Thermal Stress in a Coated Short Fibre Composite

In this paper, the model is built by using the SIP structure. The SIP structure is consist of two chips which were using QFN flip-structure. The finite element analysis is used to simulate and calculate the distribution of moisture diffusion under thermal and moisture condition, then to simulate the thermal stress and hygrothermal stress. The results showed that thermal stress are mainly distributed in the mismatch of CTE (Coefficient of Thermal Expansion) between the materials. The largest thermal stress lie in the junction of EMC, DA and chips. We can see that it has a larger thermal effect to the SIP devices used QFN packaging inside than general QPN.The distribution of hygrothermal stress, which is similar as thermal stress in trend, is mainly in mismatch of CTE and CME Coefficient of Moisture induced Expansion). However, moisture gradient, which is caused by structure and the uneven diffusing of moisture become larger. It due to moisture difference that hygrothermal stress is much higher than the thermal stress, its stress concentration appears obviously at the junction between the materials. If the interface of the two materials exist defects, they would lead to cracks and cause the failure of the device. In the area of inside device there is less moisture diffused, its hygrothermal stress distribution is similar to general thermal stress, which is due to moisture transfer firstly through the EMC materials then into the device inside. Moreover, the internal bonding material (DA) is thin, only can absorp moisture from EMC, resulting in only a little moisture can diffuse into the DA material, then cause higher stress gradient. From the simulation results, we can see that the greater stress gradient will lead to warpage of the device, it will prone to more damage in the areas stress concentration. It is larger effect on reliability on SiP device in which used QFN packaging than the general QPN device. Therefore, the effect of EMC and DA’s moisture on the device’s stress can not be ignored. And the reaults show that the maximum hygrothermal stress is 1.56 times as thermal stress. Therefore, the effect of moistrue and thermal in reliability on the device can’t be ignored.

Solder joints in electronic packages are frequently exposed to thermal cycling in both real-life applications and accelerated thermal cycling tests. Cyclic temperature leads the solder joints to be subjected to cyclic mechanical loading and often accelerates the cracking failure of the solder joints. The cause of stress generated in thermal cycling is usually attributed to the coefficients of thermal expansion (CTE) mismatch of the assembly materials. In a die-attach structure consisting of multiple layers of materials, the effect of their CTE mismatch on the thermal stress at a critical location can be very complex. In this study, we investigated the influence of different materials in a die-attach structure on the stress at the chip–solder interface with the finite element method. The die-attach structure included a SiC chip, a SAC solder layer and a DBC substrate. Three models covering different modeling scopes (i.e., model I, chip–solder layer; model II, chip–solder layer and copper layer; and model III, chip–solder layer and DBC substrate) were developed. The 25–150 °C cyclic temperature loading was applied to the die-attach structure, and the change of stress at the chip–solder interface was calculated. The results of model I showed that the chip–solder CTE mismatch, as the only stress source, led to a periodic and monotonic stress change in the temperature cycling. Compared to the stress curve of model I, an extra stress recovery peak appeared in both model II and model III during the ramp-up of temperature. It was demonstrated that the CTE mismatch between the solder and copper layer (or DBC substrate) not only affected the maximum stress at the chip–solder interface, but also caused the stress recovery peak. Thus, the combined effect of assembly materials in the die-attach structure should be considered when exploring the joint thermal stresses.

