Search

In recent years, the field of electronic packaging has been rapidly developing. The traditional lead-containing solders have been abandoned because of their pollution to the environment and harm to human body. Therefore, lead-free solders are gradually beginning to be used in the electronic packaging industry, among which, the Sn-3.0Ag-0.5Cu (SAC305) solder is the most frequently used material in lead-free solder. As the fundamental elements, solder joints in electronic components are affected by periodical mechanical stress and strain for the while service life, so it is important to understand the fatigue damage about them. In this paper, SAC305 solder specimens were experimentally investigated to uniaxial cyclic loading at different temperatures from 288.15K to 373.15K under the same strain rate of $\mathbf{10^{-3}/\mathrm{s}}$ and the same maximum strain of 0.10. In order to explain the accumulative damage due to fatigue at different temperatures and the mechanical properties, SAC305 solder specimens after cyclical loading were tested through tensile force until fracture at the same temperature. Furthermore, the differences of microstructure were observed by a scanning electron microscope (SEM). It could be known that the fatigue damage of the specimens is different at different temperatures, and the failure morphology of the solder specimens also varies.

Flexible Printed Circuits (FPC) have been extensively used in daily life. It is inevitable to perform high-frequency reciprocating bending when using FPC., which induces deviation of the circuit board structure., separation of the adhesive layer and finally damage of the device. To solve the problem., it is suggested to change the FPC laminate design and adjust the internal structure and materials of the FPC so that the stress of the Cu layer is concentrated in the neutral layer during the bending process of the FPC and reducing the strain of the adhesive layer. In this paper., we systematically investigate the impact factors to the bending process of the FPC by using the finite element analysis (FEA). The characterized factors include the stacked structure and elastic modulus of each layer., the thickness of the glue/Cu layer., and the bending radius of the FPC. We find that different stack structures drastically influence the position of the stress neutral layer; the stress increases as the elastic modulus of material increases., but decreases with reducing the adhesive layer. Whereas., the elastic modulus of the material and the thickness of the adhesive layer have little effect on the position of the stress neutral layer of the FPC. Furthermore., the increasement in the thickness of the Cu layer increases the stress of the FPC and moving the position of the stress neutral layer downward. As the bending radius increases., the stress on FPC decreases., while the position of the neutral layer hardly changes. We also find that the smaller elastic modulus and thinner glue/Cu layer reduce the stress of the Cu layer. These simulation results provide a guidance for the design of structures and material selection of FPC., which may help solving the mechanical problems during the bending process.

Electronic packaging structures go through complicated loading history such as power on-off, vibration and drop impact during the storage and serving conditions processes. All these loading applications result in the deformation and possible damage accumulation in the packaging structure of the electronic device. Considering that the thermal cycling is a standard procedure for evaluating the mechanical reliability of a packaging, we systematically investigate the combination effect of the drop and thermal cycling on the reliability of the packaging structures by using the finite element analysis. Compared with the transient analysis of drop impact, the analysis duration for thermal cycling is significantly prolonged. A representative Ball Grid Array (BGA) packaging structure is adopted to perform the sequential analysis. The strain rate and temperature dependent visco-plastic properties are considered for the Sn-3.0Ag-0.5Cu (SAC305) solder material at the solder joints. Those material properties have been validated against published experimental data. Firstly, the numerical analysis of the packaging structures subject to drop impact is conducted to obtain the distribution of plastic deformation and strain. As the plastic deformation and strain are not recoverable after the loading application, those distributions can be utilized as initial strain conditions to be imported into the subsequent analysis under thermal cycling. In the second analysis step, a temperature profile between −50°C and 125°C is applied to the packaging structure to predict the deformation and strain during the thermal cycling. The thermal fatigue life is evaluated using the Coffin-Manson model based on the predicted increment of equivalent plastic strain in the critical solder joints. More importantly, the effect of drop-impact damage on the thermal deformation and thus on the fatigue life is discussed by correlating the drop impact, thermal cycling and fatigue life as a closed loop.

In this study, nanoindentation technology is applied to evaluate the elastic-plastic law of mechanical properties of small-sized in-situ packaging materials. As one of the most appealing advantage, the proposed method requires only a single nanoindentation, which means the nanoindentation experiments can be performed at minor effort. Combined with nanoindentation technology, the mechanical behavior of packaging materials in single-point nanoindentation loading process was simulated by the finite element method. The effect of surface stress on nanoindentation was studied numerically. In the finite element simulations, after applying prestress to the substrate material, the single-point nanoindentation was numerically analyzed up to the same indentation depth. Based on the finite element predictions, the mechanical properties of materials subjected to prestressing in different directions are studied. By comparing the applied load-penetration depth curves, stress contour, plastic zone and pile-up behavior, the influence of surface stress on the elastic-plastic material is elucidated.

