Yansong Tan, Xin Li, Xu Chen, Zhenwen Yang and Guo-Quan Lu
This paper aims to use nano-silver paste to design a new bonding method for super-large-area direct-bonded-aluminum (DBA) plates. It compared several frequently used bonding…
Abstract
Purpose
This paper aims to use nano-silver paste to design a new bonding method for super-large-area direct-bonded-aluminum (DBA) plates. It compared several frequently used bonding methods and proved the feasibility of an optimized low-pressure-assisted double-layer-printed silver sintering technology for large-area bonding to increase the thermal conductivity of power electronic modules with high junction temperature, higher power density and higher reliability.
Design/methodology/approach
The bonding profile was optimized by using transparent glasses as substrates. Thus, the bonding qualities could be directly characterized by optical observation. After sintering, the bonded DBA samples were characterized by nondestructive X-ray computed tomography system, scanning electron microscopy equipped with an energy dispersive spectrometer. Finally, bonding stress evolution was characterized by shear tests.
Findings
Low-pressure-assisted large-area double-layer-printed bonding process consisting of six-step was successfully developed to bond DBA substrates with the size of 50.8 × 25.4 mm. The thickness of the sintered-silver bond-line was between 33 and 74 µm with the average porosity of 12.5 per cent. The distribution of shear strength along the length of DBA/DBA bonded sample was from 9.7 to 18.8 MPa, with average shear strength of 15.5 MPa. The typical fracture primarily propagated in the sintered-silver layer and partially along the Ni layer.
Research limitations/implications
The bonding stress needs to be further improved. Meanwhile, the thermal and electrical properties are encouraged to test further.
Practical implications
If nano-silver paste can be used as thermal interfacial material for super-large-area bonding, the thermal performance will be improved.
Social implications
The paper accelerated the use of nano-silver paste for super-large-area DBA bonding.
Originality/value
The proposed bonding method greatly decreased the bonding pressure.
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Guangcheng Dong, Xu Chen, Xinjian Zhang, Khai D.T. Ngo and Guo-Quan Lu
The purpose of this paper is to study the phenomenology of Al2O3-DBC substrate thermal-cracking under different high temperature cyclic loadings. The extremely low cycle fatigue…
Abstract
Purpose
The purpose of this paper is to study the phenomenology of Al2O3-DBC substrate thermal-cracking under different high temperature cyclic loadings. The extremely low cycle fatigue (ELCF) life prediction model for ductile materials was used to describe the thermal fatigue life of Al2O3-DBC substrates.
Design/methodology/approach
Four groups of thermal cycling tests using Al2O3-DBC substrates with 0.65 mm thick copper were conducted using different peak temperatures. The failure samples were observed by optical microscope. The thermal plastic strain distribution in the Al2O3-DBC substrates was analyzed using a finite element method with the Chaboche model for describing plastic deformation of copper. The ELCF life prediction model was used to predict the life of Al2O3-DBC substrates under high temperature cyclic loadings.
Findings
Interface cracking was observed to initiate at the short edge of the bonded copper and deviated into the ceramic layer when the crack grew beyond the critical length of 0.1-0.8 mm. The interface crack deviated into the ceramic layer at different thickness and grew parallel to the interface layer between the ceramic layer and copper layer. The crack propagation stopped after certain cycles. The copper layer with 10-20 μm thick alumina inside was not split away totally from the ceramic layer. The ELCF life prediction model could predict the life of Al2O3-DBC substrates well under high temperature cyclic loading. The material constants in the extremely low fatigue life prediction model were obtained using thermal fatigue tests results.
Research limitations/implications
The influence of copper layer thickness and ceramic layer thickness on thermal cracking characteristics of DBC substrate should be studied in the future. Failure models should also be further investigated.
Originality/value
The failure model of Al2O3-DBC substrates under high temperature cyclic loading was studied. A method for predicting the life of the substrate samples under high temperature cyclic loading was proposed.
