Reliability assessment and mechanical characterization of Cu and Au ball bonds in BGA package

Extended reliability and mechanical characterisation of Au and Pd-coated Cu (Cu) ball bonds are useful technical information for Au and Cu wire deployment in semiconductor device packaging. This paper discusses the influence of wire type on the package reliability and mechanical performance after several component reliability stress tests. Failure analysis has been conducted to identify the associated failure mechanisms of Au and Cu ball bonds after reliability tests. Wire pull strength and ball bond shear (with its break modes) of both wire types are investigated after unbiased highly accelerated temperature and humidity stress test (UHAST), temperature cycling (TC) and high temperature storage life test (HTSL) at various aging temperatures. Reliability Weibull plots have been plotted for each reliability stresses. Obviously Au ball bond is found with longer time-to-failure in Unbiased HAST stress compare to Cu ball bonds. Cu wire exhibits equivalent package and or better reliability margin compare to Au ball bonds in TC and HTSL tests. Failure mechanisms of UHAST and HTSL have been proposed and its mean-time-to failure (t₅₀), characteristics life (t_63.2, η) and shape parameter (ß) have been discussed in this paper.     

Direct Writing of Spot and Line Bonds for Microsystem Packaging Using Transmission Laser Bonding Technique

The transmission laser bonding (TLB) technique has been developed for making both spot and line bonds for microsystem packaging applications. The formation of spot bonds is first presented, while the line bond is generated by overlapping a series of spot bonds. Spot overlapping is achieved through varying the laser scanning speed as it moves across the wafers, resulting in a wide range of laser energy density or fluence. An analytical model has been developed to understand the TLB process, especially the relationship between the energy intensity distribution and the processing parameters, including the laser scanning speed and pulse rate. Also, guided by this model, experiments have been conducted to bond Pyrex-to-Si wafers at various bonding conditions with or without overlapping. To demonstrate the reliability of the TLB technique, the strength of the resulting spot and line bonds have been evaluated by microtensile tester while the surface characteristics of the bonding pair is quantified by atomic force microscopy. The strength of bonded spots can reach a stable value of 10.5 MPa, while the bonded lines can have strength at 9.2 MPa. A bonding strength higher than 9.2 MPa should be considered to be comparable to those obtained by other major bonding processes. Finally, recommendations for future study are presented.     

Fracture characterisation of green-glued polyurethane adhesive bonds in Mode I

Unseasoned (green) spruce timber side boards of size 25 × 120 × 600 mm were flatwise-glued with a one-component PUR adhesive. Glued pairs of boards were then kiln-dried to 12 % moisture content. A special small-scale specimen for testing the fracture properties of the adhesive bond in Mode I was developed in order to evaluate the adhesive bond properties. The complete force versus deformation curve, including both the ascending and the descending parts, could be obtained with these small-scale specimens, enabling the strength and fracture energy of the bond line to be calculated. In addition, the fractured specimens were examined by scanning electron microscope. Results show that both the tensile strength and the fracture energy of the green glued PUR adhesive bonds were equal to those of the dry glued bonds. The methodology developed and used in the present study gives new possibilities for analysis of the mechanical behaviour of wood adhesive bonds, and particularly of their brittleness and its correlation with the type of fracture path. This is in sharp contrast to the use of standardised test methods (e.g. EN 302, ASTM D905) with specimens having relatively large glued areas. Using such types of specimens, it is not possible to obtain the complete force versus deformation response of the bond. In addition, when using such test methods, failure takes place in the wood or in the fibres near the bond, thus making it impossible to obtain detailed information about the bond line characteristics.     

