现代电子产品焊接质量的检测技术

介绍了现代电子产品焊接质量的检测技术,以及用于检查焊点质量和组装板(SMA)功能的测试仪器的工作原理及测试能力.目前常见和广泛使用的焊接质量检测方法是人工目测检查,其主要问题是其主观性,只能用来检测电子元件的形状、尺寸、颜色、表面特征及焊点的外观质量;自动光学检测(AOI)可以在整个过程中发现和纠正缺陷,但只是外在的焊点缺陷;激光/红外线组合式检测系统、X射线检测系统可以分析焊点微米级水平的缺陷,能找出其他检测所不能可靠地发现的缺陷,包括空洞、焊点形状差和冷焊锡点等;另外,X射线检测系统能一次测试单面或双面电路,准确地定位缺陷,获取工艺参数,例如锡膏厚度等.     

以寻呼区分析为基础的PHS网络优化方法

通过分析PHS网络中各寻呼区的统计指标,剖析指标劣化的根本原因,采取修改相应的参数配置、移站、调整天线等措施进行网络优化,优化实施后再次收集统计指标做对比分析,总结经验。     

Fixed mesh front‐tracking methodology for finite element simulations

A direct front‐tracking method using an Eulerian–Lagrangian formulation is developed in two space dimensions. The front‐tracking method is general in that it can track any type of interface once its local velocity is specified or has been determined by calculation. The method uses marker points to describe the interface position and tracks the interface evolution on a fixed finite‐element mesh, including growth, contraction, splitting and merging. Interfacial conditions are applied directly at the interface position. The method is applied to three scenarios that involve different interface conditions and are based on energy and mass diffusion. The three calculations are for the dendritic solidification of a pure substance, the cellular growth of an alloy, and the Ostwald ripening of silica particles in silicon. Numerical results show that very complicated interface morphologies and topological changes can be simulated properly and efficiently. Copyright © 2004 John Wiley & Sons, Ltd.     

Optimal resource allocation for wireless video over CDMA networks

We present a multiple-channel video transmission scheme in wireless CDMA networks over multipath fading channels. We map an embedded video bitstream, which is encoded into multiple independently decodable layers by 3D-ESCOT video coding technique, to multiple CDMA channels. One video source layer is transmitted over one CDMA channel. Each video source layer is protected by a product channel code structure. A product channel code is obtained by the combination of a row code based on rate compatible punctured convolutional code (RCPC) with cyclic redundancy check (CRC) error detection and a source-channel column code, i.e., systematic rate-compatible Reed-Solomon (RS) style erasure code. For a given budget on the available bandwidth and total transmit power, the transmitter determines the optimal power allocations and the optimal transmission rates among multiple CDMA channels, as well as the optimal product channel code rate allocation, i.e., the optimal unequal Reed-Solomon code source/parity rate allocations and the optimal RCPC rate protection for each channel. In formulating such an optimization problem, we make use of results on the large-system CDMA performance for various multiuser receivers in multipath fading channels. The channel is modeled as the concatenation of wireless BER channel and a wireline packet erasure channel with a fixed packet loss probability. By solving the optimization problem, we obtain the optimal power level allocation and the optimal transmission rate allocation over multiple CDMA channels. For each CDMA channel, we also employ a fast joint source-channel coding algorithm to obtain the optimal product channel code structure. Simulation results show that the proposed framework allows the video quality to degrade gracefully as the fading worsens or the bandwidth decreases, and it offers improved video quality at the receiver.     

Level set and geodesic active contours based measurement of material removal between serial sections

The measurement of the thickness of material removed between serial sections is a crucial step of three-dimension reconstruction. Geodesic active contours is an efficient method for contour detection of objects on an image. The indents on the SiC/Al composite micrographs are segmented by using level set and geodesic active contours. After getting the contours of the indents, we calculate the distance from the uppermost pixel to the lowermost pixel to get the vertical diagonal length, and calculate the distance from the leftmost pixel to the rightmost pixel to get the horizontal diagonal length of each contour. Then the final length of each diagonal is acquired by averaging the vertical diagonal length and the horizontal diagonal length, respectively. The Vickers’ indenter is made by a square pyramidal-shaped diamond with opposite faces at an angle of 136°, so the thickness of material removed between two serial sections and the length difference of the diagonals on the two serial sections has a definite geometrical relation. Thus the thickness of material removed between two serial sections is acquired using the geometrical relation.     

Fatigue Behavior and Retrofit Investigation of Distortion-Induced Web Gap Cracking

This paper studies a Kansas Department of Transportation welded plate girder bridge that developed fatigue cracks at small web gaps close to the girder top flange. Repair had been previously performed by softening the connection plate end with a slot retrofit, but cracks were recently found to have reinitiated at some of the repaired details and are again propagating. A comprehensive finite-element method study was performed to investigate the cracking behavior observed in the bridge and to recommend appropriate measures for future bridge retrofit. The analytical results show that stresses developed at the top flange web gaps could exceed yielding under the loading of an HS15 fatigue truck. The current slot repair used in the bridge was found to have introduced higher magnitude fatigue stresses in the web gap. To achieve a permanent repair of the bridge, it is recommended that a welded connection plate to flange attachment be used during future bridge retrofit. The web gap details should be able to withstand unlimited number of load cycles once this additional repair is performed.     

扇形段电磁搅拌对82B钢连铸坯成分和组织的影响

进行了结晶器搅拌和结晶器 +扇形段电磁搅拌连铸 82B钢 2 80mm× 380mm方坯的工业试验。结果表明 ,扇形段搅拌降低和改善了铸坯的中心偏析、疏松和夹杂物分布。 82B钢铸坯经扇形段搅拌后 ,中心部位碳含量峰值明显减小 ,碳偏析指数降低 5 7%～ 1 0 8% ,铸坯成分的方差值 (σ)减小 32 %～ 34 9%。     

高炉与风机配置的优化

根据高炉风机配置的具体情况 ,为充分发挥大风机生产优势 ,建立了适宜的送风制度及装料制度 ,对利旧风机采取了自动防喘振控制、高压水阻软启动的措施 ,风机仪表采用集散控制 ,并优化上料系统 ,提高了高炉上料能力。一系列优化方案保证了高炉稳定、安全、顺行 ,1号、2号高炉利用系数分别由 3 .3 64、3 .43 2t/m3.d提高到 3 .917和 3 .767t/m3.d。     

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以天然气为能源，用燃气机代替电动机驱动压缩机并充分利用发动机废热、工业余热以及低品位环境热量的采暖、空调装置——燃气压缩式热泵近年来发展迅速。美国燃气设备的总装机容量已达到1．2GW，日本燃气机热泵占全部燃气制冷设备的29％，总装机容量大约1GW，燃气机热泵在英国、法国、德国、瑞士等都得到了较快的发展与应用。中研究了以天然气为驱动能源的压缩式热泵的特点及能量利用率，分析并测试了该系统的运行特性及交工况特性，进行了各种工况及转速下的实验研究。结果表明，该系统具有较好的部分负荷特性、一次能源利用率及交工况特性，为一种绿色高效的供热空调系统。