bluuebunny/crossref_metadata_2025 huggingface dataset:
Effects of Population Size on Reproductive Success of the Endangered and Endemic Species Primula merrilliana
Authors: Shao Jian‐Wen , Zhang Xiao‐Ping , Zhang Zhong‐Xing , Zhu Guo‐Ping
Published: April 2008

The reproductive ability related to the population size of the endangered and endemic species Primula merrilliana Schltr. was investigated. In 26 populations observed, only four contain more than 500 flowering individuals, whereas most of them (53.8%) consist of less than 100 flowering individuals. Though the ratio of pin and thrum plants keeps its balance at 1:1 for all populations, the frequency of pin and thrum flowers was significantly biased in most small populations. Population size strongly affected reproductive success; plants in small populations produced significantly fewer flowers and fruits per plant and fewer seeds per fruit, and therefore fewer seeds per plant. The floral density was another important factor that influenced the reproductive success of P. merrilliana, because four main reproductive success parameters (fruits per plant, seeds per fruit, seeds per plant, and the proportion of flowers setting fruit) were all positively correlated with floral density. The size of plants and the number of leaves per plant (measure of habitat quality) were not influenced by the variation of population size, suggesting that the reduced fecundity in small populations may not be a consequence of lower habitat quality. Inbreeding depression and pollen limitation as a result of less attractiveness in small populations are therefore likely explanations for these patterns.
Does Education Still Pay Off in Rural China: Revisit the Impact of Education on Off‐farm Employment and Wages
Authors: Zhang Haiqing , Zhang Linxiu , Luo Renfu , Li Qiang
Published: April 2008

The present study considers how education affects off‐farm job participation and wages. We use a nationally representative dataset from a survey conducted in 5 provinces, 101 villages and 808 households by the authors in early 2005. The empirical results show that educational attainment, skill training and years of experience of rural residents have positive, statistically significant effects on off‐farm employment. The average return to a year of education is 7 percent, which is higher than those observed in previous studies. We also find the return to an additional year of schooling to be higher for post‐junior high schooling than for junior high and below schooling: 11.8 versus 3.2 percent. We conclude that not only does education still pays off in rural China, but also the rate of return to education is increasing over time.
Estimating Trade Mis‐invoicing from China: 2000–2005
Authors: Beja Edsel L.
Published: April 2008

An examination of available data reveals large trade misinvoicing between China and its trade partners. The analysis finds a net trade misinvoicing of US$287.6bn between 2000 and 2005, while the full magnitude of unrecorded trade is estimated at US$1.4tn. China needs to establish more effective management of its international trade flows. At the same time, the international community needs to provide more effective governance mechanisms to address trade misinvoicing.
Factors Influencing FDI Location Choice in China's Inland Areas
Authors: Luo Laijun , Brennan Louis , Liu Chang , Luo Yuze
Published: April 2008

Since 1978, the bulk of foreign direct investment (FDI) has gone to the south‐eastern coastal areas in China, with only a small portion received inland. With the launch of the Western Development Strategy in 2000 and the Central China Rising Strategy in 2004, the choice of investment locations has expanded to inland areas. Based on panel data covering 98 inland cities from 1999 to 2005, this study identifies location preference variables for FDI invested in China's inland areas, and finds that well‐established factors such as natural resources and low labor costs are not important factors in determining FDI locations within China's inland. Instead, policy incentives and industrial agglomeration are the most important factors. The findings of the present study have policy implications for both host country authorities and multinational corporations.
Evolutionary history of Pacific salmon in dynamic environments
Authors: Waples Robin S. , Pess George R. , Beechie Tim
Published: April 2008

Contemporary evolution of Pacific salmon (Oncorhynchus spp.) is best viewed in the context of the evolutionary history of the species and the dynamic ecosystems they inhabit. Speciation was complete by the late Miocene, leaving c. six million years for intraspecific diversification. Following the most recent glacial maximum, large areas became available for recolonization. Current intraspecific diversity is thus the product of recent evolution overlaid onto divergent historical lineages forged during recurrent episodes of Pleistocene glaciation. In northwestern North America, dominant habitat features have been relatively stable for the past 5000 years, but salmon ecosystems remain dynamic because of disturbance regimes (volcanic eruptions, landslides, wildfires, floods, variations in marine and freshwater productivity) that occur on a variety of temporal and spatial scales. These disturbances both create selective pressures for adaptive responses by salmon and inhibit long‐term divergence by periodically extirpating local populations and creating episodic dispersal events that erode emerging differences. Recent anthropogenic changes are replicated pervasively across the landscape and interrupt processes that allow natural habitat recovery. If anthropogenic changes can be shaped to produce disturbance regimes that more closely mimic (in both space and time) those under which the species evolved, Pacific salmon should be well‐equipped to deal with future challenges, just as they have throughout their evolutionary history.
Recurrent evolution of life history ecotypes in sockeye salmon: implications for conservation and future evolution
Authors: Wood Chris C. , Bickham John W. , John Nelson R. , Foote Chris J. , Patton John C.
Published: April 2008

