Mohd Nasir Tamin

Solder joint fatigue in a surface-mount assembly subjected to mechanical loading

A mechanical approach employing cyclic twisting deformation to a surface mount assembly is examined as an alternative to temperature cycling for evaluating solder joints fatigue performance. This highly accelerated test is aimed at reducing solder joint reliability testing cycle time. In this study, the mechanics of solder joints in a surface mount assembly subjected to cyclic twisting deformation on the PCB is investigated. For this purpose, a test package with 24 by 24 peripheral-array solder joints is modeled using the finite element method. Unified inelastic strain theory defines the strain-rate-dependent plastic stress-strain response of the 60Sn-40Pb solder. Cyclic twisting deformation in the range of plusmn1deg at a rate of 120 seconds per cycle was applied to the PCB assembly sub-model. The calculated stress and strain distributions in the critical solder joint are compared with those predicted for temperature cycling and accelerated temperature cycling tests. Results showed that the accumulated inelastic strain concentrates in a small region of the critical solder joint near the component side for temperature cycling and near the pad side for cyclic twisting cycles. The rate of inelastic strain accumulation per fatigue cycle in the solder joint for both thermal cycling (TC1) and mechanical twisting (MT1) tests are similar. Thus mechanical twisting test imparts similar characteristics in terms of the shear strain range to temperature cycling tests. Low cycle fatigue is dominated by localized shear effect as reflected in the largest shear strain range of the hysteresis loops. The Coffin-Manson strain-based model yielded more conservative prediction of fatigue lives of solder joints when compared to energy-based approach

Microstructure and environment-dependent fatigue crack propagation properties of Ti-48Al intermetallics

Fatigue crack propagation properties of Ti-48Al were studied by means of in-situ observation of the crack growth process under vacuum and air conditions. Three types of microstructures, namely, as-cast, duplex and fully lamellar structure were investigated in the present study. The results show that the crack growth behavior is dependent on the microstructure and the angle between the crack growth direction and orientation of lamellar colonies. The preferred path of crack propagation through the weakest phase of the structure associated with microcracking within the crack tip plastic zone has been investigated. The accelerated crack growth observed in air environment is attributed to absorption of air humidity at the crack tip plastic zone. This phenomenon is apparent through the observation of crack propagation process and the resulting fractured surface.

Time-dependent behavior of continuous-fiber-reinforced metal matrix composites: modeling and applications

A time-dependent approach employing a four-phase concentric cylinder model has been developed to predict the response of metal matrix composites (MMCs) subjected to thermomechanical loadings in which both plastic and creep responses of the composites are considered. The progressive development of plasticity in the matrix phase is determined using the deformation theory of plasticity while the creep deformation of this phase is estimated using the Bailey-Norton equation with an Arrhenius-type expression for the time-dependent creep coefficient. The model is applied to SCS-6/Ti-β21S composite to study the evolution of the stress and strain states in the constituents of the composite during initial cool-down and subsequent thermal cycles. The model is then employed to examine the influence of several critical parameters on the composite internal stress and strain states. These parameters include the thickness of the equivalent composite media, the type of fiber coating material, the thickness of the reaction zone, cooling rate during initial cool-down, and the kinetics of creep process during thermal cyclic loading. Results of these applications indicated that the process-induced thermal stresses in the matrix phase can be relaxed due to creep following initial cool-down from fabrication. This stress reduction is enhanced at a slower cooling rate. Comparison of different fiber coating materials shows that the use of carbon coating induces compressive stress state in the brittle interfacial region. TiB2-coated fibers, however, are found to be less affected by the growing interphase thickness in preserving the compressive radial stress component in the matrix and the interphase zone. Furthermore, it is found that the matrix activation energy for creep, Q, is history-dependent and can be correlated with the level of creep strain accumulated in the matrix phase. In addition, the residual thermal stresses induced in the matrix phase during initial cool-down can be relaxed by the application of subsequent thermal cycles.

