Bond Line Research Articles

Understanding Criticality of Thermal Performance in Thermal Interface Material Applications

Thermal interface material (TIM) is used in between a heat generating component (e.g., microelectronic packaging) and a heat spreading component (e.g., heatsink or cooling plate) to create an effective path for the heat (Phonon) to travel. Standard heatsink and heat generating component surfaces are generally uneven and rough. Actual metal to metal contact is no more than 10%. These surface imperfections allow air to get trapped in between the two surfaces. Air, being a thermal insulator, prevents the heat from dissipating and thus the device/system fails to maintain the required operating temperature to meet the reliability and functionality needed. Replacing the air with the “right” thermal interface material is the focus of this research. TIM is a composite of thermally conductive fillers dispersed in a polymer matrix. A higher filler loading causes a higher bulk conductivity. Common practice among design engineers is to utilize output from thermal modelling & simulation to specify a TIM with a certain thermal conductivity to meet the system’s thermal needs. What many engineers miss is the impact of thermal boundary resistance that could have significant effect on the overall thermal management of the design. This paper discusses how to characterize thermal performance of TIM beyond the bulk thermal conductivity. Boundary/interfacial thermal resistance and impedance will be explored as a function of TIM wetting ability, bond line thickness and surface conditions. This paper will discuss the importance of thermal conductivity, thermal resistance/impedance in a real application scenario.

Thermal Performance of Liquid Metal Pastes Containing low Content of Metal Particles for Thermal Interface Materials in IC Module Electronics

In recent years, considerable attention has been paid to gallium-based liquid metal alloys in their use as thermal interface materials for GPU, APU, and HPC in PC and super computers due to its considerably high thermal conductivity and low contact thermal resistance. Gallium-based Liquid metal paste containing low content of metal particles (LMP-MPs) to enhance thermal performance and controllable BLT of the prepared liquid metal composites for thermal interface materials (TIMs) prepared by in-situ introducing gallium oxide into the liquid metal alloys will be reported in our recent research. In this paper, we will report effect of the composition, particle size/types, and bond line thickness on thermal properties of the LMP-MPs. Thermal properties of the LMP-MPs were measured at 50o C using a thermal tester, TIMA5, based on ASTM-D5470 test methodology. It was found in the present study that the composition and types of metal particles, and BLT have significant influence on thermal conductivity, and the conductivity values increase with an increase of the content of metal particles and BLT. However, thermal resistance varies with the change of composition and BLT of LMP-MPs. Scanning electron microscope (SEM) was used to characterize microstructure of LMP-MPs with different metal particles and sizes. Micrographic morphology of the LMPs-MPs shows continuous frame structure observed with SEM, which could keep liquid metals from spreading out and stabilize the phase. We eventually found the LMP-MPs have the better wetting and adhesion properties on bare Cu, glass and bare Si surface than liquid metal alloys that endows good printability to the LMP-MPs. Continuous monitoring of thermal resistance of LMP-MPs at 45o C and 89o C using a house-made thermal tester shows stable thermal resistance after a few months, which indicates LMP-MPs have excellent thermal reliability in ambient atmosphere.

Development of all new structure of Dicing Die Attach Film (DDAF) for Stealth Dicing and Cool Expansion processes

Mobile devices, including smart phones and tablets, are increasing in complexity and require ever greater amounts of memory storage. In the past the challenge of higher memory densities and increase functionality has been met through miniaturization of device features, but as we reach the physical limits of feature size, packaging is being relied on more and more to achieve the necessary higher densities in memory and additional device functionality. Thin dies reduce overall package height, however, to achieve high memory densities in low profile packages the die attach material must have an even, thin bond line thickness. DAF (Die Attach Film) is an excellent solution. LINTEC developed an all new DDAF (Dicing Die Attach Film) with thinner DAF for CE (Cool Expansion) process. Conventional tape has a large area of exposed DAF surface which creates the risk of wafer and / or equipment contamination if DAF flies off during the CE process. We developed an all-new tape structure which is compatible with the SD (Stealth Dicing) and CE processes. The structure consists of DAF, IB (Intermediate Basefilm), and dicing tape. The diameter of the DAF and IB is close to the wafer diameter size to significantly reduce the risk of DAF separation and ejection during CE. We considered IB to improve DAF separation and it was found that a thinner IB layer is a good approach to separate DAF effectively. We can adjust adhesion between the DAF and the IB by changing the material properties of the IB, as an effective means to control die-fly and pick-up.

