Solder Joint Reliability Research Articles

Impact of electromigration and isothermal ageing on lead-free solder joints of chip-sized SMD components

This paper presents the effect of electromigration and isothermal ageing in lead-free SAC305 (Sn3Ag0.5Cu) solder joints of chip-size surface mounted (SMD) components, where different track-to-pad joining angles define the current density in the joint. The samples were based on 0402 and 0603 size components and were loaded for an extensive period of 4000 h with 2 and 2.5 A, respectively, in an increased temperature environment (40 °C). The solder joint reliability was assessed at selected times during the loading, with shear tests and cross-section analysis on metallurgic structures. Optical and scanning electron microscopy were used to analyse the microstructure of the solder joints. The shear strength of the solder joints was found to decrease with the increasing loading times. Intermetallic compound (IMC) was found on the 4000 h current stressed samples fracture surface, contrary to the other samples. The IMC layer thickness increased with the loading times and was slightly higher for the anode side of the current stressed components. The current stressing resulted in extensive copper dissolution from the pad and IMC layer accumulation at the component metallisation at the cathode side, affecting the reliability of the joints and pointing to electromigration.

The Reliability of SAC305 Individual Solder Joints during Creep–Fatigue Conditions at Room Temperature

The failure of one solder joint out of the hundreds of joints in a system compromises the reliability of the electronics assembly. Thermal cycling is a result of both creep–fatigue mechanisms working together. To better understand the failure process in thermal cycling, it is crucial to analyze both the effects of creep and fatigue mechanisms in a methodical manner. In this work, individual solder junctions are subjected to accelerated shear fatigue testing to investigate the effects of creep and fatigue on joint dependability at room temperature. A modified fixture is used to conduct fatigue tests on an Instron 5948 micromechanical tester. SAC305 joints with an OSP surface finish were cycled under stress control at first, and then the strain was maintained for a set amount of time. In this investigation, three stress amplitudes of 16, 20, and 24 MPa are used, together with varying residence periods of 0, 10, 60, and 180 s. The fatigue life of solder junctions is described for each testing condition using the two-parameter Weibull distribution. Additionally, as a function of stress amplitude and residence time, a dependability model is created. For each testing scenario, the progression of the stress–strain loops was studied. By quantifying relevant damage metrics, such as plastic work per cycle and plastic strain at various testing circumstances, the damage due to fatigue is distinguished from creep. To investigate the relationships between plastic work and plastic strain with fatigue life, the Coffin–Manson and Morrow Energy model is used. The results indicate that using greater stress magnitudes or longer dwell periods significantly shortens fatigue life and dramatically increases plastic work and plastic strain. The housing impact is significant; in some circumstances, testing with a longer dwelling period and lower stress amplitude resulted in more damage than testing with a shorter dwelling period and higher stress levels. When illustrating the fatigue behavior of solder junctions under various stress amplitudes or dwellings, the Coffin–Manson and Morrow Energy model were both useful. In the end, general reliability models are developed as functions of plastic work and plastic strain.

Understanding the surface segregation of solute atoms in Sn-Bi–based solder from first principles

Low-temperature Sn-Bi solder has wide application in the field of electronic packaging due to its low melting point and good wettability. The formation of Bi-rich phase and intermetallic compound is the major concern for the reliability of Sn-Bi solder joints. We employed first-principles calculations to understand the segregation of Bi and the third elements to the surface of Sn. The effects of alloying elements on inhibiting the Bi surface segregation were described. Our calculations show that the Bi surface segregation could be effectively alleviated by the addition of Ag, Ga, Ni, and In, along with the reduction of further possible formation of intermetallic compounds in the Sn-Bi–based solders. The results could be interpreted by the enhanced bond orders between Bi and its neighboring Sn, alloying elements.

