Guna S Selvaduray

Lead-free solders in microelectronics

Abstract Practically all microelectronic assemblies in use today utilize Pb–Sn solders for interconnection. With the advent of chip scale packaging technologies, the usage of solder connections has increased. The most widely used Pb–Sn solder has the eutectic composition. Emerging environmental regulations worldwide, most notably in Europe and Japan, have targeted the elimination of Pb usage in electronic assemblies, due to the inherent toxicity of Pb. This has made the search for suitable “Pb-free” solders an important issue for microelectronics assembly. Approximately 70 Pb-free solder alloy compositions have been proposed thus far. There is a general lack of engineering information, and there is also significant disparity in the information available on these alloys. The issues involved can be divided into two broad categories: manufacturing and reliability/performance. A major factor affecting alloy selection is the melting point of the alloy, since this will have a major impact on the other polymeric materials used in microelectronic assembly and encapsulation. Other important manufacturing issues are cost, availability, and wetting characteristics. Reliability related properties include mechanical strength, fatigue resistance, coefficient of thermal expansion and intermetallic compound formation. The data available in the open literature have been reviewed and are summarized in this paper. Where data were not available, such as for corrosion and oxidation resistance, chemical thermodynamics was used to develop this information. While a formal alloy selection decision analysis methodology has not been developed, less formal approaches indicate that Sn-rich alloys will be the Pb-free solder alloys of choice, with three to four alloys being identified for each of the different applications. Research on this topic continues at the present time at a vigorous pace, in view of the imminence of the issue.

Solder Joint Fatigue Models – Review and Applicability to Chip Scale Packages

Abstract A review of fourteen solder joint fatigue models is presented here with an emphasis on summarizing the features and applications of each fatigue model. The models are classified into five categories: stress-based, plastic strain-based, creep strain-based, energy-based, and damage-based. Fatigue models falling outside these categories are categorized as ‘other empirical models’. Each model is presented under one category with the relevant parameters and applicable packages. Following each category, common issues such as thermal cycling conditions, solder joint geometry, and coverage are addressed. Two fatigue model application scenarios are discussed. In the first scenario, a set of existing fatigue test data is given to the engineer who must determine how best to interpret the data and which fatigue model(s) best apply. In the second scenario, a test scheme must be devised for a new chip scale package product. The number of cycles to failure (Nf) or fatigue life must be determined. A general procedure is presented for choosing an appropriate fatigue model(s) based on the package conditions and limited Finite Element Analysis time. This procedure is summarized in a flowchart.

Effects of surface treatment on the surface chemistry of NiTi alloy for biomedical applications

Alloys of NiTi have gained popularity in biomedical applications; however, Ni is known to cause toxic and allergic reactions in the body, and concerns have been expressed regarding the materials biocompatibility. In this study, coupons of equiatomic NiTi were prepared by four methods, namely, mechanically polishing to a mirror finish, electropolishing, chemical etching and argon plasma etching, to produce various levels of roughness, and then examined by atomic force microscopy (AFM), XPS and AES. The resulting surface chemistry depended upon the method of preparation and was found not to be a function of surface roughness. The mechanically polished samples, although having the smoothest surface, showed the highest level of Ni in the near-surface region. The other preparation methods produced surfaces that were predominantly TiO2, with the electropolished surfaces showing the next smoothest surface and the least Ni in the near-surface region. The correlation between method of preparation, surface roughness and surface chemistry may be important in the preparation of NiTi for biomedical applications.

Effect of CO2 on the processing of Y-Ba-Cu-O superconductors

The superconducting properties of YBa{sub 2}Cu{sub 3}O{sub 6+{ital x}} reacted with various known ratios of O{sub 2}/CO{sub 2} gas mixtures during sintering at different temperatures were studied. {ital J}{sub {ital c}} was found to decrease drastically upon reaction with CO{sub 2}, becoming zero at certain CO{sub 2} activities. The stability region for the 123 superconductor, as a function of CO{sub 2} activity and temperature, was empirically formulated as follows: log {ital p}{sub CO{sub 2}}{lt}({minus}45,000)/{ital T}+33.4 . The grain boundaries in sintered samples with {ital J}{sub {ital c}}=0 were investigated with HRTEM in conjunction with EDS. Two distinct types of grain boundaries were observed. Approximately 10% of the grain boundaries were wet by a thin layer of a second phase, deduced to be BaCuO{sub 2}. The remaining boundaries were sharp grain boundaries. The grain structure near the sharp grain boundaries were tetragonal. These two types of grain boundaries are thought to be responsible for {ital J}{sub {ital c}} being zero.