Numerical modeling investigation of cross-laminated timber connections consisting of multiple glued-in rods

Glued-in rods are stiff and high-capacity connections that are used in timber structures. Regardless of the many advantages of glued-in rods, research on the glued-in rods in CLT is limited, especially on numerical modeling. This paper provides the numerical modeling principles, validation details, and parametric studies of glued-in rods embedded on the edge of CLT. The numerical modeling was performed on a 3D finite element basis, and the cohesive surface method was used to define the bond lines along the rods. To validate the numerical models, 24 replica experiments were performed and investigated the effect of the rod diameter and of the embedment length on the pull-out strength of rods embedded perpendicular, parallel, and on the boundary of two cross-wised layers. The numerical models were validated according to the maximum load, stiffness, as well as the failure mode of the glued-in rods. The numerical models showed high reliability in simulating the experiments and predicted the strength of the glued-in rods with a maximum of 4% difference. After validation of the numerical models, the parametric study covered the pull-out strength investigation of rods with 20, 24, and 30 mm diameter and anchorage lengths of 250, 350, and 450 mm embedded in the perpendicular, parallel, and on the boundary of cross-wised layers on the edge of 5 and 7 layer CLT panels. The findings showed that increasing the embedment length and rod diameter enhanced the pull-out strength of the glued-in rods, regardless of where the rod was positioned on the CLT's edge. Rods embedded perpendicular to the grain showed the highest pull-out strength, and two rods with 30 mm diameter and 450 mm embedment lengths reached 961 kN pull-out strength. Numerical models showed that for the same diameter and embedment length of two rods embedded on the parallel and on the boundary of two cross-wised layers reached 836 and 738 kN pull-out strength, respectively. Finally, A design equation is provided to determine the pull-out strength of the glued-in rods embedded in CLT based on the shear strength of the bondline.

Analysis of Wood Bonding Failures that Initiated Before Adhesive Solidification: Air Fingers and Cavitation

Bond line openings while an adhesive is still viscous (liquid or paste-like) leave characteristic marks. Air fingers and cavitation develop on the surface of the adhesive when joint parts are fully or partially separated before solidifying of the adhesive. Therefore, the observation of air fingers or cavitation provides important clues on the root cause of the failure of bond lines. There is still limited knowledge about the factors that lead to the formation of either air fingers or cavitation. Additionally, a resoftening of thermoplastic adhesives by high temperatures may be confused with air fingers or cavitation that developed before the initial curing. To improve the understanding of the adhesive structures, we assemble some experiments on this phenomenon. Air fingers dominate when air easily penetrates the adhesive, the adhesive thickness is high, and the adhesive has a low viscosity (early in the process). Cavitation dominates when the adhesive layer is thin, when the viscosity is high (late in the process) or when the air ingress is restricted.

Microstructural characteristics and mechanical response of diffusion bonding Inconel 617 superalloy

This study focuses on the microstructural characteristics and mechanical response of the diffusion-bonded joints of Inconel 617 superalloy. The joints exhibited two types of interfacial microstructure involving the straight interface (i.e., bond line) and the interface with partial migration of grain boundaries (GB). The straight interface was composed of the GB of γ-Ni, M23C6 carbide and α-Al2O3 oxide. The M23C6 carbide was precipitated with a semi-coherence of 33̅3M23C6//111̅γ−Ni, and an amorphous interface was detected between the α-Al2O3 and γ-Ni phase. The migration of GB across the interface was mainly caused by dynamic recrystallization and strain-induced GB migration. In addition, the α-Al2O3 oxide was remained inside the migrated grain. The formation of the M23C6 and α-Al2O3 was governed by diffusion mechanism, and the γ′-Ni3Al phase was formed surrounding the α-Al2O3 due to the abundant diffusion of Al. The straight interface, particularly embedded with the hard particles (i.e., the M23C6 carbide and α-Al2O3 oxide), restricted GND slipping, resulting in concentration of stresses and initiation of cracks. The migrated interface, in contrast, effectively prevented crack propagation and induced crack deflection, supporting more continuous and uniform plastic deformation. The elongation of 51.2 % was obtained for the joint with partial migrated interface, reaching 71 % of the base metal. Nevertheless, the optimum impact toughness of the joints (20.38 J/cm2) was merely 24 % of the base metal, mainly affected by the remained straight interface.

