Accelerated Stress Test Research Articles

WLCSP Fully Protected Fan-In Reliability

A strong supply chain requires higher performance and lower cost processes. DCA conventionally referred to as WLCSP has been used for low density devices, however for higher density devices reliability concerns have emerged. Six-side protected, aka “6S or fully protected”, CSP is a rapidly growing market. Fully protected CSP was first implemented with processes such as M-series utilizing FO. These processes require costly and complex die reconstitution, expensive tapes, molding, and other operations. These steps can be eliminated in a P-WLCSP process to provide the cost-effective high reliability 6S format needed for high-performance higher pin count and/or thin silicon chips. American Semiconductor’s Semiconductor-on-Polymer (SoP) 300mm SoP-TM, a P-WLCSP process, is an advanced packaging process optimized for protected CSP, fan-in, and chiplets. Protected FI process innovations can improve performance in power devices, RF switches, photonic IC (PIC), die stacking and thin board applications. The P-WLCSP process incorporates the best of DCA (simplicity) and FO (protection) while incorporating advanced manufacturing processes to lower cost and improve performance. This paper builds on the SoP-TM process release announced at the 2023 Device Packaging Conference earlier this year with the addition of reliability characterization beyond first silicon data for electrical test chips. Reliability data such as accelerated stress tests, die strength, operating lifetime and thermal testing are key areas of investigation for the new P-WLCSP technology that will be presented. The presentation will also include and update for ultra-thin device reliability test methods under consideration for new NIST standards.

Gold Coated Silver Bonding Wire and Its Consistent Reliability Performances

Thermal aging of gold coated silver bonded ball with aluminium bond pads exhibited a stable interface without Kirkendall voids until 4000h at 200°C for an un-molded device tested under vacuum. Ball and stitch pull remains in strength and remained constant until 4000 hours of aging. The growth of aluminides at the bond interface is about 4.5µm for 4000 hours of aging. The electrical resistance of the bonded ball remains the same until 4000 hours at 175°C for an epoxy molded device, molded with 5 to 7 pH green epoxy molding compound containing low chlorine and sulfur. Interestingly, the bonded ball interface of epoxy molded device has sustained 480h on biased Highly Accelerated Stress Test (bHAST) at +20V bias, 130°C, 85%RH. The bonded ball interface is stable until 1000 hours on unbiased HAST (uHAST) at 130°C, 85% RH as well. Cross-sectional ovservations of the bonded balls revealed the presence of intermetallic phases at the bond interface. However, formation of Kirkendall voids or crack propagatioon due to galvanic corrosions are absent.

Development of Wafer Level Package Platform for High Reliability Crossover MCUs

Fan in and fan out wafer level packages are primarily used in the industry for applications in handheld consumer electronic products. The key benefits of wafer level package (WLP) are small form factor, reduced cost, improved electrical and thermal performance. WLP investigated in this study are directly surface mounted on PCB and have no intermediate substrate. Direct mount of package on PCB, leads to significant CTE mismatch between the package and board which stresses the package solder joint, leading to cyclic fatigue induced solder joint cracking. To overcome the solder joint cracking, stiffer solder alloys have been evaluated. However, higher stiffness of the solder alloy shifts the cyclic stress induced failure, from the solder joint to the package metal interconnect layers. Moreover, WLP on advanced silicon node have fragile low-k and extremely low-k back end of line dielectric layers that also crack or delaminate in cyclic fatigue. In this work extensive WLP platform development has been done with following DOE variables; multi-layer package routing, dielectric materials, solder alloys, solder ball pitch and diameter. Developed packages passed and outperformed the following product reliability qualification conditions: high temperature storage life (150°C, 2000 hours), highly accelerated stress test (110°C / 85%RH / Bias, 528 hours), application level temperature cycling ((-40 °C to 125 °C, 600 cycles), board level temperature cycling (-40 °C to 125 °C, 500 cycles) and board level drop test (1500g/0.5ms, 30 drops). Excellent reliability, functional performance, and successful chip package integration was achieved for WLP for crossover MCU products.

