Thermoelectric Stress Research Articles

Research on Reliability of Ni/Sn/Cu(Ni) Copper Pillar Bump Under Thermoelectric Loading

Abstract With the development of packaging devices toward high performance and high density, electronic devices are subjected to thermo-electric stresses under service conditions, which has become a particularly important reliability problem in micro-electronics packaging. The reliability of the chip under thermo-electric stresses is studied in this paper. First, thermo-electric coupling experiments were carried out on two solder joint structures of Ni/Sn3.5Ag/Cu and Ni/Sn3.5Ag/Ni. The interface evolution of solder joints under different current densities was analyzed. The reliability of the two structures under thermo-electric stresses was compared and analyzed. After that, three-dimensional finite element analysis was employed to simulate the current density, Joule heat, and temperature distribution of the flip chip. Finally, through the combination of experiment and simulation, the distribution of Joule heat and temperature of the chip was analyzed. The results show that the Ni/Sn3.5Ag/Ni structure has better reliability than the Ni/Sn3.5Ag/Cu structure under thermal–electric coupling. In addition, when the Ni layer was used as the cathode side, the constant temperature applied on the chip was 150 °C, and the current density was higher than 5 × 104 A/cm2, the dissolution failure of the Ni layer occurred in two structures. Because the higher current density generated a large amount of Joule heat where the current was crowded, resulting in excessively high temperature and rapid dissolution of the Ni barrier layer.

Behavior of XLPE for HVDC Cables under Thermo-Electrical Stress: Experimental Study and Ageing Kinetics Proposal

The present work deals with the study of the electrical behavior of cross-linked polyethylene (XLPE) used for HVDC cable insulation. The aim is to better understand the influences of electrical and thermal stresses on the insulating material in order to provide useful information for designing HVDC cables. This study was carried out on Rogowski samples made of XLPE insulation with semiconductive electrodes, aged for more than 3 years (1220 days) at three different temperatures (70, 80 and 90 °C) under two DC electric fields (30 and 60 kV/mm). Dielectric loss factor, volume resistivity and space charge accumulation were measured. Results are analyzed and cross-correlated, in order to propose possible ageing kinetics.

Reliability Assessment of IGBT Through Modelling and Experimental Testing

Lifetime of power electronic devices, in particular those used for wind turbines, is short due to the generation of thermal stresses in their switching device e.g., IGBT particularly in the case of high switching frequency. This causes premature failure of the device leading to an unreliable performance in operation. Hence, appropriate thermal assessment and implementation of associated mitigation procedure are required to put in place in order to improve the reliability of the switching device. This paper presents two case studies to demonstrate the reliability assessment of IGBT. First, a new driving strategy for operating IGBT based power inverter module is proposed to mitigate wire-bond thermal stresses. The thermal stress is characterised using finite element modelling and validated by inverter operated under different wind speeds. High-speed thermal imaging camera and dSPACE system are used for real time measurements. Reliability of switching devices is determined based on thermoelectric (electrical and/or mechanical) stresses during operations and lifetime estimation. Second, machine learning based data-driven prognostic models are developed for predicting degradation behaviour of IGBT and determining remaining useful life using degradation raw data collected from accelerated aging tests under thermal overstress condition. The durations of various phases with increasing collector-emitter voltage are determined over the device lifetime. A data set of phase durations from several IGBTs is trained to develop Neural Network (NN) and Adaptive Neuro Fuzzy Inference System (ANFIS) models, which is used to predict remaining useful life (RUL) of IGBT. Results obtained from the presented case studies would pave the path for improving the reliability of IGBTs.

Sub-100 nm2 Cobalt Interconnects

Co has elicited a strong interest to replace Cu for future interconnect applications in microelectronic circuits due to its potentially lower resistivity and better reliability at scaled dimensions. Here, we demonstrate Co wires with electrical cross-sectional areas as small as 34 nm2 using a simple subtractive patterning technique. Semiclassical resistivity modeling of the wire resistivity indicates that grain boundary scattering is the dominant contribution for cross-sectional areas of the order of 100 nm2, while the contribution of surface scattering increases drastically below 50 nm2. Furthermore, wafer-level reliability tests of the Co wires show the high fusing current densities and a strong resistance to thermoelectric stress demonstrating the potential for robust reliability. The resistivity and the reliability of the Co wires are comparable with those of Ru wires.

