Behavior Of Solder Joints Research Articles

MICROSTRUCTURE EVOLUTION AND MECHANICALBEHAVIOR OF INTERMETALLIC COMPOUNDSIN SOLDER JOINT

Thickness and roughness evolution of the intermetallic compounds (IMC) layer between Sn3.0Ag0.5Cu and Cu substrate are examined under various isothermal aging times at 150 . The effect of the microstructure evolution of the IMC on the mechanical behavior of solder joints is experimentally investigated. The results show that the growth of the IMC follows the Fick’s law that predicts the total mean thickness increases linearly with the square root of the aging time. With the increase of the isothermal aging time, the initial scallop morphology of the solder/IMC interface changes to a more planar type. Both the thickness and roughness of the IMC layer affect the tensile strength and fracture mode of the solder joints. The tensile strength decreases with the increase of either the IMC thickness or the solder/IMC interfacial roughness. With the increase of the isothermal aging time, the IMC thickness increases and the solder/IMC interfacial roughness decreases, and the fracture mode migrates from ductile fracture in bulk solder to brittle fracture in the IMC layer.

Effects of Multiple Reworks on the Accelerated Thermal Cycling and Shock Performance of Lead-Free BGA Assemblies

In this paper, the effect of multiple rework cycles (up to 5×) on reliability of lead-free assemblies is investigated. The test vehicle is designed to capture the reliability impact of rework processes on interconnects of ball grid array (BGA) devices, the solder joints of its adjacent components, and BGA devices of clamshelled devices. The effects on high aspect ratio plated through hole (PTH) vias and microvias are also considered. A variety of BGA packages, such as flip-chip BGA, plastic BGA, and chip array BGA are selected to represent different package technologies and stress levels. The effect of multiple reworks on the behavior of lead-free solder joints during accelerated thermal cycling (ATC) and mechanical shock is extensively explored in this paper. The experimental and analytical findings showed that multiple reworks significantly degraded the ATC performance of the reworked assemblies by up to 50% in terms of characteristic life. It is also found that multiple reworks affect the PTH vias significantly. The combination of increased lead-free rework temperature excursions and high-end printed circuit board (PCB) aspect ratios may incur higher stresses in the PTH copper barrel. Early failures are observed post multiple reworks and the subsequent ATC test due to PTH barrel cracking. However, the effects of multiple reworks are less severe for devices with microvias compared to those with PTH vias. Multiple reworks also cause severe degradation of long-term performance of devices with clamshell designs. With proper control in rework process, multiple reworks have minimum effects on the adjacent BGA solder joints. In addition, failure analysis is performed in order to study the impact of multiple rework on the microstructure of SnAgCu solder joints. It is found that the intermetallic compounds (IMC) layer grows thicker as the number of reworks increases; however, the IMC growth rate and morphology are dependent on the surface finish of the package substrate. The failure mode and failure mechanism after ATC are also compared for different number of rework cycles. The implications of these results for the reliability of lead-free solder joints are discussed in this paper.

Abnormal accumulation and rotation of Sn in Cu-cored Sn solder joints under current stressing

The electromigration behavior of Cu-cored Sn solder joints under a current density of 1.3 × 104 A cm−2 was investigated in this work. Finite element simulation was performed to obtain the distribution of the current density in the solder joint. The Cu core was chosen primarily as the path for current flux, as it has a lower resistivity than Sn. It is found that most morphology changes appeared at the regions where current crowding occurred, demonstrating the consistency of the experimental and simulation results. Taking the special structure into consideration, the Cu core contributed additional electrodes to the solder joint at the interfaces, resulting in abnormal accumulation and rotation of Sn at both the cathode and the anode. The IMC growth was quite slow in the Cu-cored solder joint.

