Soldering

Summary

Soldering (USˈsɒdərɪŋ; UKˈsoʊldərɪŋ) is a process of joining two metal surfaces together using a filler metal called solder. The soldering process involves heating the surfaces to be joined and melting the solder, which is then allowed to cool and solidify, creating a strong and durable joint. Soldering is commonly used in the electronics industry for the manufacture and repair of printed circuit boards (PCBs) and other electronic components. It is also used in plumbing and metalwork, as well as in the manufacture of jewelry and other decorative items. The solder used in the process can vary in composition, with different alloys used for different applications. Common solder alloys include tin-lead, tin-silver, and tin-copper, among others. Lead-free solder has also become more widely used in recent years due to health and environmental concerns associated with the use of lead. In addition to the type of solder used, the temperature and method of heating also play a crucial role i

Official source

https://en.wikipedia.org/wiki/Soldering

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Deformation and Damage in Lead-free Solder Joints

Milad Maleki

For environmental reasons, lead has been banned from microelectronic solder materials since the adoption of the RoHS legislation in 2006. A large number of investigations have lead to the development of new lead-free solder materials like the SnAgCu alloy which can satisfy most of the technological requirements. However, the long-time reliability of the lead-free candidates is still a crucial issue and in several cases, failure of electronic assemblies has been attributed to that of the solder joints. To overcome the present challenge in the soldering technology, the damage mechanisms occurring along the service life of lead-free solder joints like microstructure evolution, viscoplastic deformation as well as interfacial cracking must be better understood. The main scope of this thesis is to shed light on the complex interdependency of microstructures, deformation property and the interfacial fracture of solder joints in order to explain the change in the failure mode of solder joints from ductile to brittle interfacial fracture as well as the link between microstructural evolution and the change in the mechanical property of the solders. The first part of this research focused on characterization and macroscopic modeling of the fracture at the interface of SnAgCu solder and Cu substrate with a special attention on the viscoplastic nature of the bulk solder. For this purpose, the viscoplastic behavior of the solder joint was initially identified and the material parameters related to a unified constitutive viscoplastic model were determined. The interfacial fracture behavior of the joint was characterized by performing stable fracture tests at different strain rates. The results confirmed that the failure mode of the solder joint is a function of the loading rate. It was shown that an increase in the loading rate causes a decrease in the load-bearing capacity of the joint by enhancing the tendency to develop an interfacial failure. The interfacial crack propagation in the fracture tests were simulated by developing a detailed three-dimensional (3D) finite element (FE) model containing a cohesive interface and considering the viscoplastic behavior of the bulk solder. The influence of strain rate and viscoplasticity on the load-bearing capacity and failure behavior of the joint were pointed out by analyzing the simulation results. It was shown how the normal stress generated at the interface is limited by the significant viscoplastic relaxation developed in the bulk solder at low strain rates, while a large portion of the external work is transferred to the interface at higher rates which gives rise to interfacial fracture. The second part of this thesis focused on developing microstructure-based models in order to simulate the change in the macroscopic mechanical response of the SnAgCu solder caused by the microstructural evolution during thermal ageing. For this purpose, first, the 3D configurations of microstructures in different ageing conditions were visualized using a combination of synchrotron X-ray and FIB/SEM tomography techniques. Analyses showed that although the size and orientation of crystalline grains as well as morphology of dendrites in SnAgCu solder remain stable during ageing, two main morphological changes occur in the Ag3Sn and Cu6Sn5 intermetallic particles: spheroidizing of needle-shape intermetallics and coarsening due to bulk diffusion. The mechanical property of the solder and its phases at different ageing conditions were experimentally characterized. A significant decrease in the yield strength of the solder joint was identified because of thermal ageing and it was shown that the deformation behavior of the solder is governed by the morphometrics of the dispersed intermetallics. The link between the microstructures and the deformation behavior of solder was successfully simulated by developing a hybrid microstructure-based modeling approach. The average indirect strengthening of the intermetallics was modeled by incorporating the effects of geometrically necessary dislocation on the in-situ deformation behavior of the tin matrix. Subsequently, a micromechanical FE homogenization was considered to model the load-sharing behavior of the intermetallics. For this purpose, the tomographic images were directly used to generate 3D FE meshes of the actual microstructures and then the constitutive deformation behavior of the solder were determined by adopting two levels of numerical homogenization in the eutectic phase and over the whole joint. The decrease in the yield strength of solder due to ageing was explained through analyses of indicators in the models. It was demonstrated how the change in the morphometrics of intermetallics during ageing reduces the contribution of intermetallics in load-sharing as well as the in-situ strength of the matrix by causing a significant change in the strain distribution as well as particle size effects. The simulation results were experimentally verified and the capabilities as well as limitations of the developed approach were pointed out. Overall, the results of this research led to a proper understanding of the critical reliability issues in lead-free solders during isothermal ageing, i.e., microstructural coarsening and interfacial fracture. In addition, through this work, a set of methodologies for multi-scale modeling and experimental characterization of the damage and deformation in solder joints were developed and validated which can be extremely helpful for efficient use of the SnAgCu solders in electronic industry as well as designing new solder alloys with higher strength and better reliability for a safe transition to lead-free solders in critical applications.

