Solder Joint Thickness Research Articles

Machine learning aided modelling of thermomechanical fatigue of solder joints in electronic component assemblies

Printed board assemblies, i.e. components soldered on printed circuit boards (PCBs), are exposed to thermal cycles responsible for fatigue cracking of solder joints as a result of thermal expansion mismatch between the constituting elements. Advanced finite element simulations are performed using a traction–separation law to represent the cracking process and a temperature-dependent elasto-viscoplastic model for the joint response. Predictions are successfully assessed towards machine learning processed experimental data. In particular, the high sensitivity of thermal ageing reliability to geometric dimensions and solder joint thickness is properly captured. Additional parameters, related to the PCB substrate, are also studied, opening new avenues towards design optimization.

Reliability Enhancement of a Power Semiconductor With Optimized Solder Layer Thickness

This article deals with the reliability of a power semiconductor exposing to the severe thermal stresses. The importance of solder joint thickness on the power semiconductor's useful lifetime is demonstrated in this article. Solder layer thickness has knock on effects both on the creep accumulated strain and thermal characteristics of the power semiconductors. Since, these effects are in contradictory of each other, a trade-off seems to be essential to optimize the solder layer thickness. Thereby, thermo-mechanical behavior of a discrete power semiconductor under the thermal mission profile was simulated and the results were integrated to the actual conditions. The simulation results reveal that after thermal cycling, some creep strain is produced in the solder layer especially at the corners. The thinner the solder joint was, the greater accumulated creep strain was observed leading to the faster degradation. On the contrary, the thicker the solder layer was, the larger thermal resistance was observed leading to the higher junction temperature. Accordingly, the article is concentrated on optimizing the solder layer thickness based on these two issues. The scanning electron microscope micrographs, the EDS maps and X-ray diffraction analysis were also taken to indicate the solder layer thickness effects on the number of voids and their propagations in the different solder layer thicknesses. The experimental tests validate the expected results in the extracted simulations.

Advanced Statistical Model Calibration to Determine Manufacturing-Induced Variations of Effective Elastic Properties of SAC Solder Joints in Leadless Chip Resistor Assemblies

The unknown statistical distributions of two effective elastic properties of Sn-3.0Ag-0.5Cu solder joint of leadless chip resistors (LCRs), induced by an assembly condition, are determined by the advanced statistical model calibration. Two key elements are involved in the model calibration: an uncertainty propagation (UP) analysis and an optimization process using a calibration metric. In this paper, the UP analysis utilizes an advanced approximate integration method, which allows us to take into account the statistical variations of six additional known input variables, including die thickness, solder joint height, termination length, and thickness and elastic moduli of a printed circuit board. The cyclic bending test results of LCR assemblies are used in conjunction with the maximum-likelihood metric to obtain the statistical distributions of the effective properties. The cycles-to-failure distribution of the identical LCR assemblies subjected to a different loading level is predicted accurately by the calibrated model, which corroborates the validity of the proposed approach.

Effect of Solder Joint Thickness on Intermetallic Compound Growth Rate of Cu/Sn/Cu Solder Joints During Thermal Aging

The sandwich structure Cu/Sn/Cu solder joints with different thicknesses of the solder layers (δ) are fabricated using a reflow solder method. The microstructure and composition of the solder joints are observed and analyzed by scanning electron microscopy (SEM). Results show that the thickness of intermetallic compound (IMC) and Cu concentration in the solder layers increase with the decrease of δ after reflow. During thermal aging, the thickness of IMC does not increase according to the parabolic rule with the increase of aging time; the solder joint thickness affects markedly the growth rate of IMC layer. At the beginning of thermal aging, the growth rate of IMC in the thinner solder joints (δ ≤ 25 μm) is higher than that in the thicker ones (δ ≥ 30 μm). The growth rate of IMC (δ ≤ 25 μm) decreases in the thinner solder joints, while increases in the thicker solder joints (δ ≥ 40 μm) and is nearly invariable when the δ equals to 30 μm with aging time extending. The growth rate of IMC increases first and then decreases after reaching a peak value with the increase of δ in the later stage during aging. The main control element for IMC growth transfers from Cu to Sn with the reduction of size.