Size and constraint effects on interfacial fracture behavior of microscale solder interconnects

The fracture behavior of solder joints has long been an important issue in the reliability evaluation of electronic components and packages. In this study, experimental and finite element methods were used to characterize the interfacial fracture behavior of “Cu-wire/solder/Cu-wire” sandwich-structured microscale Sn–3.0Ag–0.5Cu solder joints with different diameters (100–575μm) and thicknesses (35–325μm) under quasi-static micro-tension loading, with systematic comparison to Sn–37Pb solder joints. Dynamic stress intensity factors for the microcracks in Cu, Sn–3.0Ag–0.5Cu, and Sn–37Pb under pulse tensile loading and high speed propagation were calculated. The numerical simulation results showed that the interface stress intensity factors KII and KI for the crack set at the solder/IMC interface increased with increasing the thickness of the joints from 35μm to ∼125μm, and thereafter decreased and then became stable. With increasing the diameter of the joints, basically KI increases while KII decreases. Under the quasi-static micro-tension loading, the crack driving forces in Sn–3.0Ag–0.5Cu solder joints are lower than that in Sn–37Pb joints, and this means that fracture is less likely to occur in Sn–3.0Ag–0.5Cu solder joints than in Sn-joints. The rapid expansion of a high hydrostatic pressure region may enhance the mechanical performance of interconnects when the diameter-to-thickness ratios of the joints are very large. As the diameter of the solder joints increases, the evolution of the energy release distribution results in a change of the fracture position and mechanism in the interconnects. Compared to Sn–37Pb solder, the Sn–3.0Ag–0.5Cu lead-free solder shows a lower resistance to rapid crack propagation under constant tensile loading.

Reliability behavior of lead-free solder joints in electronic components

With more consumer products moving towards environmentally friendly packaging, making solder Pb-free has become an urgent task for electronics assemblies. Solder joints are responsible for both electrical and mechanical connections. Solder joint does not have adequate ductility to ensure the repeated relative displacements due to the mismatch between expansion coefficients of the chip carrier and the circuit board. Materials behavior of solder joints involves a creep–fatigue interaction, making it a poor material for mechanical connections. The reliability of solder joints of electronics components has been found playing a more important role in service for microelectronics components and micro-electro-mechanical systems. So many researchers in the world investigated reliability of solder joints based on finite element simulation and experiments about the electronics devices, such as CR, QFP, QFN, PLCC, BGA, CSP, FCBGA and CCGA, which were reviewed systematically and extensively. Synchronously the investigation on reliability of solder joints was improved further with the high-speed development of lead-free electronic packaging, especially the constitutive equations and the fatigue life prediction equations. In this paper, the application and research status of constitutive equations and fatigue life prediction equations were reviewed, which provide theoretic guide for the reliability of lead-free solder joints.

Improved reliability of copper-cored solder joints under a harsh thermal cycling condition

This study simulated the performance of Cu-cored solder joints in microelectronic components subjected to the extreme thermal cycling conditions often encountered in the automobile industry by comparing the thermal cycling behavior of Cu-cored solder joints containing two different coating layers of Sn–3.0Ag and Sn–1.0In with that of a baseline Sn–3.0Ag–0.5Cu solder joint under a severe temperature cycling range of −55 to +150°C. Both Cu-cored solder joints can be considered a potential solution to interconnects in microelectronic semiconductor packages used under harsh thermal conditions on account of their greater resistance to thermal stress caused by the severe temperature cycling than the baseline Sn–3.0Ag–0.5Cu solder joint.

Electromigration analysis of solder joints under ac load: A mean time to failure model

In this study, alternating current (ac) electromigration (EM) degradation simulations were carried out for Sn95.5%Ag4.0%Cu0.5 (SAC405- by weight) solder joints. Mass transport analysis was conducted with viscoplastic material properties for quantifying damage mechanism in solder joints. Square, sine, and triangle current wave forms ac were used as input signals. dc and pulsed dc (PDC) electromigration analysis were conducted for comparison purposes. The maximum current density ranged from 2.2×106A/cm2 to 5.0×106A/cm2, frequency ranged from 0.05 Hz to 5 Hz with ambient temperature varying from 350 K to 450 K. Because the room temperature is nearly two-thirds of SAC solder joint’s melting point on absolute temperature scale (494.15 K), viscoplastic material model is essential. Entropy based damage evolution model was used to investigate mean time to failure (MTF) behavior of solder joints subjected to ac stressing. It was observed that MTF was inversely proportional to ambient temperature T1.1 in Celsius and also inversely proportional to current density j0.27 in A/cm2. Higher frequency will lead to a shorter lifetime with in the frequency range we studied, and a relationship is proposed as MTF∝f-0.41. Lifetime of a solder joint subjected to ac is longer compared with dc and PDC loading conditions. By introducing frequency, ambient temperature and current density dependency terms, a modified MTTF equation was proposed for solder joints subjected to ac current stressing.

