Processing and material issues related to lead-free soldering

The European requirement for lead-free electronics has resulted in higher soldering temperature and some material and process changes. Traditional tin–lead solder melts at 183°C, where as the most common lead-free alternatives have a much higher melting temperature—tin–copper (227°C), tin–silver (221°C) and tin–silver–copper (217°C). These have challenged the ingenuity of the materials and process engineers. This chapter will explore some of the issues that have come up in this transition, and which these engineers have understood and addressed. As we enter the lead-free era, we see changes as printed wiring board (PWB) substrates which were designed for lower soldering temperatures are being replaced by newer materials. Factors such as glass transition temperature (T _g), decomposition temperature (T _d) and coefficient of thermal expansion must be considered. Many electronic components are made for lower peak temperatures than those required by the new solders. Solder flux chemistries are changing to meet the needs of the new metal systems, and cleaning of flux residues is becoming more of a challenge. Finally, there is a potential reliability problem—an increased potential for the growth of conductive anodic filament (CAF), an electrochemical failure mechanism that occurs in the use environment.     

Processing of whisker-reinforced ceramics

Processing methods for whisker-reinforced ceramic composites are investigated which have been developed from known methods used for monolithic ceramics. Techniques for the removal of particulate impurities from whiskers by sedimentation and for mixing whiskers and ceramic powder are described with reference to three types of commercially available whiskers.     

Thermal conductivity of calcium-doped aluminium nitride ceramics

Aluminium nitride ceramics were prepared with the addition of up to 12wt% of calcium oxide as a sintering aid. Both the oxygen and the calcium content of the samples decreased during sintering with increasing sintering temperature and soaking time. Higher amounts of calcium oxide resulted in higher thermal conductivities, with values up to 142 W m^–1 K^–1. Moderate sintering temperatures, short temperature soaking times and the use of inexpensive Ca-based sintering additives should enable the production of aluminium nitride ceramics with sufficiently high thermal conductivity at relatively low cost.     

Processing of spodumene-modified mullite ceramics

The use of β-spodumene (Li₂O·Al₂O₃·4SiO₂) has been investigated as a liquid-phase sintering aid for the densification of mullite processed from fumed silica and ESP (alumina) dust. XRD, DTA, SEM and Vickers indentation were used to characterize the effect of spodumene on the phase relations, sintering behaviour, microstructure and mechanical properties of mullite. The results show that the presence of spodumene significantly reduces the porosity, improves the sintering behaviour and enhances the formation of mullite at 1550 °C. Spodumene-modified mullite ceramics also have better physical and mechanical properties. This revised version was published online in November 2006 with corrections to the Cover Date.     

碳化硅陶瓷导热性能的研究进展EI北大核心

SiC陶瓷具有优异的力学性能、热学性能、抗热震性能、抗化学侵蚀性能和抗氧化性能,是热交换器设备的常用基体材料。由于原料、成型工艺、烧成工艺和烧结助剂等因素制约,SiC陶瓷含有较多气孔、晶界、杂质和缺陷,导致其常温热导率(≤270 W·m^(-1)·K^(-1))低于碳化硅单晶材料(6H-SiC,490 W·m^(-1)·K^(-1)),且不同制备工艺下热导率存在较大差异。本文主要分析了温度、气孔、晶体结构和第二相对SiC陶瓷导热性能的影响,归纳了热压烧结法、放电等离子烧结法、无压烧结法、重结晶烧结法和反应烧结法制备高导热SiC陶瓷的特点,对优化烧结助剂种类及含量、高温热处理和添加高导热第二相等改善SiC陶瓷导热性能的主要措施进行阐述,并展望了未来高导热SiC陶瓷的研究方向,为未来制备低成本、高导热SiC质热交换器提供理论参考。     

Meeting the challenges related to material issues in chemical industries

Reliable performance and profitability are two important requirements for any chemical industry. In order to achieve high level of reliability and excellent performance, several issues related to design, materials selection, fabrication, quality assurance, transport, storage, inputs from condition monitoring, failure analysis etc. have to be adequately addressed and implemented. Technology related to nondestructive testing and monitoring of the plant is also essential for precise identification of defect sites and to take appropriate remedial decision regarding repair, replacement or modification of process conditions. The interdisciplinary holistic approach enhances the life of critical engineering components in chemical plants. Further, understanding the failure modes of the components through the analysis of failed components throws light on the choice of appropriate preventive measures to be taken well in advance, to have a control over the overall health of the plant. The failure analysis also leads to better design modification and condition monitoring methodologies, for the next generation components and plants. At the Indira Gandhi Centre for Atomic Research (IGCAR), Kalpakkam, a unique combination of the expertise in design, materials selection, fabrication, NDT development, condition monitoring, life prediction and failure analysis exists to obtain desired results for achieving high levels of reliability and performance assessment of critical engineering components in chemical industries. Case studies related to design, materials selection and fabrication aspects of critical components in nuclear fuel reprocessing plants, NDT development and condition monitoring of various components of nuclear power plants, and important failure investigations on critical engineering components in chemical and allied industries are discussed in this paper. Future directions are identified and planned approaches are briefly described     

