength of 300μm SAC305/Cu and SAC305-0.1Ni/Cu solder joints before and after aging were explored in this study. The grain structure of SAC305 solder revealed to be beach ball-like grain, while SAC305-0.1Ni solder exhibited multiple grain with interlaced sections in both aging conditions. Ni spread in solder acted as nucleation sites for the IMC precipitates and thus affected the growth of β-tin. With the aid of XYZtec bonding test, no matter before or after aging, the higher peak force of Ni-doped solder joints was detected due to the inhibition of dislocation suggested by Hall-Petch and dispersion strengthening mechanism. Thus, it is demonstrated that the mechanical reliability of SAC305-0.1Ni/Cu solder joint is superior to that of SAC305/Cu solder joint regardless of aging conditions.

3:20 PM Break

3:40 PM  
Electrical and Mechanical Properties of Sn-Ag-Cu Solder Pastes for Reverse-offset Printing Depending on Particle Concentration: Min-jung Son1; Minwoo Kim1; Taik-Min Lee1; Hoo-Jeong Lee2; Inyoung Kim1; 1Korea Institute of Machinery & Materials (KIMM); 2Sungkyunkwan University
    Nowadays, various printing techniques far surpassing the limitation of stencil printing are receiving a lot of attention in semiconductor packaging industry. Among them, reverse-offset printing has shown a high printing resolution of 1~3 μ m and smooth film surfaces for Ag nanoparticle inks. To apply the reverse-offset printing to a flip chip packaging, we fabricated Sn-Ag-Cu solder pastes consisting of flux medium and solder particles. The viscoelastic property for printability was controlled by changing the particle concentration without additional additives. After 2-step-reflow treatment, electrical and mechanical properties of solder bumps depended on the particle concentration. Soldering behaviors of the pastes, their microstructural development and organic residue were analyzed to explain the change of electrical and mechanical properties depending on the particle concentration. As a result, the solder paste with a particle concentration of 85 wt% represented a resistance and a shear strength of 0.5 Ω and 5.9 MPa, respectively.

4:00 PM  
The Strengthening Effects of Bismuth in Aged Lead-Free Solder Alloys Characterized using Transmission Electron Microscopy (TEM): André Delhaise1; Doug Perovic1; 1University of Toronto
    One of the main concerns with lead-free solder alloys is the degradation of mechanical properties after aging, which has been shown to be detrimental towards reliability. Several studies have concluded that the inclusion of bismuth in the alloy may retard this degradation and subsequently improve reliability. While it has been speculated that these effects may be due to solid solution strengthening and precipitation hardening, the exact mechanisms have not been characterized in any significant detail. The most effective tool for such an analysis is transmission electron microscopy (TEM). This paper discusses preliminary work using TEM to investigate the strengthening effects of bismuth on aged binary tin-bismuth alloys. Sample preparation techniques are shown and described. Specific techniques utilized include Weak Beam Dark Field Microscopy (WBDFM) to study defects brought about by the inclusion of bismuth, and in situ microscopy to observe metallurgical changes that occur during aging in real time.