Die Stress Research Articles

The optimization design of pre-forming for forging automobile parts using Taguchi method based grey relational analysis

This study used finite element analysis software DEFORM-3D to analyze the preform formability and die stress in the pre-forming process of forging automobile transmission components. The preform and die materials used are AISI- 5120 alloy steel and JIS-SKD61 mould steel, respectively. This research investigated the phenomenon of folding defects in the preform at the end of the final forging process. First, as the die shape under the pre-formed channel was designed to change, its die shape was close to the final forged hole shape. Then, three quality characteristics were optimally analyzed by the Taguchi method for pre-formed passes, namely, “maximum tensile stress of the upper die, ”“maximum stretch stress of the lower die, ” and “ the degree of filling of the preform in the die cavity.” Then, this study applied grey correlation analysis to identify the experimental parameter configuration that could achieve a balance between the three quality characteristics, as compared with the results obtained by the Taguchi method. The results show that the lower die after the design change of the pre-forming pass can indeed improve the phenomenon of folding defects in the preform. In terms of the overall experimental data, the experimental data obtained by grey correlation analysis can balance the experimental results of the Taguchi method based on the optimization of the three quality characteristics. The experimental result of this set of parameters not only obtained the maximum tensile stress value of 716.614 MPa in the upper die but also considered the degree of the preform filling and maximum tensile stress in the lower die. The two values were 102.56% and 836.843 MPa. Although these two values are not optimal, compared to the data obtained by the Taguchi method, the preform was filled completely in this case. The maximum tensile stress value of the lower die is also the lowest.

Investigation on Die Crack in a Stacked Die Package Using Finite Element Analysis

Die crack is one of the problems in stacked die semiconductor packages. As silicon dies become thinner in such packages due to miniaturization requirement, the tendency to have die crack increases. This study presents the investigation done on a die crack issue in a stacked die package using finite element analysis (FEA). The die stress induced during the package assembly processes from die attach to package strip reflow was analyzed and compared with the actual die crack failure in terms of the location of maximum die stress at unit level as well as strip level. Stresses in the die due to coefficient of thermal expansion (CTE) mismatch of the package component materials and mechanical bending of the package in strip format were taken into consideration. Comparison of the die stress with actual die crack pointed to strip bending as the cause of the problem and not CTE mismatch. It was found that the die crack was not due to the thermal processes involved during package assembly. This study showed that analyzing die stress using FEA could help identify the root cause of a die crack problem during the stacked die package assembly and manufacturing as crack occurs at locations of maximum stress. The die crack mechanism can also be understood through FEA simulation and such understanding is very important in coming up with robust solution.

Analysis of stress pins for the local prestressing of cold forging tools

The trend towards lightweight design leads to an increasing demand for sophisticated part geometries with high functional integration. In order to use the advantages of cold forging regarding the time- and resource-efficient production of high-quality parts, high local stresses causing fatigue failure in geometrically complex tools have to be controlled. The objective of this manuscript is to analyse the use of stress pins for a local influence on the stress state within forging, especially in non-circular symmetrical cold forging dies. For this purpose, a closed-die forging process for elliptical parts is designed and analysed regarding the die stresses. Distinct areas with local compressive and tensile stresses occur in the process. To counteract the tensile stresses critical for fatigue failure, the effect of stress pins pressed into the die creating a local prestress is analysed. Around the pins, compressive radial and tensile tangential stresses occur. While large pin diameters, interferences and close positioning to the tensile area lead to an increasing prestressing effect, too high values of these parameters cause a detrimental superposition of tensile process and pin stresses. If used correctly, there is high potential to improve the stress state and tool life especially for locally stressed complex tools.

Die stress analysis and improvement of the welding valve fastener in multi-stage forging

This study explores the multi-stage cold forming die of a welding valve fastener using simulation software. It is possible to understand the various stress intensities of the die core bore and the corresponding distributions during each forging stage so as to improve the service life of the die. These stresses include radial stress, axial stress, hoop stress, and maximum principal stress, as well as the different types of stresses that could cause different fractures of the die core. Therefore, it is necessary to use different die design methods to improve the fracture issues for different die cores. For example, shrink fit can be used between the die core and die case. By adjusting the size of the shrink fit, tensile hoop stress can be converted into compressive hoop stress, which can avoid the generation of axial cracking of the die during the forging formation. In addition, drastic changes in axial stress caused by the stress concentration on the die core can yield a transverse crack of the die core. Thus adopting preventative measures by split such a stress concentration into two sections reduces the drastic changes in axial stress on that section.