A one-dimensional numerical model of carbon corrosion in catalyst layers of proton exchange membrane fuel cells

Understanding interaction between carbon corrosion and physicochemical processes inside proton exchange membrane fuel cells (PEMFCs) is important. A one-dimensional carbon corrosion model considering cathode catalyst layer (CCL) structure evolutions is established and coupled with one-dimensional PEMFC performance model. The coupled model is validated and employed to predict CCL structure evolution and PEMFC performance degradation during accelerated stress test. The results find that high relative humidity can enhance performance but accelerate carbon corrosion. Increasing carbon loading and decreasing ionomer weight can improve both performance and durability. Low Pt loading leads to reduced performance and durability. There exists an optimal carbon diameter leading to the best performance. Small carbon diameter can enhance cell durability. Proper gradient CCL design can enhance durability at cost of small initial performance reduction. Finally, an optimized structure is proposed with 21.7 % higher current density at 0.6 V and 5.2 % lower performance loss compared with base case.

Accelerated stress test protocol for solid oxide cell based on ex-situ artificial aging of the fuel electrode via redox-cycling - In memory of Prof. A. Milchev

Accelerated stress test protocol for solid oxide cell based on ex-situ artificial aging of the fuel electrode via redox-cycling - In memory of Prof. A. Milchev

Thermal Performance and Reliability of Liquid Metal Pastes with Solid Metal Particles as Thermal Interface Materials for the Cooling of Computing Electronics

In recent years, we have developed a series of gallium-based liquid metal pastes containing low content solid metal particle additives (LMPMPs) to enhance thermal performance and control bondline thickness (BLT) of prepared liquid metal composites as thermal interface materials (TIMs). This new material is synthesized via in-situ by introducing gallium oxide into liquid metal alloys. In our recent research, we intensively investigated thermal properties of different primary liquid metal alloys containing small amounts of metal additives, and ran extensive reliability tests including power cycling, thermal cycling, and aging tests on the synthesized LMPMPs. In this paper, the thermal properties of the synthesized LMPMPs with different liquid metal alloys are studied with reliability tests that focus on power cycling by monitoring thermal resistance of LMPMPs over time using a custom power cycler. We have found power density, BLT, and different liquid metal alloys have a significant impact on thermal properties and reliability of LMPMPs. We also conducted thermal aging tests on Cu-LMPMPs-Cu sandwich specimens at 85°C in an ambient atmosphere to study the deterioration of thermal conductivity of LMPMPs over time, measured by laser flash analyzer using the layered structure method. Highly accelerated stress test (HAST) with a profile of 110°C, 85RH%, and 264 hours was used to study the decomposition of liquid metal alloys. Thermal cycling test with a profile of -40 to 125°C was employed to investigate the pump-out of LMPMPs. It has been proven that LMPMPs have much better thermal performances than polymer-based thermal grease.

Dynamic accelerated stress test and coupled on-line analysis program to elucidate aging processes in proton exchange membrane fuel cells

The application of hydrogen proton exchange membrane fuel cells (PEMFC) in greenhouse gas emission free heavy-duty vehicles requires extremely durable PEMFC components with service lives in the range of 30,000 h. Hence suitable test and analysis methods are required that reflect realistic operation scenarios, but significantly accelerate aging. For this purpose, a dynamic accelerated stress test was developed, which is coupled with a comprehensive in-depth in-situ and ex-situ analysis program to determine the aging processes of a PEMFC membrane electrode assembly (MEA). The test comprehends dynamic cycling between low, moderate and high load, different temperature and humidity conditions as well as recovery sequences to distinguish between reversible and irreversible failure modes. All phases of the PEMFC system (i.e. solid, liquid and gaseous) are monitored on-line during aging by sophisticated electrochemical, mass spectrometric and ion chromatographic analytical methods. The structural and elemental composition of the MEA before and after the aging program (post-mortem) are investigated by X-ray fluorescence, scanning and transmission electron microscopy. This program was able to age a commercial PEMFC to end-of-life in 1000 h, while providing an accurate picture of the aging processes involved.