Uncertainty analysis of solder alloy material parameters estimation based on model calibration method

Quantification of the uncertainties in the material characterization of solder joint has been one of the major concerns in the microelectronics packaging industry to predict fatigue failure accurately. Therefore, in this study, a model calibration method based on Bayesian approach is proposed to quantify these uncertainties arising in the material parameter estimation of the solder alloy. A specimen is fabricated to this end, which closely simulates solder joint behavior of the actual package under a thermal cycle. Experiment is conducted to examine the deformation by using Moiré interferometry. Viscoplastic finite element analysis procedure is constructed for the specimen based on the Anand model. The uncertainties which include inherent experimental error and insufficient data of experiments are addressed by using the likelihood estimation. Two materials, one being conventional solder of Sn36Pb2Ag and the other the lead-free solder of Sn3.0Ag0.5Cu, are considered to illustrate the approach. As a result, material parameters are identified in the form of credible interval (CI), and the displacements and strains using these parameters are given by the predictive interval (PI). The results suggest that the proposed approach can be a useful tool in the probabilistic estimation of the unknown material parameters of solder joint by accounting for the uncertainties due to the experimental data.

Fracture behavior of Cu-cored solder joints

Copper-cored solder can be regarded as the next-generation solder for microelectronic semiconductors exposed to harsh operating conditions owing to its excellent sustainability under extreme thermal conditions, e.g., in microelectronic semiconductors used in transportation systems. Cu-cored solder joints with two different coating layers, Sn–3.0Ag and Sn–1.0In, were compared with the baseline Sn–3.0Ag–0.5Cu solder. The fracture strength and failure mode were examined using the high-speed ball-pull and normal-speed shear tests. The Cu-cored solder joint with the Sn–1.0In plating layer exhibited the highest ball-pull and shear strengths. In addition, it showed a much lower percentage of interface fracture between the Cu-core and plating layer than the interface fracture percentage in the Sn–3.0Ag plating layer due to the improved wettability between the Cu-core and Sn–1.0In plating layer.

Effects of microstructure and temperature on corrosion behavior of Sn–3.0Ag–0.5Cu lead-free solder

The heterogeneous microstructure of solder could be obtained when cooling rate of the solder joint was not even, which would affect the corrosion behavior of solder during service. The ambient temperature would also affect the corrosion behavior of solder joint. In this paper, the effects of microstructure and temperature on the corrosion behavior of Sn–3.0Ag–0.5Cu (SAC305) lead-free solder were investigated. The various microstructures of SAC305 lead-free solder were obtained by cooling specimens in air and furnace. Compared to the fine-fibrous Ag3Sn phase inside the commercial SAC305 solder, platelet-like Ag3Sn formed as cooling speed decreasing. The polarization behavior of SAC305 solders in 3.5 wt.% NaCl solution was not significantly affected by various microstructures, but sensitive to temperature.

High temperature reliability of lead-free solder joints in a flip chip assembly

The visco-plastic behaviour of solder joints of two models of a flip chip FC48D6.3C457DC mounted on a printed circuit board (PCB) via SnAgCu solder is investigated using Anand's model. While the bumps of one of the models are realistic with 6 μm thickness of intermetallic compound (IMC) at interconnects of solder and bond pads, the other are made up of conventional bumps without IMC at these interconnects. The solder bump profiles were created using a combination of analytical method and construction geometry. The assembled package on PCB was accelerated thermally cycled (ATC) using IEC standard 60749-25. It was found in the result of the simulation that IMC does not only impact solder joint reliability but also is a key factor of fatigue failure of solder joints. The IMC sandwiched between bond pad at chip side and solder bulk is the most critical and its interface with solder bulk is the most vulnerable site of damage. With reference to our results, it is proposed that non inclusion of IMC in solder joint models composed of Sn-based solder and metalized copper substrate is one of the major causes of the discrepancy on solder joint fatigue life predicted using finite element modelling and the one obtained through experimental investigation.

