Thermal Cycling Life Prediction of Sn-3.0Ag-0.5Cu Solder Joint Using Type-I Censored Data

Because solder joint interconnections are the weaknesses of microelectronic packaging, their reliability has great influence on the reliability of the entire packaging structure. Based on an accelerated life test the reliability assessment and life prediction of lead-free solder joints using Weibull distribution are investigated. The type-I interval censored lifetime data were collected from a thermal cycling test, which was implemented on microelectronic packaging with lead-free ball grid array (BGA) and fine-pitch ball grid array (FBGA) interconnection structures. The number of cycles to failure of lead-free solder joints is predicted by using a modified Engelmaier fatigue life model and a type-I censored data processing method. Then, the Pan model is employed to calculate the acceleration factor of this test. A comparison of life predictions between the proposed method and the ones calculated directly by Matlab and Minitab is conducted to demonstrate the practicability and effectiveness of the proposed method. At last, failure analysis and microstructure evolution of lead-free solders are carried out to provide useful guidance for the regular maintenance, replacement of substructure, and subsequent processing of electronic products

Evaluation of Anisotropic Conductive Films Based on Vertical Fibers for Post-CMOS Wafer-Level Packaging

In this paper, we investigate the mechanical and electrical properties of an anisotropic conductive film (ACF) on the basis of high-density vertical fibers for a wafer-level packaging (WLP) application. As part of the WaferBoard, a\ud reconfigurable circuit platform for rapid system prototyping,\ud ACF is used as an intermediate film providing compliant and\ud vertical electrical connection between chip contacts and a top surface of an active wafer-size large-area IC. The chosen ACF is first tested by an indentation technique. The results show that the elastic–plastic deformation mode as well as the Young’s modulus and the hardness depend on the indentation depth. Second, the efficiency of the electrical contact is tested using a uniaxial compression on a stack comprising a dummy ball grid array (BGA) board, an ACF, and a thin Al film. For three bump diameters, as the compression increases, the resistance values decrease before reaching low and stable values. Despite the BGA solder bumps exhibit plastic deformation after compression, no damage is found on the ACF film. These results show that vertical fiber ACFs can be used for nonpermanent bonding in a WLP application

Micromechanical Behaviour of Lead-free Solder Joints Developed for 3D-IC Packaging Applications

Ph.DDOCTOR OF PHILOSOPH

Performance of SAC305 and SAC305-0.4La lead free electronic solders at high temperature

Purpose – Tin-Silver-Copper is widely accepted as the best alternative to replace Tin-Lead solders in microelectronics packaging due to their acceptable properties. However, to overcome some of the shortcomings related to its microstructure and in turn, its mechanical properties at high temperature, the addition of different elements into Tin-Silver-Copper is important for investigations. The purpose of this paper is to analyse the effect of lanthanum doping on the microstructure, microhardness and tensile properties of Tin-Silver-Copper as a function of thermal aging time for 60, 120 and 180 h at a high temperature of 150°C and at high strain rates of 25, 35 and 45/s. Design/methodology/approach – The microstructure of un-doped and Lanthanum-doped Tin-Silver-Copper after different thermal aging time is examined using scanning electron microscopy followed by digital image analyses using ImageJ. Brinell hardness is used to find out the microhardness properties. The tensile tests are performed using the universal testing machine. All the investigations are done after the above selected thermal aging time at high temperature. The tensile tests of the thermally aged specimens are further investigated at high strain rates of 25, 35 and 45/s. Findings – According to the microstructural examination, Tin-Silver-Copper with 0.4 Wt.% Lanthanum is found to be more sensitive at high temperature as the aging time increases which resulted in coarse microstructure due to the non-uniform distribution of intermetallic compounds. Similarly, lower values of microhardness, yield strength and ultimate tensile strength come in favours of 0.4 Wt.% Lanthanum added Tin-Silver- Copper. Furthermore, when the thermally aged tensile specimen is tested at high strains, two trends in tensile curves of both the solder alloys are noted. The trends showed that yield strength and ultimate tensile strength increase as the strain rate increase and decrease when there is an increase in thermal aging. Originality/value – The addition of higher supplement (0.4Wt.%) of Lanthanum into Tin-Silver-Copper showed a lower hardness value, yield strength, ultimate tensile strength, ductility, toughness and fatigue in comparison to un-doped Tin-Silver-Copper at high temperature and at high strain rates. Finally, simplified material property models with minimum error are developed which will help when the actual test data are not available