Impact of Pt-Loading and Carbon Support of Pt/C Anode Catalysts on Membrane Degradation Rate in Fuel Cells

In order to clarify the effect of both the Pt-loading and carbon supports in various commercial Pt/C anode catalysts on the chemical degradation rate of polymer electrolyte membranes (PEMs) in fuel cells, the membrane lifetime and fluoride emission rate (FER) were examined in a single cell with an accelerated stress test (AST) at 90 °C, while the H2O2 production rates, j(H2O2), at the Pt/C catalysts were measured in a half cell with 0.1 M HClO4 solution at 90 °C. It was demonstrated that the PEM degradation rate decreased with decreasing amount of adsorbed hydrogen atoms (Had) at the Pt/C−PEM interface by lowering the Pt-loading and using a porous carbon support (locating Pt nanoparticles within pores). The use of a graphitized carbon black (GCB) support was found to be very effective to increase the lifetime due to it having the lowest j(H2O2), although this was accompanied by the disadvantage of decreased mass activity for the hydrogen oxidation reaction. We also found good correlations between the j(H2O2) measured in a half cell and FER, as well as lifetime in a single cell, so that the lifetime of the PEM can be estimated in a shorter time.

Catalyst-Specific Accelerated Stress Tests in Proton Exchange Membrane Low-Temperature Electrolysis for Intermittent Operation

Device-level stress tests are developed that focus on anode catalyst layer degradation and future anticipated operating conditions, including intermittent load and reduced platinum group metal content. Square-wave cycles with an upper load limit of 2.5 A cm‒2 are utilized to screen commercial iridium (Ir) materials. Performance losses are primarily due to decreasing kinetics and are accompanied by catalyst migration into the membrane, worsening catalyst/ionomer integration, and weakening of the catalyst/membrane interface. For ruthenium-containing catalysts, the in situ performances are higher but durabilities lower than Ir baselines, and any performance advantage is lost within the test. Increased loss is likely due to the higher dissolution rate; microscopy confirmed greater degrees of ruthenium migration. For Ir metal or mixed oxides, ex situ activity improvements generally did not translate to in situ performance. The durability, however, is significantly lower and the loss rate increased from 3 (oxide) to 9 (metal) μV cycle‒1. These results are consistent with historical findings in literature, rationalize the continued use of iridium oxide as a baseline catalyst, and demonstrate that traditional catalyst development approaches may not improve device-level durability when focused on low-cost applications. A shift in focus may therefore be more effective at improving catalyst utilization and lessening load requirements.

Field‐representative evaluation of PID‐polarization in TOPCon PV modules by accelerated stress testing

AbstractPotential‐induced degradation‐polarization (PID‐p) can reduce module power, but how to project the extent to which PID‐p may occur in field conditions considering the factors of system voltage, condensed moisture, temperature, and illumination has not been clarified. Using tunnel oxide passivated contact (TOPCon) modules, this work demonstrates a method to test full‐size crystalline silicon PV modules for PID‐p to provide field‐representative results. In initial screening tests with positive or negative 1000 V electrical bias applied at 60°C for 96 h using Al foil electrodes on the glass surfaces, the module type exhibited reversible PID‐p only on the front face when the cell circuit was in negative voltage potential. No PID was detected on the rear after testing in either polarity. We then evaluated the PID‐p sensitivity on the front side under different UV irradiances while maintaining the glass surface wet to estimate real‐world susceptibility to PID‐p. The magnitude of the observed behavior was fit using a previously developed charge transfer and depletion by light model. Whereas power loss with −1000 V applied to the cell circuit at 60°C for 96 h in the dark was about 30%, testing the module front under 0.051 W·m−2 nm−1 at 340 nm UVA irradiation using fluorescent tubes, the mean degradation was only 3%. When the modules were tested in the dark for PID‐p with in situ dark current–voltage (I‐V) characterization, the thermal activation energy for degradation was 0.71 eV; for recovery in the dark, it was 0.58 eV. Whereas recovery from the degraded state at 60°C in the dark without voltage bias was 5% absolute in 38 h, rapid recovery of about 5% absolute was observed with 1000 W·s/m2 exposure at 25°C using a flash tester.