Superior sinterability of copper oxalate-coated Cu particles in a double reductant system and rapid compression sinter-bonding characteristics between Cu finishes

High-speed die attachment by compression (2 MPa) sinter-bonding at 300 °C in air was accomplished using a paste containing bimodal-sized (average sizes: 2 μm and 340 nm) copper oxalate-coated Cu particles, yielding a Cu-to-Cu bonding owing to the formation of a Cu bond line that can provide mechanical durability at high temperatures and excellent thermal conductivity. The particles were fabricated by immersing Cu particles in an oxalic acid solution, and a double reductant recipe was adopted to minimize the oxidation of the Cu particles and Cu finishes. The copper oxalate coatings inhibited the oxidation of Cu particles until decomposition at 290 °C, and the in-situ decomposition induced the generation of active Cu atoms, which significantly elevated the sinterability. Therefore, a sufficient shear strength exceeding 20 MPa and a near-full-density bond-line structure were achieved after bonding for only 1 and 3 min, respectively. Furthermore, the bond line did not contain an organic residue, differently observed in a paste directly including carboxylic acid. Therefore, the suggested paste formulation demonstrates the feasibility of a rapid solid-state sinter-bonding technique using low-cost Cu particles.

Performance of Tri-Adhesive Joints to Improve the Shear Stress Distribution of Lap Joints

The adhesive lap joints are extensively used in various engineering fields. Various methods were proposed to increase the strength of the lap joint. This paper presents the lap joint's characterization by applying three grades of adhesives in different material properties along the bond line. The stiffest adhesive is employed in the middle bond line, then gradually, those with a lower modulus of elasticity are placed at the ends of the lap joint. This technique reduces the stress concentration at the joining ends, so the stress distribution becomes smoother. Finite element analysis is used to model this problem in two dimensions. One of the adherent edges is applied to the fixed support, and the other end is subjected to tension. The analysis results show that the use of tri-adhesive changes the shear stress distribution along the bond line flatter and increases the strength of tri-adhesive lap joints compared to those single type adhesive applied individually.

Experimental deformation analysis of an adhesively bonded multi‐material joint for marine applications

AbstractThe strength and deformation of full‐scale adhesively bonded multi‐material joints is studied in this paper. Four joints with a thick layer of methyl methacrylate adhesive (MMA) have been manufactured in shipyard conditions. In two specimens, cracks have been introduced at steel–adhesive and composite–adhesive interfaces. One cracked and one un‐cracked specimen were subjected to quasi‐static tensile testing; the two remaining specimens were stepwise loaded/unloaded with increasing load until failure. The strain in the adhesive layers was measured with digital image correlation (DIC). This showed a predominant shear deformation and dissimilar shear strain patterns for different bond lines. Fibre Bragg (FBG) sensors were used to monitor strains at steel and composite constituents and to detect the onset and evolution of damage in the un‐cracked specimen. Strains measured by FBG sensors correspond well with DIC results at nearby regions. All specimens failed by delamination of the composite panel near the composite–adhesive interface.