We examine the evolutionary history and speculate about the evolutionary future of three basic life history ecotypes that contribute to the biocomplexity of sockeye salmon (Oncorhynchus nerka). The ‘recurrent evolution’ (RE) hypothesis claims that the sea/river ecotype is ancestral, a ‘straying’ form with poorly differentiated (meta)population structure, and that highly structured populations of lake‐type sockeye and kokanee have evolved repeatedly in parallel adaptive radiations between recurrent glaciations of the Pleistocene Epoch. Basic premises of this hypothesis are consistent with new, independent evidence from recent surveys of genetic variation in mitochondrial and microsatellite DNA: (1) sockeye salmon are most closely related to pink (O. gorbuscha) and chum (O. keta) salmon with sea‐type life histories; (2) the sockeye life history ecotypes exist as polyphyletic lineages within large drainages and geographic regions; (3) the sea/river ecotype exhibits less genetic differentiation among populations than the lake or kokanee ecotypes both within and among drainages; and (4) genetic diversity is typically higher in the sea/river ecotype than in the lake and kokanee ecotypes. Anthropogenic modification of estuarine habitat and intensive coastal fisheries have likely reduced and fragmented historic metapopulations of the sea/river ecotype, particularly in southern areas. In contrast, the kokanee ecotype appears to be favoured by marine fisheries and predicted changes in climate.
Changes in selection and evolutionary responses in migratory brown trout following the construction of a fish ladder
Authors: Haugen Thrond Oddvar , Aass Per , Stenseth Nils Christian , Vøllestad Leif Asbjørn
Published: April 2008

Brown trout (Salmo trutta) are extensively harvested and its habitat highly influenced by human encroachments. Using a 40‐year time series of mark–recapture data we estimate vital rates for a piscivorous trout population. This population spawns upstream of a waterfall, which historically acted as a migration barrier for smaller trout. In 1966, the waterfall was dammed and a fish ladder constructed. All fish ascending the fish ladder were individually tagged and measured for a variety of traits. The fish ladder overall favoured access to upstream spawning areas for middle‐sized trout, resulting in stabilizing selection acting on size at spawning. Over time, natural and fishing mortality have varied, with fishing mortality generally decreasing and natural mortality increasing. The average and, particularly, variance in size‐at‐first‐spawning, and growth rates during the first years of lake residence have all decreased over the 1966–2003 period. These changes are all consistent with a shift from directional to stabilizing selection on age and size at spawning. Estimated rates of phenotypic change are relatively high, in particular for size at first spawning, adding further support for the growing notion that human interference may lead to rapid life‐history trait evolution.
Microsatellite loci isolated from the tropical tree Hymenaea courbaril L. (Fabaceae)
Authors: BUONAMICI ANNA , CAVERS STEPHEN , VENDRAMIN G. G.
Published: April 2008

We developed 11 microsatellite markers for Hymenaea courbaril for the purpose of studying spatial genetic structure and gene flow. The microsatellite loci were screened in 44 trees from two populations. All loci were polymorphic, exhibiting between two and 16 alleles, and levels of expected heterozygosity from 0.174 to 0.909. Departures from Hardy–Weinberg equilibrium were detected for all loci in one population. The estimated null allele frequency is low or moderate. No locus combinations exhibited linkage disequilibrium.
Isolation and characterization of polymorphic microsatellite markers from Asian malaria mosquito Anopheles sinensis (Diptera: Culicidae)
Authors: MA YAJUN , FAN YONG
Published: April 2008

Microsatellite‐containing region were isolated and characterized in Anopheles sinensis, a primary vector of malaria parasites in Asia. An enrichment protocol yielded 252 microsatellite sequences. We designed primers to amplify 20 unique microsatellites, 14 of which amplify cleanly and were polymorphic. A survey of 24 individuals showed that 12 loci are highly variable with the number of alleles ranging from two to 11, and expected heterozygosity ranging from 0.116 to 0.903. These markers will be useful for population genetic studies and genome mapping in A. sinensis.
Polymorphic tetranucleotide microsatellites for Cope's giant salamander (Dicamptodon copei) and Pacific giant salamander (Dicamptodon tenebrosus)
Authors: STEELE C. A , BAUMSTEIGER J , STORFER A
Published: April 2008

We present primers and amplification conditions for 15 microsatellite loci developed for the Cope's giant salamander (Dicamptodon copei), 14 of which are tetranucleotide repeats. Cross‐species amplification revealed 10 of these loci to also be polymorphic in the Pacific giant salamander (Dicamptodon tenebrosus). Several loci produced nonoverlapping allelic ranges between the two species and may be useful in species identification. These polymorphic microsatellite loci are potentially useful for future studies of population genetics in dicamptodontid salamanders.
Timeline and distribution of melanocyte precursors in the mouse heart
Authors: Brito Flavia Carneiro , Kos Lidia
Published: April 2008