Evaluation of Solder Joint Reliability in 3D Packaging Memory Devices under Thermal Shock

In 3D packaging memory devices, solder joints are critical links between the chip and the printed circuit board (PCB). Under severe working conditions, cracks inevitably occur due to thermal shock. If cracks grow in the solder joint, the chip will be disconnected with the PCB, causing its function failure. In this paper, the reliability of solder joints under thermal shock are evaluated for 3D packaging memory devices by means of the SEM and finite element analysis. As microscopically studied by the SEM, it is found out that the main failure mechanism of solder joints in such test is the thermal fatigue failure of solder joints. Finite element analysis shows that cracks are caused by the accumulation of plastic work and creep strain. The initiation and growth of cracks are mainly influenced by the inelastic strain accumulation. The trends of cracks are influenced by the difference between the coefficient of thermal expansion (CTE) of epoxy resin and that of the chip.

Effects of Interconnect Shape and Thermal-Electro-Migration on Solder Creep of Copper-Pillar Flip Chip Joints

With the development of electronic products toward high density and high reliability, the size of interconnect solder joints and interconnect pitch continues to decrease. Under normal service conditions, solder joints are subjected to thermal–electric–mechanical loads simultaneously, resulting in reliability problems for many package structures. This article investigated the effects of interconnect shape and thermal-electro-migration on solder creep of copper-pillar flip chip joints and analyzed the corresponding failure modes and mechanisms. In this article, solder joints with different shapes were obtained by adjusting the thermal compression bonding process parameters, and creep experiments were performed on the solder joints under different temperature and stress conditions. In addition, we have performed thermoelectric coupling experiments and finite element simulations for different shapes of solder joints. Moreover, creep experiments were conducted on three types of solder joints after 10 and 40 h of thermoelectric coupling pretreatment. The results show that the three solder joints exhibit typical creep characteristics under different experimental conditions and the creep strain rate increases with the increase of shear stress and temperature. Under the same conditions, the creep strain rate of the hourglass-shaped solder joint is always higher than that of the other two solder joints. The electromigration reliability of the hourglass-shaped solder joint is the worst, and the electromigration reliability of the cylinder-shaped solder joint is the best. With the increase of thermoelectric coupling pretreatment time, the creep strain rate of solder joints increased significantly, and the fracture mode of solder joints gradually changed from ductile fracture to brittle fracture. Among the three types of solder joints, the creep resistance of the hourglass-shaped solder joints is greatly affected by the thermoelectric coupling pretreatment.

Effect of Solder Ball Geometry on Solder Joint Reliability under Solder Reflow Cooling Process

Solder joint reliability has become an increasingly important factor in electronic industries to obtain sustainable and reliable electronic packages. The simulation study of 3D finite elements on BGA test assembly models with geometries of SMD-NSMD and NSMD-NSMD is conducted through ABAQUS software and is applied with Anand model equation. The applied loading onto the test assembly is set with reflow cooling temperature of 220 °C to 25 °C. The purpose of this research is to obtain the package warpage, stress, and inelastic equivalent strain throughout the package and solder joints and to develop a predictive finite element model for mechanics and deformation of solder joint in BGA package under reflow cooling. The results obtained showed that solder joints with NSMD-NSMD pad geometry has a greater inelastic equivalent strain and has a greater potential in failing than SMD-NSMD pad geometry. Therefore, it can be concluded that SMD-NSMD pad geometry is more preferable for obtaining a more reliable solder joint.

The impact of intermetallic compound on microstructure, mechanical characteristics, and thermal behavior of the melt-spun Bi-Ag high-temperature lead-free solder

