Thermomechanical reliability of underfilled BGA packages

The effect of underfill on various thermomechanical reliability issues in super ball grid array (SBGA) packages is studied in this paper. Nonlinear finite element models with underfill and no underfill are developed taking into consideration the process-induced residual stresses. In this study, the solder is modeled as time and temperature-dependent, while other materials are modeled temperature and direction-dependent, as appropriate. The stress/strain variations in the package due to thermal cycling are analyzed. The effect of underfill is studied with respect to magnitude and location of time-independent plastic strain, time-dependent creep strain and total inelastic strain in solder balls. The effect of copper core on the solder ball strains is presented. The possibility of delamination at the interposer-underfill interface as well as substrate-underfill interface is studied with the help of qualitative interfacial stress analysis. Results on SBGA packages indicate that the underfill does not always enhance BGA reliability, and that the properties of the underfill have a significant role in the overall reliability of the BGA packages. The predicted number of thermal cycles to solder joint fatigue are compared with the existing experimental data on similar nonunderfilled BGA packages.     

Distributed Hashing for Scalable Multicast in Wireless Ad Hoc Networks

Several multicast protocols for mobile ad hoc networks have been proposed, which build multicast trees by using location information that is available from the Global Positioning System (GPS) or localization algorithms and use geographic forwarding to forward packets down the multicast trees. These stateless multicast protocols carry encoded membership, location, and tree information in each packet and are more efficient and robust than stateful protocols (for example, ADMR and ODMRP), as they avoid the difficulty of maintaining distributed state in the presence of frequent topology changes. However, current stateless multicast protocols are not scalable to large groups because of the per-packet encoding overhead, and the centralized group membership and location management. We present the hierarchical rendezvous point multicast (HRPM) protocol, which significantly improves the scalability of stateless multicast with respect to the group size. HRPM consists of two key design ideas: 1) hierarchical decomposition of a large group into a hierarchy of recursively organized manageable-sized subgroups and 2) the use of distributed geographic hashing to construct and maintain such a hierarchy at virtually no cost. Our detailed simulations demonstrates that HRPM achieves significantly enhanced scalability and performance due to hierarchical organization and distributed hashing.     

Experimentally feasible measures of distance between quantum operations

We present two measures of distance between quantum processes which can be measured directly in laboratory without resorting to process tomography. The measures are based on the superfidelity, introduced recently to provide an upper bound for quantum fidelity. We show that the introduced measures partially fulfill the requirements for distance measure between quantum processes. We also argue that they can be especially useful as diagnostic measures to get preliminary knowledge about imperfections in an experimental setup. In particular we provide quantum circuit which can be used to measure the superfidelity between quantum processes. We also provide a physical interpretation of the introduced metrics based on the continuity of channel capacity.     

干燥法制备聚丙烯腈初生纤维样品的研究

采用湿法纺丝得到聚丙烯腈(PAN)初生纤维,干燥后得到PAN初生纤维样品.利用电阻法确定初生纤维的共晶温度;获得初生纤维的冻干曲线;通过压汞法对初生纤维孔结构进行表征,并对比考察了鼓风干燥、自然风干和冷冻干燥对样品截面形貌的影响.结果表明:初生纤维的共晶温度为-55℃;鼓风干燥会使样品收缩,纤维间发生粘连,原有形貌被破坏;自然风干使样品的截面积和周长分别收缩了20.66%、10.67%,形状系数变化率为3.52%.而冷冻干燥时样品的截面积和周长仅收缩了1.21%、1.15%,形状系数变化率为1.13%.冷冻干燥有效地保留了PAN初生纤维的固有原始结构,可用于制备各种分析的样品.