Apart from the well‐studied melanocytes of the skin, eye and inner ear, another population has recently been described in the heart. In this study, we tracked cardiac melanoblasts using in situ hybridization with a dopachrome tautomerase (Dct) probe and Dct‐LacZ transgenic mice. Large numbers of melanoblasts were found in the atrioventricular (AV) endocardial cushions at embryonic day (E) 14.5 and persisted in the AV valves into adulthood. The earliest time Dct‐LacZ‐positive cells were observed in the AV endocardial cushions was E12.5. Prior to that, between E10.5 and E11.5, small numbers of melanoblasts traveled between the post‐otic area and third somite along the anterior and common cardinal veins and branchial arch arteries with other neural crest cells expressing CRABPI. Cardiac melanocytes were not found in the spotting mutants Ednrbs‐l/s‐l and Kitw‐v/w‐v, while large numbers were observed in transgenic mice that overexpress endothelin 3. These results indicate that cardiac melanocytes depend on the same signaling molecules known to be required for proper skin melanocyte development and may originate from the same precursor population. Cardiac melanocytes were not found in zebrafish or frog but were present in quail suggesting an association between cardiac melanocytes and four‐chambered hearts.
Thermal Stresses in Compliantly Joined Materials
Authors: Glaser J. C.
Published: April 2008

In the past several years there have been a number of papers published which provide closed-form solutions for the stresses in bonded layers of materials. These closed-form solutions offer a rapid method to obtain first-order stresses for materials which are joined together and the compliant layer between them. However, before using them, it is desirable to have some feeling as to the accuracy of the results from these closed-form equations. Comparisons between these analytical relations and other approaches found in published works on bonding and to finite element solutions for several example problems are given. An attempt is made to qualify these closed-form equations in terms of their accuracy, as compared to other methods of analysis. The effects of finite element mesh refinement on the material interface stress results are also given.
Structural Analysis For Circuit Card Systems Subjected to Bending
Authors: Engel P. A.
Published: April 2008

The determination of stress in a typical module/lead/card system is generally a very complex task. In the present paper a new engineering modeling approach is given, high-lighting simple, approximate calculations of the most significant system responses. Simplified patterns of plate action are construed using strips, including the module, in both x and y directions. Relationships were obtained for various shapes and sizes of card and module (e.g., square and rectangular); various systems of load and support conditions; various designs of modules (e.g., double-sided and stacked) and leads; clustered groups of modules, etc. The finite element method was used both to corroborate results of the theoretical approach, and to provide an “experimental” technique for the major response quantities.
Thermoplastic Problem of a Thick Orthotropic Beam on Two-Directional Elastic Foundation
Authors: Birman V.
Published: April 2008

The thermoelastic problem of an orthotropic beam supported by an elastic foundation which resists lateral deformations as well as displacements parallel to the beam axis is considered. The solution for a relatively thick beam is obtained based on the first-order shear deformation theory. The problem is reduced to the solution of an ordinary seventh-order differential equation which can be obtained in the closed form. The case of a slender beam is treated as a particular case of a more general solution for a shear deformable beam.
Performance of a Conduction Cooling Module
Authors: Song Tae-Ho , Bang Se Yoon
Published: April 2008

A piston-type conduction cooling module for PCB-mounted IC’s has been developed and tested. Analysis of performance has been made using a finite difference scheme for the conduction piston. The effect of direct contact area between the piston and the IC as well as the effect of thermal conduction grease between them have been investigated. An analytical solution for the heat transfer coefficient of coolant flowing along the side of piston has been obtained. Excellent agreement between experimental and analytical results has been found. The overall thermal resistance from the IC to the coolant can be lowered to 3° C/W or less with negligible coolant-side pressure drop for practical applications.
Free Convection in a Discretely Heated Vertical Enclosure: Effects of Prandtl Number and Cavity Size
Authors: Prasad V. , Keyhani M. , Shen R.
Published: April 2008

Natural convection in a discretely heated vertical cavity by three uniform flux heat sources on one side wall is experimentally and numerically investigated. The other vertical wall is considered to be isothermally cooled and the ends are adiabatic. The flow visualization pictures for the ethylene glycol-filled cavity reveal a multicellular flow pattern consisting of primary, secondary, and tertiary flows. The growth of these secondary and tertiary cells with Rayleigh number and aspect ratio is well-demonstrated by the numerical results for Prandtl number of 25. Although the flow structure varies significantly when the Prandtl number is reduced from 25 to 1, the variation is negligible for an increase in Pr from 25 (dielectric liquid) to 166. The numerically obtained temperature fields indicate that the temperature of the heated sections are strongly influenced by the thermal stratification and the flow pattern in the cavity. Numerical predictions are in excellent agreement with the flow visualization pictures and the measured heat transfer coefficients. The effect of aspect ratio on Nusselt number is minimal in the case of tall cavities, A ≥3. Bounds for minimum and maximum temperatures are discussed and several Nusselt number correlations are presented.
Mechanical Evaluation of the “Euler” Test Probe
Authors: Suhir E. , Benedetto W. E.
Published: April 2008