Finite Element Modeling of Shrink-Fit Design for Improvement of Die-Service Life in Hot Forging Process of a Bevel Gear

In a hot forging process, die-service life is one of the most important factors affecting the production cost. This paper illustrates about a shrink-fit design used to improve the die-service life in the hot forging process of a bevel gear. In this process, there are two-operation steps of forming, preform and finishing steps. The major problem was that the forging die in finishing step prematurely fractured at small fillet radius due to mechanical fatigue. In order to eliminate the problem, the forging die was redesigned to shrink fit into the optimum shrink ring. Furthermore, Finite Element Modeling (FEM), which was set up and verified by the existing process, was used as a tool to study the effect of a shrink-fit ratio and diameter of a shrink ring on the die stress. The FEM results showed that both parameters influent on the die stress directly and the maximum principle stress of the new design at the fracture area significantly reduces because of pre-compression stress from the shrink ring. Finally, the FEM result was proved by experiment in the actual process and then it was found that the die-service life is improved approximately 17 times when comparing to the initial design.

Improving Forgings Formability by Warm Complexing Processes

Hand-tool is one of the important products which are widely used in manufacturing industry as well as in human life. The torx is one kind of hand-tools which is considered as a case study in this research. There are two trial preforms designed to analyze their influence on forgings formability. Firstly, DEFORM-3D software was used to simulate and analyze forming processes of different preform shapes for observing the formability of each trial preform and defects during the forming processes. Die stress analysis was also conducted to investigate the die stress of the forming stages in different forming processes. Secondly, according to the formability and the die stress, the most suitable forming processes and preform were selected for actual experiments. Finally, to verify the simulation results accuracy, the feature matching and hardness test were performed. The result of this research show that appropriate forming processes and preform applied improve the quality of product compared to original product.

Plastic Flow Analysis in Hot Aluminum Extrusion of Asymmetric Parts

In this study, a design guideline for the die cavity and die bearing length distribution is proposed during an extrusion process of an asymmetrical thin sheet, such as clips of cell-phones. The plastic flow pattern of the billet inside the die cavity is analyzed using a commercial finite element package “DEFORM 3D”. The Module of Die Stress Analysis in the finite element software “DEFORM 3D” is also used to simulate the stress, strain, and the displacement distributions of the die during extrusion of clips of cell-phones. The extrusion load, the stress and strain distributions of the die, the temperature distribution, and thickness distribution of the extruded product are investigated. Furthermore, hot extrusion experiments using A6061 as the specimen are executed. The experimental results of temperature, thickness distribution of the product, extrusion force, etc, are compared with the analytical values to verify the validity of the proposed die design guideline.

Multi-step forming simulation and experiment of swing arms for torsion beam

Numerical simulations of bending, performing, and hydroforming processes of automotive swing arms for torsion beam were conducted using finite element analysis. The process parameters such as bending radius, mandrels, loading paths, and calibration pressure were optimized in the simulation. In addition, closing force was predicted by die stress–strain analysis using the elastic finite element method. Subsequently, the bending, performing, and hydroforming experiments were performed based on the simulation results. The experimental results indicated that swing arms could be fabricated when the bending radius, calibration pressure, and closing force would be of 120 mm, 150 MPa, and 15,000 kN, respectively. Finally, swing arms with high thickness uniformity were obtained and the thickness distribution between numerical and experimental results was well consistent.

Formation ability welding seams and mechanical properties of high strength alloy AA7075 when extrusion hollow square tube

Porthole die extrusion has a great advantage in the forming of hollow section of aluminum alloy tube. The proposed paper aims at the development of an extruding seam square tube of AA7075 high strength aluminum alloy by simulation and experiment. There are several factors related to extrusion load such as: billet temperature, billet dimensional, flow stress, die cavity and product geometry. The extrusion loads and die stresses for a hot extruding square tube of AA7075 with respect to different product have been investigated. This study used DEFORM 3D software based on finite element analysis to simulate two different types of hollow extruding die configurations in the square tube extrusion process, namely: Type I welding position at the corners of the square tube and Type II on the side faces. The welding strengths of these two kinds of extruded tube are studied by expanding test and tensile test. The die life of these two extrusion die components are studied based on the die stress and die wear information. The results of FE analysis are compared with those experiments.