Debonding model for nonlinear Fe-SMA strips bonded with nonlinear adhesives

The application of adhesively-bonded joints for strengthening of structures using iron-based shape memory alloys (Fe-SMAs) has recently emerged in construction. Fe-SMAs and the majority of structural adhesives exhibit a pronounced nonlinear material behavior, which may result in a favorable ductile failure mechanism. The development, however, of a mechanical model to predict the structural behavior of the joint is non-trivial due to the presence of nonlinearity in the adherent and adhesive. This study aims to propose a semi-analytical and semi-numerical model for describing the mechanical behavior of Fe-SMA-to-steel adhesively bonded joints. The developed model serves three main functions: (i) estimating the bond capacity for a given interfacial fracture energy, and vice versa; (ii) processing the bond–slip (τ−s) behavior directly from the load–displacement (F−Δ) curve, and vice versa; and (iii) delivering a numerical method to simulate the full-range mechanical behavior of the bonded joints, namely the behavior at different loading stages. The model is validated using the experimental testing of 26 Fe-SMA-to-steel lap-shear joints, as well as 24 further bonded joints subject to shear with different adherents (e.g., stainless steel strips and Nickel–Titanium SMA wires) and base materials (e.g., concrete and composite polymer). An experimental data processing protocol, on the basis of the experimentally measured force–displacement (F−Δ) behavior and the distributed displacement along the bond line (s−x) via the Digital Image Correlation (DIC) technique, is further proposed to assess the full-range behavior of bonded joints.

A novel cohesive zone modelling approach to represent mixed mode loading and bond line thickness effects

ABSTRACT Accurate representation of the traction–separation response for mixed mode loading in a cohesive zone model (CZM) is critical to predicting the response of adhesive joints in a number of applications, including transportation and vehicle crashworthiness. Traditionally, the Mode I and Mode II responses are treated independently, with mixed mode response determined by relationships between the degree of mode mixity and separation, potentially leading to overprediction of the plateau traction and underprediction of the plateau length in mixed mode loading. This poor fit is due to the indirect relationship between mixity and traction and having minimal fitting options for separation-to-plateau and softening. To address this limitation, a mixed mode CZM approach is proposed, based on measured mixed-mode traction–separation results for a toughened epoxy adhesive. The effects of bond-line thickness were considered, to examine the ability of the proposed approach to include additional effects (beyond mode mixity) that are known to affect the traction–separation response. The CZM implementation was assessed using the original test data and was shown to capture the measured experimental traction–separation response across a range of mixed mode loading and bond line thickness more accurately compared to traditional CZM treatments.

SHORT NOTES: THE TEA LEAVES AS A FILLER FOR UF RESIN PLYWOOD PRODUCTION

The addition of green tea leaves fractions < 0.315 mm as afiller to urea-formaldehyde resin to reduction of emission of formaldehyde from three-layer birch plywood was investigated. Moreover, other properties such as bonding quality and water resistance of plywood were investigated. It was found that green tea had an ability to absorb formaldehyde. This phenomenon was caused by phenolic compounds contained in green tea leaves. The addition of green tea in the amount of 20% and 25% resulted in a decrease in formaldehyde emission fromplywood comparing to the reference sample. Thebonding quality test showed that the addition of tea leaves as a filler did not affect thestrength properties of the tested bond lines.

Mechanical heterogeneities in cured structural wood adhesives

Wood adhesive bond lines are typically considered chemically and mechanically homogeneous and the occurrence of heterogeneities in cured wood adhesives has rarely been reported in the literature. Nanoindentation, SEM-EDX, and confocal Raman imaging were used to distinguish phases in cured adhesive bond lines that differ in their mechanical and chemical properties. In filler-containing adhesive systems, differences in elastic modulus and chemical composition were revealed that indicated local variations in their filler distribution. Different filtration mechanisms of the wood adherend or porous barriers formed by filler particles are assumed to be responsible for such differences in filler concentration and the particle size between the bulk bond line and the wood–adhesive interphase. The occurrence of such differences shows that assuming the wood bond line as a homogeneous material in the micromechanical modeling of adhesive bonds cannot be generalized.