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Yunhui Mei, Gang Chen, Xin Li, Guo‐Quan Lu and Xu Chen
The purpose of this paper is to determine: how much the residual curvature could be formed in sintered nano‐silver assembly when it is cooled to room temperature from the…
Abstract
Purpose
The purpose of this paper is to determine: how much the residual curvature could be formed in sintered nano‐silver assembly when it is cooled to room temperature from the sintering temperature (normally 275°C); how the cyclic temperature load affects the residual curvature or stresses in sintered joint. Then the stress level and the reliability of sintered nano‐silver for high‐temperature applications can be understood.
Design/methodology/approach
5 mm * 2.5 mm silicon chip was bonded with 96 per cent Al2O3 substrate by sintering nanosilver paste. An optical system was developed to measure the curvature of the sintered assemblies. Reliability of the sintered assemblies was evaluated by temperature cycling of −40∼125°C. Finite element analysis was employed to simulate the behavior of the joint subjected to the temperature cycling from −40°C to 125°C by ANSYS. SEM images were taken to investigate the impact of temperature cycling on the reliability of sintered silver attachment.
Findings
This residual bending at room temperature was found concave towards the substrate (alumina) side. Also, with the bondline thickness increasing, the residual curvature decreases obviously. The severity of the residual bending in all the structures was mitigated to some extent with increasing number of cycles. There is no crack in the joint with the thickness of 25 μm. The drop of the residual curvature of the samples with bondline of 25 μm is caused mainly by stress relaxation in sintered silver before 300 cycles. Sample with thicker bondline is more susceptible to thermal cycling for the structure bonded with nanosilver than that with thinner bondline. The poor quality of bonding is due to the thicker sintered joint, which means that sintered nanosilver is not suitable for die‐attachment requiring thick bondline.
Originality/value
The paper describes: how a precise optical system was developed to measure the residual curvature of the sintered assemblies; how the evolution of the residual curvature of the sintered assembly with the temperature cycling was obtained by both experiment and simulation; and how microstructures of the sintered silver joint were analyzed for as‐sintered assembly and the sintered assembly after temperature cycling.
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Xin Li, Gang Chen, Xu Chen, Guo‐Quan Lu, Lei Wang and Yun‐Hui Mei
The purpose of this paper is to evaluate the mechanical properties of nano‐silver paste sintered lap shear structures and to discuss the effects of loading rate and ambient…
Abstract
Purpose
The purpose of this paper is to evaluate the mechanical properties of nano‐silver paste sintered lap shear structures and to discuss the effects of loading rate and ambient temperature on shear strength and fracture mechanism.
Design/methodology/approach
Single lap shear joints with an area of 2 mm2 and thickness of 50 μm were fabricated by joining two copper substrates with nano‐silver paste. The lap shear tests were carried out under strain control mode on a micro uniaxial fatigue testing system with four loading rates and temperatures. The fracture sections were analyzed by SEM observation to determine the effect of temperature on the fracture mechanism.
Findings
Results from the study highlighted that the shear strain rate and temperature can have a significant impact on the shear behaviour of nano‐silver paste sintered lap shear joints. The shear strength increased with shear strain rate, but decreased with increasing ambient temperature. The lap shear joints displayed excellent ductility at higher temperatures due to the grain plastic flow.
Originality/value
So far, the investigation of the mechanical behaviour of low‐temperature sintered nano‐silver paste was restricted to a film form. No work had been done on nano‐silver paste connected structures. The findings presented in this paper give a basic understanding of the mechanical properties of nano‐silver sintered joints when sheared under different loading rates and temperatures.
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Kun Qi, Xu Chen and Guo‐Quan Lu
Traditional chip‐level interconnection materials show many weaknesses given the development trend of microelectronic packaging technology. In order to meet the needs of…
Abstract
Purpose
Traditional chip‐level interconnection materials show many weaknesses given the development trend of microelectronic packaging technology. In order to meet the needs of high‐temperature packaging for wide‐bandgap semiconductors, low‐temperature sintered nano‐silver as a novel semiconductor device‐metallized substrate interconnection material is being developed. One phenomenon that larger interconnection area would cause poor interconnection quality had been found in the industry butut the mechanisms were never previously studied. This paper aims to address these issues.
Design/methodology/approach
The changes in the shear strengths and microstructures of nano‐silver joints induced by the changes of interconnection areas were investigated by shear tests and scanning electron microscopy.