Performance of High-Voltage Polymeric Insulators Under Simulated Environmental Conditions in Desert Areas of Southern Libya

One of the primary aims for any type of electrical utility is the minimization of active power losses. In terms of cost and reliability, high-voltage (HV) polymeric insulators are used worldwide. This study considered the performance and reliability of insulators under accelerated aging conditions with the simulation of the environmental conditions of southern Libya. These issues with insulators have been a major problem for power utilities in southern Libya for many years. In order to check the HV performance, the International Electrochemical Commission (IEC) Standard-61109 was used to age the two insulators. Other samples were maintained under the accelerated aging conditions, i.e., the inland temperature and the actual ultraviolet radiation intensity; the two thermoelectric stresses and UV radiations were applied on the polymeric insulators, aged as per the IEC standard and modified IEC standards, simulating the desert of Libya. The experimental data revealed that the thermoplastic insulators and their dielectric response under the tested thermoelectric cum ultraviolet-irradiations performed better than the silicone rubber insulators. Hence, the assessment of the data provided vital information regarding the stability and performance of these composite insulators which allows for an informed decision about the best materials for the application.

Interfacial reaction and failure mechanism of Cu/Ni/SnAg1.8/Cu flip chip Cu pillar bump under thermoelectric stresses

Micro-interconnection copper pillar bumps are being widely used in the packaging areas of memory chip and high performance computer due to their high density, good conductivity and low noise. Studying the interfacial behavior of copper pillar bump is of great significance for understanding its failure mechanism and microstructure evolution in order to improve the reliability of flip chip package. The thermoelectric stress test, in-situ monitor, infrared thermography test, and microstructure analysis method are employed to study the interfacial reaction, life distribution, failure mechanism and their effect factors of Cu/Ni/SnAg1.8/Cu flip chip copper pillar interconnects under 9 groups of thermoelectric stresses including 2104-3104 A/cm2 and 100-150℃. Under thermoelectric stresses, the interfacial reaction of Cu pillar can be divided into three stages:Cu6Sn5 growth and Sn solder exhaustion; the Cu6Sn5 phase transformation, exhaustion and the Cu3Sn phase growth; voids formation and crack propagation. The rate of Cu6Sn5 phase transforming into Cu3Sn phase is positively correlated with the current density. There are four kinds of failure modes including Cu pad consumption, solder complete consumption and transformation into Cu3Sn, Ni plating layer erosion and strip voids. An obvious polar effect is observed during the dissolution of Cu pads on the substrate side and the Ni layer on the Cu pillar side. When Cu pad is located at the cathode, the direction of electron flow is the same as that of the heat flow, and it can accelerate the consumption of Cu pad and the growth of Cu3Sn. When Ni layer serves as the cathode, the electron flow can enhance the consumption of Ni layer. Under 150℃ and 2.5104 A/cm2, the local Ni barrier layer is eroded after 2.5 h, which results in the transformation of Cu pillar on the Ni side into (Cux, Niy)6Sn5 and Cu3Sn alloy. The life of Cu pillar interconnection complies well to the 2-parameter Weibull distribution with a shape parameter of 7.78, which is a typical characteristic of cumulative wear-out failure. The results show that the intermitallic growth behavior and failure mechanism at Cu pillar interconnects are significantly accelerated and changed under thermoelectric stresses compared with the scenario under the single high temperature stress.

Modelling polarization and conduction processes in poly(ethylene naphthalate)

Challenges in the field of solid organic insulators are linked to the development of more compact, reliable and eco-friendly systems where these materials are employed as dielectrics. One way to increase the reliability of such systems is to understand and model the behavior of these materials under thermo-electrical stress, in order to prevent any failure. Fluid charge transport models have been developed to predict the charge accumulation in such materials when submitted to an external stress, mostly electrical. However, these kind of models are not enough to predict the global behavior of the material, as most of these dielectrics are polar materials, subjected to a non-negligible dipolar response. This paper shows the first simulation results produced using a global charge transport model including polarization, and compares simulated to experimental current measurements. For moderate fields and temperatures, the model is able to reproduce the experimental results, qualitatively and quantitatively.