The study contains a theoretical and experimental evaluation of the mechanical behavior of wires in a flexible (“Euler”) test probe, including buckling, axial and lateral deflections and stresses. The developed formulas are simple, easy-to-use and clearly indicate the role of the major factors affecting the wire performance. Calculations, executed for an Everett-Charles 32 point probe, are in good agreement with the measured data. Some recommendations for smaller stresses in the wires are presented. The obtained results could be of help in the analysis and physical design of “Euler” test probes.
Fracture of Glass Seals in Surface Mount IC Packages
Authors: Stevens Karl K. , Wong Tin-Lup , Renavikar Ajit , Chen Wayne
Published: April 2008

Results of a study to investigate the cracking of glass seals in a surface-mount integrated circuit package with ceramic covers are presented. Finite element analyses were combined with principles of fracture mechanics to estimate values of lead tip displacement (or applied force) at the onset of seal cracking. Such cracking provides points of entry for moisture, and otherwise degrades package reliability. Limiting values of lead tip displacement were estimated by three different approaches. The results obtained were in close agreement, and appear to be consistent with physical observations.
Strength Analysis of Surface Mounted Assemblies Under Bending and Twisting Loads
Authors: Wong Tin-Lup , Stevens Karl K. , Wang Junshi , Chen Wayne
Published: April 2008

Simulations of bending and twisting of surface mounted assemblies have been performed using the hybrid analytical/experimental analysis approach, and the results are presented. Analytical analyses were combined with experimental load-deformation characteristics of the surface mounted assemblies to predict the maximum allowable loadings and deflections that the surface mounted assemblies can withstand before incurring failures. The simulation results were in close agreement with the real loading situations.
Creep in Shear of Experimental Solder Joints
Authors: Tribula D. , Morris J. W.
Published: April 2008

Thermal fatigue failures of solder joints in electronic devices are a great concern in the electronics industry. Since the fatigue load is often in shear the details of thermal fatigue failure in shear are of particular interest. Recent work indicates that similar failure mechanisms operate in both thermal fatigue in shear and unidirectional creep in shear. Additionally, since the operative temperatures during thermal fatigue represent high solder homologous temperatures, creep deformation is certainly involved. These factors and the relative case of conducting creep experiments encourage the study of solder joints under shear creep conditions. This work presents steady state shear creep rate vs. shear stress data for several solder compositions, including the binary eutectic alloy and Pb-Sn alloyed with small amounts of Bi, Cd, In, and Sb, in a joint configuration. These data indicate that conventional creep mechanisms operate in the temperature and shear strain rate ranges studies. Extensive microstructural information is also reported. The microstructural evolution under creep conditions indicates that the instability of the as-cast binary Pb-Sn eutectic microstructure initiates creep failure. Changes of the as-solidified microstructure with the third element addition are reported as are the microstructural responses of each of these alloys to creep deformation. The efficacy of postponing the microstructural instability with the addition of small amounts of ternary elements is discussed.
Damage Boundary Curves: A Computational (FEM) Approach
Authors: Caletka D. V. , Caldwell R. N. , Vogelmann J. T.
Published: April 2008

As the mechanical, thermal, and electrical demands on 2nd and 3rd level electronic packages increase, so does the need for early dynamic analysis of the proposed design. Estimating effect of dynamic transients (shock inputs) is of primary concern and the current topic of discussion. The computational determination of a damage boundary curve/s (DBC) for a proposed 2nd/3rd level package may seem a formidable task. With the experienced use of the finite element method (FEM), engineering insight, and some relatively simple mechanical testing, the task can be reasonably accomplished. This is not to say that every failure mechanism can be foreseen, predicted or modeled, but some failures can be avoided during qualification testing. The proposed method of DBC determination requires some initial ideas related to where failures may occur due to transient inputs. Failures may include a given maximum stress level, strain, deflection of a particular point or the force at a riveted connection. These suspect areas of failure will tend to guide the modeling technique. Models must be constructed not only to address the specific areas of concern but also to reasonably represent the overall dynamics of the package. Examples of this technique will be presented, each with varying degrees of verification.
A Comparison of the Solderability of Wrought and Plated Nickel Surfaces: The Selection of a Reliable Pin for Second Level Packaging
Authors: Woychik C. G. , Trivedi A. K.
Published: April 2008