The die stress and forging streamline analysis of stainless automotive battery fastener

The study aims at the stainless automobile battery fastener forging to perform a series of forming simulations and experimental verifications. The fastener forging analysis regarding effective stress, effective strain, velocity field, forging streamline, die effective stress and forging force has been explored by a finite element method simulation and approved by experimental results. Industrial application of mass production has been shown well too. It is noted that the die stress and the forging streamline analysis are of great importance in the mould and die design. The realistic die situation can prove the position which occurs when the maximum die stress simulated by the finite element method, and the forging streamline simulation can compare with the experimental results.

Application of a Ag Ductile Layer in Minimizing Si Die Stresses in LDMOS Packages

Lateral Diffused Metal Oxide Semiconductors (LDMOS) normally have a Cu-W flange, whose CTE is matched to Si. Low cost Cu substrate material provides 2X high thermal conductivity, and along with a AuSi eutectic solder is recommended for optimal thermal performance. However, the CTE mismatch between Cu and Si can lead to failure of the semiconductor as a result of die fracture, due to thermal stresses developed during the soldering step of the manufacturing process. Introducing a Ag ductile layer is very important in minimizing such thermal stresses and preventing catastrophic failure of the semiconductor. Ag is a ductile material electroplated on the Cu substrate to absorb stresses developed during manufacturing due to the CTE mismatch between Si and Cu. The Ag layer thickness affects the magnitude of the resulting thermal stresses. This study attempts to measure the yield strength of the Ag layer, and examines the optimal layer thickness to minimize die stresses and prevent failure. The yield stress of the ductile layer deposited on a Cu flange was measured by nanoindentation. The Oliver and Pharr method was applied to obtain modulus of elasticity and yield depth of Ag. A finite element analysis of the package was performed in order to map die stress distribution for various ductile layer thicknesses. The analysis showed that increasing the ductile layer thickness up to 0.01 - 0.02 mm, decreases the Si die stresses.

CMOS Sensor Arrays for High Resolution Die Stress Mapping in Packaged Integrated Circuits

This paper reports the design, calibration and application of multiplexed arrays of piezoresistive field-effect transistor stress sensors fabricated in a standard complementary-metal-oxide semiconductor (CMOS) process. Two complementary arrays of 256-current mirror sensor cells provide high spatial density stress mapping with approximately 300 pts/mm2 using only a 1.5 μm process. The arrays are sequentially scanned by an on-chip counter, producing efficient stress measurement, and the sensors resolve normal and shear stresses on the surface of the die with resolution below 1 MPa. The CMOS sensor chips have been used to map stress over a large portion of the die in chip-on-beam and encapsulated chip-on-beam samples, as well as a ceramic dual-in-line package with its cavity filled with underfill material. Finite-element simulation results correlate well with the measured stress distributions. The experimental results from these chips are used to validate finite-element simulation models, and the array designs can be used as subarrays in much larger test chips.

A Process Dependent Warpage and Stress Model for 3D Packages Considering Incoming Die/Substrate Warpage and Assembly Process Impacts

Warpage control and die stress are critical for 3D package assembly. Currently most of the warpage studies, either measurements or simulations, are mainly evaluated at the end-of-line (EOL) for the finished package. However, for 3D packaging such as multiple die stacking or through-silicon-via (TSV), the interaction between warpage and assembly process steps becomes far more important because: (1) Warpage at each single process step will impact the yield of the next process step. (2) Process induced warpage will accumulated and affect the final end-of-line package warpage, and (3) Die stress in the end-of-line package is highly dependent on the incoming die alone residual stress and the assembly process flow. It is required to develop capability to evaluate warpage at each process step following the process flow. In this paper, we use a 2-die TSV package as a test vehicle to collect warpage data at each assembly step. A process dependent finite element model (FEM) method is developed which is capable of capturing the process induced warpage and stress at each process step based on the process condition. Shadow moire is used to measure warpage data at following check points: (1) Incoming bare dies and bare substrate warpage; (2) Die 1 warpage after underfilled on substrate; (3) Die 2 warpage after TCNCP on Die 1; (4) Completed package after molding (end-of-line). Element birth and kill approach is developed in the FEM model to simulate the assembly process flow. In each process step, only the presented components and materials in the model are active while the others are killed. When new parts or materials are added through the assembly flow, they are activated in the model and the related process condition is applied for each step. Using this process dependent modeling approach, excellent correlation of the model results with the actual measured warpage data is achieved at each process step. One factor often been ignored is the incoming bare die warpage and bare substrate warpage effect. They are often assumed flat and stress free. However, as die and substrate become thinner and thinner, their incoming initial warpage or residual stress plays an important role. In this paper, the measured incoming substrate and die warpage data are presented, their impacts to the warpage and stress evolution through the process flow are evaluated. A novel method is also developed in the simulation model which is able to study the incoming die/substrate warpage and residual stress effect.

