Research on Power Device Structure Based on FO Package Method

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.

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

The tubular heat exchanger, being a device to transfer heat between two fluids at different temperatures, is a natural candidate for thermal expansion related problems. Indeed thermal stresses merit careful analysis in the design of all kinds of heat transfer apparatus. In shell-and-tube exchangers, the thermal stress problem usually stems from the differential expansion between the shell and the tubes. The tubes are in (turbulent) contact with both fluids undergoing the heat transfer process, whereas the shell is in contact with the shell-side fluid only. The resultant differential expansion between the shell and the tubes, sometimes abetted by the difference in the thermal expansion coefficients of the tube and the shell materials, is a major source of structural problems in fixed tubesheet heat exchangers. Chapter 9 deals with the methods to predict the stress field in fixed tubesheet exchangers. As stated in Chapter 1, this drawback of the fixed tubesheet design is eliminated by resorting to U-tube construction, wherein the longitudinal expansion of the shell is physically isolated from that of the tubes. This inherent feature of U-tube design has fostered the mistaken belief that U-tube designs are immune from axial thermal expansion problems. While the relative differential expansion between the shell and the tube bundle is certainly eliminated as a potential design problem in the U-tube construction, the thermal stress problem now manifests itself in the tube bundle. The straight tube segments in different tube passes are, in general, subject to different flow patterns and to different fluid temperatures. As a consequence, the two constituent legs of a U-tube expand or contract by unequal amounts. Since the U-tubes are clamped (“built-in”) at the tubesheet for all intents and purposes, the semicircular bends at the rear end of the bundle must absorb most of the differential expansion between the adjoining legs. Often, the attendant stresses are quite high.

Microchannel plate is widely used in the field of low light level night vision, photomultiplier, tubes, X-ray enhancer and so on. In order to meet the requirement of microchannel plate electron multiplier, we used the method of thermal oxidation to produce a thin film of silicon dioxide which could play a role in electric insulation. Silicon dioxide film has a high breakdown voltage, it can satisfy the high breakdown voltage requirements of electron multiplier. We should find the reasonable parameter values and preparation process in the oxidation so that the thickness and uniformity of the silicon dioxide layer would meet requirement. This article has been focused on researching and analyzing of the problem of oxide insulation and thermal stress in the process of production of silicon dioxide film. In this experiment, dry oxygen and wet oxygen were carried out respectively for 8 hours. The thickness of dry oxygen silicon dioxide films was 458 nm and wet oxygen silicon dioxide films was 1.4 μm. Under these conditions, the silicon microchannel is uniformity and neat, meanwhile the insulating layer's breakdown voltage was measured at 450 V after the wet oxygen oxidation. By using ANSYS finite element software, we analyze the thermal stress, which came from the microchannel oxygen processes, under the conditions of which ambient temperature was 27 ℃ and porosity was 64%, we simulated the thermal stress in the temperature of 1200 ℃ and 1000 ℃, finally we got the maximum equivalent thermal stress of 472 MPa and 403 MPa respectively. The higher thermal stress area was spread over Si-SiO₂ interface, by simulate conditions 50% porosity silicon microchannel sample was selected for simulation analysis at 1100 ℃, we got the maximum equivalent thermal stress of 472 MPa, Thermal stress is the minimum value of 410 MPa.

A numerical approach for the solution of coupled fluid–solid and thermal stress problems in crystal growth processes

Heat-transfer coefficient distributions on both surfaces of a rotating disk are optimized to minimize Mises equivalent thermal stress by the combined use of genetic algorithm and analytical solutions to transient heat conduction and thermal stress problems in the rotating disk subjected to local heat generation near the outer radius. The transient temperature field in the rotating disk with arbitrary, radial change in the heat-transfer coefficient distribution is analyzed by the application of Vodicka's method. The optimized heat-transfer coefficient distributions show three patterns of distribution depending on the heat-transfer coefficient on the outer radius, the disk thickness and the area of the heat generation. Furthermore, it is discussed whether it is possible to realize the optimized heat-transfer coefficient distributions or not.