The solderability of a wrought nickel pin directly overplated with gold and that of a wrought kovar (tradename of an Fe-Ni-Co alloy manufactured by Carpenter Technology Corporation) pin plated with thick nickel and then overplated with gold was evaluated. The kovar pin was determined to exhibit very high solderability whereas the nickel pin was found to have extremely poor solderability. The poor solderability of the nickel pin as compared with the kovar pin is attributed to the contamination of the nickel surface prior to gold plating. A primary cause of the poor wetability of the nickel pin was due to embedded alumina particles on the surface of the pin (this occurs during the tumbling operation to remove any metallic slivers) prior to gold plating. The high solderability of the kovar is related to the clean nickel plating prior to overplating with gold. The cleanliness of the nickel layer was found to strongly influence the formation of the Ni-Sn intermetallic phase layer, having a nominal thickness of 42 μin (1.1 μm) after the kovar pin was tinned with eutectic Sn-Pb solder. For the tinned wrought nickel pin, a thin but continuous Ni-Sn intermetallic layer was also present; however, the nominal thickness of this layer was only 15 μin (0.38 μm). This large variation in solderability of both types of pins is attributed to differences in the intermetallic phase layer.
Holographic Interferometry Using Self-Developing Optical Crystals for Heat Flux Evaluation
Authors: Magnusson R. , Hafiz A. , Bagby J. S. , Haji-Sheikh A.
Published: April 2008

Double-exposure holographic interferometry with crystals of iron-doped lithium niobate as the storage material is applied to study heat transfer from electronic chips. These crystals are self-developing, making the reults immediately available for use. This method of interferometry can be used to study the temperature field in heating and cooling fluids. In particular, it is a practical method for studying the heat transfer from small devices such as microelectronic chips. Double-exposure holographic interferometry is a viable experimental technique in transient as well as steady-state problems.
Indentation Method to Measure Plating Ductility
Authors: Engel P. A. , Questad D. L.
Published: April 2008

The ductility of a plating is an important measure of its durability. In the present paper an indentation test for thin platings is introduced. The test is based on the fact that tensile radial strains arising at the rim of the indentation crater tend to cause cracks in a material of insufficient ductility. By a quantitative knowledge of the radial strain the ductility – i.e., ultimate strain – of the material can be determined. We use a simple geometric analysis which assumes no tangential displacement in the contact area to determine the “average radial strain” across the indentation. In the experiments sharp conical indenters (typically having a 60 degree cone angle with a 0.025 mm (0.001 in.) tip radius) are used, and the plastically deformed crater is checked by electron microscopy (SEM) for cracks. If a crack is detected, its depth is further checked by an attendant X-ray procedure (EDX) for the presence of the underlying substrate material. If a “through-crack” is found, then the ductility of the plating has been exceeded; i.e., the ductility will be equated with the strain value measured for the last indenter loading not involving a crack.
Strains in Aluminum-Adhesive-Ceramic Trilayers
Authors: Hall P. M. , Howland F. L. , Kim Y. S. , Herring L. H.
Published: April 2008

In many of today’s high speed, high density circuits, there is a need to remove large amounts of heat. To facilitate this removal of heat, it is common to adhere a sheet of a high thermal conductivity material (such as aluminum or copper) to the substrate (which may be alumina ceramic). This can result in large expansion mismatches which cause stresses and bowing, with the possibility of delamination, cracking, stressing solder joints, loss of hermeticity, or shorting of a metal lid to wire bonds inside a cavity. One approach to this problem is to use a compliant adhesive to decouple the materials. The present paper is an experimental and theoretical study of the strains as a function of temperature from −40° C to 140° C in a trilayer structure of 0.030 in. or 0.76 mm thick aluminum, 0.006 in. or 0.15 mm thick adhesive, and 0.021 in. or 0.5 mm thick low-temperature cofired (glassy) ceramic. The strains are analyzed using E. Suhir’s theory, and they are measured using strain gages for three adhesives: an epoxy, a fabric-reinforced epoxy, and a silcone elastopolymer. If the adhesive has an elastic modulus below 10 psi or 70 kPa, theory predicts almost complete de-coupling. Between 100 and 105 psi or 700 kPa and 700 MPa, there is partial decoupling, depending on the in-plane dimensions. Above 10,000 psi or 700 MPa, the decoupling is negligible, and the same bowing results for any elastic modulus between 10,000 and 1,000,000 psi or 70 MPa and 7 GPa. For temperatures below 80° C, only the elastomer has enough compliance to provide any de-coupling. Above 80° C, the elastomer de-couples the most, and the unreinforced epoxy the least. Almost all of the observed effects are understandable in terms of the Suhir theory, along with the fact that the elastic modulus of the epoxy materials decreases with increasing temperature. In particular, when there is some decoupling of the materials, the amount of decoupling depends on the in-plane dimensions of the sample.
Maximizing Electronic System Reliability Through Optimized Distribution of System Coolant
Authors: Miner S. M.
Published: April 2008

A mathematical technique for maximizing the reliability in a parallel flow system of electronic units is presented. The failure rate is used as the measure of reliability, and reliability is maximized when the failure rate is minimized. An expression for the system failure rate as a function of individual unit flow rates is derived. This expression, subject to the constraint that the sum of the individual unit flows must equal the system flow rate, forms the basis for this analysis. The method of Lagrange multipliers is applied to minimize the failure rate and determine the optimum distribution of flow within the system. Comparisons are made to failure rates for two other flow distributions. 1. Flow based on maximum allowable junction temperature. 2. Flow proportional to unit power dissipation.
Experimental Determination of Thermal Contact in a Diode-Heat Sink Assembly
Authors: Maudgal V. K. , Kliman J. , Miles J. , Sunderland J. E.
Published: April 2008