PurposeThis study aims to study the impact of intermetallic compound on microstructure, mechanical characteristics and thermal behavior of the melt-spun Bi-Ag high-temperature lead-free solder.Design/methodology/approachIn this paper, a new group of lead-free high-temperature Pb-free solder bearing alloys with five weight percentages of different silver additions, Bi-Agx (x = 3.0, 3.5, 4.0, 4.5 and 5.0 Wt.%) have been developed by rapidly solidification processing (RSP) using melt-spun technique as a promising candidate for the replacement of conventional Sn-37Pb common solder. The effect of the addition of a small amount of Ag on the structure, microstructure, thermal and properties of Bi-Ag solder was analyzed by means of X-ray diffractometer, scanning electron microscopy, differential scanning calorimetry and Vickers hardness technique. Applying the RSP commonly results in departures from conventional microstructures, giving an improvement of grain refinement. Furthermore, the grain size of rhombohedral hexagonal phase Bi solid solution and cubic IMC Bi0.97Ag0.03 phase is refined by Ag addition. Microstructure analysis of the as soldered revealed that relatively uniform distribution, equiaxed refined grains of secondary IMC Bi0.97Ag0.03 particles about 10 µm for Bi-Ag4.5 dispersed in a Bi matrix. The addition of trace Ag led to a decrease in the solidus and liquidus temperatures of solder, meanwhile, the mushy zone is about 11.4°C and the melting of Sn-Ag4.5 solder was found to be 261.42°C which is lower compared with the Sn-Ag3 solder 263.60°C. This means that the silver additions into Bi enhance the melting point. The results indicate that an obvious change in electrical resistivity (?) at room temperature was noticed by the Ag addition. It was also observed that the Vickers microhardness (Hv) was increased with Ag increasing from 118 to 152 MPa. This study recommended the use of the Bi-Ag lead-free solder alloys for higher temperature applications.FindingsSilver content is very important for the soldering process and solder joint reliability. Based on the present investigations described in this study, several conclusions were found regarding an evaluation of microstructural and mechanical deformation behavior of various Bi-Ag solders. The effect of Ag and rapid solidification on the melting characteristics, and microstructure of Bi-Ag alloys were studied. In addition, the mechanical properties of Bi with different low silver were investigated. From the present experimental study, the following conclusions can be drawn. The addition of Ag had a marked effect on the melting temperature of the lead-free solder alloys, it decreases the melting temperature of the alloy from 263.6 to 261.42°C. Bi-Ag solders are comprised of rhombohedral Hex. Bi solid solution and cubic Ag0.97Bi0.03 IMC is formed in the Bi matrix. The alloying of Ag could refine the primary Bi phase and the Bi0.97Ag0.03 IMC. With increasing Ag content, the microstructure of the Bi-Ag gradually changes from large dimples into tiny dimple-like structures. The refinement of IMC grains was restrained after silver particles were added into the matrix. The inhibition effect on the growth of IMC grains was most conspicuous when solder was doped with Ag particles. As a result, the Vickers microhardness of the Bi-Ag lead-free solder alloys was enhanced by more than 100% ranging from 118.34 to 252.95 MPa. Bi-Ag high-temperature lead-free solders are a potential candidate for replacing the tin-lead solder (Sn-37Pb) materials which are toxic to human and the environment and has already been banned.Originality/valueThis study recommended the use of the Bi-Ag lead-free solder alloys for high-temperature applications.

Effect of the IMC layer geometry on a solder joint thermomechanical behavior

PurposeIn a printed circuit board assembly (PCBA), the coefficient of thermal expansion (CTE) mismatch between the solder joint materials has a detrimental impact on reliability. The mechanical stresses caused by the thermal changes of the assembly lead to fatigue and sometimes the failure of the solder joints. The purpose of this study is to propose a novel pad design to obtain an interrupted solder/substrate interface, to improve the PCBA reliability.Design/methodology/approachAn interruption in the continuous intermetallic compound (IMC) layer of a solder joint was implemented, by the deposition of a silicone film in the pad, changing its geometry. That change allows a redistribution of stresses in the most ductile zone of the solder joint, the solder. The stress concentration at the solder/substrate interface is reduced, as well as the general state of stress at the solder joint.FindingsA new way was developed to reduce the stress on the solder joints, caused by thermal variations, because of the different components CTEs mismatch. This new method consists of interrupting the IMC layers of the solder joint, strategically, redirecting the usual stresses to a more ductile area of the joint, the solder. This is an innovative method that allows increase the lifetime of PCBAs and the equipments.Originality/valueIn this study, a new pad design concept for higher solder joint reliability was developed to reduce the shear stress in the solder joints because of the CTE mismatch between all the solder joint components.