A Laser Shock-Based Disassembly Process for Adhesively Bonded Ti/CFRP Parts

The application of adhesively bonded joints in aerospace structural parts has increased significantly in recent years and the general advantages of their use are well-documented. One of the disadvantages of adhesive bonding is the relevant permanence, when compared to traditional mechanical fastening. End-of-life processes generally require the separation of the adherents for repair or recycling, and usually to achieve this, they combine large mechanical forces with a high temperature, thus damaging the adherents, while consuming large amounts of energy. In this work, a novel disassembly technique based on laser-induced shock waves is proposed for the disassembly of multi-material adhesively bonded structures. The laser shock technique can generate high tensile stresses that are able to break a joint, while being localized enough to avoid damaging the involved adherents. The process is applied to specimens made from a 3D-woven CFRP core bonded to a thin Ti layer, which is a common assembly used in state-of-the-art aircraft fan blades. The experimental process has been progressively developed. First, a single-sided shot is applied, while the particle velocity is measured at the back face of the material. This method proves ineffective for damage creation and led to a symmetric laser configuration, so that the tensile stress can be controlled and focused on the bond line. The symmetric approach is proved capable of generating a debonding between the Ti and the CFRP and propagating it by moving the laser spot. Qualitative assessment of the damage that is created during the symmetric experimental process indicates that the laser shock technique can be used as a material separation method.

Determination of the Bonding Strength of Finger Joints Using a New Test Specimen Geometry

In this study, a specimen geometry for testing finger joints was developed using finite element simulation and proofed by experimental testing. Six different wood species and three adhesives were used for finger-jointing specimens. With the test specimen geometry, the bonding strength of the finger joints was determined without the usual self-locking of the joint. Under load, the test specimen geometry introduces maximum stress at the beginning of the bond line (adhesive zone). However, the test specimen geometry does not generate a symmetric stress state. The main difficulty here is the flank angle of the finger joint geometry. The wood species and adhesives significantly influenced the performance of the finger joints.

INFLUENCE ON THE LOAD‐DISPLACEMENT BEHAVIOUR OF STEEL‐TO‐CONCRETE CONNECTIONS WITH POST‐INSTALLED ADHESIVE ANCHORS

AbstractDesigning frame structures requires knowing the behaviour of each member of the frame (beam, column, joint, etc.) regarding the axial forces, shear forces, and bending moments. Furthermore, the ductility of the structure under seismic loading is essential in earthquake regions. Nowadays, structural frames are built from different materials such as concrete and steel, to achieve better performance. Therefore, the behaviour of the connections between steel and concrete is essential. The steel‐to‐concrete joints were the focus of the INFASO project, where joint solutions with easy fabrication, quick assembly, applicability in old structures, sufficient ductility, and high loading capacity were developed. They proposed the use of anchor plates with welded studs or post‐installed fasteners such as adhesive anchors to connect the steel and concrete members. This paper focuses on the performance of post‐installed adhesive anchors. During their service life, post‐installed anchors are subjected to monotonic, constant, and seismic loading. Each of these loading approaches is described in the current standards. Seismic and constant loading tests are of importance to the long‐term behaviour of the anchors. Seismic loading tests are carried out using a predefined cycle pattern. According to TR049, seismic tests of category C1 (tension and shear) are performed with 140 load cycles, where the load amplitude decreases after 10, 30, and 100 cycles. On the contrary, C2 category tests increase the amplitude within 75 or 59 cycles, depending on the test. Constant loading tests apply when the anchors are installed in cracked concrete. In contrast to the mentioned patterns, this study observes the behaviour of the adhesive anchors when the amplitude of each cycle is increased by 5 % until the anchor fails. Standard short‐term pull‐out tests are carried out to determine the load increments. Various parameters such as the embedment depth, bond line thickness, hole cleaning, wet concrete, and elevated temperatures are studied. Confined and unconfined tests are performed. Overall, the reference short‐term failure loads are higher than those in the tests with incremental and cyclic loading. The anchors installed with reduced hole cleaning, in wet concrete or subjected to elevated temperatures have a lower failure load compared to the reference series. Two additional bond line thicknesses are used for comparison. The increase in the thickness influenced the failure load differently for confined and unconfined test setups.