EPFL2012

Solders are widely used as interconnect material for the electronic industry. The solder interconnect provides both electrical contact and mechanical connection between the integrated circuit devices and their substrate. Thus solders with good reliability are required to facilitate successful functioning of electronic devices and components that we encounter in daily life. However traditionally used Sn-Pb solders have to be replaced by Sn rich lead-free solders because of the toxicity concerns surrounding the use of lead. Many Sn-rich lead-free solders do exists, however they are not drop-in substitutes for Sn-Pb solders as the knowledge concerning their mechanical behavior and reliability are not well understood. A particularly critical aspect is the lead-free solders' reliability at high service temperatures required in several microelectronics applications. Such applications require solders possessing good microstructural stability, strength and creep resistance. In this regard, literature reports the alloying and the composite approach to improve the properties of the lead-free solders. Among these two methods, the composite approach wherein suitable particles are added in to the solder matrix seems to be promising. However, the scientific understanding between property improvement and characteristic of the particle reinforcement is too limited. Also the processing of the composite solder is limited by stringent process parameters that require significant improvement to increase its industrial applicability. Another important issue arises from the fact that solders as such exhibit complex elasto-plastic properties. Furthermore the solder's mechanical response is greatly influenced by specimen geometry and process parameters. Therefore novel localized strain measurement techniques that could assess and understand the mechanical response of the solder without much effect from substrate are required. The goals of the current PhD thesis are identified within the framework of the issues previously mentioned. The composite approach is demonstrated to improve the microstructural and the mechanical properties of existing lead-free solders. As reinforcing particles either Cu or Ni is used with volume fraction from 0.8-4.2 vol. %, while Sn-4.0Ag-0.5Cu (SAC405) alloy is chosen as matrix. Throughout the study the results of the composite solder is compared against the un-reinforced SAC405 reference solder. The solder material in bulk form (reflowed in ceramic crucible) is used for microstructural analysis and also as joining material between Cu substrate to investigate the mechanical response in tension and shear. The current research demonstrates a new processing method developed for producing composite lead-free solders utilizing micro and nanometer scale metallic particles. Generally the preparation of nano-composite solders requires several process parameters that restrict their industrial use. In literature even the exact process route is still not disclosed due to technical secrecy. This processing issue could be overcome by adopting the mechanical mixing method reported in this research. A significant improvement has been achieved in reducing porosity within the newly developed composite solders. This reduced porosity improves the consistency of mechanical properties in many folds. Till date no standardized mechanical test method for solder alloys has been reported. In this regard, the design of a shear test specimen proposed in this PhD thesis is novel. Extensive finite element (FE) simulations accompanied by series of experimental tests is performed to optimize the specimen geometry. This proposed specimen geometry minimizes the effect of plastic strain localization which ensures almost uniform plastic strain at the center of the joining layer. This design leads to "cleaner" shear tests minimally affected by stress concentration and with a low risk of premature failure at the substrate/solder interface. As a consequence, reliable mechanical properties can be identified from experimental results. In case of tensile specimens, the gap width is chosen such that effects of plastic constraints from rigid substrates (Cu) are kept at minimum. In-situ optical extensometry combined with Digital Image Correlation (DIC) is used to determine the stress-strain response of the solder joint. Finally, the constitutive properties of solder materials are numerically identified from the global stress-strain response of the solder joint by inverse FE modeling of the shear test. For the first time, a 3D FE