The effect of current stressing on the reliability of 63Sn37Pb solder joints with Cu pads was investigated at temperatures
of −5 °C and 125 °C up to 600 h. The samples were stressed with 3 A current (6.0 × 102 A/cm2 in the solder joint with diameter of 800 μm and 1.7 × 104 A/cm2 in the Cu trace with cross section area of 35 × 500 μm). The temperatures of the samples and interfacial reaction within
the solder joints were examined. The microstructural change of the solder joints aged at 125 °C without current flow was also
evaluated for comparison. It was confirmed that the current flow could cause the temperature of solder joints to rise rapidly
and remarkably due to accumulation of massive Joule heat generated by the Cu trace. The solder joints stressed at 125 °C with
3 A current had an extensive growth of Cu6Sn5 and Cu3Sn intermetallic compounds (IMC) at both top and bottom solder-to-pad interfaces. It was a direct result of accelerated aging
rather than an electromigration or thermomigration effect in this experiment. The kinetic is believed to be bulk diffusion
controlled solid-state reaction, irrespective of the electron flow direction. When stressed at −5 °C with 3 A current, no
significant change in microstructure and composition of the solder joints had occurred due to a very low diffusivity of the
atoms as most Joule heat was eliminated at low temperature. The IMC evolution of the solder joints aged at 125 °C exhibited
a subparabolic growth behavior, which is presumed to be a combined mechanism of grain boundary diffusion and bulk diffusion.
This is mainly ascribed to the retardant effect against the diffusion course by the sufficiently thick IMC layer that was
initially formed during the reflow soldering. 相似文献
In this article, the problem of providing a fair bandwidth allocation to the flows sharing a congested link in a router is investigated. Queue management, bandwidth share and congestion control are very important to both the robustness and fairness of the Internet. The buffer at the outgoing link is a simple FIFO, shared by packets belonging to the flows. A new transmission control protocol (TCP)-friendly router-based active queue management scheme, termed WARD, is proposed to approximate the fair queueing policy. WARD is a simple packet-dropping algorithm with a random mechanism which discriminates against flows that submit more packets per second than is allowed as their fair share. By doing this, it not only protects TCP connections from user datagram protocol flows, but also solves the problem of competing bandwidth among different TCP versions, such as TCP Vegas and TCP Reno. In addition, WARD works quite well for TCP flow isolation even with different round trip times. In other words, WARD improves the unfair bandwidth allocation properties. Furthermore, as it is stateless and easy to implement, WARD controls unresponsive or misbehaving flows with only a minimum overhead. 相似文献
An overview of the present status and future trends in electric vehicle technology is provided. The emphasis is on the impact of rapid development of electric motors, power electronics, microelectronics, and new materials. Comparisons are made among various electric drive systems and battery systems. The market size of electric vehicles in the coming years and the potential electric vehicle impacts are discussed 相似文献
This paper describes the development of an expert system in gearing design application. The knowledge in this design field is vast and involves selection of appropriate data from large amounts of engineering standards data available in engineering catalogues and design handbooks. Experts very oftens have different opinions in determining the design solution. The theme of the work is to organise, systemise, conserve and spread this knowledge relating to gearing design and configuration.
There is a large number of design parameters involved in gearing design. The design of gears also requires an iterative approach to optimize these design parameters that govern the kinematic, as well as the strength performance. Owing to the complex combinations of these aspects, conventional design office practice tends to become complicated and time consuming. Hence, in this piece of work, attempts are made to formulate a design system capable of designing a required type of gear and its configuration under a specified set of working conditions.