In this paper, the thermal performance analysis and optimization of the package of a specific multi-heat source power device is carried out. Firstly, the finite element analysis software ANSYS Icepak is used as a tool to build a simplified model of the array type multi-power device package module. Secondly, the impact of five variable parameters on thermal performance of multi-heat source power devices is analyzed. These parameters are chip size, chip power consumption, die attach material, packaging material, package size. Finally, through the orthogonal test analysis method, the junction temperature and thermal resistance of the package are selected as the test indexes, and five parameters are selected as the orthogonal test factors to analyze the influence of the five parameters on the thermal performance. The results show that the thermal conductivity of the die attach material is most significant regardless of the package junction temperature or thermal resistance. By summarizing the influence of various factors on the thermal performance of power devices, the suggestions for optimizing the thermal performance of multi-heat source power devices are proposed. The results obtained have important guiding significance for the thermal performance optimization of power devices.

When a coated short fibre composite is subjected to temperature change, thermal stresses in and around the coated fibres are induced due to the mismatch of thermal expansion coefficients of the constituents, resulting in a possibility of cracking in the coating. The problem of the above thermal stresses in a coated short fibre composite is solved by using the Boussinesq-Sadowsky stress functions. The present results are compared with Eshelby’s solutions for an extreme case and good agreement between the two methods is obtained. A parametric study is then conducted to examine the effect of the geometry and thermo-mechanical properties of the coating on the stress field in and around a coated short fibre. It is found in this study that the stress field in the coating is sensitive to the properties and geometry of the coating.

PurposeThe purpose of the present study was to review the thermo-mechanical challenges of reflowed lead-free solder joints in surface mount components (SMCs). The topics of the review include challenges in modelling of the reflow soldering process, optimization and the future challenges in the reflow soldering process. Besides, the numerical approach of lead-free solder reliability is also discussed.Design/methodology/approachLead-free reflow soldering is one of the most significant processes in the development of surface mount technology, especially toward the miniaturization of the advanced SMCs package. The challenges lead to more complex thermal responses when the PCB assembly passes through the reflow oven. The virtual modelling tools facilitate the modelling and simulation of the lead-free reflow process, which provide more data and clear visualization on the particular process.FindingsWith the growing trend of computer power and software capability, the multidisciplinary simulation, such as the temperature and thermal stress of lead-free SMCs, under the influenced of a specific process atmosphere can be provided. A simulation modelling technique for the thermal response and flow field prediction of a reflow process is cost-effective and has greatly helped the engineer to eliminate guesswork. Besides, simulated-based optimization methods of the reflow process have gained popularity because of them being economical and have reduced time-consumption, and these provide more information compared to the experimental hardware. The advantages and disadvantages of the simulation modelling in the reflow soldering process are also briefly discussed.Practical implicationsThis literature review provides the engineers and researchers with a profound understanding of the thermo-mechanical challenges of reflowed lead-free solder joints in SMCs and the challenges of simulation modelling in the reflow process.Originality/valueThe unique challenges in solder joint reliability, and direction of future research in reflow process were identified to clarify the solutions to solve lead-free reliability issues in the electronics manufacturing industry.

Fan-out (FO) packages are widely used in handheld, mobile consumer, and Internet of Things (IoT) devices due to the facility they provide for a greater I/O density and the integration of multiple components in a single package. Various types of FO package are available, including embedded wafer-level BGA (eWLB), fully molded (FM), and a flip-chip-based structure referred to as a fan-out chip-last package (FOCLP). In this paper, ANSYS simulations are performed to examine the warpage, extreme low-k (ELK) interconnect stress and board-level solder joint reliability of the various package types. The validity of the simulation model is confirmed by comparing the numerical results for the warpage of the eWLB and FM packages with the experimental observations. Further simulations are then performed to investigate the heat dissipation performance and electrical crosstalk properties of the three packages. Taguchi experiments are conducted to examine the effects of eight control factors [namely, the inclusion (or otherwise) of a backside laminate film, the board thickness, the joint standoff distance, the ball joint diameter, the under bump metallurgy (UBM) diameter, the joint pitch, the die thickness, and the Cu pillar height] on the thermomechanical reliability of the FM package under accelerated thermal cycling testing. Finally, the Taguchi analysis results are used to determine the optimal geometry design of the FM package.