Optimization of thermo-mechanical reliability of solder joints in crystalline silicon solar cell assembly

A robust solder joint in crystalline silicon solar cell assembly is necessary to ensure its thermo-mechanical reliability. The solder joint formed using optimal parameter setting accumulates minimal creep strain energy density which leads to longer fatigue life. In this study, thermo-mechanical reliability of solder joint in crystalline silicon solar cell assembly is evaluated using finite element modelling (FEM) and Taguchi method. Geometric models of the crystalline silicon solar cell assembly are built and subjected to accelerated thermal cycling utilizing IEC 61215 standard for photovoltaic panels. In order to obtain the model with minimum accumulated creep strain energy density, the L9 (33) orthogonal array was applied to Taguchi design of experiments (DOE) to investigate the effects of IMC thickness (IMCT), solder joint width (SJW) and solder joint thickness (SJT) on the thermo-mechanical reliability of solder joints. The solder material used in this study is Sn3.8Ag0.7Cu and its non-linear creep deformation is simulated using Garofalo-Arrhenius creep model. The results obtained indicate that solder joint thickness has the most significant effect on the thermo-mechanical reliability of solder joints. Analysis of results selected towards thermo-mechanical reliability improvement shows the design with optimal parameter setting to be: solder joint thickness — 20μm, solder joint width — 1000μm, and IMC thickness — 2.5μm. Furthermore, the optimized model has the least damage in the solder joint and shows a reduction of 47.96% in accumulated creep strain energy density per cycle compared to the worst case original model. Moreover, the optimized model has 16,264cycles to failure compared with the expected 13,688cycles to failure of a PV module designed to last for 25years.

Assessment of Constitutive Properties of Solder Materials Used in Surface-Mount Devices for Harsh Environment Applications

To perform a thermomechanical simulation of surface-mount devices (SMD), relevant solder constitutive properties are needed. This paper reviews some promising Pb and Pb-free solders that are candidates to harsh environment applications. It presents the discrepancy in their common mechanical properties reported in the literature. The mechanical test systems, solder microstructure under test, and the test conditions (temperature and strain rate) are among the major causes for the discrepancy. The use of such variations in the solder thermomechanical properties as input data for simulation studies may cause reliability prediction less meaningful. A novel and reliable test methodology to extract relevant solder properties for surface mount applications is presented in this paper. To fulfill the methodology requirements, lap shear tests with accurate control of the reflow profile and dimensionally accurate solder joint thickness need to be conducted to reproduce the SMD solder joint microstructures for determining solder joint properties for elastic, plastic, and creep model simulations. Two case studies with high-Pb and Pb-free solders are presented. Time-independent yield strength and ultimate tensile strength data obtained from our studies were found to be higher than the reported literature data. This methodology can be extended for other solder material systems with any surface finish plating materials.

Investigation on the effect of Sn grain number and orientation on the low cycle fatigue deformation behavior of SnAgCu/Cu solder joints

Generally, lead-free solder joints in electronic device are near single or multi crystalline rather than poly crystalline, and the fatigue failure behavior of solder joints is greatly influenced by the solder joint thickness. In this study, the grain number and orientation of solder joints with four solder thicknesses (0.1, 0.2, 0.3, 0.6 mm) were analyzed and the effect of that on the low cycle fatigue deformation behavior of solder joint was also investigated. The orientation analysis was conducted by means of electron backscatter diffraction and the fatigue test was performed under room temperature with a fixed frequency of 1 Hz. Moreover, the deformation evolution on side surface of solder joint was observed by utilizing scanning electron microscope. It is shown that there is no preferred solidification orientation for lead-free solder joints. However, grains in one joint seem to have a similar orientation when the solder thickness is <0.3 mm. Solder joint grain number increases with solder thickness. Besides, the slip direction and slip band density are different for grains with different orientations. Consequently, the deformation on both sides of the grain boundary is inhomogeneous and the fatigue crack is prone to initiate and propagate in this area.

GS27 改良型ラップジョイント試験片によるはんだ接合部のせん断疲労試験方法

This paper presents the shear fatigue test method of the solder joint by the advanced lap joint specimen. The advanced lap joint specimen was developed for purpose of stiffness improvement and manufacturing of solder joint thickness equivalent to an actual solder joint of an electric board. The usefulness of the advanced lap joint specimen was checked by FEM analysis and the tensile test, and the test method was considered.

Size effects of lead-free solder joint thickness under shear creep based on micro-electrical-resistance strain

Single shear lap creep specimens with a 1 mm2 cross sectional area between thin copper strips were developed and fabricated using a lead-free solder (Sn–3.5Ag) to quantify their electrical resistance. The electrical-resistance strain of solder joints with different thicknesses (0.05, 0.10, 0.15, 0.20, 0.25, 0.30, 0.40, 0.50 mm) were measured by a tailor-made in situ micro-electrical-resistance measurement. The results showed that the solder joints with 0.25 mm thickness yielded a minimum electrical-resistance strain and the longest creep life. Thicker solder joints produced a larger electrical-resistance strain and a shorter lifetime if the thickness >0.25 mm. In contrast, thinner solder joints produced a larger electrical-resistance strain and a shorter lifetime if the thickness <0.25 mm. A quantificational relationship between the electrical-resistance strain and the solder joint thickness was obtained according to the experimental data. On the other hand, solder joints with different thicknesses (corresponding to those used in the experiment) were studied by the finite element method (FEM). The results showed that the creep strain of solder joints was the lowest with 0.25 mm thickness, and the creep strain increased with the increment in the thickness if the thickness >0.25 mm but decreased with the increment in the thickness if the thickness <0.25 mm. The quantificational relationship between the creep strain and solder joints thickness was obtained according to the data of FEM. Finally, the quantificational relationship between the creep strain and the electrical-resistance strain was obtained by combining the experimental data and the FEM data.