Design of the Contact Metallizations for Gold-Tin Eutectic Solder-A Thermodynamic-Kinetic Analysis

This dissertation focuses on the design of reliable interconnections using Au-20wt.%Sn solder with the assistance of thermodynamic calculations. In this work, three commonly encountered contact metallizations, namely Ni, Cu, and Pt, have been selected. In order to assess the reliability of the Au-20wt.%Sn|X (X=Ni, Cu and Pt) interconnections from the metallurgical viewpoint, firstly, the phase diagrams of the Au-Sn-X ternary systems have been thermodynamically established with the Calphad method. Secondly, the diffusion couple method was employed to study the interfacial reactions experimentally. The microstructures of the as-soldered and subsequently aged Au-20wt.%Sn|X interconnections were characterized by means of scanning electron microscopy equipped with energy-dispersive X-ray spectroscopy (EDX) and scanning transmission electron microscopy with EDX. The observed interfacial reaction phenomena have been rationalized by combining the experimental results with the thermodynamic considerations. Emphasis has also been placed on collecting the mechanical properties of the IMCs formed at the solder/Cu and Ni interfaces since these values are essential for evaluating the reliability of the interconnections.  The results in this dissertation show that Au-20wt.%Sn|Pt was more thermally stable than Au-20wt.%Sn|Ni and Au-20wt.%Sn|Cu when these as-soldered reaction couples were subsequently aged at 150 °C for a long-term. When a short bonding time is employed, Pt contact metallization is superior to the Ni and Cu contact metallizations for the Au-20wt.%Sn solder

ELECTROMECHANICAL INTERACTION ON THE DEFORMATION BEHAVIOR OF METALLIC MATERIALS

Metallic materials play important roles in providing electrical, thermal, and mechanical functions in electronic devices and systems. The understanding of the electrical-thermal-mechanical interaction caused by the passage of electric current with high density is important to improve the performance and reliability of electronic assembly and packaging. The electromechanical interaction on the deformation behavior of copper and tin is studied in this work. The electromechanical response of Cu strips was studied by passing a DC electric current. The electric resistance linearly increased with time before the occurrence of electric fusing. The electrothermal interaction led to the buckling of the Cu strips with the maximum deflection increasing with the increase of the electric current density. The total strain was found to be proportional to the square of the electric current density. A power law relation was used to describe the dependence of the time-to-fusing on the electric current density. Using the nanoindentation technique, the effect of electric current on the indentation deformation of copper and tin was studied. The reduced contact modulus of copper and tin decreased with increasing the electric current density. With the passage of a DC electric current, the indentation hardness of copper increased slightly with increasing electric current density. With the passage of an AC electric current, the indentation hardness of copper decreased with increasing the indentation deformation. With the passage of a DC electric current, the indentation hardness of tin decreased with increasing the indentation load, showing the normal indentation size effect. Both the limit of infinite depth and the characteristic length were dependent on the electric current density. Using the tensile creep technique, the creep deformation of pure tin was studied with the passage of a DC electric current. The steady state creep rate increased with the increase in temperature, tensile stress and electrical current density. For the same tensile stress and the same chamber temperature, the steady state creep rate increased linearly with the square of the electric current density. The electric current density has no significant effect on the stress exponent and activation energy of the tensile creep of tin for the experimental conditions

