Process and reliability of die attachment by time-reduced sintering of nanosilver film

This study focused on the time-reduced sintering process of nanosilver film and power cycling reliability of the sintered die attachments. The aim is to demonstrate whether the time-reduced sintered joints formed within seconds have sufficient strength to survive the subsequent assembly processes such as wire bonding and whether the reliability of the time-reduced sintered joints is still better or at least comparable to that of the high lead solder joints without losing the competitiveness on the processing time. The sintered joints were prepared by commercial available nanosilver dry film and a recently developed high accuracy die bonder. The whole process for each die attachment takes only a few seconds and the sintering time is reduced to less than 9 s. The sintering behaviour of nanosilver film was studied by uniform design-of- experiments for considering three critical sintering parameters. Analysis of the sintered joints was conducted by die shear strength test and porosity measurement. Results of parametric study show that use of nanosilver dry film together with a die bonder for accurate control over sintering parameters can produce high strength, reproducible sintered joints within seconds of sintering time. At the same time, the relationship between these sintering parameters and the shear strength and porosity/density has been established and processing windows for time-reduced sintering processes at different manufacturability conditions have been drawn. The power cycling reliability of the sintered die attachment for attaching SiC diodes was carried out and compared with Pb5Sn soldered die attachment by using constant temperature swing of 50 to 200 ºC. The thermal performance and microstructure evolution of the sintered die attachments were characterized regularly and correlated with each other. The results showed that the sintered joints formed by time-reduced sintering processes demonstrated a similar power cycling reliability when the bonding strength varied from 20.5 to 40.5 MPa or the porosity changed from 35.5 to 50.9 %. Their thermal effective conductivity decreased nearly linearly from ~100 to ~13 W/(m2·K) as the power cycling test proceeded from zero to ~600k power cycles. On the other hand, the sintered die attachment with a higher bonding strength (~52.7 MPa)/low porosity (~24.7 %) could maintain its effective thermal conductivity of ~100.9 W/(m2·K) nearly unchanged up to ~100k power cycles. By contrast, the effective thermal conductivity of Pb5Sn degraded rather rapidly from 43.7 to 12.5 W/(m2·K) as the power cycling test proceeded from zero to 60k power cycles. All the thermal performance results can be well correlated with the evolution of their microstructures during power cycling

Time-efficient sintering processes to attach power devices using nanosilver dry film

Pressure-assisted sintering processes to attach power devices using wet nanosilver pastes with time scales of minutes to a few hours have been widely reported. This paper presents our work on time-efficient sintering, using nanosilver dry film and an automatic die pick and place machine, resulting in process times of just a few seconds. The combined parameters of sintering temperature 250 °C, sintering pressure 10 MPa and sintering time 5 s were selected as the benchmark process to attach 2 mm × 2 mm × 0.5 mm dummy Si devices. Then the effects of either the sintering temperature (240 to 300 °C), time (1 to 9 s) or pressure (6 to 25 MPa) on the porosity and shear strength of the sintered joints were investigated with 3 groups and a total of 13 experimental trials. The average porosities of 24.6 to 46.2% and shear strengths of 26.1 to 47.7 MPa are comparable with and/or even better than those reported for sintered joints using wet nanosilver pastes. Their dependences on the sintering temperature, time and pressure are further fitted to equations similar to those describing the kinetics of sintering processes of powder compacts. The equations obtained can be used to not only reveal different mechanisms dominating the densification and bonding strength, but also anticipate the thermal-induced evolutions of microstructures of these rapidly sintered joints during future reliability tests and/or in service

Improved reliability of planar power interconnect with ceramic-based structure

This paper proposes an advanced Si3N4 ceramic-based structure with through vias designed and filled with brazing alloy as a reliable interconnect solution in planar power modules. Finite element (FE) modeling and simulation were first used to predict the potential of using the proposed Si3N4 ceramic-based structure to improve the heat dissipation and reliability of planar interconnects. Power cycling tests and non-destructive microstructural characterization were then performed on Si3N4 ceramic-based structures, flexible printed circuit boards (PCB) and conventional Al wire interconnect samples to evaluate the FE predictions. Both the FE simulations and experimental tests were carried out on single Si diode samples where both the ceramic-based structures and flexible PCBs were bonded on the top sides of Si diodes with eutectic Sn-3.5Ag solder joints. The results obtained demonstrate that Si3N4 ceramic-based structures can significantly improve the reliability of planar interconnects. The experimental average lifetimes and FE simulated maximum creep strain accumulations for the ceramic-based structure and flexible PCB interconnect samples can reasonably be fitted to existing lifetime models for Sn-3.5Ag solder joints. Discrepancies between the models and experimental results can be attributed to defects and poor filling of the brazing alloy in the vias through the Si3N4 ceramic

Towards Integrated Traffic Control with Operating Decentralized Autonomous Organization

With a growing complexity of the intelligent traffic system (ITS), an integrated control of ITS that is capable of considering plentiful heterogeneous intelligent agents is desired. However, existing control methods based on the centralized or the decentralized scheme have not presented their competencies in considering the optimality and the scalability simultaneously. To address this issue, we propose an integrated control method based on the framework of Decentralized Autonomous Organization (DAO). The proposed method achieves a global consensus on energy consumption efficiency (ECE), meanwhile to optimize the local objectives of all involved intelligent agents, through a consensus and incentive mechanism. Furthermore, an operation algorithm is proposed regarding the issue of structural rigidity in DAO. Specifically, the proposed operation approach identifies critical agents to execute the smart contract in DAO, which ultimately extends the capability of DAO-based control. In addition, a numerical experiment is designed to examine the performance of the proposed method. The experiment results indicate that the controlled agents can achieve a consensus faster on the global objective with improved local objectives by the proposed method, compare to existing decentralized control methods. In general, the proposed method shows a great potential in developing an integrated control system in the ITSComment: 6 pages, 6 figures. To be published in 2023 IEEE 26th International Conference on Intelligent Transportation Systems (ITSC