Inherent voids or defects at the interface between die pad and plastic encapsulant become stress concentration sites when the IC package absorbs moisture or undergoes reflow soldering. If the stress intensity factor at the interfacial voids exceeds the critical fracture toughness of the pad-encapsulant interface, delamination occurs, eventually leading to popcorn cracking. In this paper, the mechanics of interfacial delamination under hygrothermal stress during solder reflow are discussed. A methodology for computation of the resultant total stress intensity factor due to the combined effects of hygro- and thermal stresses is presented. The methodology is then applied to study the effect of prebaking on delamination during solder reflow. To verify the methodology, a series of experiments was conducted in which several plastic IC packages were fabricated with a very small defect at the pad-encapsulant interface and preconditioning at 85/spl deg/C/85% RH for 168 hours. The packages were then prebaked at 150/spl deg/C for varying periods such as 30 minutes, 6.5 hours and 24 hours before being placed in varying high temperature environments in an oven. Oven temperatures ranged from 185/spl deg/C to 200/spl deg/C. The packages were then examined for delaminations by scanning acoustic microscopy. It was found that when the prebaking period was short, delamination occurred. The methodology developed was applied to predict the minimum period of prebaking required to avoid delamination during solder reflow. Good agreement was obtained between predicted and observed values.

In this paper, we consider the problems of thermal stress during selective laser melting. Using scanning electron microscopy, the average particle size and shape of the AlSi10Mg powder alloy were revealed. Details of complex geometric shapes have been grown by various construction strategies using the selective laser melting method. In the first case, the influence of supporting structures on the geometric shape of the part was considered. An optimal set of supporting structures has been developed in order to reduce the time spent on machining after selective laser melting, as well as to exclude supports in hard-to-reach cavities. In the second case, the location of the part on the construction platform was considered. It was found that changing the orientation of the parts allows to reduce the volume and number of supporting structures. The results are obtained on thermal stresses occurring in the process of growing a part by selective laser melting.

Characterization of asymmetric ultracapacitors as hybrid pulse power devices for efficient energy storage and power delivery applications

CAST2D is a coupled thermal-stress finite element computer code for casting process modeling. This code can be used to predict the final shape and stress state of cast parts. CAST2D couples the heat transfer code TOPAZ2D and solid mechanics code NIKE2D. CAST2D has the following features in addition to all the features contained in the TOPAZ2D and NIKE2D codes: (1) a general purpose thermal-mechanical interface algorithm (i.e., slide line) that calculates the thermal contact resistance across the part-mold interface as a function of interface pressure and gap opening; (2) a new phase change algorithm, the delta function method, that is a robust method for materials undergoing isothermal phase change; (3) a constitutive model that transitions between fluid behavior and solid behavior, and accounts for material volume change on phase change; and (4) a modified plot file data base that allows plotting of thermal variables (e.g., temperature, heat flux) on the deformed geometry. Although the code is specialized for casting modeling, it can be used for other thermal stress problems (e.g., metal forming).

As a third-generation wide-bandgap semiconductor material, Silicon Carbide (SiC) boasts broad application prospects in power electronics, radio frequency (RF) devices, and other fields. Wafer thinning is a crucial process for improving the performance of SiC power devices. This study takes fine grinding thickness as the core control parameter to investigate its influence on the surface quality of thinned SiC wafers and optimize the process. Experimental results show that when the fine grinding thickness is increased from 10μm to 20μm, the surface quality of the wafers is significantly improved: the Total Thickness Variation (TTV) is reduced to 5.18μm, a decrease of 22.5% compared with that before Optimization; the surface roughness is reduced to 0.6407 nm, with a reduction rate of 18.7%; the warpage is suppressed to 405.93μm; and the internal stress is decreased to 24.634 MPa, a substantial reduction of 35.3%. The core mechanism lies in the reasonable increase of fine grinding thickness, which can thoroughly eliminate the damaged layer and subsurface cracks remaining from rough grinding, disperse the contact pressure of abrasive grains, and synchronously release mechanical stress and thermal stress, forming a positive Optimization cycle. This study verifies the feasibility and effectiveness of optimizing the processing quality of SiC wafers by adjusting the fine grinding thickness, providing an effective technical reference for improving the quality of subsequent processes, such as laser annealing and dicing, as well as enhancing the electrical performance and reliability of the final devices.