Electronic Packaging and Interconnections 2021: Pb-free Solder Alloys II
Sponsored by: TMS Functional Materials Division, TMS: Electronic Packaging and Interconnection Materials Committee
Program Organizers: Mehran Maalekian, Materials & Metallurgy Expertise; Christopher Gourlay, Imperial College London; Babak Arfaei, Ford Motor Company; Praveen Kumar, Indian Institute of Science; Sai Vadlamani, Intel Corporation; Kazuhiro Nogita, University of Queensland; David Yan, San Jose State University
Tuesday 2:00 PM
March 16, 2021
Room: RM 22
Location: TMS2021 Virtual
Session Chair: Christopher Gourlay, Imperial College London; David Yan, San José State University
2:00 PM Invited
Effect of Current Stress on the Microstructure of SnBiAg-SAC Mixed Solder Joints: Eric Cotts1; Faramarz Hadian1; Randy Owen1; 1Binghamton University
We examined the effect of current stressing on the distribution of Bi in SAC/Bi-SnAg solder joints (mixed assemblies), and in more homogeneous, SnBiAg solder joints. After reflow of mixed assemblies, a large gradient in the concentration of Bi in SnBiAg region of the assemblies was observed. The effects of both heat treatment and electro migration were studied, at temperatures up to 150C and current densities up to 4000 amps/cm2. In this regime, both Bi concentration gradients and the electron wind, were found to be significant driving forces for Bi diffusion. The migration of the Bi atoms could be described using a simple model, with reasonable values for the diffusion coefficient of Bi in Sn, and for the effective charge of Bi. Some indication of the current density limits of such Sn-Bi based solder joints was obtained.
2:20 PM
Effects of Antimony on the Microstructure and Reliability of Sn-Ag-Cu-based Solder Joints: Sergey Belyakov1; Richard Coyle2; Babak Arfaei3; Christopher Gourlay1; 1Imperial College London; 2Nokia Bell Laboratories; 3Ford Motor Company
Antimony is increasingly being added to Pb-free solders to improve thermal cycling performance in harsher conditions. Here, we investigate microstructure evolution and failure in harsh accelerated thermal cycling (ATC) of a Sn-3.8Ag-0.9Cu solder with 5.5 wt% antimony as the major addition in two ball grid array (BGA) packages. Similar to Sn-Ag-Cu solders, microstructure and damage evolution were generally localised in the beta-Sn near the component side where localised beta-Sn misorientations and subgrains, accelerated SbSn and Ag3Sn particle coarsening, and beta-Sn recrystallisation occurred. Cracks grew along the network of recrystallised grain boundaries to failure. The improved ATC performance is mostly attributed to SbSn solid state precipitation within beta-Sn dendrites which supplements the Ag3Sn that formed in a eutectic reaction between beta-Sn dendrites, providing populations of strengthening particles in both the dendritic and eutectic beta-Sn.
2:40 PM Invited
Sn-Ag-Cu and Sn-Bi Solder Powders for Fine Pitch Printing: Amir Nobari1; Arslane Bouchemit2; Gilles L’Espérance2; 15N Plus Inc - Micro Powders; 2École Polytechnique de Montréal
The continuing demand for smaller electronic products has driven the use of miniature components. The assembly of these miniature components requires fine solder joints; and, finer solder joints require advanced solder pastes with finer particle sizes. This paper presents our experimental results on the characterization of fine solder powders of Sn-Ag3-Cu0.5 (SAC305) and Sn42-Bi58 (Sn-Bi). The powders investigated in this study were produced with a proprietary atomising technology particularly effective in producing solder powders ranging from 25 to 1 μm which corresponds to Type 5 to Type 10. More specifically, the oxide layer formed on the powder surface is characterized. The thickness and the nature of the chemistry of the surface oxide present are analyzed using Instrumental Gas Analysis (IGA), Auger Electron Spectroscopy (AES) and Transmission Electron Microscopy (TEM). The results on SAC305 and Sn-Bi powders are then compared against each other.
3:00 PM Invited
Single Solder Joint Shear with In-situ Current Stressing: Kendra Young1; Choong-Un Kim2; Tae-Kyu Lee1; 1Portland State University; 2University of Texas, Arlington
Single solder joint shear tests were performed on Sn based solder joints with in-situ current stressing in variations of current density per solder joint. In order to investigate the current effect, the tests were design to decouple the Joule heating from current flow effect. Subsequent shear tests were performed in the absence of applied current at a range of -40oC to 100oC steady-state temperature holding condition to mimic the temperature variables induced by current stressing. Decoupled temperature effects from current reveal that the shear load decreased through elevated temperature shearing relative to their respective induced current test and demonstrate an increase in maximum shear load on the isothermally aged 700A/cm2 current density condition. Certain current density stressing caused an instantaneous thermal gradient and an inhomogeneous temperature distribution in the solder ball, which ultimately differed the shear response with in-situ current shearing.
3:20 PM
Solderability Assessment of Lead-free Alloys: Mehran Maalekian1; 1Mat-Tech
The ability of a surface to be wetted and to form a solder joint is referred to as solderability. Good solderability requires spreading and wetting of molten solder on the substrate. Solderability of an alloy can be quantified by wetting balance test (immersion method), and sessile drop test (spread method). In this work, the solderability of few lead-free solders, Sn, Sn-Cu, Sn-Bi, Sn-Cu-Bi and Sn-Cu-Bi-Ag systems, is assessed. Kinetics of wetting, wetting force and contact angle are measured and the effects of alloying elements are discussed. It is demonstrated that to measure and compare the solderability of alloys, it is essential to consider the thermal behaviour of the solder alloys as well.
3:40 PM
Finite Element Analysis Modeling of Stress Evolution and Whisker Growth Under Applied Pressure: Nupur Jain1; Piyush Jagtap1; Allan Bower1; Eric Chason1; 1Brown University
Sn whisker growth is often considered to be a stress driven phenomenon. Through experiments using applied stress, we have quantified the real-time nucleation and growth kinetics in Sn thin films. In this work, we analyze the evolution of the stress under applied load in order to determine its relation to whisker formation kinetics. We have developed a finite element model which accounts for stress evolution due to material diffusion, elasticity, power-law creep and whisker growth mechanisms. The whisker growth kinetics predicted by the model show good agreement with experimental observations.
4:00 PM
Corrosion Behavior of Co-based Surface Finishes in Sulfur-containing Gas: Si-Wei Lin1; Albert T. Wu1; 1National Central University
With the development of 5G network and automobile industry, the reliability of electronic devices become important. Damaging of the devices may occur when exposed to air-pollution, such as sulfide, sulfur oxide, and nitrogen oxide. Designing effective layers to protect devices from corrosion in electronic packaging is necessary. In this study, the corrosion behaviors of different surface coatings on printed circuit board are investigated. Sample surfaces are coated with electroless Co-based surface finishing layers. The samples are put in a chamber at 80 °C and 100% relative humidity (RH) with different concentration of SO2 for corrosion test. The morphology and composition of samples after corrosion test are compared. The results show that the Co-based surface finishes effectively inhibit the diffusion of Cu from the printed circuit board and successfully protect the electronic devices.