Flip chip ball grid array (FCBGA) is one of the most broadly used package types in different types of electronic packages. In a FCBGA package, the die/ silicon is flipped and attached to substrate with help of copper pillars. The material property coefficient of thermal expansion (CTE) mismatch between die/ silicon and substrate materials like core, dielectric material results in package thermal warpage and may lead to thermo-mechanical related failures in electronic packages. CTE can be defined as the rate at which a material expands with change in the temperature. Material expansion increases with a higher CTE. The CTE of die/ silicon is lower than the overall CTE of organic substrate. During reflow, as an electronic package cools down it warps in a convex or crying shape which indicates positive package warpage, and as the electronic package heats up it warps in a concave or smiling shape which indicates negative package warpage. Induced thermal warpage during the electronic package reflow assembly process is an important factor affecting the package assembly and thermo-mechanical reliability of FCBGA packages; therefore, precise package thermal warpage prediction is one of the most significant parameters for the electronic package design processes. In this study, the effect of different electronic package parameters (core material CTE, core thickness, package type) of the FCBGA package on its warpage are assessed by using finite element analysis (FEA) mechanical simulation methodology. A bare die FCBGA package having substrate core material with lower coefficient of thermal expansion (CTE) shows lower package warpage as the mismatch between die/ silicon and substrate core material CTE becomes dominant factor to drive the package warpage. To understand the package warpage trend, warpage is evaluated at room temperature & high/ reflow temperature. The package warpage trend is reversed for larger FCBGA packages with stiffener ring. Substrate core material with higher CTE shows lower package warpage as CTE mismatch between substrate core material and stiffener ring material (steel/copper) CTE becomes the dominant factor to drive the package warpage. The counter intuitive package warpage data from mechanical simulation is compared with shadow-moire test data and it shows good correlation and validates the simulations findings. Based on this work, it is recommended that a thorough investigation for large size packages be conducted before choosing packaging materials. This work shall provide new insights for package designers to optimize thermal warpage and stress during package initial design phase.

The one-dimensional calculations of thermally induced residual stresses that are generated during the fabrication of multilayered functionally graded material plates (FGMs) because of a continuous macroscopic variation in the composition are presented. In this study, a laminated plate theory is used on the alumina/nickel FGM system to analyze the residual and thermal stresses in the multilayered FGM plates fabricated with a powder metallurgy method. Furthermore, the modeled ratios of thermo-mechanical parameters are employed to evaluate the residual thermal stress. The influences of residual thermal stress during thermal shock testing were also evaluated. The analytical results indicate that the residual thermal stress distributions can be controlled by adjusting the compositional gradient across the thickness of the FGM plates and the characteristics of thermal residual stress are more strongly dependent on the thermal expansion coefficients mismatch between ceramics and metal, in comparison with the mismatch of elastic moduli. It also indicates that during thermal shock process the thermal shock stress on the alumina surface does not change much by altering the compositional gradient in the FGM plate, and the release of thermal stress on the alumina surface is accomplished mainly by decreasing the tensile residual thermal stress.

The paper analyzes thermal stresses and effective thermal expansion coefficients of the composites in which the fiber-matrix interface is allowed to slip. Thermal stresses are evaluated by introducing an eigenstrain in the fibers, which corresponds to the strain due to the mismatch of thermal expansion coefficients. For simplicity, the effect of friction is neglected. Boussinesq-Papkovich displacement potential method is used in the analysis. Then, the results for a single sliding fiber are used to predict the average thermal expansion coefficients of the composite containing finite concentration of fibers. It is observed that sliding at the fiber-matrix interface causes higher stress concentrations and affects significantly the average coefficients of thermal expansion of the composite.

In recent years, to enhance the interlaminar fracture toughness of CFRP laminates, interlaminar-toughened laminates have been developed in which resin rich layers are placed in interlaminar regions. In the present study, stress and displacement fields in interlaminar-toughened cross-ply laminates with transverse cracks are analyzed. The analysis is based on a two-dimensional model which assumes that generalized plane strain conditions prevail. Account is taken of thermal residual stresses arising from a mismatch in thermal expansion coefficients of the 0° ply, interlaminar resin and 90° ply. The analytical results are shown for the laminates with interlaminar resin layers of various thicknesses. The analysis will be a basis for the optimal design of laminates with interlaminar resin layers.