A New Effective Methodology for Semiconductor Power Devices HTRB Testing

An advanced high-temperature reverse bias (HTRB) testing procedure for performing reliability tests on power transistors is reported. The main target is to monitor continuously the degradation trend of tested devices. Therefore, the total HTRB test time is divided into short stress cycles. Thanks to a purposely designed miniature heater, which controls the individual case temperature of devices under test (DUTs), electrical characterization, at low or high temperature, can be performed at the end of each stress period automatically. In this way, DUT's electrical parameters can be periodically measured to identify early warnings of failure, and test can be stopped for the sole out-of-specification devices. In addition, thanks to the fast thermal control, thermal runaway processes can be inhibited, freezing the degradation state to a presettled level, in order to perform appropriate postfailure analysis. Finally, the new HTRB methodology allows for evidencing anomalous behaviors, which are not considered as failures, and the application of low frequency noise measure techniques provides the evidence of the effects of the applied thermo-electrical stress. The proposed HTRB methodology together with low frequency noise measurements are presented as well as the results obtained from the experimental application of the procedure on silicon power MOSFETs.

Electroluminescence and cathodoluminescence from polyethylene and polypropylene films: Spectra reconstruction from elementary components and underlying mechanisms

The mechanisms of electroluminescence from large band gap polymers used as insulation in electric components are still under debate. It becomes important to unravel the underlying physics of the emission because of increasing thermo-electric stress and a possible relationship between electroluminescence and field withstand. We report herein on the cathodoluminescence spectra of polyethylene and polypropylene films as a way to uncover the nature of its contributions to electroluminescence emission. It is shown that spectra from the two materials are structured around four elementary components, each of them being associated with a specific process contributing to the overall emission with different weights depending on excitation conditions and on materials. The cathodoluminescence and electroluminescence spectra of each material are reconstructed from the four spectral components and their relative contribution are discussed. It is shown that electroluminescence from polyethylene and polypropylene has the same origin pointing towards generic mechanisms in both.

An explanation for catastrophic failures of GaN-based vertical structure LEDs subjected to thermoelectric stressing

Both accelerated life tests and engineering practices present a common behavior: LED products may encounter catastrophic failure under thermo-electric co-stressing. The present work proposes that the applied thermoelectric stresses themselves are not necessary conditions but pre-conditions for LED breakdown. The deformation and sliding of the metal lines on n-GaN under thermoelectric co-stressing cause electrodes to become dilapidated. Such dilapidated electrodes with rough edges will help to generate localized high electrical fields. The catastrophic failure occurs during the de-trapping process of space charges in and around threading dislocations when the applied electric field is removed and short circuits form between the charged dislocation cores and the tips of the dilapidated electrodes. A phenomenological relationship is given to describe the interactions between temperatures and applied electric currents on the diffusion-flux-accommodated sliding and deformation of patterned metal electrodes on vertical LEDs. Such a law is coincident with the practical trend of LED failure rates observed in accelerated life tests and engineering applications.

Calculation of thermoelastic stresses near the crystallization front for melt-grown single-crystal rods with a circular cross section

Approximate expressions for the thermoelectric stress tensor components in an isotropic crystal rod of a circular cross section, applicable in the entire crystal including the region near the crystallization front, are obtained. Using the resultant approximate formulas, the stress fields in leucosapphire single crystals are calculated for model temperature fields. It is shown that exactly near the crystallization front, thermoelastic stresses reach maximal values.

Testing of porous SiC with dense coating under relevant conditions for Flow Channel Insert application

Thermally and electrically insulating porous SiC ceramics are attractive candidates for Flow Channel Inserts (FCI) in dual-coolant blanket concepts thanks to its relatively inexpensive manufacturing route. To prevent tritium permeation and corrosion by Pb-15.7 a dense coating has to be applied on the porous SiC. Despite not having structural function, FCI must exhibit sufficient mechanical strength to withstand strong thermal gradients and thermo-electrical stresses during operation. This work summarizes the results on the development of coated porous SiC for FCI. Porous SiC was obtained following the sacrificial template technique, using Al2O3 and Y2O3 as sintering additives and a carbonaceous phase as pore former. Sintering was performed in inert gas at 1850–1950°C during 15min to 3h, followed by oxidation at 650°C to eliminate the carbonaceous phase. The most promising bulk materials were coated with a ∼30μm thick dense SiC by CVD. Results on porosity, bending tests, thermal and electrical conductivity are presented. The microstructure of the coating, its adhesion to the porous SiC and its corrosion behavior under Pb-17.5Li are also shown.