homogenization model for particle reinforced composites is employed to predict the solder elastoplastic behavior as a function of the particle volume fraction. This objective has been demonstrated on the example of composite solder prepared with micrometer sized Ni particles. The homogenization results are then compared with the constitutive model of the composite solder identified from the experimental data. This comparison serves as a basis for the discussion of the effects of particle reinforcement in the composite solder. Another objective of the current research is to investigate the effect of adding reactive particles to the solder alloy on the subsequent growth of the interfacial intermetallic layer and the ensuing tensile strength of the joint. To address this objective, two composite solders (solder alloy with additional particles) were fabricated for comparison against the un-reinforced alloy through aging at 150 °C for up to one week. Either micrometer scale or nanometer scale Cu particles are employed as the compositing particles to subsequently produce additional Cu6Sn5 intermetallic reinforcing phases. The expectation is that the large difference in initial particle size of this additional source of Cu will help to elucidate the rate limiting mechanism growth of the interfacial intermetallics. It is demonstrated that while a similarly reduced steady-state of intermetallic growth is obtained for the two composite samples, it is achieved sooner in the one fabricated with the nanometer scale particles. Mechanical tests indicate that the composite specimen prepared with nanometer scale particles also provides the highest tensile strength both before and after aging. As a final objective, creep tests were conducted at room temperature to assess the steady state creep strain rate of composite solders. Though many publications report creep tests conducted at various temperatures, the current study is unique with respect to the attempt made to clarify the effect of applied stress on the creep strain rate of the composite solders. Subsequent effects of particle reinforcement on the creep strain rate are discussed. It is demonstrated that composite solder prepared with Cu nanoparticles exhibit the best creep properties (lowest creep strain rate) among the solders considered in this study. The mechanisms responsible for strength increase, ductility loss and creep properties observed for various composite solders are rationalized based on the current understanding of the microstructure-mechanical response relationship. Differential Scanning Calorimetric (DSC) studies were conducted to investigate the solidification temperature of the particle reinforced lead-free solders. The distinct microstructural features obtained for composite solder are shown to be significantly influenced by disparity in solidification kinetics caused by particle addition. In this context a key result achieved during this research was the solidification mechanism proposed for the composite solders. Overall the results of this research lead to: (i) development of a viable processing method for nano composite solders with excellent creep properties and (ii) demonstration of a novel approach (experimental and modeling) to better understand the effect of particle volume fraction on the elasto-plastic response of the composite solder. This approach will provide some valuable guidelines for tailoring the properties of composite lead-free solder based on the target application.

EPFL2009

Method of manufacturing a module of photoelectrochemical cells with long-term stability

Described is a method of manufacturing a module with long-term stability even though it contains a non-heat-resistant material, in particular a photosensitizer. Two glass plates (1, 2) are connected to each other round their edges with an edge-sealing structure made from glass solder. The non-heat-resistant material is subsequently (i.e. after the glass plates have been heat-sealed) pumped into the sealed module through at least one suitably shaped feed aperture (78.1). Selective coating with glass solder enables linear ribs (10.1, 10.2, 10.3, 11.1, 11.2, 11.3) to be produced in order to form spatially discrete chambers. When being joined by heat-sealing, at least one of the glass plates is heated to a temperature in the region of, or above, the transformation temperature of the plate in order to ensure that the plates (1, 2) are levelled or the distance between them is minimized.

2007

Deformation and Damage in Lead-free Solder Joints

Milad Maleki

EPFL2012

EPFL2009