Optimizing design of catalyst layer structure with carbon-supported platinum weight ratio mixing method for proton exchange membrane fuel cells

The design of catalyst structure of proton exchange membrane fuel cells (PEMFCs) functions a crucial role in water and reactants transport. In this work, catalyst layer is designed to mixing strategy of commercial Pt/C catalysts with various weight ratios (20 wt%, 40 wt%, and 20 + 40 wt%). Our mixing strategy demonstrates beneficial effects for optimized catalyst structure, leading to improved electrochemical performance and durability. Mix weight ratio Pt/C (as we abbreviated 20 + 40 wt% to Mix wt%) confirmed the optimized morphology through physical characterization and verified through electrochemical characterization under varying relative humidity (RH) conditions. Remarkably, Mix wt% Pt/C showed the highest electrochemical performance at 40–120 % RH, with a maximum power density elevation of ∼42 % and charge transfer resistance improvement of ∼40 % under low humidity conditions. This enhancement can be attributed to the improved mass transport resulting from increased pore size and reduced distortion in transport pathways. Additionally, we performed a 5,000 cycle accelerated stress test (AST), demonstrating enhanced durability in Mix wt% Pt/C. Our strategy provides a reproducible and simplified process to achieve higher fuel efficiency. Moreover, we anticipate that this offers the potential to enhance both performance and durability in commercial PEMFC applications.

Assessing Pt and Ni dissolution mechanism and kinetics of shape-controlled oxygen reduction nanocatalysts

An electrochemical flow cell combined with inductively coupled plasma mass spectrometry (FC-ICP-MS) is a powerful tool to understand the mechanisms of metal dissolution and to develop mitigation strategies. Herein, we quantified in situ the amount of Pt and Ni atoms dissolved from PtNi/C nanocatalysts employed to electrocatalyze the oxygen reduction reaction (ORR) in proton-exchange membrane fuel cell cathode. The nanocatalysts feature similar crystallite size and Pt:Ni atomic ratio but different morphologies (spheres, octahedra, sponges). The FC-ICP-MS results reveal that the nanocatalyst morphology affects the dissolution rate of Pt and Ni but not the dissolution mechanism. They also provide analytical evidence that dissolution of Pt atoms is consistently accompanied by the dissolution of Ni atoms exceeding the stoichiometric composition. Furthermore, we demonstrate that ex situ acid leaching mitigates, but does not entirely prevent, the electrochemical dissolution of Ni atoms. Stabilized Pt and Ni dissolution rates were achieved after a one hour long accelerated stress test (AST). We provide evidence that the Pt dissolution rate remains constant before and after the AST. In contrast, the dissolution rate of Ni decreases by a factor of 10 following the AST. Among various nanoparticle shapes, spherical PtNi/C nanoparticles offer the best solution regarding the Pt and Ni retention compared to other nanoparticle shapes.

A robust protonic ceramic fuel cell with a triple conducting oxygen electrode under accelerated stress tests

Accelerated stress test of protonic ceramic fuel cells for evaluating the durability of materials and interfaces.

Effect of Engineered Cracks in Catalyst Layers on PEMFC Catalyst Layer Durability

Proton exchange membrane fuel cells (PEMFCs) are expected to play a pivotal role in decarbonizing the transportation sector, and particularly heavy-duty vehicles (HDVs). However, improvements in durability are needed for PEMFCs to compete with state-of-the-art power sources for HDVs. Here, we examine how catalyst layer (CL) cracks that are engineered affect the CL durability by using patterned silicon templates to control the CL crack density at the micrometer scale. Electrochemical analyses show that the initial PEMFC performance is relatively unaffected by crack density, but the performance after durability testing was strongly affected. Specifically, CLs with high crack density showed higher performance relative to CLs without cracks after application of a carbon corrosion accelerated stress test. Electrochemical analyses coupled with X-ray computed tomography and scanning transmission electron microscopy with energy dispersive X-ray spectroscopy showed that the cracks provide shorter oxygen diffusion pathways to reaction sites, leading to decreased oxygen transport resistance. Additionally, we observed that the catalyst durability is unaffected by cracks. Our results provide a mechanistic explanation of the role of cracks in CL durability.