The strength in thermal stress cycling is one of the most complicated in the strength of material at high temperature, as the authors pointed out in the previous report. The study on such strength is important from fundamental and also practical point of view. Practically the structure members subjected to load at elevated temperature are not in a steady condition but in unsteady condition, as for stress and temperature, especially, variation of thermal stress is the most severe in the unsteady conditions. On the other hand, in recent years, attention has been placed on such an aspect of the fatigue problem that failure results from comparatively small cycles of severe stresses where strain amplitude of every cycle is the independent variable which destines the life. Also, attention has been given to the problem of influence of temperature variation on fatigue life. These attentions are focused on the problem of thermal stress cycling.The authors have been interested in the problem regarding the meaning of strain amplitude in thermal cycling test and in the study of the relationship between strength in thermal cycling and the fundamental strength at elevated temperature, such as static tensile strength, strength in strain-cycling, strength of creep rupture and others. The study on such relationship is practically important to make predictions as to test data under thermal cycling from the informations of other fundamental tests.The test of thermal cycling descrived above is that of completely reversed thermal stress cycling. However, the test of pulsating tensile thermal stress cycling is also necessary in order to know the relationship between the strength in thermal cycling and that of creep rupture. This is necessary to obtain the information of strength of material subjected to both mechanical steady stress and cyclic thermal stresses, from a practical point of view. Therefore, in the present paper, a test of pulsating thermal stress cycling on AISI 304 type stainless steels is performed, using the same testing apparatus as in the case of conventional thermal cycling test, being remodelled in gripping device so as to apply specimen tensile thermal stress only, free from the compressive stress. The test results under pulsating tensile thermal stress cycling are discussed in comparison with those under fully reversed thermal stress cycling. Attention is also placed on the relationship between the strength in pulstaing tensile thermal stress cycling and that of static creep rupture.Following are conclusions from the present study:(1) Failure life for pulsating tensile thermal stress cycling is much smaller than in the case of fully reversed thermal stress cycling, that is, the failure life under completely reversed cycling is nearly two hundred times as much as that under pulsating cycling as for the basis of total strain amplitude.(2) The life for pulsating tensile thermal stress cycling is a little smaller than that estimated from the criterion regarding life of creep rupture of the material subjected to simultaneous periodic variation of temperature and applied stress (Eq. 4), that is, the estimated result is nearly three times as big as the experimental one. It would mainly be because of neglecting exact calculation on the increase of thermal stress amplitude expected in accordance with the number of cycles in the criterion. Thus, in such a case of prediction, the effect of increase of thermal stress amplitude on the rupture life must be taken into consideration

Displacement and stress fields in a functionally graded (FG) fiber-reinforced rotating annular disk with a non-uniform thickness profile, subjected to angular deceleration and a temperature profile were investigated. Unidirectional fibers were considered to be circumferentially distributed within the disk with fiber volume fraction changing radially. The governing equations for displacement, stress, and temperature fields were solved using finite difference method. The results indicated that thermal induced stresses were more dominate than the rotational induced stresses. Disks which were fiber rich at the inner radius, the fibers made negligible difference on the displacement and stress fields when compared to a homogenous disk made from the matrix material. In addition, it was found that the deceleration magnitude had no effect on the radial and hoop stresses, nor the temperature on the developed shear stress. The shear stress was only affected by the disk deceleration. Tsai-Wu failure criterion was applied for decelerating disks to ascertain their failure behavior. The results show that Tsai-Wu failure index is dominated by the thermal stresses.

It is important to reduce the thermal stresses for managing and extending the lives of pipes in plants. In this problem, heat conduction, elastic deformation, heat transfer, liquid flow should be considered, and therefore the problem is of a multidisciplinary nature. An inverse method was proposed by the present authors for determining the optimum thermal load history which reduced transient thermal stress considering the multidisciplinary physics. But the obtained solution had a problem that the temperature increasing rate of inner surface of the pipe was discontinuous at the end time of heat up. In this study we introduce temperature history functions that ensure the continuity of the temperature increasing rate. The multidisciplinary complex problem is decomposed into a heat conduction problem, a heat transfer problem, and a thermal stress problem. An analytical solution of the temperature distribution of radial thickness and thermal hoop stress distribution is obtained. The maximum tensile and compressive hoop stresses are minimized for the case where inner surface temperature Ts(t) is expressed in terms of the 4th order polynomial function of time t. Finally, from the temperature distributions, the optimum fluid temperature history is obtained for reducing the thermal stresses.