Die bond materials and bonding mechanisms in microelectronic packaging

Abstract The bonding mechanism for a silver-lead borate glass adhesive, used as a die bonding material in microelectronics packaging, was investigated. Silicon dies were bonded to alumina substrates with the glass adhesive, thermally cured and then sectioned, polished and analyzed. The diffusion of silicon from the die was found to be important in the development of the bond. Adhesion between the silver glass and the silicon die was found to be achieved via a glass network structure that starts at the silicon dioxide film on the die back side. Bonding between silver glass and the ceramic substrate was found to be mechanical in nature.

Characterization of silver nanoparticle-infused tissue adhesive for ophthalmic use

In this work, we demonstrate the successful enhancement of breaking strength, adhesive strength, and antibacterial efficacy of ophthalmic tissue adhesive (2-octyl cyanoacrylate) by doping with silver nanoparticles, and investigate the effects of nanoparticle size and concentration. Recent work has shown that silver nanoparticles are a viable antibacterial additive to many compounds, but their efficacy in tissue adhesives was heretofore untested. Our results indicate that doping the adhesive with silver nanoparticles reduced bacterial growth by an order of magnitude or more; nanoparticle size and concentration had minimal influence in the range tested. Tensile breaking strength of polymerized adhesive samples and adhesive strength between a T-shaped support and excised porcine sclera were measured using a universal testing machine according to ASTM (formerly American Society for Testing and Materials) standard techniques. Both tests showed significant improvement with the addition of silver nanoparticles. The enhanced mechanical strength and antibacterial efficacy of the doped adhesive supports the use of tissue adhesives as a viable supplement or alternative to sutures.

Conducting a Soft First-Story Multifamily Dwelling Survey: An Example Using Santa Clara County, California

The objective of this survey was to provide data to emergency managers in Santa Clara County on the number of soft first-story multifamily dwellings (MFD) located in their communities using a cost-effective town-gown partnership. Target areas of cities were found by identifying areas containing residential units of two or more stories that had four or more living units. The survey found that 2,630, or 36%, of the 7,391 MFD in Santa Clara County are of the soft first-story construction type. It was found that one out of every nine apartment units in Santa Clara County is located in a soft first-story building. It is estimated that approximately 83,000 persons could be affected in the event of a severe earthquake. Emergency managers were given maps indicating areas of high-, medium-, and low-volume clusters of soft first-story MFD in each city. Follow-up interviews found that the communities that contain 67% of the identified soft-story MFD are using the survey information in their post-earthquake planning.

Microelectronics packaging curriculum development at San Jose State University

This paper describes the development of an engineering education curriculum in microelectronic packaging at both the undergraduate and graduate levels, at the College of Engineering, San Jose State University. Motivation for establishing courses to support such a curriculum and the needs of the local electronics industry are discussed. The undergraduate courses are taught in the lecture/laboratory mode, thus requiring students to conduct experiments. The interdisciplinary graduate curriculum is offered as an area of specialization within the Master of Science in Engineering program. Guest speakers from industry are invited to discuss advances and challenges present in this constantly evolving field. The College also offers short courses for engineers practicing in industry. With NSF funding, workshops for skills enhancement of university faculty are currently in the planning stage; four such workshops will be offered during 1997-1998.

Survey of nuclear fuel reprocessing technologies

Abstract A comprehensive review of 32 separation technologies applicable to nuclear fuel reprocessing has been conducted. These techniques have been classified and subdivided according to their principles of separation. The input fuels, principles of separation, history, stage of development, ability to extract plutonium and other pertinent information are summarized and tabulated for easy reference and comparison. For the most part the flow sheets and technologies involved in each separation process are described and the primary advantages and disadvantages are discussed. Engineering parameters such as decontamination factors, operating temperature, number of stages, chemistry and construction materials needed are examined.

Effect of Design Factors on Microvia Reliability of Flip Chip Ball Grid Array Polymeric Substrates

Microvia failures in flip chip ball grid array (FCBGA) polymeric substrates have been a major concern in the development of reliable packages for high-performance and high-density chips. To determine the relationship between reliability and design factors of the microvias, a 10-layer substrate was used to investigate these contrasting design factors: “stack-on-core” vs. “non–stack-on-core,” “high” vs. “low” aspect ratio, “stacked” vs. “staggered,” and “fillet” vs. “non-fillet.” Temperature cycling was used to generate stresses on the microvias. Electrical resistance was measured and analyzed, using design of experiment (DOE), to determine the effects of these design factors on microvia reliability. The significant single factors for a robust microvia were “non–stack-on-core” and “staggered.” Cross-sectioning was employed to understand the failure pattern. Cracks occurred on “stack-on-core” and “stacked” designs only. All the cracks were located at the interface between the capture pad and the bottom of th...