Probing the bulk ionic conductivity by thin film hetero-epitaxial engineering

Abstract

Highly textured thin films with small grain boundary regions can be used as model systems to directly measure the bulk conductivity of oxygen ion conducting oxides. Ionic conducting thin films and epitaxial heterostructures are also widely used to probe the effect of strain on the oxygen ion migration in oxide materials. For the purpose of these investigations a good lattice matching between the film and the substrate is required to promote the ordered film growth. Moreover, the substrate should be a good electrical insulator at high temperature to allow a reliable electrical characterization of the deposited film. Here we report the fabrication of an epitaxial heterostructure made with a double buffer layer of BaZrO₃ and SrTiO₃ grown on MgO substrates that fulfills both requirements. Based on such template platform, highly ordered (001) epitaxially oriented thin films of 15% Sm-doped CeO₂ and 8 mol% Y₂O₃ stabilized ZrO₂ are grown. Bulk conductivities as well as activation energies are measured for both materials, confirming the success of the approach. The reported insulating template platform promises potential application also for the electrical characterization of other novel electrolyte materials that still need a thorough understanding of their ionic conductivity.     

Probing the bulk ionic conductivity by thin film hetero-epitaxial engineering

Highly textured thin films with small grain boundary regions can be used as model systems to directly measure the bulk conductivity of oxygen ion conducting oxides. Ionic conducting thin films and epitaxial heterostructures are also widely used to probe the effect of strain on the oxygen ion migration in oxide materials. For the purpose of these investigations a good lattice matching between the film and the substrate is required to promote the ordered film growth. Moreover, the substrate should be a good electrical insulator at high temperature to allow a reliable electrical characterization of the deposited film. Here we report the fabrication of an epitaxial heterostructure made with a double buffer layer of BaZrO₃ and SrTiO₃ grown on MgO substrates that fulfills both requirements. Based on such template platform, highly ordered (001) epitaxially oriented thin films of 15% Sm-doped CeO₂ and 8 mol% Y₂O₃ stabilized ZrO₂ are grown. Bulk conductivities as well as activation energies are measured for both materials, confirming the success of the approach. The reported insulating template platform promises potential application also for the electrical characterization of other novel electrolyte materials that still need a thorough understanding of their ionic conductivity.     

Microstructure and thermal conductivity of AIN ceramics containing additives

The effect on AIN ceramic of the addition of Y₂O₃, Yb₂O₃, Er₂O₃ and CaO were investigated using scanning electron microscopy (SEM), transmission electron microscopy (TEM) and thermal conductivity measurements. The effect of grain boundary segregation and second phase distribution on the thermal conductivity are discussed. The Er₂O₃-CaO-and the Yb₂O₃-CaO-AIN ceramics have a higher thermal conductivity than the CaO-and the Y₂O₃-CaO-AIN ceramics. This is explained on the basis of the free energy of formation (G°), the vaporization of the sintering additives and the microstructural development. Oxidation of freshly cleaned surfaces of those AIN ceramics was studied.     

磁控溅射制备ZrN_x薄膜离子导体性能研究

采用射频反应磁控溅射工艺,以纯Zr为靶材,在WO3/ITO/Glass基片上采用不同工艺参数沉积ZrNx薄膜,用紫外-可见分光光度计、循环伏安法、X射线衍射仪、扫描电镜等研究了ZrNx薄膜的离子导电性能。研究结果表明,所制备的ZrNx薄膜为非晶态,ZrNx/WO3/ITO/Glass复合膜的光学调节范围最大达57%以上,在离子传导过程中表现出良好的离子导电性能。     

High-temperature thermal conductivity of biomorphic SiC/Si ceramics

Thermal conductivity of biomorphic SiC/Si, a silicon carbide + silicon containing two phase material, was evaluated using the laser steady-state heat flux method. These materials were processed via silicon melt infiltration of wood-derived carbon scaffolds. In this approach, heat flux was measured through the thickness when one side of the specimen was heated with a 10.6-µm CO₂ laser. A thin mullite layer was applied to the heated surface to ensure absorption and minimize reflection losses, as well as to ensure a consistent emissivity to facilitate radiative loss corrections. The influence of the mullite layer was accounted for in the thermal conductivity calculations. The effect of microstructure and composition (inherited from the wood carbonaceous performs) on measured conductivity was evaluated. To establish a baseline for comparison, a dense, commercially available sintered SiC ceramic was also evaluated. It was observed that at a given temperature, thermal conductivity falls between that of single-crystal silicon and fine-grained polycrystalline SiC and can be rationalized in terms of the SiC volume fraction in biomorphic SiC/Si material.     

Processing and properties of nanoporous hydroxyapatite ceramics

Method for fabrication and properties of nanoporous hydroxyapatite (HA) ceramic were described in the present work. The nanoporous hydroxyapatite was derived from nano hydroxyapatite powder and polyvinyl alcohol (as a pore former). The HA nanopowder was obtained from vibro-milling for 4 h. The nanoporous ceramics were sintered at 1200 °C. Properties of the nanoporous ceramics were investigated using various methods. Average porosity of the final product was found to be 64.6 ± 1.4%. Open and interconnected pores were obtained with an average pore size less than 100 nm, confirming the nanoporous structure of this ceramic. A high bending strength of 14.7 ± 3.2 MPa for the nanoporous ceramic, shows significant promise as a potential bone repairing material.