Hygro-mechanical Stress Research Articles

Novel risk assessment tools for the climate-induced mechanical decay of wooden structures: Empirical and machine learning approaches

The microclimate in which historical buildings and objects are placed strongly influences the mechanical decay and properties of the constituent materials, especially if they are susceptible to fluctuations of temperature (T) and relative humidity (RH). For this reason, in this work, the attention is focused on the indoor microclimate of an historic building completely made of Scots pine wood: Ringebu Church (Norway). In particular, the indoor RH of Ringebu church has been analyzed by means of the European Standard EN15757, which establishes guidelines on assessing whether the RH fluctuations are risky for the conservation of historic hygroscopic materials (such as wood). However, for conservation purposes, it is useful to further study the entity of the RH fluctuations. In this framework, a novel simple strategy, named Median of Data Strategy (MoDS), for identifying RH drops is here proposed; the new approach, by scanning the RH time series and being tested on several examples, displays potential for better assessing the risk of degradation of wooden assets, objects, and artefacts. Then, based on evidence available in literature, a simple empirical model for computing the levels of hygro-mechanical stress developed due to hygric changes has been described and a novel tool for assessing the climate-induced mechanical risk for wooden structures has been proposed. Finally, a machine learning approach for predicting whether climatic fluctuations may have catastrophic effects on the historical wooden materials is presented as well. The obtained results show that the different proposed approaches may be useful for general assessments on the risk of decay of wooden samples and they open a pathway for future investigations in the fields of fracture mechanics, fatigue behavior and smart timeseries prediction for conservation and preservation purposes.

Prediction of Delamination Failure in Quad Flat No-Lead Packages Using Combined Experimental and Simulation Methods

In this research, finite-element analysis and failure analysis were performed to analyze delamination failure in quad flat no-lead (QFN) packages. Although a QFN is a traditional package and represents mature technology, the reliability standards of QFNs in automotive applications have become strict. The quality requirements of QFNs are increasing and require that no delamination or voids occur inside the package during the reflow process and precondition tests. Therefore, interfacial delamination is still a critical reliability issue for QFNs. Delamination failure weakens the internal structural strength of a QFN and affects the product reliability. In the interface delamination, initially generated small voids gradually expand due to hygromechanical stress during the moisture sensitivity level test. In addition, moisture penetration into the package can result in not only electrical failure but also chemical reactions that lead to corrosion. In this study, delamination occurs on the interface between the Cu pad and molding compound during the precondition tests. The available evidence indicates that the bonding strength between Ag plating on the Cu pad and molding compound is weak. Most delamination failure sites are located in the area of Ag plating according to scanning acoustic tomography (SAT) observations. To accurately predict the delamination failure in QFNs, the shear force on the molding compound/Ag plating and molding compound/Cu is combined with finite element modeling. Parametric analysis is conducted to determine the optimal design of the geometric structure in the QFN to combat delamination failure. This investigation can help improve the product reliability by preventing delamination failure in the QFN package through the analysis of the physics of failure (PoF).

A simulating method of moisture continuous diffusion under changing temperatures and analysis of moisture-induced stresses covering moisture desorption and reflow processes for the QFN

Moisture in the atmosphere could diffuse into plastic packaging devices, which would cause high hygro-mechanical stress and vapor stress and lead irreversible damages. In this paper, an analysis method for moisture continuous diffusion under changing temperature is proposed. Considering the vapor pressure induced strain as hygro-mechanical strain, a corresponding relationship between the parameters of hygro-mechanical stress and vapor stress is built. Subsequently, the formula of moisture-induced total stress is derived according to the superposition principle. A QFN (Quad Flat No‑lead) package model with gold wires inside is established to validate the effectiveness of the method. The moisture continuous diffusion from the absorption process to the desorption one is obtained, then the dynamic variations of the hygro-mechanical stress, vapor stress and moisture-induced total stress of the QFN during the desorption process and the entire reflow process are obtained for the first time ever. The results prove that the method in this paper can be employed for the analysis of moisture diffusion and moisture-induced stress under dynamic temperature and uneven humidity distribution.