This study examines experimentally the quality of contact of mechanical joints in a diode heat sink assembly. Steady-state contact conductance, h, is used as a quantitative measure of the quality of contact of a joint. The emphasis of the work is on determining the contact conductance, h, for a nonideal joint using a noncontact method of recording temperature distribution in a complex geometry. Thermal contact conductance for an interface is known to depend on parameters such as contact pressure, mean interface temperature and the surface roughness characteristics of the mating surfaces. The results are presented for three different conditions of the interface namely (i) the mating surfaces are bare (ii) aluminum foil is inserted between the mating surfaces, and (iii) a high thermal conductivity grease is applied to the mating surfaces. Two levels of contact pressure are used for the interface with aluminum foil. The results indicate that the contact conductance increases with the mean temperature of the interface in all the cases. At low interface temperature, the contact conductance was greatest for the bare interface conditions. At high interface temperature, the contact conductance was greatest for the aluminum foil interface condition.
Thermal Analysis of Surface Mounted Leadless Chip Carriers
Authors: Cooley W. T. , Razani A.
Published: April 2008

A thermal analysis has been made of 40 pin leadless chip carriers (LCC) surface mounted on an alumina substrate. Finite element (FE) codes have been used to numerically simulate the problem. We show both theoretically and experimentally that the junction to case temperature is not a convenient parameter for thermal characterization of the heat transfer processes in the complex geometry under consideration. The geometrical complexity of the problem together with the uncertainty of the thermophysical properties and heat transfer coefficients make accurate thermal characterization difficult for parametric study and design analysis. A method, based on homogenization or simplification of a complex region by introducing an effective homogeneous material, is proposed for thermal analysis of complex geometries and its range of applicability is presented. Using this method, different models of heat transfer can be lumped together and various available computer codes can be utilized for rapid thermal design analysis of microelectronic devices.
Piezoresistive Stress Sensors for Structural Analysis of Electronic
Packages
Authors: Bittle D. A. , Suhling J. C. , Beaty R. E. , Jaeger R. C. , Johnson R. W.
Published: April 2008

Structural reliability of electronic packages has become an increasing concern for a variety of reasons including the advent of higher integrated circuit densities, power density levels, and operating temperatures. A powerful method for experimental evaluation of die stress distributions is the use of test chips incorporating integral piezoresistive sensors. In this paper, the theory of conduction in piezoresistive materials is reviewed and the basic equations applicable to the design of stress sensors on test chips are presented. General expressions are obtained for the stress-induced resistance changes which occur in arbitrarily oriented one-dimensional filamentary conductors fabricated out of crystals with cubic symmetry and diamond lattice structure. These relations are then applied to obtain basic results for stressed in-plane resistors fabricated into the surface of (100) and (111) oriented silicon wafers. Sensor rosettes developed by previous researchers for each of these wafer orientations are reviewed and more powerful rosettes are presented along with the equations needed for their successful application. In particular, a new sensor rosette fabricated on (111) silicon is presented which can measure the complete three-dimensional stress state at points on the surface of a die
Stress Analysis of the Vertical Interconnect for Three-Dimensional
Packaging
Authors: Zouari K. , Lee Y. C.
Published: April 2008

Three-dimensional (3-D) packaging is critical to developing supercompact electronic systems. Its representative configuration consists of several circuit packs connected by pressure sensitive vertical interconnects. The sensitive characteristics of the interconnects demand a uniform distribution of acceptable pressure levels. Solutions to analyze and generate such a distribution are derived for a multilayer configuration under an isothermal assembly condition. They are proved to be insensitive to the unknown interface conditions, and applied to study 1) a five-layer model representing a surface mount assembly and a basic building block for 3-D packaging and 2) multilayer models representing a complete system. A uniform pressure distribution may result from 1) thick metal loading plates with a high modulus, 2) thick polymer interconnects with a low modulus, 3) short loading distances, and 4) addition of more layers. The last factor informs designers a good surface mount fixture design can be readily applied to 3-D packaged systems.
Experimental Modal Analysis and Dynamic Response Prediction of PC
Boards With Surface Mount Electronic Components
Authors: Wong Tin-Lup , Stevens Karl K. , Wang Gang
Published: April 2008

Use of modal testing and structural dynamic modification techniques for prediction of the modal parameters of a PC board with a surface-mount component is described. Experimentally determined natural frequencies and mode shapes of a PC board with and without an added component are presented. Several simple dynamic models of the component are evaluated. Results obtained indicate that the component can sometimes be modeled as a point mass.
A Probabilistic Approach for Predicting Thermal Fatigue Life of Wire
Bonding in Microelectronics
Authors: Hu Jun Ming , Pecht Michael , Dasgupta Abhijit
Published: April 2008

