Search

Tin electrodeposits produced from aqueous electrolytes are frequently used within the electronics industry due to their high solderability and corrosion protection. One limitation to using these deposits is their spontaneous formation of long conductive filament whiskers. These whiskers grow post-electrodeposition and increase the risk of unwanted electrical shorts within electronic devices. In this thesis, tin electrodeposits produced from a proprietary bright acid Tinmac electrolyte, currently used in industry, were studied. Electrodeposits were produced using a range of current densities with and without agitation and were characterised with respect to crystallographic orientation, topography and surface finish. Moreover, the intermetallic compound (IMC) growth produced at the copper substrate-tin coating interface was assessed over a period of time as its growth is considered to be a significant driving force behind whisker formation. In addition, a technique for the electrochemical anodic oxidation of tin electrodeposits on copper substrates was developed. This technique was used throughout this project for the study of IMC growth from tin electrodeposits as it was able to effectively remove the tin whilst leaving the IMCs and substrate unaffected. Ionic liquids exhibit promising electrochemical characteristics for electrodeposition but are still not widely utilised in industry. Their ability to deposit tin coatings has been studied in the present investigation. Trials concentrated on process optimisation to produce uniform electrodeposits by varying current density, SnCl2.2H2O concentration, and electrolyte composition. These deposits were then characterised and compared to tin coatings of similar thickness produced from Tinmac with respect to topography, surface finish, crystallographic orientation, IMC growth, and whisker propensity. Electrodeposits produced from the ionic liquid electrolyte exhibited a different crystallographic texture, topography, and IMC growth compared to those produced from Tinmac. Moreover, the deposit produced from the ionic liquid featured increased whisker growth compared to those produced from Tinmac, but in a wider context, far less growth than conventional tin electrodeposits in the literature. In addition, by exploiting other electrochemical characteristics of ionic liquids, such as their large potential window, future work may be able to produce novel tin or tin alloy electrodeposits which may further reduce the whisker propensity of deposits produced in this investigation.

As the most commonly used electrode, leadframe, and package material in electronic packaging, copper (Cu) has severe oxidation problem, preventing wetting during soldering processes and prohibiting successful wire bonding. Thus leads and bond pads of leadframes and packages are often coated with a passivation layers such as palladium, silver, or tin, which suppresses the growth of copper oxides. In this research, I investigated the use of silver-indium ((Ag)In) solid solution as a new alternative coating material for passivation. First, a reaction study of bulk (Ag)-9.5In solid solution disks with Sn was conducted to evaluate possible intermetallic compounds in the Ag-In-Sn system. Next, (Ag)-9.5In solid solution layer was coated on copper substrates by E-beam evaporation. The (Ag)9.5In-passivated Cu substrates and bare Cu substrates were aged at 150°C for up to 1000 hours in air. No oxide was detectable on (Ag)9.5In-passsivated Cu substrates while thick Cu2O3 was grown on bare Cu. This result illustrates that (Ag)9.5In solid solution is an excellent passivation coating material. Then, (Ag)9.5In-passivated Cu substrates were electroplated with tin. As a reference, bare Cu substrates were also plated with tin. These two different types of samples were reflowed and aged for different amount of time. No flux was used. The reaction results were examined by scanning electron microscope/energy dispersive X-ray spectroscopy (SEM/EDX) and compared. The experimental results show that tin wetted and reacted well with (Ag)9.5In-passivated Cu substrates. The growth of intermetallic compound (Cu6Sn5, Cu3Sn) on (Ag)9.5In-passivated Cu substrates is also suppressed compared to the growth on bare Cu substrates. The design of (Ag)In solid solution passivation is an invention and its superior performance is a discovery. It provides an economical, environmentally friendly, and valuable passivation alternative on copper electrodes, leadframes, and packages.

With the trend of big data and the Internet of things, we live in a world full of personal electronic devices and small electronic devices. In order to make the devices more powerful, advanced electronic packaging such as wafer level packaging or 3D IC packaging play an important role. Furthermore, μ-bumps, which connect silicon dies together with dimension less than 10 μm, are crucial parts in advanced packaging. Owing to the dimension of μ-bumps, they transform into intermetallic compound from tin based solder after the liquid state bonding process. Moreover, many new reliability issues will occur in electronic packaging when the bonding materials change; in this case, we no longer have tin based solder joint, instead, we have intermetallic compound μ-bumps. Most of the potential reliability issues in intermetallic compounds are caused by the chemical reactions driven by atomic diffusion in the material; thus, to know the diffusivities of atoms inside a material is significant and can help us to further analyze the reliability issues. However, we are lacking these kinds of data in intermetallic compound because there are some problems if used traditional Darken’s analysis. Therefore, we considered Wagner diffusivity in our system to solve the problems and applied the concept of chemical effect on diffusion by taking the advantage that large amount of energy will release when compounds formed. Moreover, by inventing the holes markers made by Focus ion beam (FIB), we can conduct the diffusion experiment and obtain the tracer diffusivities of atoms inside the intermetallic compound. We applied the technique on Ni3Sn4 and Cu3Sn, which are two of the most common materials in electronic packaging, and the tracer diffusivities are measured under several different temperatures; moreover, microstructure of the intermetallic compounds are investigated to ensure the diffusion environment. Additionally, the detail diffusion mechanism was also discussed in aspect of diffusion activation enthalpy and diffusion pre-factor by using lattice structure simulation. Last but not the least, X-ray photoelectron spectroscopy and First principal calculation simulation were used to observe the electron binding energies in the intermetallic compound and illustrate the partial covalent bonding behavior in the intermetallic compounds.