Groove Geometry and Mold Shrinkage Effects on Die Stress in Flip Chip Molded BGAs (FCMBGA)

Exposed die flip chip molded BGA (FCMBGA) packages are preferred for their improved thermal performance and reduced system cost. In this package type, mold compound replaces the traditional capillary underfill and also provides a better stiffening option for the package without the need for additional structural support such as lid and/or stiffening ring. In addition, it allows better utilization of the board real estate as the passive components can be placed closer to the die. Groove or an undercut is the shape of the mold around the exposed die that is formed during the molding process. To ensure a mold-free top surface of the die, a seal (soft insert) that has a larger surface area than the die is being used to cover the die top surface. This larger portion of the seal outlines the groove geometry when it is compressed on top of the die. Seal size can be designed to establish certain groove geometry. Thus, it is important to characterize/understand the effects of the groove geometry as it is a design parameter and can be adjusted to create more robust molded packages. In this study, specific groove width and depth values for various package configurations are investigated using finite elements analysis, FEA. Initially, a detailed finite element model is prepared and warpage simulation is performed. Model correlation to the actual Shadow Moiré is obtained. Then, using the correlated finite element model, die back side stress and shear stresses, where die faces mold compound, are obtained for a thermal cool down simulation from the molding temperature. Mold compound shrinkage is also considered by using an adjusted thermal expansion coefficient value. As a validation study, a test mold chase/tool for varying insert sizes has been designed and, molded packages with different groove geometries and different mold compounds were build. Using the correlation between the test data and the simulation results an order of importance (based on the groove geometry parameters and mold compound's material properties) will be presented which then, can be used as a design guideline to change the groove geometry to produce more robust molded packages.

Stress Analysis and Failure Mechanism of H13 Steel Hot-Forging Die for Automobile Engine Crankshaft

The hot forging process of automobile engine crankshaft was simulated in the present study on the basis of the finite element software DEFORM-3D. The materials of die and crankshaft workpiece are H13 steel and L1438 steel, respectively. The material flow in the die, contact pressure distribution and temperature distribution of the die were investigated. The interaction forces between the workpiece and the dies were mapped onto die faces to serve as load boundary conditions for die stress analysis. The results showed that the die stress reduces with increasing temperature and increases with increasing strain rate. Such behavior is likely associated with flow stress of the workpiece, which reduces with increasing temperature and increases with increasing strain rate. High flow stress results in high die stresses and hence more easily leads to die failure.

Characterization of Compressive Die Stresses in CBGA Microprocessor Packaging Due to Component Assembly and Heat Sink Clamping