Modified Norris-Landzberg model for Pb-free solder joint reliability evaluation

One important task for reliability engineers in new product design phase is to evaluate the long-term reliability of Pb-free solder joints under the service environment conditions. It is a common practice, due to the practical restrictions such as cost and aggressive development cycle time, to map the results under accelerated testing conditions to field conditions via well-benchmarked transform models. One of the most widely adopted models is the modified version based on Norris-Landzberg (NL) acceleration factor (AF) model (Norris and Landzberg, 1969). The popular NL AF model simultaneously makes it easy to apply but difficult to correlate with data from different sources. In fact, there is ongoing discussion that questions the validity of the model (Osterman, 2018). To overcome the shortcomings of the model, Salmela (2007) has suggested a correction term to consider package type and solder material along with the temperature and the dwell time on the solder joint. Chuang et al. (2010) have compared several models and confirmed that the prediction based on Salmela's model is closer to their testing results. Ahmad and Liu (2010) have proposed the constants of NL AF model as functions of the characteristic life under the accelerated testing conditions. All of the aforementioned variations of NL AF models, while improving the model prediction, would still need different constants for data under different situations; therefore, it is not convenient to use especially in product design phase where some critical information such as the package detail and testing results may not be available.In this paper, a unified NL AF model has been proposed by introducing an add-in impact function to factor package-related contributors such as package size, die size, joint pitch, and delta CTE. It is demonstrated that such an impact function enables NL AF model to correlate very well with a wide range of experimental data using one unified set of constants that were derived by nonlinear fitting of 111 data points from internal source and public domain publications. The robustness of the derived constants was then validated using the generic algorithm to ensure the best of the goodness-of-fit while minimizing the standard residual error. The resulting model has R2 equals 0.86 which showed a good correlation with the testing results. Furthermore, the derived model has been benchmarked with the testing results of iNEMI Pb-free solder alloy alternative project (Henshall et al., n.d.; Parker et al., n.d.; Sweatman et al., n.d.; Coyle et al., n.d.), an in-depth industry wide collaboration. The testing results are representable as two types of components with different solder alloys have been tested under multiple temperature cycling profiles. The benchmarking demonstrates a good correlation as the majority of the predicted life, from the derived model, falls into ±50 % band compared with the testing results. Hence the proposed NL AF model, with a unified constant set, can be helpful and handy in product design phase for reliability assessment.

Grain Size Effects on Mechanical Properties of Nanocrystalline Cu6Sn5 Investigated Using Molecular Dynamics Simulation.

Intermetallic compounds (IMCs) are inevitable byproducts during the soldering of electronics. Cu6Sn5 is one of the main components of IMCs, and its mechanical properties considerably influence the reliability of solder joints. In this study, the effects of grain size (8–20 nm) on the mechanical properties (Young’s modulus, yield stress, ultimate tensile strength (UTS), and strain rate sensitivity) of polycrystalline Cu6Sn5 were investigated using molecular dynamics simulations at 300 K and at a strain rate of 0.0001–10 ps−1. The results showed that at high strain rates, grain size only slightly influenced the mechanical properties. However, at low strain rates, Young’s modulus, yield stress, and UTS all increased with increasing grain size, which is the trend of an inverse Hall–Petch curve. This is largely attributed to the sliding and rotation of grain boundaries during the nanoscale stretching process, which weakens the interaction between grains. Strain rate sensitivity increased with a decrease in grain size.