Investigation of the transient freeze start behavior of polymer electrolyte fuel cells

Understanding the water management in polymer electrolyte fuel cells during sub-zero operation is crucial for designing effective freeze start strategies. This study uses sub-second operando X-ray tomographic microscopy to study the effect of warm-up rate and pre-drying on the water dynamics during non-isothermal freeze starts from −30 °C. A faster warm-up rate and cell pre-drying before freezing improved the cell performance during the freeze start. Imaging showed that during the freeze starts no new water clusters were formed in the cathode gas diffusion layer. Temporary catalyst layer detachment from the micro-porous layer was also observed between −20 to 18 °C due to the hygro-mechanical stresses during the sub-zero operation. Isothermal freeze starts were also performed to study the effect of membrane conductivity and local water flux in the catalyst layer during the sub-zero operation. Higher temperatures and lower operating current densities resulted in an increase in the operation time. Imaging results showed water clusters in the cathode gas diffusion layer for the isothermal freeze starts from −15 to 0 °C after 6 C/cm2 charge was produced.

Transient strength decrease of Graphite-Phenolic laminated composites during drying

Graphite-Phenolic (GPh) laminates, used as structural parts under hygro-mechano-thermal loadings, may suffer premature interlaminar failures due to strength reduction during service life. The aim of the present study was to show that significant transient decrease in the strength of GPh specimens can be associated with the build-up of time-dependent hygromechanical (HM) stresses, caused by high shrinkage strains. To characterize the specimens' transient strength reduction, flexure test sets were conducted during controlled drying, revealing minimal strength values. Developing a coupled HM model allowed associating the transient strength reduction with the evolution of HM interlaminar stress. Although HM coupled models were previously established for composite materials, the effect on the composite's strength was not directly measured. In the present study, substantial decrease in the composite's strength was demonstrated, with good predictability of minimal strength time. Further work is still needed to establish the failure criterion and its relation to the material's characteristics.

Hygromechanical coupling in laminate composites and its effect on interlaminar failure

Graphite-Phenolic (GPh) laminates are used as structural parts under mechanical and hygrothermal loadings. High shrinkage strains during drying may cause premature interlaminar failure which originates from the hygromechanical coupling. In this study, modified coupled hygromechanical governing equations which include non-constant coefficient of moisture expansion (CME), were derived. The model was experimentally validated by measuring transient weight and strain between various equilibrium states of humidity, obtaining relevant material parameters. Finally, a set of three point bend specimens was used to obtain the strength profile during drying, which revealed a minimal value at a characteristic time. Using a non-local failure criterion it was possible to associate the temporal strength profile with the build-up of hygromechanical stresses.

Integrated Process-Aging Modeling Methodology for Flip Chip on Flex Interconnections With Nonconductive Adhesives

This paper presents a comprehensive methodology to model the static temperature-humidity (TH) aging test ( 85 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">deg</sup> C/85%RH over 1000 h) of flip chip on flex interconnections with nonconductive adhesives (NCAs). NCAs, being a special form of conductive adhesives, are chosen, as they allow bringing the pitch further down. The methodology combines experimental techniques for material characterization, finite element modeling (FEM), and model validation. A NCA has been characterized using several techniques. The thermomechanical properties and the moisture absorption properties were obtained for the NCA. A temperature dependent viscoelastic constitutive model was also obtained for the NCA. The viscoelastic model was defined by the Prony series expansion. The shift factor was approximated by the Williams-Landel-Ferry (WLF) equation. Finite element modeling has been performed to analyze the flip chip interconnects on flex with the NCA under process condition and reliability aging conditions. The viscoelastic constitutive relation has been used to model the NCA in aging modeling. An integrated process-aging modeling methodology has been developed to combine the thermo-mechanical stress and hygro-mechanical stress, followed by stress relaxation analysis. To verify the finite element models, static TH aging tests (85 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">deg</sup> C/85%RH) were also performed. The contact resistance was monitored with high measuring resolution during the accelerated test. The simulation results are in good agreement with the experimental results. The approach developed in this paper can be used to provide guidelines with respect to adhesive material properties, assembly process parameters to achieve good reliability performance.

Two-Step Fracture Mechanics–Based Approach to Assess Early-Age Delamination Distress

The development of a comprehensive, early-age delamination model requires an integration of interfacial strength of the aggregate–mortar interface and environmental, materials, and construction effects. Previous research has led to the establishment of the model's mechanism that can be characterized with engineering mechanics. However, fracture mechanics theory, which is more suitable to represent the delamination process, has not been widely applied to concrete pavement analysis. A two-step fracture mechanics–based model was developed to predict the occurrence potential of delamination in continuously reinforced concrete pavements. The model is based on the comparison between the stress intensity factor (KI) and fracture toughness (KIC). The development of the KI of concrete at an early age was simulated through a two-step numerical analysis, including (a) a coupled thermal–hygro analysis to predict the temperature and moisture profiles and the associated material properties and (b) a coupled hygro–mechanical stress analysis to predict the differential drying shrinkage-induced stress. This model addressed the integration of effects of environment, materials, and construction factors through key parameters that included moisture diffusivity, shrinkage strain gradient, pavement thickness, and elastic modulus of concrete.

