Reliability Of Electronic Packages Research Articles

Influence of Thermal Aging on Lead-Free Solder Joints Reliability: A Review

The handheld electronic products currently turning to high density, high performances, and multifunctionality, which sets higher demands on the solder joints reliability in electronic packaging. Because of environmental conscious towards Pb-containing solders, Pb-free solder get attracted wide attention in the electronic industry. Challenges of the Pb-free solders in the microelectronics device are the microstructural evolution influence the mechanical behavior of the solder reliability. This paper analyses a short overview on the progress of the study on the relationship between structure and property, and possible concerns regarding reliability of Pb-free solders joints. Furthermore, the measures to enhance the solder joint reliability are evaluated which contribute to a further theoretical foundation for the research on the solder joints reliability of electronic devices in service life.

Experimental and finite element analysis on determining the fatigue life of pb-free solder joint (Sn-0.5Cu-3Bi-1Ag) used in electronic packages under harmonic loads

Electronic packages that are used these days are exposed to different types of vibration loadings in their usage environment. This vibration exposure can be categorized as harmonic and random vibrations. When reliability assessment of modern electronic systems is considered, vibration loading has an important role to play. One of the biggest challenges facing today is the accurate and rapid assessment of fatigue life under the vibration loading. Conventional solder joints were made of lead-tin alloy. According to many environment legislations and rules, lead is prohibited as an ingredient in the solder alloy. The reason for the prohibition of the usage of the lead is that it poisons the environment. In this study, Sn-0.5Cu-3Bi-1Ag is used as the lead-free solder alloy. Fatigue life prediction of electronic package containing SAC405 is conducted with the aid of vibration testing and Finite element analysis under harmonic vibration loading. A specially designed Plastic Ball Grid Array Package (PBGA) component is mounted on Printed Circuit Board (PCB). It is taken as a test vehicle for the vibration test. The test vehicle is excited by a sinusoidal vibration. The frequency of this excitation equals the fundamental frequency of the test vehicle and it is continued till the component fails. Since the solder balls are very small for direct measurement, Finite Element analysis (FEA) is used for noting down the stresses. The stress versus failures cycles (S-N) curve is made by relating both the stresses on the solder balls obtained and the number of failure cycles from vibration analysis. The fatigue life of the component can be estimated from the generated S-N curve. It is analyzed that the methodology is effective in predicting the component’s life. Hence, the reliability of electronic package can be improved.

Improving shear test performance of SAC305/OSP Cu solder joints by modifying microstructure with minor Ni doping before and after aging

The influence of Ni on the grain orientation of Sn-3.0Ag-0.5Cu-xNi (wt.%; x = 0, 0.05, 0.1)/organic solderability preservative (OSP) Cu solder joints and reliability of slow speed under shear test with and without aging were investigated. The interlaced structure was revealed in Ni-doped solder joints by employing a scanning electron microscope-based electron backscatter diffraction. Apart from that, with the aid of a multiple functions tester (Xyztec Sigma), Ni-doped solder joints were revealed to be more reliable under slow speed shear tests before and after aging. The enhancement of shear strength of solder joints was attributed to the modified microstructure caused by minor Ni addition. The present work explored that with slight Ni doping in solder joints, the mechanical reliability of advanced electronic packaging was improved.

The effect of Ag on the growth of intermetallics at the interface of Sn5Zn/Cu interconnects