Elastic-damage deformation response of fiber-reinforced polymer composite laminates with lamina interfaces

The single-layer and multi-layer finite element models are developed and examined for adequacy in predicting the elastic-damage response of fiber-reinforced polymer composite laminates. A new experimental-computational approach featuring a two-tier mesh convergence analysis of the finite element models is developed. A 12-ply carbon fiber-reinforced polymer composite laminate beam specimen with anti-symmetric layups is designed and loaded to induce matrix damage under significant deflection without catastrophic fracture. A constitutive model incorporating Hashin’s equations for damage initiation criteria, along with an energy-based damage propagation law is employed in the finite element simulation. The results shows that the multi-layer finite element model predicts well the load–deflection curve of the carbon fiber-reinforced polymer composite laminate, while the single-layer model overestimates the elastic flexural stiffness of the specimen by 47 %. During the flexural deformation, matrix damage initiates in the central and edge regions of the critical laminas under compressive and tensile stresses, respectively. The multi-layer finite element model also predicted the matrix-induced interface delamination along the edges of the critical laminas under tension, as observed experimentally. The model demonstrates the adequacy in representing the role of lamina interface in dictating the elastic-damage response of carbon fiber-reinforced polymer composite laminates manufactured by prepreg layups method.

Mechanics of Composite Delamination under Flexural Loading

The mechanics of interface delamination in CFRP composite laminates is examined using finite element method. For this purpose a 12-ply CFRP composite, with a total thickness of 2.4 mm and anti-symmetric ply sequence of [45/-45/45/0/-45/0/0/45/0/-45/45/-45] is simulated under three-point bend test setup. Each unidirectional composite lamina is treated as an equivalent elastic and orthotropic panel. Interface behavior is defined using damage, linear elastic constitutive model and employed to describe the initiation and progression of delamination during flexural loading. Complementary three-point bend test on CFRP composite specimen is performed at crosshead speed of 2 mm/min. The measured load-deflection response at mid-span location compares well with predicted values. Interface delamination accounts for up to 46.7 % reduction in flexural stiffness from the undamaged state. Delamination initiated at the center mid-span region for interfaces in the compressive laminates while edge delamination started in interfaces with tensile flexural stress in the laminates. Anti-symmetric distribution of the delaminated region is derived from the corresponding anti-symmetric ply sequence in the CFRP composite. The dissipation energy for edge delamination is greater than that for internal center delamination. In addition, delamination failure process in CFRP composite can be described by an exponential rate of fracture energy dissipation under monotonic three-point bend loading.

Continuum damage evolution in Pb-free solder joint under shear fatigue loadings

Progressive materials damage process in solder joint under cyclic shear deformation is examined in this study. A 3-D finite element model of a single reflowed SAC405 solder specimen is developed for this purpose. Cyclic relative displacement cycles between zero and 1.0 mm at displacement ramp rate of 2.0 mm/sec. is applied in the direction parallel to the solder/pad interface. Strain rate-dependent response of the solder is modeled using unified inelastic strain equations (Anands model) with optimized model parameters for SAC405 solder. Damage initiation is characterized by the inelastic hysteresis energy per stabilized cycle, ΔW. Elastic stiffness of the solder is then degraded progressively using prescribed damage evolution rule. Results show that extensive inelastic strain accumulation is confined to small selected edge regions near the solder/pad interface. The calculated slow inelastic strain rates in the range of 0.24 to 0.29 sec−1 resulted in bulk solder fatigue failure. In the predominantly tensile stress region near the solder/pad interface, inelastic shear strain range parallel to and stress range normal to the interface plane has the greatest magnitude. The inelastic hysteresis energy or work per stabilized cycle has demonstrated to be a better parameter over accumulated inelastic strain per cycle for describing low cycle fatigue failure of solder joints.