Molecular dynamic simulations on tensile mechanical properties of single-walled carbon nanotubes with and without hydrogen storage

Using a bond order potential, molecular dynamics (MD) simulations have been performed to study the mechanical properties of single-walled carbon nanotubes (SWNTs) under tensile loading with and without hydrogen storage. (10,10) armchair and (17,0) zigzag carbon nanotubes have been studied. Up to the necking point of the armchair carbon nanotube, two deformation stages were identified. In the first stage, the elongation of the nanotube was primarily due to the altering of angles between two neighbor carbon bonds. Young's Modulus observed in this stage was comparable with experiments. In the second stage, the lengths of carbon bonds are extended up to the point of fracture. The tensile strength in this stage was higher than that observed in the first stage. Similar results were also found for the zigzag carbon nanotube with a lower tensile strength. Hydrogen molecules stored in the nanotubes reduced the maximum tensile strength of the carbon nanotubes, especially for the armchair type. The effect may be attributed to the competitive formation between the hydrogen–carbon and the carbon–carbon bonds.     

Vacancy formation process in carbon nanotubes: first-principles approach

The electronic and structural properties of a single-walled carbon nanotube (SWNT) under mechanical deformation are studied using first-principles calculations based on the density functional theory. A force is applied over one particular C-atom with enough strength to break the chemical bonds between the atom and its nearest neighbors, leading to a final configuration represented by one tube with a vacancy and an isolated C-atom inside the tube. Our investigation demonstrates that there is a tendency that the first bond to break is the one most parallel possible to the tube axis and, after, the remaining two other bonds are broken. The analysis of the electronic charge densities, just before and after the bonds breaking, helps to elucidate how the vacancy is formed on an atom-by-atom basis. In particular, for tubes with a diameter around 11 angstroms, it is shown that the chemical bonds start to break only when the externally applied force is of the order of 14 nN and it is independent of the chirality. The formation energies for the vacancies created using this process are almost independent of the chirality, otherwise the bonds broken and the reconstruction are dependent.     

Influence of shear strength on long term biased humidity reliability of Cu ball bonds

This paper compares and discusses the influence of shear strength (in terms of shear-per mils-square) of Au and Cu ball bonds on the biased humidity reliability performance in SOIC 8LD leaded package. Biased (HAST) highly accelerated temperature and humidity stress test, 130 °C, 85 % RH) has been carried out to estimate the long term reliability of Au and Cu ball bonds. Lognormal reliability plots have been plotted for the three legs (control, leg 1 and leg 2) whereby Shear-per-mil-square of 6.68 is identified to have better mean-time-to-failure (t₅₀) compared to other two legs. Open failure from biased HAST 96, 192 h are subjected for secondary electron microscopy cross-sectioning and found typical interfacial CuAl intermetallic compound corrosion microcracking. HAST failure rates have been analyzed and the Cu ball bond lifetime has been established by using Peck model. The obtained Cu ball bond lifetime, of SPMS of 6.68 is >25 years and belongs to wearout reliability data point. This proves significant influence of SPMS on biased HAST failure rate. The higher the ball bond shear strength the lower the failure rate of biased HAST test. Hence, we should implement control on the average SPMS of ≥7.50 g/mil².     

Influence of water on bond behavior between CFRP sheet and natural calcareous stones

In this paper the effect of a long term immersion in water on bond durability is analyzed when FRPs (Fiber Reinforced Plastic) are externally applied to a masonry substrate. In the performed research a substrate made by natural calcareous stones, strengthened by CFRP (Carbon Fiber Reinforced Plastic) sheets has been analyzed. For a better comprehension of water effect on the adhesive bond between stone and CFRP, the same treatments were performed to the constituent materials, namely epoxy resins, CFRP sheets and stones. To this aim mechanical tests were carried out on stone, composite materials and epoxy resins before and after their immersion in water, evaluating the effects of this agent on the properties of the materials. The influence of the aging in water on the interface stone-reinforcement was analyzed in terms of bond strength, maximum bond stress, optimal bond length, slip-bond stress relationship and mode of failure. In addition the possibility of calibrating design relationships, taking into account the influence of environmental conditions is discussed. Detailed results on adhesives and composites aged in water have been reported in a previous paper while in the present work the significant decay of the mechanical properties of the stone is specifically investigated. With regard to the conditioning treatment a reduction of the bond strength has been observed (up to 26%) as well as a similar decrease of the maximum bond stress; in addition the aged specimens have shown a more fragile behavior. On the basis of the obtained results the empirical coefficient, reported in the available Italian Guidelines, to determine the FRP-masonry bond strength seems still effective when the system FRP-masonry is aged in water once the properties of the aged materials are considered in the provided relationships.     

