Influence of size effects on material properties and springback behavior of metal foils in micro bending : a review

With product miniaturization, the requirement on good forming quality and high dimensional accuracy of micro parts is increasing dramatically. In micro bending process, the springback, a critical factor for the accuracy of micro-bent parts, is significantly affected by size effects. In view of the strong influence of material properties on springback behavior, this paper first reviews the influences of three size-dependent factors on material properties, including foil thickness, grain size and thickness to grain size ratio. Afterwards, the review on the influences of these factors on springback behavior are presented, aiming at enhancing the understanding of relevant size effects and proposing a quantitative analysis approach to evaluate the dimensional accuracy of micro-bent parts

Size effect on the springback of CuZn37 brass foils in tension and micro W-bending

With the ever-increasing demands on miniaturization, the requirement on good forming quality and high dimensional accuracy of micro parts is dramatically motivated. As a decisive factor affecting the accuracy of micro-bent parts, the springback is significantly influenced by size effects. In order to explore size-effect associated springback behaviour and evaluate the forming quality of micro-bent parts, an experimentally-based investigation on the influence of foil thickness, grain size and their interactive effect on the springback behaviour of CuZn37 brass foils with different thicknesses and grain sizes was carried out. The experimental results obtained via micro tensile tests revealed that the yield strength, Young’s modulus and elongation had a close correlation with the thickness to average grain size ratio. A micro W-bending process was used to perform the bending tests. Both springback and negative springback phenomena were observed. It was found that how size effects may influence the amount of springback would depend on the springback behaviours, e.g. positive springback or negative sprinback, etc. In addition, scatter phenomenon of the springback behaviour was analyzed quantitatively. An increased scatter was observed for the 50 μm thick specimens when the thickness to average grain size decreased, whereas an inverse tendency of the decreased scatter of the negative springback was found for the 75 and 100 μm thick specimens with increase of the grain size. Finally, forming quality of the W-shaped micro-bent parts was assessed

Influence of Process Parameters on the Deformation of Copper Foils in Flexible-Pad Laser Shock Forming

This paper investigates a new microforming technique, Flexible-Pad Laser Shock Forming (FPLSF), to produce mi-crofeatures on metallic foils without rigid punches and dies. FPLSF uses the laser-induced shock pressure and a flexi-ble-pad to plastically deform metal foils into hemispherical microcraters. In order to understand the deformation characteristics of metal foils in FPLSF, it is necessary to analyze the influence of process parameters on the foil deformation. In this paper, the effects of parameters such as the flexible-pad thickness, confinement layer medium, confinement layer thickness and the number of laser pulses on the depth, diameter and shape of the craters formed on copper foils were investigated. It is found that the flexible-pad thickness should be greater than its threshold value to maximize the deformation of foils. By comparing two different confinement media, namely water and glass, it is observed that hemispherical craters were formed on the copper foils at different laser fluence values tested when using water as the confinement; whereas shockwave ripples were formed on the copper foil at higher laser fluence while using the glass confinement. Using water as confinement medium, an increase in confinement thickness from 4 mm to 7 mm resulted in 48% increase of the crater depth at 7.3 J/cm2. However, at 13.6 J/cm2, reduction in crater depth was observed for thickness greater than 6 mm after an initial increasing trend. Regarding the number of pulses, it is found that increasing the number of pulses from 1 to 3 resulted only in a small increase (less than 1%) in crater depth at 7.3 J/cm2 and 13.6 J/cm2 laser fluence whereas 19.3% increase in depth was observed at larger laser fluence (20.9 J/cm2). It is also observed that the optimum number of pulses to achieve maximum deformation is varying with the laser fluence