Reliability and Characterization of Nanosilver Joints Prepared by a Time-Reduced Sintering Process

This study investigates the power cycling reliability of nanosilver sintered joints formed by a time-reduced sintering process, designed for use on a die bonder. A range of sintering parameters, reflecting different levels of manufacturability, were used to produce sintered joints in respect of shear strength and porosity, within a process cycle time of a few seconds. The reliability of the sintered attachments were evaluated against Pb5Sn solder joints under constant temperature swing power cycling conditions over the range 50 to 200 °C. The thermal performance and microstructural changes of the sintered joints were monitored and evaluated non-destructively at regular intervals using transient thermal impedance and X-ray computed tomography. The results show that sintered joints with higher shear strengths (>50 MPa) and lower porosities (<25 %) tend to maintain their thermal performance up to 100k power cycles before gradual degradation occurs. Sintered joints with intermediate shear strengths (20 to 40 MPa) and with corresponding analogous porosities (35 to 51 %) also demonstrated comparable power cycling behavior; exhibiting a progressive decrease in effective thermal conductivity with increasing cycles. The evaluated lifetime of sintered joints with the highest shear strengths were found to be at least double those for the lower shear strength joints, and up to fourteen times those of a Pb5Sn solder die attachment. Even the most porous sintered joints exhibited lifetimes appreciably longer than a Pb5Sn die attachment. Degradation in thermal resistance was seen to correlate with observed microstructural changes, with a dependence on initial sintering parameters

Electrical performance and reliability characterization of a SiC MOSFET power module with embedded decoupling capacitors

Integration of decoupling capacitors in SiC MOSFET modules is an advanced solution to mitigate the effect of parasitic inductance induced by module assembly interconnects. In this paper, the switching transient behavior is reported for a 1.2kV SiC MOSFET module with embedded DC-link capacitors. It shows faster switching transition and less overshoot voltage compared to a module using an identical package but without capacitors. Active power cycling and passive temperature cycling are carried out for package reliability characterization and comparisons are made with commercial Si and SiC power modules. Scanning acoustic microscopy images and thermal structure functions are presented to quantify the effects of package degradation. The results demonstrate that the SiC modules with embedded capacitors have similar reliability performance to commercial modules and that the reliability is not adversely affected by the presence of the decoupling capacitors

Comparative Thermal and Structural Characterization of Sintered Nano-Silver and High-Lead Solder Die Attachments During Power Cycling

13.5 mm × 13.5 mm sintered nano-silver attachments for power devices onto AlN substrates were prepared at 250 ºC and a pressure of 10MPa for 5 minutes and compared with Pb5Sn solder joint die attachments under constant current power cycling with an initial temperature swing of 50-175 ºC. Both the effective thermal resistance and microstructural evolution of the samples were monitored using transient thermal impedance measurement and non-destructive X-ray computed tomography at regular power cycling intervals. The results showed a gradual increase in the effective thermal resistance of the Pb5Sn solder joints from 0.047 to 0.133 K/W from zero to 41k power cycles, followed by a rapid escalation to 0.5018 K/W at 52k cycles. This was accompanied with the formation and development of oblique cracks within the Pb5Sn die attachments until delamination occurred at the solder/device and solder/substrate interfaces. By contrast, the effective thermal resistance of the sintered Ag joints remained almost constant at 0.040 K/W up to 116k power cycles. This was explained in terms of thermally induced continuation of densification of the sintered structure and the formation and development of networked vertical cracks within the sintered Ag die attachments, some of which further extended into the Cu tracks of the AlN substrate

Shear strength of die attachments prepared using dry nanosilver film by a time-reduced sintering process

This study investigates a time-reduced sintering process for die attachment, prepared, within a processing time of several seconds using dry nanosilver film. The effects of three main sintering parameters, sintering temperature (220 to 300 °C), sintering time (1 to 9 s) and bonding pressure (6 to 25 MPa) on the resultant shear strength are investigated using uniform design and single factor experimental trials. The shear strength data series for each of the experimental trials was statistically analyzed to indicate any deviation from normal distribution. The resulting shear strength values were regressed or fitted, and further analyzed by polynomial and kinetic-like equations to estimate the relationship between the shear strength and sintering parameters. The regression analysis for shear strength may not only be used for explaining mass transportation mechanisms, but also for identifying the proposed manufacturability of the time-reduced sintering process

Comparative thermal and structural characterization of sintered nano-silver and high-lead solder die attachments during power cycling

13.5 mm × 13.5 mm sintered nano-silver attachments for power devices onto AlN substrates were prepared at 250 ºC and a pressure of 10MPa for 5 minutes and compared with Pb5Sn solder joint die attachments under constant current power cycling with an initial temperature swing of 50-175 ºC. Both the effective thermal resistance and microstructural evolution of the samples were monitored using transient thermal impedance measurement and non-destructive X-ray computed tomography at regular power cycling intervals. The results showed a gradual increase in the effective thermal resistance of the Pb5Sn solder joints from 0.047 to 0.133 K/W from zero to 41k power cycles, followed by a rapid escalation to 0.5018 K/W at 52k cycles. This was accompanied with the formation and development of oblique cracks within the Pb5Sn die attachments until delamination occurred at the solder/device and solder/substrate interfaces. By contrast, the effective thermal resistance of the sintered Ag joints remained almost constant at 0.040 K/W up to 116k power cycles. This was explained in terms of thermally induced continuation of densification of the sintered structure and the formation and development of networked vertical cracks within the sintered Ag die attachments, some of which further extended into the Cu tracks of the AlN substrate