瞬态热晕效应的数值分析

根据微扰理论,分别对准直波束和聚焦波束在短脉冲、长脉冲情况下的瞬态热晕效应进行了计算和分析。结果表明,脉冲越短,光束和大气的相互作用时间远小于形成热透镜效应的时间时,热晕效应越弱,因此短脉冲可以有效降低热晕效应;脉冲越长,热晕效应越强,当脉冲宽度远大于流体力学时间时,长脉冲热晕的结果趋向于稳态热晕的结果。     

Microstructural evolution of a lead-free solder alloy Sn-Bi-Ag-Cu prepared by mechanical alloying during thermal shock and aging

In a previous study, a lead-free solder, Sn-6Bi-2Ag-0.5Cu, was developed by mechanical alloying. The alloy shows great potential as a lead-free solder system. In the present work, the microstructural evolution during thermal shock and aging was examined. In the as-soldered joints small bismuth (1 μm to 2 μm) and Ag₃Sn (1 μm) particles were finely dispersed in a nearly pure tin matrix with a small amount of η-Cu₆Sn₅ phase in the bulk of solder. During thermal shock and aging microstructural evolution occurred with Cu-Sn intermetallic compound (IMC) layer growth at interface, bismuth phase coarsening and Ag₃Sn phase coarsening. The microstructure of the solder appeared to be stable at high temperature. The shear strength of the present solder joint is higher than that of Sn-37Pb and Sn-3.5Ag solders. Shear failure occurred Cu-Sn IMC layer-solder interface and in the bulk of solder.     

Reliability enhancement with the aid of transient infrared thermal analysis of smart Power MOSFETs during short circuit operation

The usage of novel measurement techniques enhances the capabilities of researchers and power device manufacturers to understand and address reliability problems in novel Smart Power Devices. Along this line of argument, this work describes a method to improve the reliability of the smart Power MOSFET devices by design. The design optimization process involves Silicon layout, interconnections, packaging and protection strategy as well. Accurate thermal transient analyses, made possible by the unique features of a custom infrared radiometric microscope experimental setup which allows dynamic temperature detection with a bandwidth of 1 MHz over the chip area, indicated the way to minimize peak temperature and to verify the effect of the optimization.     

Failure mode analysis of MEMS suspended inductors under mechanical shock

Micro-electromechanical system (MEMS) suspended inductors have been widely studied in recent decades because of their excellent radio frequency performance. However, few studies have been performed on the failure analysis of MEMS suspended inductors under mechanical shock. In this study, the failure of MEMS suspended inductors with a planar spiral coil is investigated through analytical and experimental methods. We present a stress and deformation analysis to study the failure mode of the suspended inductors under shock. To verify the theoretical analysis, MEMS inductors are designed and fabricated, and shock tests are carried out. The shock tests show that the failure mode of the MEMS suspended inductors is a fracture that occurs at the ends of the inductor coil, and the test results agree with the theoretical analysis.     

Location resolved transient thermal analysis to investigate crack growth in solder joints

Solder joint cracking is a common failure mechanism in microelectronic packages. To investigate interconnect integrity and reliability different inspections are established with their strengths and weaknesses: X-ray, scanning acoustic microscopy (SAM), Infrared (IR)-Thermal Imaging as well as Transient Thermal Analysis (TTA). TTA is well suited to detect changes in the thermal path, i.e. delamination in a package. However, spatial resolution in plane of an interconnection is restricted. Still, spatial resolution is necessary to analysis the crack growth in solder joints. In addition the local temperature strongly depends on the local position of a bad thermal contact. In the paper an innovative new test method, location resolved transient thermal analysis (LrTTA), is developed and its potential is investigated. LrTTA is based on transient thermal measurement (TTM). It uses several distinct diodes on a test chip to detect the thermal performance of interfaces and assemblies. The temperature is measured by the forward voltage as a function of time at different locations on the chip. Spatial resolution is obtained, e.g. cracks, voids and thickness variations can be resolved in the interface. For first experimental application of the method, a silicon thermal test chip with four differently located diodes was employed. The test chip was soldered onto an Aluminium Insulated Metal Substrate (Al-IMS) and exposed to temperature cycles. TTM were performed directly after assembly and after specific temperature shock cycle numbers (− 40 °C/+125 °C). After data processing, the increase of the thermal impedance of each diode between the initial “0” cycles and “n” cycles was obtained. The thermal data are correlated with void formation detected by X-ray. Crack or delamination is in addition detected with scanning acoustic microscopy (SAM). As a quantitative analysis, a finite element (FE) model was set up and applied to analyze the solder joint with and without voids and also the crack propagation in the solder joint during temperature shock testing. Based on the FE modelling, the thermal influence of voids can be calculated and thus these voids can be detected. Further, based on the numerical analysis, crack size and location can be identified.     