Influence of water on bond behavior between CFRP sheet and natural calcareous stones

In this paper the effect of a long term immersion in water on bond durability is analyzed when FRPs (Fiber Reinforced Plastic) are externally applied to a masonry substrate. In the performed research a substrate made by natural calcareous stones, strengthened by CFRP (Carbon Fiber Reinforced Plastic) sheets has been analyzed. For a better comprehension of water effect on the adhesive bond between stone and CFRP, the same treatments were performed to the constituent materials, namely epoxy resins, CFRP sheets and stones. To this aim mechanical tests were carried out on stone, composite materials and epoxy resins before and after their immersion in water, evaluating the effects of this agent on the properties of the materials. The influence of the aging in water on the interface stone-reinforcement was analyzed in terms of bond strength, maximum bond stress, optimal bond length, slip-bond stress relationship and mode of failure. In addition the possibility of calibrating design relationships, taking into account the influence of environmental conditions is discussed. Detailed results on adhesives and composites aged in water have been reported in a previous paper while in the present work the significant decay of the mechanical properties of the stone is specifically investigated. With regard to the conditioning treatment a reduction of the bond strength has been observed (up to 26%) as well as a similar decrease of the maximum bond stress; in addition the aged specimens have shown a more fragile behavior. On the basis of the obtained results the empirical coefficient, reported in the available Italian Guidelines, to determine the FRP-masonry bond strength seems still effective when the system FRP-masonry is aged in water once the properties of the aged materials are considered in the provided relationships.     

聚氨酯封孔材料在炮孔封堵中的应用探索

为探索聚氨酯封孔材料在炮孔封堵中的应用可行性,通过理论分析炮孔中封堵物的受力情况,对聚氨酯封孔材料的力学性能与安全性能进行了试验研究。实验测试了聚氨酯封孔材料的抗压强度、相对形变以及弹性模量,并采用拉拔法与推出法测试了聚氨酯材料的粘结力。结果显示:聚氨酯封孔材料的平均抗压强度为776.2 k Pa,压缩变形为10%,弹性模量均值为16.66 MPa;拉拔法测得的聚氨酯封孔材料的粘结力为1788 N,粘土的粘结力为835 N;推出法测得的聚氨酯封孔材料的最大推出力为952 N,粘结强度为103.8 k Pa,粘土的最大推出力为692 N,粘结强度为75.5 k Pa。显然,聚氨酯封孔材料的粘结力显著优于粘土,力学性能较好。鉴于聚氨酯材料的发热性,测定了聚氨酯发泡过程的放热性及其对塑料导爆管传爆可靠性的影响。结果显示:聚氨酯封孔材料在反应过程中温度较高,且受用量、环境温度等的影响,存在一定的危险性。     

型钢高性能纤维混凝土粘结滑移性能试验研究

为研究型钢与高性能纤维混凝土之间的粘结滑移性能,以高性能纤维混凝土强度、保护层厚度以及型钢锚固长度为变化参数,设计10个型钢高性能纤维混凝土试件,进而对其进行推出试验,观察试件的破坏过程和裂缝发展形态,获取试件的荷载-滑移曲线。基于试验结果,探讨了各设计参数对特征粘结强度的影响规律,并建立了特征粘结强度的计算公式。对有效粘结应力计算公式进行推导,得到有效粘结应力-滑移分布曲线,并对其进行全过程分析。结果表明:适当增加型钢保护层厚度和混凝土强度,能有效提高型钢高性能纤维混凝土的特征粘结强度;特征粘结强度计算值与试验值吻合较好,表明该文提出的计算公式具有较高的精度;有效粘结应力能够反映型钢与高性能纤维混凝土界面间粘结应力的发展变化过程,并得到粘结应力各组份之间的比例关系。研究成果可为型钢高性能纤维混凝土相关计算理论提供试验依据。     