The main features of the expert gearing design system are the use of semantic nets in representing the objects, a frame-based data acquisition and representation system, and production rules in representing the domain-specific knowledge base. The system provides a very user-friendly interface through the use of an expert system development tool Prolog. While Prolog is used to set up the various databases and the inference engine, the detail gear design calculations are performed by C. It is found that the communication between these two language media is excellent in terms of performance and efficiency. 相似文献
The present study in conscious rabbits with intracisternal (i.c.) catheters sought to determine the relative contribution of the I1 subtype of imidazoline receptors (IR) and alpha 2 adrenoceptors to the hypotensive effects of rilmenidine, clonidine and moxonidine with an I1-IR/alpha 2 adrenoceptor antagonist efaroxan and a specific alpha 2 adrenoceptor antagonist 2-methoxyidazoxan (2-MI). The alpha 2 adrenoceptor antagonist effect of efaroxan was compared with 2-MI by performing cumulative dose-response curves in the presence of alpha-methyldopa (400 micrograms/kg i.c.). 2-MI was 5.6 times more potent than efaroxan at reversing 75% of the hypotension elicited by alpha-methyldopa (P < .025). This dose ratio was used to match doses of efaroxan and 2-MI for similar alpha 2 adrenoceptor blockade. The effects of efaroxan (4.1, 13, 41 micrograms/kg i.c.) and 2-MI (0.74, 2.3, 7.4 micrograms/kg i.c.) were investigated on a single i.c. dose of rilmenidine (12 micrograms/kg), clonidine (0.75 microgram/kg) and moxonidine (0.51 microgram/kg). These doses of the antihypertensive agents, which were determined from cumulative dose-response curves, produce 90% of the maximum hypotension. Efaroxan was more effective at reversing the hypotension induced by moxonidine and rilmenidine than was 2-MI (P < .01). These findings suggest that rilmenidine and moxonidine act predominantly via IR. By contrast, 2-MI was more effective at reversing the clonidine-induced hypotension than was efaroxan (P < .001), suggesting that clonidine acts mainly via alpha 2 adrenoceptors in conscious normotensive rabbits. Thus, a higher selectivity of the second generation agents moxonidine and rilmenidine for I1-IR over alpha 2 adrenoceptors, compared with the first generation agent clonidine, appears to be necessary for this effect to be manifested in their hypotensive actions. 相似文献
Simultaneous measurements of weight uptake and conductivity variation during iodine vapor sorption of polyacetylenes (PA) at 20–25°C show that the sorption is a two-stages process. For the first stage, iodine diffusion to the fibrilar surface of the cis-rich and trans-rich PA is of Knudsen type pore diffusion as supported by the sorption measurements of hexane for the cis-rich PA, and has a diffusion coefficient in the order of 107 cm2/sec. Conductivity of the PA rises rapidly and reaches a maximum at end of the stage. For the second stage, the diffusion is more restrictive and has a diffusion coefficient lower than the first stage by a factor of about 10 due to multilayer sorption of iodine, which leads to a decrease in the pore diameter and therefor the diffusion rate. In addition to the restricted diffusion in the second stage, diffusion of the iodine molecules adsorped on the fibrilar surface into the interior of the fibrils is appreciable for cis-rich PA (leading to a conductivity drop) and is negliglible for tran-rich PA (leading to insignificant variation in conductivity). 相似文献
While current psychiatric taxonomies recognise a classification of amphetamine dependence, derived from the notion of an alcohol dependence syndrome, little research has validated that such a condition exists for this drug. Current amphetamine users (N = 331), were interviewed using the World Health Organization operationalisation of DSM-III-R substance dependence criteria, and a measure of the psychological components of dependence. Structural analyses indicated that a unidimensional dependence syndrome as assessed by DSM-III-R and DSM-IV criteria exists for amphetamine, and that physiological, psychological and behavioural indicators were all important in accounting for the variance in responses. It was demonstrated that the concept of a dependence syndrome is applicable to amphetamine, and that the inclusion of the amphetamine dependence syndrome in DSM-III-R and DSM-IV is valid. 相似文献
Crystalline titanium dioxide powders have been synthesized as either rutile or anatase from aqueous solutions at low temperatures
(T≤100°C) and atmospheric pressure. First, a sol is prepared by the hydrolysis of a titanium alkoxide in an acidic solution.
The sol is subsequently heated at different rates to produce the different crystalline phases of titanium dioxide. Powder
characterization was carried out using X-ray diffraction, scanning electron microscopy and high resolution transmission electron
microscopy. In general, the precipitate size was observed to be between 50 and 100 nm. Possible mechanisms involved in determining
the crystal variants are discussed.
This revised version was published online in November 2006 with corrections to the Cover Date. 相似文献