In 3D IC, μ-bumps and through silicon vias (TSVs) have been developed to achieve the vertical stacking of Si chips. Similar to the downward scaling of transistors in semiconductor industry, the development in advanced packaging also leads to the scaling down of interconnect structures. Compared to flip chip solder joints having a diameter about 100 μm, the diameter of μ-bumps is reduced 10 times and the total volume is reduced 1000 times. This size reduction of solder joints will lead to many reliability concerns. In this thesis, the finding that intermetallic compound (IMC) growth rate between solder and Cu under bump metallization increases in small μ-bumps will be analyzed and discussed. The kinetic model for small size Cu-Sn interfacial reaction has been investigated by making different size of Sn pillars on Cu substrate with the use of focused ion beam (FIB), down to 1 μm. With the shrinking size of pillars, surface diffusion becomes dominant and can no longer be neglected in the Cu-Sn interfacial reaction. A simple kinetic model of surface diffusion controlled intermetallic compound growth of Cu6Sn5 is proposed for pillars with diameter below 5 μm by combining surface diffusion and interstitial diffusion. Also, the reliability concern of electroless nickel immersion gold (ENIG) surface finishing on IMC formation when the size of solder is reduced from flip chip solder joints to μ-bumps has been studied. Since the volume of solder shrinks 1000 times in μ-bumps, a thin layer of ENIG will transform almost the entire solder joint into (Au,Ni)Sn4 intermetallic compound. The fracture reliability concern of this brittle phase in μ-bumps will be discussed.In addition to size shrinking of solder joints, the diameter of Cu lines is also reduced. In 3D IC packaging, there is a layer of Cu used to fan out electric current from flip chip solder joints to TSVs and μ-bumps, and this layer is known as redistribution layer (RDL). In electrimgration test, the weakest link in this whole packaging system is found to be located in the RDL. This is because of the design of RDL, which is composed of very thin Cu due damascene lines and has quite compact structure. Thermal dissipation becomes a problem in this design of RDL. Joule heating will be accumulated and resulting in the increase of temperature, which in turn increase the rate of electromigration depletion of atoms in Cu lines and making the lines thinner and thinner. This synergistic effect of Joule heating and EM on burn-out is a new failure mode of reliability.

The effects of intermetallic compound (IMC) formation on reliability of Pb-free Sn-based solders for flip chip and three-dimensional (3D) interconnects were studied. The dissertation is organized into four parts. In the first part, the effect of Sn grain orientation on electromigration (EM) reliability of Pb-free Sn-based flip chip solder joints was studied. The Sn grain microstructure in flip chip solder joints was characterized using the electron backscatter diffraction (EBSD) technique, and wa found to be closely related to the EM failure mechanims. The approach to grain structure optimization for improved EM reliability was also explored. In addition to the experimental work, a kinetic analysis was formulated to investigate the early EM degradation mechanism in Sn-based solder joints with Ni under-bump metallization (UMB). The aforementioned kinetic analysis, the intrinsic diffusion coefficients were not readily available in the literature. In the second part of the work, a Monte Carlo method known as simulated annealing was applied to estimate the unknown diffusion coefficients using a multi-parameter optimization method by fitting to experimental measurements. The intrinsic diffusion coefficients of Ni and Sn in Ni₃Sn₄ between 150 and 200°C, and those of Cu and Sn in Cu₃Sn and Cu₆Sn₅ between 120 and 200°C were estimatd. The activation energies for these diffusion coefficients were also determined. Together, this provides the diffusivity parameters to predict the intermetallic growth as a function of temperature. The third objective focused on the EM reliability of Sn-based microbump joints in 3D interconnects with through-silicon vias (TSVs). No EM-induced bump failure was observed, showing a robust EM reliability in microbumps. High temperature thermal annealing test was also performed on microbumps with three different metallizations in an effort to explore structural and process optimization. Finally, interfacial reaction induced stress in IMC microbumps was investigated. A numerial analysis was formulated to study the concurrent diffusion, phase transformation, and deformation in the process of IMC formation. Stress generation due to unbalanced diffusion rates and volumetric change upon phase transformation was considered. The coupled analysis was applied to investigate Ni₃Sn₄ growth in the Ni-Sn microbumping system. A simulation approach based on finite difference method with moving boundaries was employed to numerically solve stress evolution in Ni₃Sn₄. The equilibrium stress was also investigated using a modified model with a finite thickness of solder. Simulation predictions were found to be in good qualitative agreement with experimental observations.