Microstructure and mechanical strength of snag-based solid liquid inter-diffusion bonds for 3 dimensional integrated circuits

Solid liquid inter-diffusion is a bonding technology that has been proposed at microscale for fabrication of ultrafine interconnects in high density and 3 dimensional integrated circuit packages. Due to very small size, bonds that are formed using this technology are very susceptible to process variations and thus reliability and quality of the bonds may deteriorate by small variations in the process. This study examines the effect of process parameters; temperature, bonding time, and pressure on the bond strength, microstructure, and intermetallic formation at the bonds. A full factorial experiment is designed and implemented on the process. Bonds are characterized using a micro-tester for mechanical strength and ductility. The microstructure of bonds formed under different process conditions are explored using scanning electron microscope, energy dispersive X-ray spectroscopy, and X-ray diffraction pattern to determine the bond thickness, elemental composition, copper concentration, and extent of copper diffusion and intermetallic compound. 1-dimensional phase lag modeling technique was used to estimate the time required for complete bond transformation to intermetallics. Results of these analyses show that temperature plays the strongest role in intermetallics formation and thickness. Bonds formed at higher temperatures and longer bonding time behaved more brittle and had higher strength. Pressure does not show a significant effect, and bonds that are formed under higher pressure show a slight reduction in bond strength with the same brittle behavior. This reduction is associated with formation of ε phase of intermetallic and the lower strength between ε and η phase. Modeling intermetallics formation for different bond thicknesses show that smaller bond thickness does not translate into much shorter time for complete transformation to intermetallics.     

An investigation on loose cemented granular materials via DEM analyses

This paper presents the results of a numerical study carried out by 2D discrete element method analyses on the mechanical behavior and strain localization of loose cemented granular materials. Bonds between particles were modeled in order to replicate the mechanical behavior observed in a series of laboratory tests performed on pairs of glued aluminum rods which can fail either in tension or shear (Jiang et al. in Mech Mater 55:1–15, 2012). This bond model was implemented in a DEM code and a series of biaxial compression tests employing lateral flexible boundaries were performed. The influence of bond strength and confinement levels on the mechanical behavior and on the onset of shear bands and their propagation within the specimens were investigated. Comparisons were also drawn with other bond models from the literature. A new dimensionless parameter incorporating the effects of both bond strength and confining pressure, called BS, was defined. The simulations show that shear strength and also dilation increase with the level of bond strength. It was found out that for increasing bond strength, shear bands become thinner and oriented along directions with a higher angle over the horizontal. It also emerged that the onset of localization coincided with the occurrence of bond breakages concentrated in some zones of the specimens. The occurrence of strain localization was associated with a concentration of bonds failing in tension.     

Mechanical properties of functionalized carbon nanotubes

Carbon nanotubes (CNTs) used to reinforce polymer matrix composites are functionalized to form covalent bonds with the polymer in order to enhance the CNT/polymer interfaces. These bonds destroy the perfect atomic structures of a CNT and degrade its mechanical properties. We use atomistic simulations to study the effect of hydrogenization on the mechanical properties of single-wall carbon nanotubes. The elastic modulus of CNTs gradually decreases with the increasing functionalization (percentage of C-H bonds). However, both the strength and ductility drop sharply at a small percentage of functionalization, reflecting their sensitivity to C-H bonds. The cluster C-H bonds forming two rings leads to a significant reduction in the strength and ductility. The effect of carbonization has essentially the same effect as hydrogenization.     