The reliability of electronic packages is primarily affected by the growth of the intermetallic compound (IMC) between the solder and the copper interconnection. It is therefore necessary to study the effect of element addition on the growth of the IMC interface with respect to an improvement in the package reliability. In this study, the influence of Ag on the growth of intermetallic compounds in the Sn5Zn solder/copper interconnection interface was investigated. The results revealed that in comparison with Sn5Zn/Cu, the interfacial IMC thickness decreased initially and then increased in accordance with an increase in the doping amount of alloying elements in the solder. Further, it was observed that the doping of the alloying element (Ag) can significantly reduce the IMC growth rate at the interface. Compared with the Cu5Zn8 unit cell formation energy (-0.632eV), one dopant atom and two dopant atoms of Ag can reduce the formation energy of a Cu5Zn8-based unit cell to the minimum values of -0.636 eV and –0.640 eV, respectively; consequently, the stability is increased. The maximum diffusion activation energies of Cu atoms and Zn atoms in a Cu5Zn8-based unit cell are 3.419 eV and 1.692 eV, which increase to 3.512 eV and 1.971 eV, respectively, after doping with Ag.

Residual Thermal Strain Distribution Measurement of Underfills in Flip Chip Electronic Packages by an Inverse Approach Based on the Sampling Moir\xe9 Method

Residual deformation evaluation of underfill (UF) materials in flip chips is crucial to improve the reliability of electronic packages. In this study, we propose to evaluate the residual thermal strain distributions using an inverse method based on the sampling moiré technique. Even if a grid pattern is fabricated on the specimen at room temperature, the residual strain distributions at an arbitrary temperature relative to the specimen formation temperature can be successfully calculated. The residual strain distributions relative to the free contraction state at an arbitrary temperature can also be measured when the coefficient of thermal expansion is available. A thermal chamber for flip chips was designed under a laser scanning microscope. Using the proposed method, the normal, shear and principal internal strain distributions and deformation characteristics of two kinds of UFs in flip chips were investigated relative to 150 °C. The strains of the UF with low glass transition temperature (UF-A) concentrate near the die material, especially at the die corner, while the strain concentration of the underfill with high glass transition temperature (UF-B) mainly occurs at the die corner and the buffer layer. The maximum principal strain of UF-A is greater than that of UF-B around the die corner. The residual maximum principal strain distributions relative to the free contraction state at 25 °C were compared with the simulation results by the finite element method. The residual strain distribution trends from experiments are consistent with those from simulations.

Investigations of Material Properties of Underfill Material for Mechanical Reliability Improvement of Electronic Devices

In this paper, two types of underfill with different additive contents were formulated to investigate the effects of the underfill properties on the mechanical reliability of electronic packages in portable electronic devices. Dynamic mechanical analyses (DMA), tensile tests and shear tests were conducted to examine the material properties of the underfill, such as the storage modulus (E′), glass transition temperature (Tg), mechanical strength, stiffness and modulus of toughness. In the DMA and tensile test results for cured underfill specimens, Underfill 1 without a flexible additive showed a low modulus of toughness, and Underfill 2 with a small amount of flexible additive exhibited a high modulus of toughness. For underfills with a similar Tg value, the underfill with the higher modulus of toughness exhibited better mechanical reliability than the underfills with high strength, stiffness and low modulus of toughness, due to their fracture resistance to crack pro- pagation. Therefore, the modulus of toughness of the underfill should be predominantly considered, when developing or selecting an underfill to enhance the mechanical reliability of electronic devices, rather than excessively high mechanical strength or stiffness. Key words: Area array packaging, Electronic device, Mechanical strength, Reliability, Toughness, Underfill

Three-Dimensional Thermal Modeling and Management of System on Package with Gold and Nano Based Material