Mechanism and thermal effect of delamination in light-emitting diode packages

This work reports on the mechanism of delamination in light-emitting diode (LED) packages and its effects on thermal characteristics of LEDs. The LED samples were subjected to moisture preconditioning followed by heat block testing. Transient thermal measurements were performed to investigate the thermal behavior of the delaminated LEDs. Increase of thermal resistance with the degree of delamination was observed from the transient measurement. The thermo-mechanical and hygro-mechanical stress distributions calculated from coupled-field FEA simulation agree well with the micrographical evidence. It was found that the thermo-mechanical stress plays more important role than the hygro-mechanical stress for the development of delamination in the LED packages. Moisture preconditioning for 3 and 6h under 85°C/85RH conditions was found to make little contribution to the delamination between the chip and lead frame.

Numerical and Analytical Scale-Transition Prediction of Hygro-Mechanical Stresses in Multidirectional Carbon-Epoxy Laminates

Moisture diffusion within an epoxy matrix based composite structure exposed to humid environmental conditions induces two types of internal stresses: the macroscopic stresses (at the scale of the composite plies) and the microscopic stresses (experienced by the elementary constituents of a considered ply). Macroscopic stresses induced by gradients of moisture concentration and/or the heterogeneity of the coefficients of moisture expansion are determined using continuum mechanics classical formalism. This method enables taking into account both space and time effects on moisture diffusion in the composite structure. Localization of the macroscopic mechanical states at microscopic scale lead to different stresses in the matrix and the fiber. The discrepancies come from the strong heterogeneities of elastic properties, coefficients of moisture expansion and moisture content of the composite plies constituents. Scale transition models are often used in order to achieve the localization procedure.

The effect of hygro-mechanical and thermo-mechanical stress on delamination of gold bump

The reliability of the FC–CSP (flip chip–chip scaled package) package with gold bump at the MRT (moisture resistance test) reflow temperature, was evaluated by using the finite element method. The moisture properties of EMC (epoxy molding compound) obtained from the test described in JEDEC standard, were used to characterize the local moisture concentration analysis by transient moisture diffusion, the hygro-mechanical analysis by CME, the vapor pressure analysis and the thermo-mechanical analysis by CTE mismatch. Also, after precondition, the package reliability under the reflow process was predicted, by comparing and integrating each factors, package swelling and stress due to by vapor pressure, as well as thermo-mechanical stress. Consequently, the result showed that the effects on hygro-mechanical stress and vapor pressure in a package could not be negligible, when it is compared with that of the thermo-mechanical stress by CTE mismatch, which is recognized as the main effect on the package crack under reflow temperature. The stress was concentrated at interface between gold bump and die, where most of delamination occurred.

Integrated vapor pressure, hygroswelling, and thermo-mechanical stress modeling of QFN package during reflow with interfacial fracture mechanics analysis

In this paper, a comprehensive and integrated package stress model is established for quad flat non-lead package with detailed considerations of effects of moisture diffusion, heat transfer, thermo-mechanical stress, hygro-mechanical stress and vapor pressure induced during reflow. The critical plastic materials, i.e., moldcompound and die attach are characterized for hygroswelling and moisture properties, which are not easily available from material suppliers. The moisture absorption during preconditioning at JEDEC Level 1, and moisture desorption at various high temperatures are characterized. The moisture diffusivity is a few orders higher at reflow temperature than moisture preconditioning temperature. Due to coefficient of moisture expansion mismatch among various materials, hygro-mechanical stress is induced. The concept is analogous to coefficient of thermal expansion mismatch which results in thermo-mechanical stress. Thermal diffusivity is much faster than the moisture diffusivity. During reflow, the internal package reaches uniform temperature within a few seconds. The vapor pressure can be calculated based on the local moisture concentration after preconditioning. Results show that the vapor pressure saturates much faster than the moisture diffusion, and a near uniform vapor pressure is reached in the package. The vapor pressure introduces additional strain of the same order as the thermal strain and hygrostrain to the package. Subsequently, the interfacial fracture mechanics model is applied to study the effect of crack length on die/mold compound and die/die attach delamination.