Borate Treatment of CLT Panels Using Vacuum: A Proof of Concept

Abstract Cross-laminated timber (CLT) is an increasingly popular wood-based alternative for large building applications. CLT panels are typically not treated due in part to a perceived lack of efficient and effective methods for treating large panels and a lack of information on what effect treatment processes may have on the panels' mechanical properties. We propose that treating CLT panels with borate solution, applied under vacuum in flexible bags, could provide a practical and effective option for providing preservative protection for interior applications. Samples were cut from commercially produced CLT panels and treated with borate solution using vacuum. The samples were then evaluated for preservative retention, swelling, and degradation of mechanical properties. Initial treatments resulted in a wide range of preservative retentions and property effects among the products tested. In subsequent adjustments, the treatment parameters were changed to provide consistent and sufficient retention among the products. The vacuum treatment method effectively penetrated cracks in the lumber and the bond lines (adhesive joint between adjacent lamellae). Swelling and effects on mechanical properties were minimal in the adjusted samples. In addition to these samples that were treated in a rigid pressure vessel, a larger sample was successfully treated in a flexible plastic bag. Data from this study support the concept that vacuum treatment of CLT panels with borate can provide sufficient levels of preservative retention, can be adjusted to the material being treated, and has minimal effects on mechanical properties.

AI-based 3D Metrology and Defect Detection of HBMs in XRM Scans

In this paper, we employ the latest developments in 3D semi-supervised learning to create cutting-edge deep learning models for 3D object detection and segmentation of buried structures in high-resolution X-ray semiconductor scans. We illustrate our approach to locating the region of interest of High Bandwidth Memory (HBM) structures and their individual components and identifying various defects. We showcase how semi-supervised learning is utilized to capitalize on the vast amounts of available unlabeled data to enhance both detection and segmentation performance. Additionally, we explore the benefits of contrastive learning in the data pre-selection for our detection model and multi-scale Mean-Teacher training paradigm in 3D semantic segmentation to achieve better performance compared to the state of the art. We also provide an objective comparison for metrology-based defect detection with a 3D classification network. Our extensive experiments have shown that our approach outperforms the state of the art by up to 16% on object detection and 7.8% on semantic segmentation. Our fully-automated custom metrology package shows a mean error of less than 2 [Formula: see text]m for key features such as bond line thickness and provides better defect detection performance than the direct 3D classification approach. Overall, our method achieves state-of-the-art performance and can be used to improve the accuracy and efficiency of a wide range of failure analysis applications in semiconductor manufacturing. Finally, we also increase the segmentation models flexibility and adaptability to new data. We propose a generic training strategy and a new loss function that reduces the training time by 60% and the required amount of data by 48% making the training process more efficient.

Numerical Investigation of Interfacial Stress Distribution in Adhesive-Bonded Joints for Different Adhesive Materials

The widespread use of adhesive bonded connections has been used in a range of technical fields. In this paper, the interfacial stress distribution of adhesive bonded joint is presented. When determining whether or not a structure is dependable for use in operation, the stresses that act along the bond line of an adhesively bonded lap joint are of the utmost importance. The purpose of this study is to develop finite element solutions that are able to anticipate shear and peel stresses using the theory of elasticity as their foundation. The effect of adhesive properties on stress distribution is investigated by using different adhesive materials. By analysing five different adhesive materials it is concluded that ‘adhesive I’ is more reliable for operation.

Analysis of failure mechanisms of adhesive joints modified by a novel additive manufacturing-assisted method

The research presented in this paper used an innovative method to modify the configuration of adhesively bonded joints for improved mechanical performance. Additive manufacturing was employed to produce sacrificial support structures with a water-soluble filament (Polyvinyl Alcohol). The design freedom offered by additive manufacturing makes it easy to tailor fixtures to any geometry, which can be used to accurately make the desired fillet shape at the end of the adhesive bond line. In addition to the experimental tests, the finite element method (FEM) was used to study the stress distribution along the bond line for four different modified bonded joints, whilst the discrete element method (DEM) was used to estimate the joint failure load and crack path in the adhesive bond line due to its strength in describing the initiation and progression of micro-cracks. The results show that the novel manufacturing method can produce an accurate fillet at the end of the bond line, regardless of the adhesive type. The mechanical performance of the joints with the modified features increased significantly. Furthermore, the failure load and crack path obtained from the DEM model is in close agreement with experimental and finite element (FE) results. Hence, the failure mechanism of the hybrid joints is then summarised.