Abstract A detailed computational fluid dynamic based thermal model and simulation of exposed pad SOIC package on PCB is developed with Nano and gold-based material to characterize the thermal behavior or cooling capability of the package. Achieving lower thermal resistance of electronic packaging is one of the important points of interest in electronic application. In this paper, exposed small outline IC package on PCB is compared with different composition of Au-Sn die attach materials and Nano-foil preform material to understand the thermal behavior of the package which will provide cooling through low thermal resistance between junction-to-case. Thermal reliability of electronic packaging depends on the package design and selection of materials. Package material design plays an important role in thermal management of the product. Nano-based package material can be a good choice to improve the thermal performance of system on package. Developed a 3 D model of exposed small outline IC (E-SOIC) package on a printed circuit board (PCB). Au-Sn solder paste is used as die attach material and Nano-Foil preform is used to develop the system on package (SOP) model of exposed pad SOIC package on PWB. Different types of composition of Au-Sn solder paste, such as 78Au22Sn and 80Au20Sn are considered for die attach purpose. The reason for choosing the AuSn solder paste is good reliability and performance. Au-Sn has higher thermal conductivity than other solder materials, such as Pb-free solder paste, 96.5Sn3.5Ag and leaded solder paste, 37Pb63Sn. Two types of thermal model developed to compare the package material and design of the different die attach materials and Nano-Foil preforms. In this paper simulation and data analysis will show how the optimum thermal management depends on the material selection and design of the system on package (SOP). Surface mount type SOIC package with exposed pad design is selected to enhance the thermal purpose. Gold-tin and Nano-Foil preform will be used as alternative of lead-free package materials.

The Variation of Microstructure and the Improvement of Shear Strength in SAC1205-xNi/OSP Cu Solder Joints Before and After Aging

The correlations between different Ni contents in Sn-1.2Ag-0.5Cu-xNi (wt.%; x = 0, 0.05, 0.1)/organic solderability preservative Cu solder joints and shear test performance before and after aging were probed. With the aid of electron back scattering diffraction analysis and a scanning electron microscope, complexity in microstructure of Ni-doped solder joints was investigated. The results of a slow speed shear test revealed that the peak force of solder joints was efficiently enhanced by the addition of Ni before and after aging. The improvement of mechanical strength was ascribed to the modification of microstructure by the introduction of minor Ni addition. Spread and tiny intermetallic compounds, and the oriented interlaced structure in Ni-doped solder joints acted as hindrances for propagation of cracks and dislocations. It is revealed that solder joints with minor Ni doping tend to exhibit better mechanical reliability in advanced electronic packaging no matter before or after aging.

Formation Mechanism of Novel Sidewall Intermetallic Compounds in Micron Level Sn/Ni/Cu Bumps

A new kind of intermetallic compounds (IMC) were found around copper pillar in micron level bumps. To investigate the formation mechanism, three different sized Sn/Ni/Cu bumps (10 μm, 20 μm, 50 μm) were electroplated then reflowed at 230 °C for 100 s. After reflow process, a thin layer of IMC was formed around copper pillar, which is attributed to surface wetting behavior. After aging at 170 °C and 200 °C for different times, the growth mechanism of sidewall IMC was observed by scanning electron microscopy combined with electron backscatter diffraction (EBSD) technology. Surface diffusion was considered to be the main driving force for sidewall IMC growth for the activation energy of them was found to be much smaller than that in previous studies. The EBSD results showed a preferred orientation of sidewall Cu3Sn grains being perpendicular to copper periphery, which indicated direction of Cu atoms flux during Cu3Sn growth. Formation mechanism of this novel sidewall IMC was proposed based on surface wetting and surface diffusion. The findings contribute to the failure mechanism study in small size bumps and provide insights into the reliability of 3D electronic packaging.

Reliability Physics Approach for High-Density Ball Grid Arrays in Autonomous Vehicle Applications

<div class="section abstract"><div class="htmlview paragraph">Integration of advanced sensing systems in autonomous vehicles is possible due to high performance processors that utilize high-density ball grid array (HD-BGA) packaging. The configuration of advanced sensors within autonomous vehicles requires the placement of processing modules within non-conventional vehicles compartments that can drastically influence the reliability of HD-BGAs. Durability of HD-BGAs to different loads depend on their location within the vehicle as well as the form factor of the package itself. Reliability Physics Approach (RPA) combines simulation tools and empirical models to predict the reliability of advanced electronic packages under complex environmental and operational loads by identifying the susceptibility of electronic components to the dominant failure mechanism. In this paper, the reliability of an electronic module will be investigated under various vibration loads at different vehicle compartments and the influence of mounting configuration of electronic module to housing under thermal loads. In addition, the BGA form factor will be investigated to demonstrate the implementation of RPA early in the design process to develop such mitigation techniques as underfill selection to improve the reliability of HD-BGAs in autonomous vehicle applications.</div></div>