Acoustic emission testing of electrical stressed copper trace

Abstract Copper becomes more and more important in the electronic industry. This metal is more resilient than conventional materials (like aluminium) due to the higher electrical and thermal conductivity. During several electrical stress pulses the copper changes. The surface can melt up and cracks and voids can develop inside the copper specimen. In this paper we use acoustic emission testing to analyse copper degradation especially of copper and copper traces. It is evaluated if an acoustic wave excited by a thermo-electrical stress pulse can be detected by a simple measurement setup and if a correlation between the acoustic signal and the degradation exists. The idea is that during the degradation, the behaviour of the material changes which in turn influences the emitted acoustic signal.

Degradation of High Voltage Polymeric Insulators in Arid Desert's Simulated Environmental Conditions

Problem statement: High Voltage (HV) polymeric insulators are replacing ceramic insulator commonly used for HV outdoor networks due to their ease of handling, reliability and cost. However, their long term performance and reliability are major concerns to power utilities. Approach: To investigate their performance in arid desert's conditions, two types of HV composite insulators were aged as per International Electrochemical Commission (IEC) standard-61109. Additional test samples were subjected to accelerated aging conditions simulating the actual Ultraviolet (UV) radiation intensity and temperature in the inland desert. Results: This study described the experimental results of the effects of thermo electric stress and UV radiations on the polymeric insulators aged under two conditions i.e., as per IEC standard and modified IEC standard that simulates the inland arid desert. The tests results after the artificial accelerated aging indicated that the dielectric response of thermoplastic insulators under the tested thermo-electric cum UV-irradiations outperforms Silicone rubber insulators. Conclusion: From the obtained results it will be easy to assess the performance and suitability of composite insulators for their applications in arid desert environments.

Insulation characterization in multistress conditions by accelerated life tests: An application to XLPE and EPR for high voltage cables

This paper focuses on accelerated testing procedures that can be applied to XLPE and EPR for high voltage cables and that are aimed to achieve the thermo-electrical endurance characterization of insulation. Several methods and techniques for single and combined thermo-electrical stress are illustrated; indices for the evaluation of electrical, thermal, and multi-stress endurance are reported and their derivation is shown. Indications on the feasibility of the design-stress inference from tests on laboratory specimens also are given. Experimental results mainly refer to cable models, but data relevant to full-sized cables, as well as to simple, flat specimens, are also considered.

In-situ study of the degradation behaviour of GaAs MESFETs for hi-rel applications

In this paper, an accelerated ageing experiment on GaAs MESFET's is presented applying a novel in-situ technique on a reduced time scale. The ageing behaviour of several dc parameters is monitored continuously while thermoelectrical stress is applied. As different ageing processes leave distinct fingerprints in the degradation curves, the increased measurement resolution and data density, which are typical for in-situ measurements, provide additional information compared with conventional ex-situ tests. The result of the numerical analysis of the experimental data is presented, which provide an initial basis for comparison with conventional data and lifetime prediction.

A comparative investigation of electrothermal endurance models for insulating materials and systems characterization

Inference of ageing mechanisms becomes a very complex matter in insulations, where the presence of electrical field concentrations, temperature gradients, various insulating materials, inhomogeneities, etc., complicates tremendously any physical investigation. This paper focuses on recent developments in modeling insulation behavior under thermo-electrical stresses. In particular, a comparison between phenomenological and physical models, with the relevant limits and constraints, is presented. Moreover, a probabilistic approach, which may hold for materials and systems as well, is discussed, referring to both parametric and non-parametric models.

The role of trapped space charges in the electrical aging of insulating materials

An investigation of the effect of trapped space charges on the aging of polymeric insulating materials subjected to thermo-electrical stress is reported in this paper. Possible scenarios of degradation mechanisms, thermally activated, but accelerated by the presence of space charges, are examined. The model which derives from this approach has some interesting features: it is characterized by electrical and thermal thresholds, its parameters have a physical background, it can be cast into a probabilistic framework. Acceleration of aging due to space charges is attributed to a reduction of the free-energy barrier to degradation, seen as a local partially-reversible reaction, which is caused by energy stored in space-charge centers. The validity of the model is limited to dc voltage, and to the time of formation of microcavity-crazes, rather than to breakdown times, since other mechanisms will occur under electrical field once large enough cavities are formed in the insulation. The model is applied to the results of thermo-electrical life tests performed on PET, showing very good fitting, as well as interesting relationships between parameter estimates and insulation morphology. It is shown that the model can also fit well to ac life data, where it takes on a phenomenological meaning.

Thermoelectric stresses in a cylinder heated from its end face

Thermoelectric stresses in a cylinder heated from its end face