Fuzzy Approach for Reliability Modeling of Lead-Free Solder Joints in Elevated Temperature Environmental Conditions

The mechanical and fatigue properties of microelectronic interconnection materials are important issues in critical applications in the life of electronic assemblies. Due to the growth in the applications of electronic components in new technological products used in tough conditions, evaluating the reliability of solder alloys has become crucial to the prediction of product life. SAC (Sn-Ag-Cu) solder alloys are common lead-free alloys used to form solder joints. In the current study, the reliability of individual SAC305 solder joints in actual setting conditions with an organic solderability preservative (OSP) surface finish was examined using an accelerated shear fatigue test at different load levels (16 MPa, 20 MPa, and 24 MPa). Four operating temperature levels (−10, 25, 60, and 100 °C) were also used. Seven samples were utilized at each experimental condition. An orthogonal array (L12) was employed for this experiment. The fatigue behavior of the SAC305 solder joints in actual setting conditions was described by a two-parameter Weibull distribution. The characteristic life and the shape parameter were extrapolated from the Weibull distribution for each experimental condition. Characteristic life was employed to represent the fatigue resistance of the solder joints. For each sample, the inelastic work per cycle and plastic deformation were determined. The results indicated a notable increase in the inelastic work per cycle and the plastic strain as a result of increasing either the stress load or the working temperature. Combinations of stress amplitude, working temperature, inelastic work, and plastic deformation were applied as inputs to the fuzzy system for computing a comprehensive output measure (COM-Value) using the Mamdani method. A negative relationship was observed between the solder life and the four fuzzy inputs. To fuzzify the inputs of the fuzzy system, two membership functions, “MFs”, were formed for each input, and five MFs were set for the output. The center-of-gravity (COG) theorem was utilized as a defuzzification method in the fuzzy inference system. The characteristic life of the solder joints was predicted by the COM-Value, which was used as an independent variable. Finally, The COM-Value was used to generate a general predicted reliability model for SAC305 solder joints with an acceptable model adequacy index.

Machine learning for board-level drop response of BGA packaging structure

Board-level drop responses are critical to evaluate the mechanical reliability of solder joints to serve as electrical and mechanical connections in electronic devices to resist failure due to drop impact. A machine learning (ML) model based on the back propagation (BP) method is proposed for predicting three-dimensional board-level drop responses of the ball grid array (BGA) packaging structures. Compared with conventional finite element (FE) simulations with solid elements, the proposed approach with a deep neural network is more than three orders of magnitude faster in terms of computational efficiency. More importantly, detailed stress and strain of all solder joints and also the warpage of the printed circuit board (PCB) can be accurately predicted throughout the drop process. According to the contact type between BGA packaging structure and rigid ground, the drop conditions are divided into three types: surface contact, line contact and point contact. After training the ML model, the obtained nonlinear mapping relation can well predict the mechanical response of solder joints in BGA packaging structures. To significantly extend the application scope, the trained ML model could reasonably predict the dynamic responses of BGA structures with the drop angles which are beyond the training samples. Compared with FE results, the prediction accuracy of the proposed ML model is objectively measured by the Pearson correlation coefficient which is found to be above 0.95 after estimating the stress, strain and energy density of solder joints and PCB warpage. Except for those stress, train and energy density values close to 0, the prediction errors are mostly less than 10% compared with the finite element simulation values. Therefore, it is affirmed that the computational efficiency and accuracy of the proposed ML method are satisfactory to replace the traditional time-consuming FE modeling for predicting the dynamic responses of board-level BGA packaging structures even under more extreme and complicated loading conditions.

Research on the reliability of Micro LED high-density solder joints under thermal cycling conditions

For the purpose of studying the effect of high-density, small-size solder joints on the thermal fatigue life of display devices, a 3D model of the corresponding device structure is established, using finite element analysis methods, and using Anand constitutive equations and Coffin-Manson life prediction models. The equivalent stress and strain change law of an indium solder joint with a diameter of 10 μm under thermal cycling loading conditions is explored, and the key position of the solder joint damage is inferred based on the simulated strain cloud diagram. In addition, the influence of different bump height, bump contact area and Under-Bump Metallization (UBM) thickness on the thermal fatigue life of solder joints is studied separately. The final simulation results show that the location where the solder joint may be damaged first is at the point on the outer side where the solder joint is connected to the UBM on the edge away from the center point. The height of the bump and the contact area of the bump have a significant effect on its thermal fatigue life. In the actual experiment, we should focus on the impact of this aspect.