Experimental Study on the Reliability of PBGA Electronic Packaging under Shock Loading

Plastic Ball Grid Array (PBGA) one of the most important electronic packaging methods, is widely used in aeronautical industry field. According to the JEDEC standard, shock tests of PBGA assemblies are conducted under different loading conditions. Several important parameters, such as the fatigue life of PBGA assemblies, the relationship between solder joint positions and fatigue life, the relationship between strain energy density and fatigue life, are analyzed based on experiment results. The failure modes of PBGA assemblies are studied by optical microscope (OM). The results show that during the shock tests, the strains of the solder joints near the center of the specimen are larger than other positions, and these solder joints are prone to form micro cracks. With the increase of the shock times, these micro cracks extend rapidly which will eventually cause the failure of the PBGA electronic packaging.

Empirical Solutions and Reliability Assessment of Thermal Induced Creep Failure for Wafer Level Packaging

The accelerated thermal cycling test (ATCT) is a method of testing the electronic packaging reliability characteristic. A component must pass this test before being launched into the market. According to many studies, under thermal loading, the excessive thermal strain and stress occur between the package and substrate because of the coefficient of thermal expansion mismatch, which damages the solder bump of electrical packaging. In the ATCT, −40° C to 125° C is the regular thermal range of packages tested. Furthermore, the homologous temperature exceeds one-third of the melting point (K) of solder, and one should consider the accumulation of creep strain and estimate the creep behavior during the loading procedure; however, the creep strain is very sensitive to mesh size in finite element simulation. Obtaining a stable and reliable creep stain for different packaging structures becomes a must for solder joint reliability assessment. In this paper, we study a feasible and fixed element size, which should be chosen carefully for the same type of electronic packaging structure, for example, wafer level packaging (WLP), as it can affect simulation results and cause the prediction of the packaging life cycle to deviate. In ATCT simulation, there is a big difference in reliability estimation for different element mesh size selection, even if all material properties and structures remain the same. It is essential to find an appropriate and fixed element size for packaging reliability prediction to yield precise and reliable simulation results. This paper aims to find an appropriate element mesh size that can consistently predict the reliability life of various WLPs in an accurate range, and the reliability results of different WLPs under the thermal cycling loading have been validated by experiments. Moreover, the simulation results are summarized by using the Anand and hyperbolic sine creep models with suitable empirical reliability assessment equations, and the results show both creep models can predict the reliability life of WLPs in an accurate range with appropriate mesh control.

A Review of Interface Microstructures in Electronic Packaging Applications: Soldering Technology

This report examines the role of interfaces in electronic packaging applications with the focus placed on soldering technology. Materials and processes are described with respect to their roles on the performance and reliability of associated interfaces. The discussion will also include interface microstructures created by coatings and finishes that are frequently used in packaging applications. Numerous examples are cited to illustrate the importance of interfaces in physical and mechanical metallurgy as well as the engineering function of interconnections. Regardless of the specific application, interfaces are non-equilibrium structures, which has important ramifications for the long-term reliability of electronic packaging.