Findings
The increased interconnection area blocks the organic components to be burnout and causes a higher pore ratio. Thus, it reduces the bonding quality. To ensure a good and steady sintering quality, the interconnection area should be limited to 3 × 3 mm2.
Research limitations/implications
A sintering technology or paste with oxygen agent will be studied in the future.
Originality/value
A relationship of shear strength and interconnection area of sintering joints with nano‐silver paste was observed.
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Guangcheng Dong, Guangyin (Thomas) Lei, Xu Chen, Khai Ngo and Guo‐Quan Lu
Direct‐bond‐copper (DBC) substrates crack after about 15 thermal cycles from −55 to 250°C. The purpose of this paper is to study the phenomenology of thermal‐cracking to determine…
Abstract
Purpose
Direct‐bond‐copper (DBC) substrates crack after about 15 thermal cycles from −55 to 250°C. The purpose of this paper is to study the phenomenology of thermal‐cracking to determine the suitability of DBC for high‐temperature packaging.
Design/methodology/approach
The thermal plastic strain distribution at the edge of the DBC substrate was analyzed by using a finite element method with the Chaboche model for copper. The parameters of the Chaboche model were verified by comparing with the three‐point bending test results of DBC substrate. The thermal analyses involving different edge tail lengths indicated that susceptibility to cracking was influenced by the edge geometry of the DBC substrate.
Findings
Interface cracking was observed to initiate at the short edge of the bonded copper and propagated into the ceramic layer. The interface crack was caused by the accumulation of thermal plastic strain near the short edge. The edge tail can decrease the thermal strain along the short edge of the DBC substrate. Thermal cycling lifetime was improved greatly for the DBC substrate with 0.5 mm edge tail length compared with that without edge tail.
Research limitations/implications
The thermal cracking of DBC substrates should be studied at the microstructure level in the future.
Originality/value
Thermal cycling induced failure of DBC was analyzed. A method of alleviating the thermal plastic strain distribution on the weakest site and improving the thermal fatigue lifetime of DBC substrates under thermal cycling was proposed.
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Guo‐Quan Lu, Xingsheng Liu, Sihua Wen, Jesus Noel Calata and John G. Bai
In this paper, some strategies taken to improve the reliability of solder joints on power devices in single device and multi‐chip packages are presented. A strategy for improving…
Abstract
In this paper, some strategies taken to improve the reliability of solder joints on power devices in single device and multi‐chip packages are presented. A strategy for improving solder joint reliability by adjusting solder joint geometry, underfilling and utilization of flexible substrates is discussed with emphasis on triple‐stacked solder joints that resemble the shape of an hourglass. The hourglass shape relocates the highest inelastic strain away from the weaker interface with the chip to the bulk region of the joint, while the underfill provides a load transfer from the joints. Thermal cycling data show significant improvements in reliability when these techniques are used. The design, testing and finite‐element analyses of an interconnection structure, termed the Dimple‐Array Interconnect, for improving the solder joint reliability is also presented.
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Hui-Wen Deng and Kwok Wah Cheung
The National People’s Congress (NPC) of People’s Republic of China, the highest organ of state power, is popularly seen as a rubber-stamp entity. However, it has been…
Abstract
Purpose
The National People’s Congress (NPC) of People’s Republic of China, the highest organ of state power, is popularly seen as a rubber-stamp entity. However, it has been substantially evolving its roles to accommodate the governance discourses within China’s political system over the decades. This study aims to explore the changes of governance discourse of the NPC within China’s political system through which to offer a thorough understanding of the NPC’s evolving substantial role in current China.
Design/methodology/approach
This study deploys a historical approach to explore the changes of governance discourse of the NPC that has seen a growing importance in China’s political agenda, as argued by this study.
Findings
The authors find that the NPC has been substantially evolving its role within China’s political system in which the Chinese Communist Party has created different governance discourses. Besides, the NPC and its Standing Committee have asserted its authority as a substantial actor within China’s political system. The NPC is no longer functioned as a rubber-stamp institution, though it is still popularized as a rubber stamp by many scholars.
Research limitations/implications
This study is a historical elaboration on the development of NPC under three governance discourses. It might be, to some extent, relatively descriptive in nature.
Originality/value
This study, therefore, sheds some light on a revisit on the governance discourses in current China.