Application of Green's functions for analysis of transient thermal states in electronic circuits

This paper presents an approach to the modelling of transient thermal states in electronic circuits using an analytical solution of the heat equation. Fully three-dimensional analytical time dependent solutions are determined with the help of Green's functions. The solution method is illustrated in detail on a practical example, where the results of transient thermal simulations of a real hybrid circuit are compared with infrared measurements.     

Increasing the accuracy of thermal transient measurements

Recent developments in the methodology of thermal transient measurements and evaluation are discussed in the paper. All of them are aimed at increasing the accuracy of these measurements and their evaluation. After a short summary of the evaluation methodology a procedure is presented for the correction of the nonconstant powering. New methods are presented for the compensation, evaluation and modeling of the nonlinearities. Various aspects of pulsed powering are discussed in detail.     

基于红外热像与电学测试法的OLED热学分析

采用红外 热成像与瞬态热学测试技术,测试并对比分析了有机电致发光二极管(OLED)的光电热学特性 。研究表明,OLED结温的升高会导致输入 电压的降低。在输入电流为100mA时,整个面板上呈现非 常明显的温度梯度,最高点与最低点 温差可高达30℃。在相同的工作条件下,采用红外热像测试获得的峰值温度比采用瞬态热 学测试获 得的温度要高,且温度梯度会随输入电流的大小发生变化。结合红外热像与电学测试法固有 特点的分析,指出两者的结合可以为OLED面板的热学设计与分析提供更具有指导意义的信息 。     

辊面激光强化过程的瞬态应力分析

轧辊表面在激光扫描过程中经历剧烈的热循环,由温度场的集中效应造成辊面应力呈现动态变化的趋势,扫描瞬时的光斑内部应力场以压应力为主。由于组织比容的变化,辊面处理后的残余应力分布更加复杂,采用实验手段测量应力分布状况的难度较大,因此,利用数值分析方法是合理的选择。     

激光淬火热致残余应力的数值分析

在分析温度、相变、应力的相互耦合关系的基础上,讨论了相变条件的数学模型和相应材料性能改变的方程,提出了反映相变影响的温度场控制方程,导出了含相变的热弹塑性本构方程,应用有限元进行了温度场、应力场的计算.     

A numerical approach based on transient thermal analysis toestimate the safe operating frequencies of thyristors

The spatio-temporal distribution of temperatures in high-power SCRs used for switching high di/dt current pulses were simulated using the finite element method (FEM). Two types of SCRs, with amplifying gate (unshorted device), and without amplifying gate (shorted device) structures, were analyzed. The details of the numerical simulation, such as the meshing strategy, the heat source model and the boundary conditions are discussed. Based on the analysis, the failure temperature of the unshorted device was computed to be 1100°C. The peak temperature in the shorted device was, however, found to be 335°C. The instantaneous cooling cycles of the devices and their cooling time constants, as obtained from the simulations, are presented. Based on these parameters, the safe operating frequencies of these devices were estimated     

Characterization of solar cells by thermal transient testing

The current paper deals with the application of thermal transient testing as a characterization tool for solar modules. Based on the measurement of different samples (concentrator solar cell, single junction silicon solar cell) we prove the applicability of this measurement technique and address some specific issues of the characterization of solar cells by the thermal transient method.From the measurement metrics such as junction-to-base plate thermal resistance and thermal capacitance(s) can be derived and can serve as a basis of a multi domain solar cell model. The used technique also enables us to verify the quality of attachment layers in a solar module allowing fair quality control and reliability analysis of these devices. Finally a method is proposed to regain the data that is covered by the initial electric transient following the power step. This initial electric transient can be high in large surface devices like solar cells, and covers valuable data describing the structure near to the p–n junction. To eliminate this, simulated transients were fitted to the part of the actual measured thermal transient where the electric transient already decayed. This way the part of the thermal transient that was covered by the electric transient can be reconstructed.     