Catalyst durability in electrocatalytic H2O2 production: key factors and challenges.

On-demand electrocatalytic hydrogen peroxide (H2O2) production is a significant technological advancement that offers a promising alternative to the traditional anthraquinone process. This approach leverages electrocatalysts for the selective reduction of oxygen through a two-electron transfer mechanism (ORR-2e-), holding great promise for delivering a sustainable and economically efficient means of H2O2 production. However, the harsh operating conditions during the electrochemical H2O2 production lead to the degradation of both structural integrity and catalytic efficacy in these materials. Here, we systematically examine the design strategies and materials typically utilized in the electroproduction of H2O2 in acidic environments. We delve into the prevalent reactor conditions and scrutinize the factors contributing to catalyst deactivation. Additionally, we propose standardised benchmarking protocols aimed at evaluating catalyst stability under such rigorous conditions. To this end, we advocate for the adoption of three distinct accelerated stress tests to comprehensively assess catalyst performance and durability.

Active and highly durable supported catalysts for proton exchange membrane electrolysers.

The design and development of supported catalysts for the oxygen evolution reaction (OER) is a promising pathway to reducing iridium loading in proton exchange membrane water electrolysers. However, supported catalysts often suffer from poor activity and durability, particularly when deployed in membrane electrode assemblies. In this work, we deploy iridium coated hollow titanium dioxide particles as OER catalysts to achieve higher Ir mass activities than the leading commercial catalysts. Critically, we demonstrate state-of-the-art durabilities for supported iridium catalysts when compared against the previously reported values for analogous device architectures, operating conditions and accelerated stress test profiles. Through extensive materials characterisations alongside rotating disk electrode measurements, we investigate the role of conductivity, morphology, oxidation state and crystallinity on the OER electrochemical performance. Our work highlights a new supported catalyst design that unlocks high-performance OER activity and durability in commercially relevant testing configurations.

Fatigue-Resistant Polymer Electrolyte Membranes for Fuel Cells.

In a hydrogen fuel cell, an electrolyte membrane conducts protons, but blocks electrons, hydrogen molecules, and oxygen molecules. The fuel cell often runs unsteadily, resulting in fluctuating water production, causing the membrane to swell and contract. The cyclic deformation can cause fatigue crack growth. This paper describes an approach to develop a fatigue-resistant polymer electrolyte membrane. The membrane is prepared by forming an interpenetrating network of a plastic electrolyte and a rubber. The former conducts protons, and the latter enhances fatigue resistance. The introduction of the rubber modestly reduces electrochemical performance, but significantly increases fatigue threshold and lifespan. Compared to pristine plastic electrolyte, Nafion, an interpenetrating network of Nafion and perfluoropolyether (PFPE) reduces the maximum power density by 20%, but increases the fatigue threshold by 175%. Under the wet/dry accelerated stress test, the fuel cell with the Nafion-PFPE membrane has a lifespan 1.7 times that of a fuel cell with the Nafion membrane.

Impacts of characteristics of carbon black comprising microporous layer on performance and durability of gas diffusion layer under newly developed accelerated stress tests

The gas diffusion layer (GDL) is an essential component of a proton-exchange membrane fuel cell (PEMFC) that supplies reactant gases to catalysts by facilitating the discharge of water generated via electrochemical reaction. The GDL is primarily composed of a substrate and a microporous layer (MPL), of which the latter increases the specific contact area for electrochemical reactions with catalysts and forms a capillary pressure gradient within the GDL. The MPL primarily comprises carbon black and polytetrafluoroethylene. Carbon black is categorized into the main and additive carbon blacks. Additive carbon black is utilized to augment the specific surface area of the MPL. This study examines the effects of four GDLs with varying additive carbon black contents constituting 20%–30 % of the total carbon black content on the performance of PEMFCs. In addition, the effects of the MPL characteristics on the durability of the GDL is investigated systematically, particularly in relation to carbon corrosion and freeze/thaw conditions. Furthermore, the deterioration in the performance and properties of the GDL are investigated via an acidic solution immersion accelerated stress test.