Overstressed interatomic bonds in stressed polymers

Infrared (IR) spectroscopy is used to find how the applied mechanical stress imposed upon a sample is distributed among the interatomic bonds. The distribution is highly heterogeneous: 80–95% of the bond population experience stresses close to the applied stress, the stress on the rest of the bond population varies over a wide range and reaches 1000–2000 kg/mm². The overstressed interatomic bonds lie in the amorphous regions of the polymer and are oriented in the direction of the mechanical force. The maximum stress on interatomic bonds is determined by the magnitude of the breaking stress on them. The breaking stress is shown to be a function of the applied stress, time, and temperature. This dependence is due to scission of stressed bonds induced by thermal fluctuations.     

Fatigue failure and molecular machine design

Sophisticated molecular machines have evolved in nature, and the first synthetic molecular machines have been demonstrated. With our increasing understanding of individual operating cycles, the question of how operation can be sustained over many cycles comes to the forefront. In the design of macroscale machines, performance and lifetime are opposing goals. Similarly, the natural evolution of biological nanomachines, such as myosin motor proteins, is likely constrained by lifetime requirements. Rather than bond rupture at high forces, bond fatigue under repeated small stresses may limit the mechanical performance of molecular machines. Here, the effect of cyclic stresses using single and double bonds as simple examples are discussed. Additionally, it is demonstrated that an increase in lifetime requires a reduction in mechanical load and that molecular engineering design features, such as polyvalent bonds capable of rebinding, can extend the bond lifetime dramatically. A universal scaling law for the force output of motors is extrapolated to the molecular scale to estimate the design space for molecular machines.     

Effect of thermo-hygrometric exposure on frp,natural stone and their adhesive interface

As well known, the performance of Fiber Reinforced Polymer (FRP) materials as external strengthening technique is strongly dependent on the bond behavior between FRP and substrate. Several experimental studies have been performed on this topic, however limited attention has still focused on the bond durability. In this paper, the effect of a thermo-hygrometric environment on the interface behavior FRP-calcareous natural stones isinvestigated. Each utilized materials (natural stone, adhesive, FRP sheets) was firstly exposed to the same thermo-hygrometric atmosphere; a relevant decay of mechanical properties has been found for the analyzed substrates (Lecce stone and Neapolitan tuff) while a negligible influence of the exposure has been observed for the composite reinforcements (CFRP and GFRP). The results regarding the variation of mechanical properties of the resins evidenced that the effect of the performed exposure is strictly correlated to the specific materials properties: a relevant degradation or even an improvement of mechanical performances has been,in fact, registered. The bond strength and the kind of failure were both analyzed as a function of the treatment used, as well as the strain and stress distribution at the interface. The kind of failure changed in some cases when passing from unconditioned to conditioned specimens; the bond strength, the maximum bond stress and the interface stiffness were affected by the treatment, manly depending on the adhesive resin deterioration. Finally, on the basis of the provisions given by the CNR-DT 200 R1/2013 document, the possibility of defining design relationships, able to take into account also durability aspects, is discussed.     

Molecular resolution of cell adhesion forces

Recently, AFM-based force spectroscopy has been used to quantify single-molecule adhesion forces on living ameboid cells. Force spectroscopy was used to measure the rupture forces of single receptor-ligand bonds which can occur rapidly between the cell types used, a metastasising B16 melanoma cell and a vascular bEnd.3 endothelial cell. Parameters which influence the critical experimental conditions are discussed to discriminate between multiple bond ruptures and single bonds. Under physiological conditions of temperature and pH the force measurements show an average rupture force of 33 pN (SD = 12 pN) for single bonds. Single-molecule force spectroscopy will be very useful to study the regulation of cell adhesion on a molecular level in normal processes, such as leukocyte homing, and in major human disorders, including tumor metastasis, autoimmune diseases and atherosclerosis.     