Fracture of Sn-Ag-Cu Solder Joints on Cu Substrates: I. Effects of Loading and Processing Conditions

During service, microcracks form inside solder joints, making microelectronic packages highly prone to failure on dropping. Hence, the fracture behavior of solder joints under drop conditions at high strain rates and under mixed-mode conditions is a critically important design consideration for robust joints. This study reports on the effects of joint processing and loading conditions on the microstructure and fracture response of Sn-3.8%Ag-0.7%Cu (SAC387) solder joints attached to Cu substrates. The impact of parameters which control the microstructure (reflow condition, aging) as well as loading conditions (strain rate and loading angle) are explicitly studied. A methodology based on the calculation of the critical energy release rate, G C, using compact mixed-mode (CMM) samples was developed to quantify the fracture toughness of the joints under conditions of adhesive (i.e., interface-related) fracture. In general, higher strain rate and increased mode-mixity resulted in decreased G C. G C also decreased with increasing dwell time at reflow temperature, which produced a thicker intermetallic layer at the solder–substrate interface. Softer solders, produced by slower cooling following reflow, or post-reflow aging, showed enhanced G C. The sensitivity of the fracture toughness to all of the aforementioned parameters reduced with an increase in the mode-mixity. Fracture mechanisms, elucidating the effects of the loading conditions and process parameters, are briefly highlighted.

Influence of indium addition on electromigration behavior of solder joint

Abstract

Cracking of the Intermetallic Compound Layer in Solder Joints Under Drop Impact Loading

The significant difference between failure modes of lead-containing and lead-free solder joints under drop impact loading remains to be not well understood. In this paper, we propose a feasible finite element approach to model the cracking behavior of solder joints under drop impact loading. In the approach, the intermetallic compound layer/solder bulk interface is modeled by the cohesive zone model, and the crack driving force in the intermetallic compound layer is evaluated by computing the energy release rate. The numerical simulation of a board level package under drop impact loading shows that, for the lead-containing Sn37Pb solder joint, the damage in the vicinity of the intermetallic compound layer initiates earlier and is much greater than that in the lead-free Sn3.5Ag solder joint. This damage relieves the stress in the intermetallic compound layer and reduces the crack driving force in it and consequently alleviates the risk of the intermetallic compound layer fracturing.

Three-dimensional (3D) microstructural characterization and quantification of reflow porosity in Sn-rich alloy/copper joints by X-ray tomography

Abstract In this paper high resolution X-ray tomography was used to characterize reflow porosity in Sn–3.9Ag–0.7Cu/Cu solder joints. The combination of two segmentation techniques was applied for the three-dimensional (3D) visualization of pores in the joints and the quantification on the characteristics of reflow porosity, including pore size, volume fraction and morphology. The size, morphology and distribution of porosity were visualized in 3D for three different solder joints. Since the results are relatively similar for all three, only the results of one joint are presented. Solder reflow porosity was mostly spherical, segregated along the solder/Cu interface, and had an average pore size of 30 μm in diameter. A few large pores (larger than 100 μm in diameter) were present, some of which had lower sphericity, i.e., they were more irregular. The presence of these large pores may significantly influence the mechanical behavior of solder joints.