Overview Study of Solder Joint Reliablity due to Creep Deformation

AbstractThe effective lifetimes of electronic packages are affected by various thermos-mechanical deformations. Creep is considered the most salient mechanism in the failure of solder joints. Many researchers have conducted reasonable studies to portray the behavior of creep deformation using numerical models and further extended their research scope to forecast the lifetimes of packages with the results obtained from creep models. Many studies have identified particular creep and lifetime models to be nominal based on experimental data.In this study, the characteristics of familiar creep models were examined in detail, and their significance was made known. Lifetime prediction models that seem prominent among researchers were discussed in detail. Finite element analysis of a wafer level chip-scale package (WLCSP) used to figure out the engagement of different creep models and their capability of materializing creep deformation was investigated via simulation. The results from the simulation were applied to different lifetime prediction models, and their predictions were examined carefully. After considering the various factors that affected the reliability study of the solders, the Garofalo-Arrhenius creep model and modified strain energy density model seemed to be convincingly productive for studying the reliability of various electronic packages.

Conductive Mechanism of Bi-Doped Thick-Film Dielectric in High-Temperature Aging With Bias

The increase in high-temperature electronic applications is a challenge for the reliability of electronic packaging. Thick-film technology has the potential of operating at 300 °C. In a previous study of electrical reliability for multilayer thick films, bismuth (Bi) from the binder of a commonly used PtPdAu thick-film conductor diffused into the dielectrics during the multiple firing steps. If the dielectric was thin and the Bi was continuous through the dielectric, a significant increase in leakage current was observed between the two layers of conductor during 300 °C aging with bias. This paper studied the mechanism of leakage current increase in Bi-doped thick-film dielectrics. Two dielectrics, $D1$ and $D2$ , were intentionally doped with Bi in various weight concentrations. Capacitors fabricated with these Bi-doped dielectrics had a significant leakage current increase in a short time during aging at 300 °C with a 100-V bias in either polarity. Insulating substrate (sapphire) and conductive substrate (Pt foil) were used to replace the 96% Al2O3 ceramic substrates to study the leakage current mechanism by separating the influences of leakage current and bias voltage (electric field). Doping Bi into the multilayer dielectric created traps during the firing process. Without leakage current, the traps were not activated during aging. When bias was applied to the top electrode, the electron flow through the dielectric activated the traps, resulting in the increase of leakage current. Traps created by doping with Bi in the thick-film dielectric, $D1$ , were measured by ultraviolet and visible spectroscopy, and the trap energy was at 3.2–3.3 eV in $D1$ with 3 wt% of doped Bi.

Reliability assessment of electronic assemblies under vibration by statistical factorial analysis approach

PurposeThis paper aims to present a reliability performance assessment of electronic packages subjected to harmonic vibration loadings by using a statistical factorial analysis technique. The effects of various geometric parameters, the size and thickness of the printed circuit board and component and solder interconnect dimensions on the fundamental resonant frequency of the assembly and the axial strain of the most critical solder joint were thoroughly investigated.Design/methodology/approachA previously published analytical solution for the problem of electronic assembly vibration was adopted. This solution was modified and used to generate the natural frequency and solder axial strains data for various package geometries. Statistical factorial analysis was used to analyze these data.FindingsThe results of the present study showed that the reliability of electronic packages under vibration could be significantly enhanced by selecting larger and thicker printed circuit boards and thinner and smaller electrical components. Additionally, taller and thinner solders might also produce better reliability behavior.Originality/valueThe results of this investigation can be very useful in the design process of electronic products in mechanical vibration environments.

Growth behavior of tin whisker on SnAg microbump under compressive stress

Tin whiskers with different morphologies form on SnAg microbumps under long-term compressive stress. High resolution transmission electron microscopy results reveal that dislocations and stacking faults can be found inside tin whiskers and the neighboring grains, and the twin boundaries are irregular regions with twinning dislocations. The dislocations can slip from the adjacent grains into the tin whisker grains through the twin boundaries, resulting in whisker growth under compressive stress. The findings are of great significance to the study of tin whisker on isolated Sn-based solders and provide insights into the reliability of 3D electronic packaging.