While moving ahead with science and technology., the service conditions of microelectronic devices are becoming more and more complicated., and the performance requirements of packaging materials are also increasing especially for hash applications. The dependability of solder joints in electronic chips under high temperatures and high strain rates have attracted much attention. In this paper., experimental studies are carried out on the dynamic behavior of Sn-3.0Ag-0.5Cu (SAC305) materials. The dynamic compression experiments are operated by using the Split-Hopkinson Pressure Bar (SHPB) with a heating furnace to achieve the target temperature condition. The true stress- strain curves are received by this system at a temperature of 343K and the strain rates from 959 ${\mathrm{s}^{-1}}$ to ${1961\ {\mathrm{s}^{-1}}}$ . Additionally., in order to investigate the deformation mechanism under impact scenarios., an optical microscopy is utilized to characterize the microstructure of the SAC305 lead-free solder specimen at high temperatures and high strain rates.

Due to the cyclic loading of thermal stress, the solder joints work in the process of cyclic fatigue, and the failure behavior of solder joints in the whole process is actually a process of damage accumulation. It is feasible to study the fatigue properties of solder joints under cyclic loading. In order to achieve accurate numerical simulations, solder constitutive models have been widely concerned. The method of coupling damage variables with a constitutive model provides an effective way to achieve this goal. Under the condition of thermal cycle loading, the damage evolution corresponding to deteriorate material properties meets the basic principle framework of thermodynamics. In this framework, the damage evolution model can be derived by using continuum damage mechanics. This model is greatly simplified by averaging the micro defects of materials, as long as the established model and its derived evolution model can solve engineering problems. Based on the theoretical framework of damage constitutive model, a UMAT user material subroutine suitable for ABAQUS finite element software is completed. Through the secondary development interface of ABAQUS, the viscoplastic constitutive model parameters of coupling damage are continuously optimized, so as to achieve better simulations of the solder fatigue process. Based on the study of the coupled damage constitutive model, the UMAT is applied to the calculation of BGA packaging structure to further reflect the optimization capacity of the model for solder fatigue performance with simulation results as a good fit to the test results.

The microstructure and mechanical properties of Sn-3.0Ag-0.5Cu (SAC305) under different working conditions were studied by static compression tests. The constitutive behaviour of SAC305 material with wide ranges of temperature and strain rate was obtained and correlated with the micromorphology on the specimen surface in this paper. With the temperature varies, the micromorphology of the solder specimens changed significantly. Specifically, as the temperature increases, more compression fringes along the loading direction appeared on the surface of the specimen, and the length of the fringes was elongated. Moreover, the mechanical properties of the solder showed obvious softening phenomenon, that is, the compressive strength decreased gradually. With the increase of the loading strain rate, the micromorphology showed the opposite evolution law compared with the consequences when the temperature increases. That is, the compression fringes gradually decrease and shorten, and the strength increased continuously, which shows an obvious hardening phenomenon. Under the condition of high temperature and low strain rate, a macro crack with the degree of 45° with respect to the compression axis can also be found on the surface of static compression specimen, and its length and width changed regularly with temperature and strain rate.

With the development of miniaturization of electronic chips and their packaging structures, electronic devices are becoming highly integrated. The thickness or area of packaging materials required for device packaging structures is significantly reduced, so that the properties of those small-sized packaging materials cannot be measured using the conventional testing methods. However, for the measurement of Young's modulus of thin film by nanoindentation technology, with the increase of indentation depth, the elastic-plastic deformation of the film and the substrate will change constantly. Therefore, the mechanical property evaluation of the composite is a complex function of the film and substrate's mechanical properties. Based on finite element simulations, the influence of Young's modulus of film and substrate on the mechanical properties of composite materials is studied. The accuracy of Young's modulus calculation of film is verified and summarized by four different weight functions which are used to simulate a variety of film/substrate combinations. At the same time, for the cases with Young's modulus of the film larger or smaller than that of the substrate, the stress distribution and indentation morphology of the film/substrate composite are also discussed. Generally, the substrate modulus is taken as a known parameter in the fitting process. The weight function is applied to a variety of real composite materials to verify the effectiveness of the conclusions drawn by unknow materials.

With the development of electronic industry, electronic packaging structures and systems are developing towards high miniaturization, integration, low power consumption and high reliability. The corresponding electronic packaging structure is more complex, and the serving conditions are increasingly challenging. Therefore, the study on the material properties and the corresponding constitutive model are of great values to describe the mechanical properties of solder joints for the numerical analysis of solder joint and the mechanical reliability of electronic packaging structures. In order to accurately reflect the deterioration of the mechanical properties of the solder joints under the service conditions subjected to thermal cycling, the numerical simulation of SAC305 solder was carried out at the both levels of material and solder joints. The unified creep and plasticity constitutive model was used to simulate the uniaxial tensile test data of SAC305 solder under different displacement loading rates. According to the published data of SAC305 solder uniaxial tensile test in the literature, the parameter identification method is proposed as follows: firstly, the fixing multiple parameters are used, and then the fitting effect of individual parameters on the experimental curve is studied, and finally all constitutive model parameters are identified. Moreover, the rationality of the constitutive model is verified by the electronic packaging structure. The results show that the optimized parameters can reasonably describe the mechanical behavior of solder joints under typical service conditions.