晶闸管的瞬态热阻抗及其结温温升的研究

系统地阐述了晶闸管的瞬态热阻抗 ,针对晶闸管承受不同的功率脉冲 ,利用不同的计算方法计算瞬态热阻抗值 ,从而确定结温温升 ,并比较了它们的原理、精度及其应用范围。特别是利用了一种新的瞬态热阻抗计算模型 ,当晶闸管承受持续时间极短且周期、占空比均变化的任意波形功率脉冲时 ,它能够比较精确地分析晶闸管的热特性 ,计算半导体结的最大温升     

Evaluation of short pulse and short time thermal transient measurements

Thermal transient recording and the time constant spectrum analysis are widely used methods in the thermal testing and qualification of IC packages. A limitation of these methods is that recording of the complete transient response requires long time. This limitation, however, can be overcome by sophisticated procedures. The first method is to apply short power pulse for excitation; the second one is the interruption of the transients long before the thermal equilibrium is reached. The paper offers algorithms to evaluate these short pulse and short time measurements. The presented methods are suitable if the extraction of the little time constants is needed. This is the case if the transient method is used e.g. for die attach quality checking.     

The influence of the phosphor layer as heat source and up-stream thermal masses on the thermal characterization by transient thermal analysis of modern wafer level high power LEDs

In the last years Waver Level LED Packages (WLP-LEDs) were developed. They are thin film flip chips where the sapphire substrate remains attached on top of the epitaxial light emitting layer (EPI) which can be assembled directly on a printed circuit board. The thermal resistance and the thermal path of WLP-LED packages are measured by transient thermal analysis and transient finite element simulation. This study investigates the impact of the upstream thermal masses, i.e. the sapphire (SP), phosphor layer (PL) and the side coating (SC) on the transient thermal impedance curve and the cumulative structure function. It is shown that the standard approach to extract thermal properties by features (steps) within the structure function is misleading for thermal networks with upstream thermal load and distributed heat source (EPI and PL) because they influence the shape of the structure function. By transient thermal measurements and finite element (FE) simulation the transient thermal measurements are analysed to extract information about the thermal parameters and the thermal path. Starting from the analysis of the blue flip chip LED (FC-LED, no PL and no SC) the FE-model is set up. Stepwise the FE model is extended and the influence of the PL and the SC on the transient thermal measurement is investigated. A FE model is validated and calibrated which allows simulating the transient thermal curves of these modern LEDs. Using the model the impact of structural changes in the LED package on the transient thermal curves can be identified for reliability analysis.     

Effects of twin grain boundaries on the subgrain rotation of the solder joint during thermal shock

The ball grid array (BGA) component was selected as the specimen with the outmost edge row cross-sectioned to investigate subgrain rotation behavior influenced by grain boundaries under thermal shock cycles condition. To study the subgrain rotation, the crystal orientation was obtained by electron backscattered diffraction(EBSD). The results showed that a lot of subgrains were generated in the solder joints due to cyclic stress caused by the high mismatched coefficient of thermal expansion (CTE) under the thermal shock cycles condition. And the subgrains near the chip-side and the twin grain boundaries were analyzed in detail to estimate the impact of the grain boundaries on the subgrain rotation behavior. The results showed that a large number of subgrain boundaries and several newly generated subgrains appeared at the tilted twin grain boundaries and the chip-side in the solder joint after 200 thermal shock cycles. Meantime, the subgrain rotation axes and misorientation angles were both calculated, and the dislocation slip was recognized to closely relate to subgrain rotation by comprehensively analyzing the rotation axes and misorientation angles. The subgrain rotation axes of the chip-side was about Sn [101] and [001], while the subgrains rotation near the tilted grain boundaries in the same dominant grain was different from that of the subgrains near the chip-side, and subgrain rotation axes were [101], [100] and [110]. There were also a large difference in the direction of subgrain rotation between the chip-side and the tilted twin grain boundaries. The subgrain rotation axes at both sides of twin grain boundaries were similar, but the rotation directions were opposite.     

Impact of nonlinearities on electronic device transient thermal responses

This paper investigates the influence of nonlinearities on electronic device thermal transient responses. The discussions in the paper are based on practical examples where thermal responses of a power device are recorded in various boundary conditions for different values of dissipated power. Then, the measurement results are analysed using the Network Identification by a Deconvolution method and the differences between particular cases are discussed in detail. The presented experimental results demonstrate that the nonlinearities due to the temperature dependence of thermal model parameters might have important influence on the results, especially when still air cooling is applied. In addition, in selected cases the simulations results obtained with compact thermal models were compared with measurements.