Wire bond connections are susceptible to fatigue failure as a result of various thermo-mechanical damage mechanisms during the component operation life. Because of uncertainties arising from the random nature of temperature fluctuations, the assembly of wirebonds, and fatigue properties of bonding materials, a probabilistic analysis and reliability estimation are appropriate. This paper presents such an approach for predicting thermal fatigue life and reliability of wire bonds in electronic packages. To determine the behavior of wire bonds under thermal cycles, the stress amplitude due to the temperature fluctuation in each fatigue failure mechanism is derived based on stress analysis models, the uncertainties involved in stress determination and life prediction are analyzed, and a life prediction equation is proposed. Examples illustrating the application of the approach are discussed.
Temperature Mapping of Localized Hot Spots on Microelectronic Chip
Surfaces
Authors: Heng S. , Black W. Z.
Published: April 2008

This paper describes the use of digital image processing in conjunction with an infrared imaging apparatus to locate and quantify “micro” hot spot temperatures on the surface of energized microelectronic chips. Briefly, the temperature mapping/processing procedure creates emissivity maps for the surface of the chip at different isothermal conditions. The emissivity map images are digitized and stored as a 512 × 512 pixel array, of which 400 lines contain IR information. Apparent temperature measurements are then collected with the chip energized in its normal operating environment. These apparent temperature data are digitized and stored as a 512 × 512 integer array using the same format as the digitized emissivity data. Before correcting for emissivity variations, the apparent temperature images are rectified using digital image processing to precisely overlay the spatial coordinates of the emissivity map. Finally, actual temperature maps are obtained by correcting the apparent temperature data for the local emissivity variations and background reflections. The computer driven measurement technique has been applied to the task of measuring localized temperatures on areas as small as 30 μm on the surface of an energized chip to an accuracy of ±1°C once the surface emissivity is accurately known. The infrared equipment, image processing hardware and supporting software are used to measure the temperature distribution on the surface of a 4.7 mm × 4.7 mm energized chip. IR measured temperatures at isolated locations on the chip are compared with results obtained by the resistance-temperature technique. Since the resistance-temperature technique provides an area-averaged temperature for the energized region, the result obtained from the high resolution IR measurements yields higher localized temperatures. Results are presented for peak surface temperatures up to 100°C and maximum heat flux values of 7.9x106 W/m2. A separate set of infrared measurements are used to predict the influence of surface emissivity on the accuracy of the temperature measurements.
Natural Convection Cooling of a Horizontally Oriented Component Board
Mounted in a Low-Aspect-Ratio Enclosure
Authors: Krane R. J. , Phillips T. J.
Published: April 2008

This investigation consisted of an experimental study of the natural convection heat transfer from a square, horizontally oriented electronic component board mounted in a low-aspect-ratio enclosure with a square planform. The component board was simulated by an upward-facing constant heat flux surface. A variety of experiments were performed to determine the effects of systematically varying the enclosure geometry on the maximum surface temperature on the board for two values of the modified Rayleigh number (heater power setting).
Optimal Thermal Design of Forced Convection Heat
Sinks-Analytical
Authors: Knight R. W. , Goodling J. S. , Hall D. J.
Published: April 2008

For fully developed flow in closed finned channels used to augment heat transfer, there exists an optimal geometrical design of the size and number of cooling channels. In this paper, the problem is generalized with a statement of dimensionless thermal resistance in terms of • the number of channels • a fin to channel thickness ratio • the length to width (planar dimensions) ratio of the heat source, and • a specified fin efficiency or fin length • a fluid to fin thermal conductivity ratio • the Prandtl Number of the coolant • a dimensionless pressure term, which incorporates the maximum allowable pressure drop through the cooling channels or alternatively, • a dimensionless work rate term, which incorporates the maximum allowable coolant pumping power required, An optimization scheme is described and used for comparison with two previously published cases wherein both designs were restricted to a fixed fin to channel thickness ratio and laminar flow; one by Goldberg (1984) using air and copper and a second one only by Tuckerman and Pease (1981) for water-cooled Silicon wafers. Results from the present optimization scheme show that upon reexamination of the first study by Goldberg, significant reduction of thermal resistance can be obtained by using fin/channel dimensions other than unity. A similar reduction is found in the second instance (Tuckerman and Pease) with the relaxation of the laminar limitation.
Analysis of Transient State Temperature Distribution in the Thyristor
Packaging
Authors: Skorek A. , Jordan A. J. , Rajagopalan V.
Published: April 2008

Transient state temperature distribution in a power electronics module comprising two thyristors was computed using a finite element method (FEM). An optimal positioning of one of the thyristors with respect to another was obtained based on the minimization of two norms ‖T‖2 and ‖Tt‖∞. The results obtained from the simulation are compared with those measured using thermocouple techniques; the agreement between these results is found to be good.
Thermal Stresses in Layered Electrical Assemblies Bonded With Solder
Authors: Morgan H. S.
Published: April 2008