Characterization of elasto-plastic behavior of actual SAC solder joints for drop test modeling

This paper focuses on the characterization of the elasto-plastic properties of actual Sn–1.0 wt.%Ag–0.5 wt.%Cu (SAC105), Sn–3.0 wt.%Ag–0.5 wt.%Cu (SAC305) and Sn–4.0 wt.%Ag–0.5 wt.%Cu (SAC405) solder joints for drop test modeling. Several actual ChipArray® BGA (CABGA) packages were cross-sectioned, polished and used as the test vehicles. The drop tests were performed with various impact amplitudes using a specially designed mini drop table along with a conventional drop table to generate plastic deformations in the solder joints. The plastic deformations in the solder joints were measured after each drop using microscope imaging in conjunction with the digital image correlation (DIC) technique. The coefficients of the Ramberg–Osgood elasto-plastic model for the solder alloys were extracted from the experimental results using a finite element method (FEM) modeling-based iteration process. An application and validation of the developed model were carried out using pendulum tests.

Effect of geometry on the fracture behavior of lead-free solder joints

Copper bars were soldered along their length with a thin layer of lead-free Sn3.0Ag05.Cu alloy under standard surface mount processing conditions to prepare double cantilever beam (DCB) specimens. The geometry of the DCBs was varied by changing the thickness of the solder layer and the copper bars. These specimens were then fractured under mode-I and two mixed-mode loading conditions. The initiation strain energy release rate, G ci , increased with the relative fraction of mode-II, but was unaffected by the changes in either the substrate stiffness or the solder layer thickness. However, the steady-state strain energy release rate, realized after several millimeters of crack growth, was found to increase with the solder layer thickness at the various mode ratios. The crack path was found to be influenced by mode ratio of loading and followed a path that maximizes the von Mises strain rather than maximum principal stress. Finally, some preliminary results indicated that the loading rate significantly affects the G ci .

Reliability and Simulation of Lead-Free Solder Joint Behavior in 3D Packaging Structure

This paper deals with behavior of lead-free solder joint applied in 3D electronic packaging structure. There are plenty of factors which influences reliability and life-time of solder compound connection. Type of substrates, selection of materials and process arrangement are the main areas to investigate. The content consists from three main parts. The first deals with introduction of designed test pattern, material, and process arrangement and applied testing methods. Second part continues with ANSYS software simulation of solder interconnections during thermo-mechanical stress tests and third part is aimed to experimental evaluation of results in the comparison to simulated results.

Mechanical behavior of solder joints under dynamic four-point impact bending

Reliability of solder joints under drop impact loading is important to mobile electronic products. In this paper, dynamic four-point impact bending tests of board level electronic packages are carried out to investigate mechanical behavior of solder joints. In the test, strain gauges, a high speed camera and the digital image correlation method are used to acquire strain and deflection of the printed circuit board (PCB). After validated by the test data, a finite element model of the dynamic four-point impact bending test is used to obtain strain and stress in the solder joints. Then, failure predictions of the solder joints are made by strain index, and the predictions are compared with the experimental observations. Furthermore, a strain rate dependent Johnson–Cook material model and rate independent elastic–plastic model of lead-free solder are used to investigate the effect of strain rate on behavior of solder joints under drop impact loading. We find that the material model has insignificant influence on the deflection of the PCB during the drop impact but severely affect the stress and strain in solder joints. The rate independent elastic–plastic solder material model always underestimates the stress and overestimates the strain of the solder joints. The index of equivalent plastic strain computed by the strain rate dependent Johnson–Cook model can predict more realistic failure behavior of the solder joints.

Stress relaxation behavior of Cu/Sn/Cu micro-connect after electrical current

The effect of electromigration on stress relaxation behavior of pure tin solder joints was investigated. It was found that the stress relaxation rate was accelerated significantly after the sample was subjected to current stressing. The accelerating effect increased with the current stressing time. Measurements of the activation energy and stress exponent suggested that the dominant mechanism of the stress relaxation of pure tin solder joint went from dislocation climb to grain boundary diffusion after electromigration. As a result of grain boundary diffusion and sliding, grain boundary grooves were observed on the surface of the tin solder joints after electromigration. The groove was associated with the divergence of vacancy concentration at the grain boundaries. The vacancy concentration at the grain boundaries, which increased with the current stressing time, promoted the atomic diffusion along the grain boundaries, resulting in a higher stress relaxation rate.