Unraveling the Effects of Carbon Corrosion on Oxygen Transport Resistance in Low Pt Loading Proton Exchange Membrane Fuel Cells.

Cost and durability have become crucial hurdles for the commercialization of proton exchange membrane fuel cells (PEMFCs). Although a continuous reduction of Pt loading within the cathode catalyst layers (CCLs) can lead to cost savings, it also increases the oxygen transport resistance, which is further compounded by key material degradation. Hence, a further understanding of the mechanism of significant performance loss due to oxygen transport limitations at the triple phase boundaries (TPBs) during the degradation process is critical to the development of low Pt loading PEMFCs. The present study systematically investigates the impact of carbon corrosion in CCLs on the performance and oxygen transport process of low Pt loading PEMFCs through accelerated stress tests (ASTs) that simulate start-up/shutdown cycling. A decline in peak power density from 484.3 to 251.6 mW cm-2 after 1500 AST cycles demonstrates an apparent performance loss, especially at high current densities. The bulk and local oxygen transport resistances (rbulk and Rlocal) of the pristine cell and after 200, 600, 1000, and 1500 AST cycles are quantified by combining the limiting current method with a dual-layer CCL design. The results show that rbulk increased from 1527 to 1679 s cm-2, Rlocal increased from 0.38 to 0.99 s cm-1, and the local oxygen transport resistance with the normalized Pt surface area (rlocal) exhibited an increase from 18.5 to 32.0 s cm-1, indicating a crucial impact on the structure collapse and changes in the chemical properties of the carbon supports in the CCLs. Further, the interaction between the ionomer and carbon supports during the carbon corrosion process is deeply studied via electrochemical quartz crystal microbalance and molecular dynamics simulations. It is concluded that the oxygen-containing functional groups on the carbon surface could impede the adsorption of ionomers on carbon supports by creating an excessively water-rich layer, which in turn aggravates the formation of ionomer agglomerations within the CCLs. This process ultimately leads to the destruction of the TPBs and hinders the transport of oxygen through the ionomer.

(Invited) Modeling of Catalyst Degradation in Toyota's Mirai and Its Application to Target-Setting for Future PEFC's Durability

To popularize Polymer electrolyte fuel cells (PEFCs) as a power source for various types of mobility, improving their durability is necessary. One of the factors that determines the durability of PEFCs is the electrochemical surface area (ECSA) loss of the cathode catalyst which causes a drop in PEFC’s output power and generation efficiency. The loss of ECSA has been modeled by a mechanism based on Ostwald ripping, and the model has been validated by accelerated stress testing (AST) [1]. Using AST, it is possible to evaluate the durability of PEFCs with short time, but in actual applications, such as mobility, the inputs for PEFCs are complex rather than simple repetitive inputs, and the relationship with AST is not clear.In this study, we proposed a method for setting AST conditions that correspond to the stresses of actual applications. In order to correlate the AST with the real driving test, a simple model based on Ostwald ripping was developed, and the model was validated by experimental results for both actual vehicle tests and ASTs. Validation for the actual vehicle was performed between simulated values from Mirai's real driving data and post-driving measurements. Since the model represents well both the experimental results of both the AST and the actual vehicle, it is considered that the model can convert the stress intensity between the actual vehicle test and the AST.Using this model and PEFC vehicle model [2], we set a target durability required for future heavy-duty trucks, and the AST conditions to evaluate it. In the presentation, we will discuss the details of the proposed target-setting procedure and models.[1] R. Darling et al., J. Electrochem. Soc., 2003, 150, A1523[2] T. Watanabe, et.al, 242nd ECS Meeting, MA2022-02, 1468 (2022)