Thermo-mechanical behavior of low-dimensional systems: The local bond average approach

With the miniaturization of a solid, effects of surface strain and quantum trapping become increasingly important in determining its properties. As a result, low-dimensional materials manifest unusual features, especially in their energetic and mechanical behavior. The establishment of a consistent understanding on an atomic-level of the mechanism behind the fascinating behaviors of low-dimensional systems, which include monatomic chains, hollow tubes, liquid and solid surface skins, nanocavities, nanowires, and nanograins, as well as interfaces, has long been a great challenge. In this report, a literature survey is presented, followed by a theoretical analysis culminating in the development of a local bond average (LBA) approach that may complement existing approximations in terms of continuum medium and quantum computations. The LBA approach correlates the measurable quantities of a specimen to the identities of its representative bonds, and the energetic responses of these bonds (bond nature, order, length and strength) to external stimuli, such as changes in temperatures and coordination environments. It is shown that the shortened and strengthened bonds between under-coordinated atoms and the consequent local strain and quantum trapping dictate, intrinsically, the mechanical behavior of systems with a high proportion of such atoms. The thermally driven softening of a substance arises from bond expansion and lattice vibrations that weaken the bonds. The competition between the energy density gain and the residual atomic cohesive energy in the relaxed surface of skin depth determines, intrinsically, the mechanical performance of a mesoscopic specimen; the competition between the activation and inhibition of the motion of atomic dislocations motion dominates, extrinsically, the yield strength of the specimen during plastic deformation. Therefore, the mechanical behavior of a specimen depends on its shape, size, the nature of the bonds involved, surface and interface conditions, and the temperature at which the physical properties of the specimen is measured. Excellent agreement with existent measurements of temperature dependence of surface tension, size and temperature dependence of elasticity and extensibility, and the inverse Hall-Petch relationship in nanograins have been established. Furthermore, these agreements have led to quantitative information regarding the bond identities in monatomic chains and carbon nanotubes, as well as the factors dominating the sizes at which a grain is strongest. In addition, the interface electric repulsion between nanocontacts due to the skin trapping and the associated local charge densification may provide feasible mechanism for the superfluidity, superlubricity and superhydrophobicity as widely observed. The progress made insofar evidences the essentiality of the LBA approach from the perspective of bond formation, dissociation, relaxation and vibration and the associated energetics for the exposition of thermo-mechanical behavior of low-dimensional materials. Extending the application of the approach to junction interfaces, liquid surfaces, defects and impurities, chemically adsorbed systems, amorphous states, and substances under other applied stimuli such as pressure and electric field would contribute to better knowledge of such systems and could lead to the development of even more fascinating and profitable materials.     

Solid state metal-ceramic bonding of platinum to alumina

The formation and resulting mechanical strength of solid state metal-ceramic reaction bonds of alumina to platinum are investigated in terms of the effects of three main parameters — bonding temperature, time at temperature and contact pressure. Also the effect of the subsequent operating temperature on the bond strength is examined. An optimum bonding regime can be devised to create platinum-alumina bonds of optimum strength and durability, suitable for use in practical bonding applications.     

Crack formation in sapphire/niobium/sapphire joints under compression

Compression tests were carried out on UHV diffusion bonded single crystalline sapphire/Nb/sapphire joints to investigate their mechanical properties, the mechanisms that lead to the failure of the joints and the dislocation-interface interaction. The tests were performed for different orientation relationships (OR) at the interface to study the influence of different bond strength on the mechanical behavior. Additionally, the metal layer thickness was varied for each OR to alter the influence of the interface. The experimental results showed, that with decreasing metal layer thickness the stress needed to form a crack increases drastically, whereas for the Nb/sapphire system the bond strength at the interface seems to have no significant influence on the mechanical behavior of the joint. A theoretical model will be presented that explains the experimentally observed relationship between metal layer thickness and crack stress.