Study on temperature cycling reliability of Sn-5Sb-0.5Cu-0.1Ni-0.5Ag/Cu micro solder joints

PurposeThe purpose of this paper is to study the temperature cycling reliability of Sn-5Sb-0.5Cu-0.1Ni-0.5Ag/Cu micro solder joints compared with Sn-5Sb/Cu and SAC305/Cu micro solder joints, which has important engineering and theoretical significance for the research of micro solder joint reliability. This paper also aims to provide guidance for the selection of solder for third-generation semiconductor power device packaging.Design/methodology/approachThe shear strength, plasticity, bulk solder hardness and creep performance of three kinds of micro solder joints before and after temperature cycling were studied by nanoindentation and micro shear experiments. Scanning electron microscopy and energy dispersive spectrometry were used to analyze the fracture mode, fracture position and compound composition of the solder joints.FindingsThe bulk solder hardnesses and shear strengths of Sn-5Sb-0.5Cu-0.1Ni-0.5Ag/Cu solder joints were higher than those of Sn-5Sb/Cu and SAC305/Cu solder joints before and after temperature cycling. The indentation depth, creep displacement and creep rate of bulk solders of Sn-5Sb-0.5Cu-0.1Ni-0.5Ag/Cu solder joints were the smallest compared with those of Sn-5Sb/Cu and SAC305/Cu solder joints after the same number of cycles. In addition, the fracture mode and fracture position of the micro solder joints changed before and after temperature cycling.Originality/valueA new type of solder was developed with excellent temperature cycling performance.

Study of Lead-Free Solder Joint Reliability

Solders are essential for the construction of many engineering materials. The use of soldering in electronic packaging has become increasingly critical in today's world. In the electronic manufacturing industry, alternative soldering materials based on tin metal have developed gradually over the last two decades. Manufacturing companies are primarily concerned about removing lead from electronic manufacturing and waste recycling by switching to the lead-free solder. Environmental and human health are adversely affected by exposure to high lead levels in consumer products. The manufacturing industry has switched to lead-free soldering to support their social responsibility initiatives. The three categories of lead-free solder are classified according to temperature range: high-temperature solder, medium-temperature solder, and low-temperature solder. This research is conducted based on the approach of study literature, which is obtained from some papers that generally explain lead-free solder. To provide additional knowledge which is expected to add new insights about the importance of lead-free usage and its behavior.

The Effect of Acrylic Conformal Coating in the Reliability of Solder Joints

During the product life, electronic components are subjected to environmental conditions, such as humidity or temperature fluctuations. Solder joints, which allow the electrical and mechanical connection, are one of the most critical spots to have failures due to environmental conditions. Conformal coating, a thin layer that encapsulates all components and consequently solder joints, is one of the techniques that allows the protection of the component against these detrimental effects. Nevertheless, the application of conformal coating has not been recommended in some components, namely in ball grid arrays (BGAs) and quad-flat-no-leads (QFN), since it can compromise the reliability of their solder joints when exposed to temperature fluctuations. Herein, by using an accelerative test, the reliability of BGA- and QFN-coated solder joints with an acrylic conformal coating was studied. A thermal cycle test of temperature ranging from −40 °C to +125 °C comprising 837 cycles demonstrated no failures (confidence level of 0.99; <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$R = 4$ </tex-math></inline-formula> ppm/year; <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\beta = 3$ </tex-math></inline-formula> ) to occur due to acrylic conformal-coating application. However, the solder joints analysis indicated that conformal coating causes an excess of deformations on solder joints, compromising their reliability. Therefore, this study suggests that this acrylic conformal coating is not suitable for these electronic components.