An evaluation of the lap-shear test for Sn-rich solder/Cu couples: Experiments and simulation

The lap-shear technique is commonly used to evaluate the shear, creep, and thermal fatigue behavior of solder joints. We have conducted a parametric experimental and modeling study, on the effect of testing and geometrical parameters on solder/copper joint response in lap-shear. It was shown that the farfield applied strain is quite different from the actual solder strain (measured optically). Subtraction of the deformation of the Cu substrate provides a reasonable approximation of the solder strain in the elastic regime, but not in the plastic regime. Solder joint thickness has a profound effect on joint response. The solder response moves progressively closer to “true” shear response with increasing joint thickness. Numerical modeling using finite-element analyses were performed to rationalize the experimental findings. The same lap-shear configuration was used in the simulation. The input response for solder was based on the experimental tensile test result on bulk specimens. The calculated shear response, using both the commonly adopted far-field measure and the actual shear strain in solder, was found to be consistent with the trends observed in the lap-shear experiments. The geometric features were further explored to provide physical insight into the problem. Deformation of the substrate was found to greatly influence the shear behavior of the solder.

Damage accumulation under repeated reverse stressing of Sn-Ag solder joints

To better understand the effect of repeated reverse stress in solder joints, a new testing method was developed. Tin-silver solder joints were fabricated, constrained between Cu blocks, and then subjected to repeated shear loading in a tensile tester. Constant strain amplitudes were applied to simulate service conditions. However, large loads were used to accelerate the damage accumulation. Microstructural features of the damage were very similar to those found with studies on thermomechanical fatigue (TMF) of small, single shear lap samples. Concentrated-shear banding or striations were observed to form along Sn dendrites. The load behavior of the solder with each cycle and during hold times at the extreme strain amplitude was consistent with damage accumulating with each successive cycle. Effects of strain amplitude, hold times at the stress extremes, number of cycles, and solder-joint thickness were found to play significant roles on the stress-strain behavior and surface damage.

A dynamic model for solder and the problem of accelerated life testing

Solder joint reliability under thermal cycling is a key problem in electronic packaging. Accelerated life testing (few cycles, larger temperature excursions) is often a practical necessity in predicting fatigue life in field environments (many cycles, smaller temperature excursions). Complex solder behavior with marked temperature dwell and cycle time influence at slower frequencies makes this a difficult problem. A dynamic model is presented which couples the effect of instability of coarsened grain shear band evolution in microstructure with the change in macroscopic constitutive behavior. Key features of the model include effects of shear band thickness compared with total solder joint thickness, pertinent to small scale design, and frictional resistance at slow deformation rates. Model correlation with test data is discussed and applied to the accelerated life test design.

Investigation of high-concentration photovoltaic cell packages after three years field service

Abstract As part of a controlled, step-wise development program, silicon cells from an early production run were assembled into packages and installed in a prototype electrical module. After three years field service, the module was disassembled and the cell packages were examined in detail. Although all 48 cell packages were still functional, significant cracking was discovered in the solder layers between ceramic stand-offs and copper heat-spreaders. A simplified analysis of thermal fatigue as a cyclic creep process predicts useful lives of about twelve years for cell packages of this configuration and solder joint thickness.

Parameters affecting thermal fatigue behavior of 60Sn-40Pb solder joints

Solder joints in electronic packages experience cyclical thermally induced strain when temperature fluctuations are encountered in service. This study investigates three parameters that affect the microstructure and therefore the thermal fatigue behavior of 60Sn-40Pb solder joints. These parameters are: 1) the effect of a tensile component in thermal fatigue, 2) solder joint thickness variations, and 3) hold time variations at the elevated temperature portion of the thermal cycle. Solder joints were thermally fatigued in a tension/compression deformation mode. Cracks developed both in the interfacial intermetallic layer (early in thermal fatigue) and in the coarsened regions of the microstructure of the solder joint (after many more cycles). The effect of joint thickness on solder joints thermally fatigued in shear was also explored. Solder joint thickness was found not to significantly affect fatigue lifetimes. The effect of an increase in the hold time at the elevated temperature portion of the thermal fatigue cycle was also investigated. It was found that time spent at the high temperature end of the fatigue cycle does not determine solder joint lifetime, rather it is the combination of the amount of deformation induced during thermal fatigue in concert with the elevated temperature.