Ductile fracture of solder-Cu interface and inverse identification of its interfacial model parameters

Interfacial failure is a crucial issue in the study of reliability in electronic packaging. The failure mechanism is complicated since ductile failure of the solders is generally accompanied with brittle fracture of the intermetallic compounds(IMCs). Besides, it is also greatly influenced by loading conditions. In this paper, fractures of SAC305-Cu interfaces under mode I and mode II loading conditions have been experimentally studied. Their interfacial micro-structures before and after failures have been analyzed using SEM and EDS. At the same time, numerical simulations of interfacial failure have also been performed using an improved cohesive zone model. Finally, the failure mechanisms for both mode I and mode II fractures were disclosed. In addition, though the cohesive zone models(CZM) were widely used to investigate the interfacial failure, their parameter identification is still an important concern, especially when considering both the accuracy and efficiency in the analysis. In our work, a parameter inverse analysis method has been developed, based on the Kalman filter algorithm and the response surface interpolation technique. This interpolation method can hugely improve the computational efficiency, but it will also introduce a large numerical error in the present cases, in which the interfacial failure is accompanied by large plastic deformation of solder, or the parameter searching ranges are set over-large. Aiming at this issue, a range-reduced inverse analysis scheme was further developed in our work. The developed method and scheme were validated robust and accurate through verifications of both pseudo and real experimental data. The cohesive zone model parameters acquired can be effective to simulate the interfacial failure of Cu and Pb-free solder. The whole inverse analysis is automatically achieved and can be easily applied to other complicated interfacial failure problems.

The variation of grain structure and the enhancement of shear strength in SAC305-0.1Ni/OSP Cu solder joint

In this study, the slow speed shear test in both Sn-3.0Ag-0.5Cu (wt%)/OSP Cu and Sn-3.0Ag-0.5Cu-0.1Ni (wt%)/OSP Cu assembly with the ball heights of 300 μm and the corresponding grain structures were investigated. With the aid of Electron Back Scattering Diffraction (EBSD) analysis, single grain structure was observed in Sn-3.0Ag-0.5Cu/OSP Cu. Besides, Ni was found to control the grain structure in Sn-3.0Ag-0.5Cu-0.1Ni solder balls, showing multiple grains with partially interlaced structure. The grain variation resulted from larger undercooling caused by smaller ball size and Ni-dopant induced tiny intermetallic compounds (IMCs). IMCs serve as heterogeneous nucleation sites for β-tin and thus alter the grain structure of solder balls. The results of shear test reveal that the peak force of solder joints was efficiently enhanced by the addition of Ni. The enhancement of mechanical strength was attributed to the modification of grain structure by the introduction of Ni dopant. It is believed that the smaller grains, tiny intermetallic compounds, and the oriented interlaced area in Ni-doped solder joints became energy barriers for propagation of cracks and dislocations. It is demonstrated that Ni-doped solder joints tend to exhibit better mechanical reliability in advanced electronic packaging.

Evaluation of the Stress Distribution by the Stepped Load Indentation Test with Constant Depth

For accurate evaluation of the reliability of electronic package, FEM analyses considering the creep deformation of solder joint in-situ should be conducted. It is well known that the indentation creep test has an advantage to evaluate the creep deformation in microscopic region although there are the problems. Authors already modified the indentation creep test and proposed the method to estimate the steady state creep deformation by the modified test. For lead free solders generally used for the solder joint, the transient creep deformation should be measured by the indentation test in addition to the steady state creep deformation. The transient creep strain occurs in the indentation process. Therefore, it needs to separate the strain into the elastic-plastic strain and the creep strain. In this paper, the method to obtain the stress-strain relation using the indentation test is proposed. New indentation test used the stepped load was proposed to obtained the stress variation during the indenter was pushed and conducted by the numerical test. The suitable measurement point to obtain the relationship between the stress and the strain was identified. The stress variation estimated by proposed area well coincides with the equivalent stress variation of the nodal solution. Therefore, if the method to obtain the strain variation during the indentation process by the indentation in future is developped, it may be possible to estimate the stress-strain curve expressed the uniaxial deformation in the microscopic region.