Fracture Behavior and Reliability of Low-Silver Lead-Free Solder Joints

Abstract Low-silver solders are increasingly being used because silver improves the tensile strength. In this study, the variation in fracture behavior with silver content in lead-free solder joints was studied using double cantilever beam specimens. Fracture tests were done with solder joints made with Sn-0.7Cu, SACX0307, and SAC305 solder materials. The critical energy release rate for crack-initiation (Gci) of the joint was correlated with the plastic zone just ahead of the precrack tip, intermetallic compound layer thickness, energy dispersive spectroscopy analysis, and scanning electron microscopy based fractography study. The Gci for Sn-0.7Cu solder joint was observed to be significantly higher than the other two solder joints. The fractography study revealed that the failure was ductile for Sn-0.7Cu and a mix of ductile and brittle for the other two solder joints. The extent of the plastic zone ahead of the crack tip, obtained from finite element modeling, was found to be significantly larger and the intermetallic compound layer was relatively thinner for Sn-0.7Cu solder joint compared to the other two solder joints. The ductile failure, significantly larger plastic zone size and thinner intermetallic compound layer resulted in significantly higher Gci for Sn-0.7Cu solder joint.

Experimental and Statistical Study of the Fracture Mechanism of Sn96.5Ag3Cu0.5 Solder Joints via Ball Shear Test.

Ball shear testing is an efficient approach to investigate the mechanical reliability of solder joints at the structural level. In the present study, a series of low-speed ball shear tests were conducted to study the deformation and fracture characteristics of Sn96.5Ag3Cu0.5 solder joints at continuous speeds from 10 μm/s to 200 μm/s. In order to account for randomness, the quantity of tests was repeated for each shear rate. The relationship between mechanical properties and shear speeds was calculated in detail via effective statistical analysis. In addition, by utilizing SEM imaging and ingredient analysis the interfacial effect and fracture mechanism of solder balls were obtained and their fracture mode classified into two types, viz., bulk fracture and interface fracture. Furthermore, by means of statistical analysis and approximate calculation it was proven that bulk fracture balls have greater adhesive powers and reliability compared with interface fracture balls.

New phenomenon: Cellular boundary during the isothermal stage in the Sn-3Ag/(0 0 1)Cu and Sn-3.5Ag/(0 0 1)Cu joint

The Cu6Sn5 intermetallic, which commonly forms at the solder interconnects, is a critical component contributing to the reliability of solder joints. In this paper, the growth behavior during the isothermal stage of Cu6Sn5 grains in the Sn-xAg/(001)Cu (x = 0, 3, 3.5) joint was investigated. A new phenomenon, the Cu6Sn5 with cellular boundary, is found in the isothermal stage of Sn-3Ag/(001)Cu and Sn-3.5Ag/(001)Cu joint, but not found in the Sn/(001)Cu joint. The result suggests that the Ag3Sn play an important role in the formation of the cellular boundary. High-pressure air blowing was used to uncover the morphology of Cu6Sn5 at the isothermal stage. Scanning electron microscopy and electron backscatter diffraction were employed to characterize the morphology and grain orientation, respectively. The result has a great meaning in understanding the growth of Cu6Sn5 during the isothermal stage and improving the reliability of solder joints.

Study of shear locking effect on 3D solder joint reliability analysis

Abstract The fabrication process of WLP (wafer-level packaging) has become more mature, and using the finite element method (FEM) to predict the reliability life of electronic packaging can shorten the WLP design cycles. This study adopted the NSMD (non-solder mask defined) solder joint structure in WLP and used an energy method to predict the solder joint geometry. A fixed mesh size will be determined for the critical region of the solder joint to evaluate the inelastic strain due to the thermal loading in finite element analysis. However, under the influence of the solder geometry of some tested vehicles, especially when the solder contact angle of the lower pad is less than 20 degrees, a shear locking phenomenon will occur at the solder joint. This phenomenon causes abnormal energy transfer. The excessive strain is concentrated on the upper part of the solder joint, resulting in an incorrect estimation of reliability life. Reduced integration can prevent shear locking and has improved strain results compared to full integration. The results of this research demonstrate that the FEM using proper mesh size control and the reduced integration point element in critical regions can deliver accurate reliability life prediction for WLP.