The nucleation and growth of Cu6Sn5 in solders

Microstructure formation and evolution in Pb-free solder alloys and solder joints on Cu substrates depend on the nucleation and growth of primary Cu6Sn5 and beta-Sn during solidification and thermal cycling in service. This thesis explores the mechanisms responsible for microstructure evolution at different stages during the lifetime of a solder joint. Cu6Sn5, a common intermetallic in Pb-free soldering, is usually first to nucleate and past work showed that aluminium additions can cause significant refinement of primary Cu6Sn5. In this work, it is showed that the mechanism of refinement is heterogeneous nucleation of Cu6Sn5 on either deltaCu33Al17 or gamma1Cu9Al4 coupled with significant constitutional supercooling ahead of growing Cu6Sn5 crystals. Cu-Al particles are shown to be effective catalytic nucleant particles in both hyper-eutectic Sn-4Cu-0.02Al and hypo-eutectic Sn-0.7Cu-0.05Al/Cu joints and share reproducible orientation relationships with Cu6Sn5. The growth of primary Cu6Sn5 also plays a role in determining the final microstructure. A deeper understanding of crystal growth mechanisms and transitions between different Cu6Sn5 morphologies is developed. It is shown that, for different composition and cooling rate combinations, Cu6Sn5 crystals undergo a faceted to non-faceted growth transition as a result of a kinetic interface roughening transition and a gradual change in mechanism from lateral growth governed by anisotropic attachment kinetics to continuous growth governed by diffusion and curvature. As the majority phase in most solder joints, betaSn nucleates at a later stage of solidification after Cu6Sn5 has nucleated. The nucleation of betaSn on the Cu6Sn5 layer in solder joints is studied in detail. It is shown that primary Cu6Sn5 is not a potent nucleant for Sn, but the Cu6Sn5 layer plays a key role in betaSn nucleation and microstructure formation in solder joints. Thermal contraction of Cu6Sn5, betaSn and other common phases in soldering is another important phenomenon that affects the performance of solder joints in service. Directional data on the anisotropic coefficient of thermal expansion (CTE) of Cu6Sn5 and other non-cubic intermetallics are measured and correlated with the directional Young’s modulus.Open Acces

Metallurgical Issues in Low-Temperature Joining of Silver Nanowires

Silver nanowires (Ag NWs) have a wide range of applications in the electronic industry and are attracting growing world-wide interest because of their unique thermal, chemical, electrical and mechanical properties. Understanding of mechanical properties of Ag NWs and joining processes for them at a nano scale is urgently needed to support exploitation of their applications. Particularly, study of processing-structure-property relationships is of much significance. In the present thesis, the following research works were conducted. Ag NWs were synthesized using the polyol method. Joining of individual Ag NWs in an end-to-end orientation at room-temperature without assistance of external pressure was investigated. Selective surface activation of Ag NWs provided surface free of protective organic layers for metallurgical joining. A similar crystal orientation was maintained between the NWs, and diffusion along the boundary contributed to the nanojunction formation. Monocrystalline V-shaped or zig-zag silver prisms were formed after nanojoining, terminated by twin boundaries and free surfaces. The feasibility of room-temperature pressure-free joining of copper (Cu) substrates using Ag NW paste was conducted and demonstrated for flexible electronic packaging applications. The organic content in water-based Ag NW pastes was largely reduced by a repeated washing process to decrease the joining temperature. The formation of end-to-side or side-to-side joints between Ag NWs was observed concurrently with those joined end-to-end. The mechanical and electrical properties of Ag NW joints were examined. It was found that self-generated local heating within the Ag NW paste and Cu substrate system promoted the joining of Ag-to-Ag and Ag-to-Cu without any external energy input. The localized heat energy could be delivered in-situ to the interfaces and promoted atomic diffusion and metallic bond formation while the bulk component temperature maintaining near room-temperature. The organic layer on the side surfaces of the Ag NWs could be broken down through consumption of the residual PVP by a CuO-PVP reaction and which produced localized heating, increasing activated surface sites dramatically and making three-dimensional networks feasible. Ag NWs were introduced into Ag nanoparticle (NP) matrices joined at low-temperature. Joining was facilitated by solid state sintering of the Ag nanomaterials and metallic bonding at Cu-Ag interfaces. It was found that Ag NWs in a Ag NP matrix acted as a second reinforcement phase. In addition to improving the fracture toughness of joints, the introduction of Ag NWs affected the path of fracture propagation, where necking, breakage and pullout of Ag NWs occurred during loading

Rheology

This book contains a wealth of useful information on current rheology research. By covering a broad variety of rheology-related topics, this e-book is addressed to a wide spectrum of academic and applied researchers and scientists but it could also prove useful to industry specialists. The subject areas include, polymer gels, food rheology, drilling fluids and liquid crystals among others