Hybrid Experimental-Computational Approach for Solder/IMC Interface Shear Strength Determination in Solder Joints

Damage-based models for solder/intermetallics (IMC) interface often require the interface properties such as tensile and shear strengths. The minute size of the solder joint renders direct experimental determination of these properties impractical. This paper presents a hybrid experimental-computational approach to determine the shear strength of solder/IMC interface. Displacement-controlled ball shear tests are performed on as-reflowed and thermally-aged solder specimens. The observed sudden load drop in the load-displacement curve corresponds to the crack initiation event and the load is indicative of the shear strength of the solder/IMC interface. Finite element simulation of the ball shear test is then employed to establish the complex stress states at the interface corresponding to the onset of fracture. The finite element model consists of Sn40Pb solder, Ni3Sn4 intermetallic and Ni layers, copper pad and a rigid shear tool. Unified inelastic strain theory describes the strain rate-dependent response of the solder while other materials are assumed to behave elastically. Quasi-static ball shear test is simulated at 30°C with a prescribed displacement rate of 0.5mm/min. Results show that steep stress gradients develop in the shear tool-solder contact and solder/IMC interface regions indicating effective load transfer to the interface. The bending (normal) stress is found to be of the same order of magnitude as the maximum shear stress. Higher stress values are predicted near the leading edge of the solder/IMC interface. The equivalent shear stress condition to the triaxial stress state at the interface, represented by the absolute maximum shear stress, τmax,abs should have reached the shear strength of the interface at fracture. The resulting shear strength of Sn40Pb/Ni3Sn4 interface is determined to be 87.5 MPa.

Thickness-dependent non-Fickian moisture absorption in epoxy molding compounds

The objective of this research is to characterize the relationship between the moisture uptake behavior and the thickness in epoxy-based molding compounds (EMCs). Experimental results from the literature were adopted for this purpose. A thickness-dependent moisture uptake model was proposed to describe the moisture uptake behavior. In order to apply the model, a methodology to develop the fictitious Fickian curve was suggested. Subsequently, the relationships between the non-Fickian parameters and the thickness were correlated and compared. Results showed that the apparent diffusivity of the fictitious curve was sensitive to the environmental conditions but not the thickness. In addition, when combining all data, it was found that each normalized non-Fickian parameter could be described by a single equation with respect to the normalized thickness. Based on the thickness-dependent model, the moisture concentration across the thickness was further characterized. In conclusion, the model proposed in this study allows the prediction of moisture uptake behavior at various thicknesses of EMCs. This could greatly reduce the time and cost of extensive experimental works.

Computer Aided Design and Analysis of Conical Forming Dies Subjected to Blast Load

In this paper design and analysis a die meant to cone explosive forming have been investigated. Since the explosive forming dies are subjected to blast loading, failure is too likely to pass. Likewise, the special geometry such as existing the several holes, sealing grooves, vacuum channel and fillets of this type of dies under explosion wave makes their analysis complicated. In the present work, the die was designed according to the final product dimension assisting a design software. In the next step the die under blast loading was analyzed using finite element method utilizing FEM software. The outcomes exhibit that the die is capable to withstand the explosion load. Besides, the trend of this paper is recommended as a routine for the designers who are going to design these types of dies.

Experimental and Numerical Studies of Fiber Metal Laminate (FML) Thin-Walled Tubes Under Impact Loading

Fiber metal laminate (FML) in form of tubular structures is a modern light-weight structure fabricated by incorporating metallic and composite materials. This present study deals with the impact characteristics and energy absorption performances of fiber metal laminate (FML) thin-walled tubes subjected to impact loading. Dynamic computer simulation techniques validated by experimental testing are used to perform a series of parametric studies of such devices. The study aims at quantifying the crush response and energy absorption capacity of FML thin-walled tubes under axial impact loading conditions. A comparison has been done in terms of crush behaviour and energy absorption characteristics of FML tubes with that of pure aluminium and composite tubes. It is evident that FML tubes are preferable as impact energy absorbers due to their ability to withstand greater impact loads, thus absorbing higher energy. Furthermore, it is found that the loading capacity of such tubes is better maintained as the crush length increases. The primary outcome of this study is design information for the use of FML tubes as energy absorbers where impact loading is expected particularly in crashworthiness applications.