Electromigration (EM) and thermomigration (TM) reliability of Pb-free solder joints are emerging as critical concerns in advanced packages. In this study, EM and TM phenomena in Sn-2.5Ag solder joints with thick Cu or thin Ni under-bump metallurgy (UBM) were investigated. A series of EM tests were performed to obtain activation energy (Q) and current density exponent (n), and to understand failure mechanisms. Joule heating was also taken into account. Q and n values were determined as follows: for Cu UBM solders, Q = 1.0 eV and n = 1.5; for Ni UBM solders, Q = 0.9 and n = 2.2. Important factors limiting EM reliability of Pb-free solder joints were found to be UBM dissolution with extensive intermetallic compound (IMC) growth and current crowding. IMC growth without current stressing was found to follow the parabolic growth law whereas linear growth law was observed for Cu₆Sn₅ and Ni₃Sn₄ under high current stressing. For Cu UBM solders, the apparent activation energy for IMC growth was consistent with the activation energy for EM, which supports that EM failure was closely related to IMC growth. In contrast, for Ni UBM solders the apparent activation energy was higher than the EM activation energy. It was suggested that the EM failure in the Ni UBM solders could be associated with more than one mass transport mechanism. The current crowding effect was analyzed with different thicknesses of Ni UBM. It was found that the maximum current density in solder could represent the current density term in Black's equation better than the average current density. FEM studies demonstrated that current crowding was mainly controlled by UBM thickness, metal trace design, and passivation opening diameter. A large temperature gradient of the order of 10³ °C/cm was generated across the sample to induce noticeable TM and to compare its effect against that of EM. TM-induced voiding was observed in Ni UBM solders while UBM dissolution with IMC formation occurred in Cu UBM solders. However, the relative effect of TM was found to be several times smaller than that of EM even at this large temperature gradient.

Soldering has become the predominant and established technique in electronic packaging industry for joining electronic components. The industry is aiming for the use of environment friendly lead-free solders. All the lead-free solders are high tin-containing alloys. During the soldering process, an intense interaction of metallization on PCB and tin from the solder occurs at the joint interface. Intermetallic compound (IMC) is formed at the interface and subsequently PCB bond-metal (substrate) is dissolved into the molten solder. The formation of the IMC is desirable for the joint to occur but excessive metal dissolution and uncontrolled growth of the IMC is detrimental to quality of joint. In the present study the terms bond-metal and substrate will be used interchangeably and the term ‘substrate’ refers to the top layer of the PCB which comes in contact with the molten solder during soldering reaction. During the wet phase of soldering process, the IMC exhibits scalloped morphology. The growth of scalloped IMC during the solder/substrate interaction entails complicated physics especially during early stages of the soldering process. Understanding of the physics and the kinetics involved in the formation of IMC phase and metal dissolution is important in achieving the desired control of the process. This study presents an in-depth analysis of the physics involved in the formation of the scalloped intermetallic phase. To substantiate the physics of the process and to verify the kinetics involved, a mathematical model is setup. The actual physics involved in the dissolution of metal layer and growth of IMC during soldering process is translated into mathematical model based on the fundamental equations and known material properties and thermodynamic constraints. The model was applied to study the interaction of Cu with Sn3.5%Ag solder as a representative example however, the model can be applied to any substrate/solder system by incorporating the relevant physical properties of material. The model served as a valuable tool to closely understand the physics involved in the process of diffusion of various species across the joint, phase change and kinetics effects during soldering process. Extensive parametric study was conducted to explore the relative effect of various parameters like interface reaction kinetics and physical properties such as the diffusivity of solder and IMC. The effect of these parameters on the dissolution behavior of metallization layer and the growth of IMC with time during soldering process was also studied. The results from the model were also compared with experimental data for IMC thickness and metal dissolution available in literature. Effects of non-isothermal processing conditions were also considered in the model. A constant value of IMC diffusivity was able to accurately predict the IMC growth for the cases involving planar IMC layer, however, the single value of IMC diffusivity did not work very well for scalloped IMC layer. Due to the complicated evolution of the scalloped IMC growth in wet phase of soldering, it was suggested that the diffusivity in the IMC phase changes with time and a varying value of IMC diffusivity be used in the model. The varying IMC diffusivity gave satisfactory predictions for the IMC thickness and metallization layer lost during reflow process. The model was also applied to study the dissolution behavior of the micro-size copper particles in lead-free composite solders during reflow process.