Among through silicon via (TSV) technologies, for replacing Cu filling method, the method of molten solder filling has been proposed to reduce filling cost and filling time. However, because Sn alloy which has a high coefficient of thermal expansion (CTE) than Cu, CTE mismatch between Si and molten solder induced higher thermal stress than Cu filling method. This thermal stress can deteriorate reliability of TSV by forming defects like void, crack and so on. Therefore, we fabricated SiC composite filling material which had a low CTE for reducing thermal stress in TSV. To add SiC nano particles to molten solder, ball-typed SiC clusters, which were formed with Sn powders and SiC nano particles by ball mill process, put into molten Sn and then, nano particle-dispersed SiC composite filling material was produced. In the case of 1 wt.% of SiC particle, the CTE showed a lowest value which was a <TEX>$14.8ppm/^{\circ}C$</TEX> and this value was lower than CTE of Cu. Up to 1 wt.% of SiC particle, Young's modulus increased as wt.% of SiC particle increased. And also, we observed cross-sectioned TSV which was filled with 1 wt.% of SiC particle and we confirmed a possibility of SiC composite material as a TSV filling material.

Creep-fatigue behaviour of aluminum alloy-based metal matrix composite

A comprehensive ESPI based system for combined measurement of shape and deformation of electronic components

In recent years, to enhance the interlaminar fracture toughness of CFRP laminates, interlaminar toughened laminates have been developed with resin-rich layers in the interlaminar regions. In the present study, stress and displacement fields in interlaminar-toughened cross-ply laminates with transverse cracks are analyzed. The analysis is based on a two-dimensional model which assumes that generalized plane strain conditions prevail. Thermal residual stresses arising from a mismatch in thermal expansion coefficients of the 0°, interlaminar resin and 90°plies are considered. The analytical results are shown for the laminates with interlaminar resin layers of various thicknesses. The analysis will provide a basis for the optimal design of laminates with interlaminar resin layers.

Recently, the downsizing and high-performing semiconductor packages have been developed and 3D packaging has been spurred research into reliability of TSV (through silicon via) [l]-[9]. In conventional SiP (System in Package), several semiconductor chips had been arranged in a plate. It is difficult to correspond to downsizing and high-performing of electronic devices because that large area is occupied by the package. The semiconductor chips have been stacked and TSVs have been used to communicate between the chips in order to solve above problems. The 3D stacked structure which was developed by ASET (Association of Su per-Advanced Electronics Technologies) [10] is shown in Fig.1. TSV (copper) is electrode which passes through silicon chip and micro bump (copper) [11] is arranged between chips. Heat is generated in package due to operation of semiconductor device, and it has been reported that silicon is broken by thermal stress which is depend on temperature increment. In case of 3D-SIP, higher stress has to be considered in the most outer part of TSV and micro bump structure due to warp and mismatch of CTE (coefficient of thermal expansion) of material. Therefore simulations with detail model must be desired for more accurate evaluation of stresses in Si chips and TSVs. However it is difficult that all of TSVs and micro bumps in chips were modeled due to large number of numerical data even if the latest computing systems have been adopted.

With merits like high I/O density, superior electrical and thermal performance, and small form factor, flip chip has become more and more widely used in electronic packages. Although the flip chip packaging process has been fairly improved, the conventional CUF (Capillary Underfill) process is a bottleneck that results in lower productivity and higher cost. MUF (Molded Underfill) process offers an alternative solution with advantages such as simplification of process, reduction of material cost and higher productivity. However, the extremely narrow gap between substrate and die makes it a big challenge to get optimal MUF material set and appropriate process conditions to cope with the serious void trapping issue. Besides, the process-induced warpage is also a problem that affects yield and reliability. In this paper, the MUF process of a flip chip package has been studied using numerical analysis method. The cure-kinetic, rheological and chemical shrinkage properties of MUF compound were measured by DSC (Differential Scanning Calorimeter), DMA (Dynamic Mechanical Analyzer) and universal testing machine, respectively. A global model and a sectional model were developed for MUF injection analysis to simulate the melt front advancement and estimate possible mold void distribution. Warpage analysis considering both chemical shrinkage and CTE (Coefficient of Thermal Expansion) mismatch was performed afterwards. The MUF injection analysis showed mold void generated near the center of underfill area. The warpage analysis indicated that stresses caused by chemical shrinkage and CTE mismatch acted in opposite directions, and the overall warpage was dominated by CTE mismatch. Simulation results of injection analysis were validated by short shot experiment and SAM (Scanning Acoustic Microscopy) test, respectively, and the warpage prediction was verified by measurement data. Finally, MUF process setting with four parameters was optimized based on the established numerical models, three indicators of mold quality had been improved by 9.10%, 8.95% and 20.6% respectively.