Reliability analysis of foil substrate based integration of silicon chips

Flexible electronics has attracted significant attention in the recent past due to the booming wearables market in addition to the ever-increasing interest for faster, thinner and foldable mobile phones. Ultra-thin bare silicon ICs fabricated by thinning down standard ICs to thickness below 50 μm are flexible and therefore they can be integrated on or in polymer foils to create flexible hybrid electronic (FHE) components that could be used to replace rigid standard surface mount device (SMD) components. The fabricated FHE components referred as chip foil packages (CFPs) in this work are ideal candidates for FHE system integration owing to their ability to deliver high performance at low power consumption while being mechanically flexible. However, very limited information is available in the literature regarding the reliability of CFPs under static and dynamic bending. The lack of such vital information is a major obstacle impeding their commercialization. With the aim of addressing this issue, this thesis investigates the static and dynamic bending reliability of CFPs. In this scope, the static bending reliability of CFPs has been investigated in this thesis using flexural bending tests by measuring their fracture strength. Then, Finite Element Method (FEM) simulations have been implemented to calculate the fracture stress of ultra-thin flexible silicon chips where analytical formulas may not be applied. After calculating the fracture stress from FEM simulations, the enhancement in robustness of ultra-thin chips (UTCs) against external load has also been proved and quantified with further experimental investigations. Besides, FEM simulations have also been used to analyse the effect of Young’s Modulus of embedding materials on the robustness of the embedded UTCs. Furthermore, embedding the UTCs in polymer layers has also been experimentally proven to be an effective solution to reduce the influence of thinning and dicing induced damages on the robustness of the embedded UTCs. Traditional interconnection techniques such as wire bonding may not be implemented to interconnect ultra-thin silicon ICs owing to the high mechanical forces involved in the processes that would crack the chips. Therefore, two novel interconnection methods namely (i) flip-chip bonding with Anisotropic Conductive Adhesive (ACA) and (ii) face-up direct metal interconnection have been implemented in this thesis to interconnect ultra-thin silicon ICs to the corresponding interposer patterns on foil substrates. The CFP samples thus fabricated were then used for the dynamic bending reliability investigations. A custom-built test equipment was developed to facilitate the dynamic bending reliability investigations of CFPs. Experimental investigations revealed that the failure of CFPs under dynamic bending was caused mainly by the cracking of the redistribution layer (RDL) interconnecting the chip and the foil. Furthermore, it has also been shown that the CFPs are more vulnerable to repeated compressive bending than to repeated tensile bending. Then, the influence of dimensional factors such as the thickness of the chip as well as the RDL on the dynamic bending reliability of CFPs have also been studied. Upon identifying the plausible cause behind the cracking of the RDL leading to the failure of the CFPs, two methods to improve the dynamic bending reliability of the RDL have been suggested and demonstrated with experimental investigations. The experimental investigations presented in this thesis adds some essential information to the state-of-the-art concerning the static and the dynamic bending reliability of UTCs integrated in polymer foils that are not yet available in the literature and aids to establish in-depth knowledge of mechanical reliability of the components required for manufacturing future FHE systems. The strategies devised to enhance the robustness of UTCs and CFPs could serve as guidelines for fabricating reliable FHE components and systems

A Review on Mechanics and Mechanical Properties of 2D Materials - Graphene and Beyond

Since the first successful synthesis of graphene just over a decade ago, a variety of two-dimensional (2D) materials (e.g., transition metal-dichalcogenides, hexagonal boron-nitride, etc.) have been discovered. Among the many unique and attractive properties of 2D materials, mechanical properties play important roles in manufacturing, integration and performance for their potential applications. Mechanics is indispensable in the study of mechanical properties, both experimentally and theoretically. The coupling between the mechanical and other physical properties (thermal, electronic, optical) is also of great interest in exploring novel applications, where mechanics has to be combined with condensed matter physics to establish a scalable theoretical framework. Moreover, mechanical interactions between 2D materials and various substrate materials are essential for integrated device applications of 2D materials, for which the mechanics of interfaces (adhesion and friction) has to be developed for the 2D materials. Here we review recent theoretical and experimental works related to mechanics and mechanical properties of 2D materials. While graphene is the most studied 2D material to date, we expect continual growth of interest in the mechanics of other 2D materials beyond graphene

A parametric study on the accuracy of bending in micro W-bending using Taguchi method

High dimensional accuracy of micro-bent parts, particularly the desired bent angle, is often required. In the study reported in this paper, a micro W-bending process was used for the study addressing this issue. Four main parameters affecting the bending accuracy of the micro W-bent parts were considered: foil thickness, grain size, foil orientation and punching frequency. Based on Taguchi L8 orthogonal array (OA), a micro-sheet-metal forming machine equipped with W-shaped punch and die was used to conduct the micro W-bending experiments. The experimental results were analyzed using signal-to-noise (S/N) ratio and the analysis of variance (ANOVA). It was identified that the extent of the effect by these parameters on the micro W-bending process depends on springback behaviours. The foil thickness had highest influence on the springback amount of the bent parts. However, the negative springback was influenced mostly by the grain size, closely followed by the foil thickness. Furthermore, the optimum bending conditions for different types of the springback were obtained. Confirmation experiments were then performed not only to validate the improved bending accuracy but also to verify the extent of the contribution from each parameter to the amounts of the springbacks. Finally, mathematical models for both, positive springback and negative springback, were developed using the regression analysis. It was observed that the predicted values fit well with the experimental results, indicating the adequacy of the established models

Size effects and dislocation patterning in two-dimensional bending

We perform atomistic Monte Carlo simulations of bending a Lennard-Jones single crystal in two dimensions. Dislocations nucleate only at the free surface as there are no sources in the interior of the sample. When dislocations reach sufficient density, they spontaneously coalesce to nucleate grain boundaries, and the resulting microstructure depends strongly on the initial crystal orientation of the sample. In initial yield, we find a reverse size effect, in which larger samples show a higher scaled bending moment than smaller samples for a given strain and strain rate. This effect is associated with source-limited plasticity and high strain rate relative to dislocation mobility, and the size effect in initial yield disappears when we scale the data to account for strain rate effects. Once dislocations coalesce to form grain boundaries, the size effect reverses and we find that smaller crystals support a higher scaled bending moment than larger crystals. This finding is in qualitative agreement with experimental results. Finally, we observe an instability at the compressed crystal surface that suggests a novel mechanism for the formation of a hillock structure. The hillock is formed when a high angle grain boundary, after absorbing additional dislocations, becomes unstable and folds to form a new crystal grain that protrudes from the free surface.Comment: 15 pages, 8 figure