Thermal stresses in a layered electrical assembly joined with solder are computed with plane strain, generalized plane strain, and three-dimensional (3D) finite element models to assess the accuracy of the two-dimensional (2D) modeling assumptions. Cases in which the solder is treated as an elastic and as a creeping material are considered. Comparison of the various solutions shows that, away from the corners, the generalized plane strain model produces residual stresses that are identical to those computed with the 3D model. Although the generalized plane strain model cannot capture corner stresses, the maximum stresses computed with this 2D model are, for the mesh discretization used, within 12 percent of the corner stresses computed with the 3D model when the solder is modeled elastically and within 5 percent when the solder is modeled as a creeping material. Plane strain is not a valid assumption for predicting thermal stresses, especially when creep of the solder is modeled. The effect of cooling rate on the residual stresses computed with creep models is illustrated.
Planar Liquid Jet Impingement Cooling of Multiple Discrete Heat Sources
Authors: Schafer D. , Incropera F. P. , Ramadhyani S.
Published: April 2008

Average heat transfer measurements have been made for discrete heat sources located under a liquid jet issuing from a rectangular slot. The heat sources were flush mounted in a plane wall of low thermal conductivity, while the jet emanated from a slot in the opposite wall. The two walls formed a plane channel which simulated a multichip module cooled by direct liquid immersion. Heaters were positioned directly below the jet, as well as at locations offset from the jet midplane. The measurements revealed a secondary peak in the heat transfer coefficient at an offset of approximately four jet widths, and the magnitude of the secondary peak increased with increasing Reynolds number. Depending on Reynolds number, the secondary peak may be due to formation of a recirculation zone and/or to boundary layer transition. The effect of separation distance between the nozzle and the impingement plate was small for the conditions of the study, as was the effect of upstream heating on heat sources located near the stagnation line of the jet. The effect of the outflow manifold location was also found to be negligible, except when it was positioned directly over a heater.
Fluid Dynamic Characteristics of the Flow Over an Array of Large Roughness Elements
Authors: Garimella S. V. , Eibeck P. A.
Published: April 2008

Flow visualization and measurements of velocity and turbulence intensity using laser Doppler velocimetry are used to investigate separation and reattachment processes in the flow over an array of protruding elements mounted on the bottom wall of a rectangular water channel. The concept of an array shear layer is introduced to demarcate the region of influence over which the resistance of the array retards the flow. Flow separation at the leading edge of the elements is documented. The confined or interacting nature of the flow in the cavities between elements is established as a function of element spacing. The reattachment length downstream of the element varies from 4 to 1.5 element heights, decreasing both with an increase in Reynolds number and a decrease in channel height.
Effects of Configuration on Plastic Package Stresses
Authors: Nguyen L. T. , Gee S. A. , Bogert W. F. v. d.
Published: April 2008

This paper examines the effects of device and leadframe configurations on the stresses encountered in typical Dual-in-Line plastic packages. The parameters studied include the die size, the die pad size, the location of the die with respect to the die pad center, and the die coating configuration. Special piezoresistive strain gages deposited on dies of varying sizes are used to map the stress profile across the die surfaces after molding. Finite element simulation of these effects is also conducted. Results indicate that the compressive stresses from the molding compounds are governed with diminishing influence by the size of the die. Furthermore, rather high compressive stresses are observed in the vicinity of the edges of large dies. More subtle effects are found for the influence of the die pad size, the aspect ratio of the die, and the extent of the die offsetting with respect to the die pad center. Finally, by surrounding the die with a thin trail of silicone gel to provide for lateral cushioning, stress reduction is slightly more effective than in the standard “glob-top” coating.
Mechanical Deformation of Leadframe Assemblies in Plastic Packages During Molding
Authors: Suhir E. , Manzione L. T.
Published: April 2008

We evaluate the maximum stresses and deflections in electric leads subjected to flow induced forces during transfer molding of plastic packages. It is found that lead deflection is proportional to the fifth power of the lead length and the inverse third power of lead thickness. These dependencies explain why it is more difficult to mold high pin count packages which have significantly longer and thinner leads to accommodate the higher interconnection density. In addition we find that the maximum value of the elastic lead deformation is about 0.9 mm (35 mils) for large state-of-the-art packages; exceeding this value causes permanent (plastic) lead distortion that compromises the symmetry of the package. We find also that minimizing the difference in flow front locations through careful gate design, and lowering the velocity over the leads by using balanced mold filling can make a significant difference in lead deformation. The results obtained are useful in establishing the appropriate geometry of the leads, as well as in choosing the materials and process parameters, so that excessive stresses and lead deflections can be avoided.
Thermal Reliability of a Bilayer Slab Subjected to a Local Heat Source
Authors: Elperin T. , Rudin G.
Published: April 2008

The paper considers the distribution of temperature and thermal stresses in a bilayer slab which consists of a substrate and a thin coating subjected to a local short heat pulse. The heat source is distributed uniformly over a circular spot on a surface of the coating. The temperature and thermal stresses distributions in a slab are found analytically in a closed form using Laplace and Hankel integration transforms. The analytic relation is obtained for the evaluation of components of stress tensor in a coating as a function of a Fourier number and radius of heat source.