Microprocessor packaging in modern workstations and servers often consists of one or more large flip chip die that are mounted to a high performance ceramic chip carrier. The final assembly configuration features a complex stack up of flip chip area array solder interconnects, underfill, ceramic substrate, lid, heat sink, thermal interface materials (TIMs), second level ceramic ball grid array (CBGA) solder joints, organic printed circuit board, etc., so that a very complicated set of loads is transmitted to the microprocessor chip. Several trends in the evolution of this packaging architecture have exacerbated die stress levels including the transition to larger die, high coefficient of thermal expansion (CTE) ceramic substrates, lead free solder joints, higher levels of power generation, and larger heat sinks with increased clamping forces. Die stress effects are of concern due to several reasons including degradation of silicon device performance (mobility/speed), damage that can occur to the copper/low-k top level interconnect layers, and potential mechanical failure of the silicon in extreme cases. In this work, test chips containing piezoresistive stress sensors have been used to measure the buildup of mechanical stresses in a microprocessor die after various steps of the flip chip CBGA assembly process. The utilized (111) silicon test chips were able to measure the complete three-dimensional stress state at each sensor site being monitored by the data acquisition hardware. Special test fixtures were developed to eliminate any additional stresses due to clamping effects. The developed normal stresses are compressive (triaxial compression) across the die surface, with significant in-plane and out-of-plane (interfacial) shear stresses also present at the die corners. The compressive stresses increase with each assembly step (flip chip solder joint reflow, underfill dispense and cure, and lid attachment). The experimental observations from this study show clearly that large area array flip chips are subjected to relatively large compressive in-plane normal stresses after solder reflow. We also observed that the majority of the die compressive stress is accumulated during the underfilling assembly step. Typical increases in the stress magnitude were on the order of 300% (relative to the stresses due to solder joint reflow only). As a general “rule of thumb,” approximately two-thirds (∼66%) of the final die stress magnitudes were observed to be developed during the underfill dispense and cure, with the second largest contribution coming from the die attachment, and the smallest contribution coming from lid attachment. The experimental test chip stress measurements were correlated with finite element simulations of the packaging process. A sequential modeling approach has been utilized to predict the build-up of compressive stress. The utilized method incorporates precise thermal histories of the packaging process, element creation, and nonlinear temperature and time dependent material properties. With suitable detail in the models, excellent correlation has been obtained with the sensor data throughout all packaging processes. Finally, CBGAs with the stress sensing chips were soldered to organic printed circuit board (PCB) test boards. A simulated heat sink loading applied, and the stresses were measured as a function of the clamping force. Compressive stress changes of up to − 60 MPa were observed for a 1000 N applied clamping force. The experimental test chip stress measurements were correlated with finite element simulations of the clamping process. With suitable detail in the models, excellent correlation has been obtained for the stress changes occurring during simulated heat sink clamping.

The Model for Coupled Simulation of Thin Profile Extrusion

The paper presents the latest development of the numerical model for extrusion of industrial profiles having complex shapes. The model provides the material flow analysis coupled with mechanical and thermal problems in the tooling set. The simulation predicts possible shape deterioration due to uneven material flow through the bearing zone and helps to equalise it by means of optimisation of the bearing design, chamber and feeding channels. The locations of welding zones in the die are clearly predicted. It allows to modify the die design for better welding conditions and to provide optimal location of welding seams in the product. Die stress analysis shows the ways to extend the tool life by means of reducing of fatigue failure and selecting proper die materials as well as to correct the influence of the die deformation on the material flow. The described model is implemented in an especially dedicated program QForm-Extrusion that effectively simulates the extrusion of hollow and solid profiles with very high elongation ratios. The experimental verification of the model is illustrated by model and industrial experiments as well as by series of case studies performed in production environment.

Analysis for Fracture Behavior of Compactor Grinding Tooth Forging Die Based on Finite Element Method

For forging die fracture behaviors during the actual forging process of compactor grinding tooth, the fully forging process has been simulated based on FEM. Die stress changes and distributions were analyzed in details through two related simulation processes. The maximum load acting on the die which type is ideal rigid body predicted firstly. Then, the die stress can be obtained by the elastic-plastic analysis when the die bears maximum load. Results show that Local stress concentration beyond ultimate strength of material causes the fracture of lower die.

The Effect of Underfill Fillet Geometry to Die Edge Stress for Flip Chip Packaging

The possible source of die edge cracking for Flip Chip Ceramic Ball Grid Array (FC-CBGA) package due to thermal cycling have been investigated in this study. Finite Element Analysis (FEA) models were used to analyze the effect of underfill fillet geometry on interfacial stresses between die edge and the underfill fillet. The input parameters of FC-CBGA from industry was used for simulation and the properties of commercial underfill were extracted by using Thermal Mechanical Analyzer (TMA) and Dynamic Mechanical Analyzer (DMA). Die stress distribution for different fillet height were generated to depict variation of stress due thermal loading. The variation of tensile stress due different fillet height and width were discussed for parameters optimization.

前方後方押出しによる複合ギア成形 : 成形性と金型応力に及ぼすスラグ形状の影響

前方後方押出しによる複合ギア成形 : 成形性と金型応力に及ぼすスラグ形状の影響