MICROSTRUCTURAL CHARACTERIZATION AND THERMAL CYCLING RELIABILITY OF SOLDERS UNDER ISOTHERMAL AGING AND ELECTRICAL CURRENT

Solder joints on printed circuit boards provide electrical and mechanical connections between electronic devices and metallized patterns on boards. These solder joints are often the cause of failure in electronic packages. Solders age under storage and operational life conditions, which can include temperature, mechanical loads, and electrical current. Aging occurring at a constant temperature is called isothermal aging. Isothermal aging leads to coarsening of the bulk microstructure and increased interfacial intermetallic compounds at the solder-pad interface. The coarsening of the solder bulk degrades the creep properties of solders, whereas the voiding and brittleness of interfacial intermetallic compounds leads to mechanical weakness of the solder joint. Industry guidelines on solder interconnect reliability test methods recommend preconditioning the solder assemblies by isothermal aging before conducting reliability tests. The guidelines assume that isothermal aging simulates a "reasonable use period," but do not relate the isothermal aging levels with specific use conditions. Studies on the effect of isothermal aging on the thermal cycling reliability of tin-lead and tin-silver-copper solders are limited in scope, and results have been contradictory. The effect of electrical current on solder joints has been has mostly focused on current densities above 104A/cm2 with high ambient temperature (≥100oC), where electromigration, thermomigration, and Joule heating are the dominant failure mechanisms. The effect of current density below 104A/cm2 on temperature cycling fatigue of solders has not been established. This research provides the relation between isothermal aging and the thermal cycling reliability of select Sn-based solders. The Sn-based solders with 3%, 1%, and 0% silver content that have replaced tin-lead are studied and compared against tin-lead solder. The activation energy and growth exponents of the Arrhenius model for the intermetallic growth in the solders are provided. An aging metric to quantify the aging of solder joints, in terms of phase size in the solder bulk and interfacial intermetallic compound thickness at the solder-pad interface, is established. Based on the findings of thermal cycling tests on aged solder assemblies, recommendations are made for isothermal aging of solders before thermal cycling tests. Additionally, the effect of active electrical current at 103 A/cm2 on thermal cycling reliability is reported

NASA-UVA light aerospace alloy and structures technology program

The report on progress achieved in accomplishing of the NASA-UVA Light Aerospace Alloy and Structures Technology Program is presented. The objective is to conduct interdisciplinary graduate student research on the performance of next generation, light weight aerospace alloys and associated thermal gradient structures in close collaboration with researchers. The efforts will produce basic understanding of material behavior, new monolithic and composite alloys, processing methods, solid and fluid mechanics analyses, measurement advances, and a pool of educated graduate students. The presented accomplishments include: research on corrosion fatigue of Al-Li-Cu alloy 2090; research on the strengthening effect of small In additions to Al-Li-Cu alloys; research on localized corrosion of Al-Li alloys; research on stress corrosion cracking of Al-Li-Cu alloys; research on fiber-matrix reaction studies (Ti-1100 and Ti-15-3 matrices containing SCS-6, SCS-9, and SCS-10 fibers); and research on methods for quantifying non-random particle distribution in materials that has led to generation of a set of computer programs that can detect and characterize clusters in particles

Earthquakes: from chemical alteration to mechanical rupture

In the standard rebound theory of earthquakes, elastic deformation energy is progressively stored in the crust until a threshold is reached at which it is suddenly released in an earthquake. We review three important paradoxes, the strain paradox, the stress paradox and the heat flow paradox, that are difficult to account for in this picture, either individually or when taken together. Resolutions of these paradoxes usually call for additional assumptions on the nature of the rupture process (such as novel modes of deformations and ruptures) prior to and/or during an earthquake, on the nature of the fault and on the effect of trapped fluids within the crust at seismogenic depths. We review the evidence for the essential importance of water and its interaction with the modes of deformations. Water is usually seen to have mainly the mechanical effect of decreasing the normal lithostatic stress in the fault core on one hand and to weaken rock materials via hydrolytic weakening and stress corrosion on the other hand. We also review the evidences that water plays a major role in the alteration of minerals subjected to finite strains into other structures in out-of-equilibrium conditions. This suggests novel exciting routes to understand what is an earthquake, that requires to develop a truly multidisciplinary approach involving mineral chemistry, geology, rupture mechanics and statistical physics.Comment: 44 pages, 1 figures, submitted to Physics Report

Study of the general mechanism of stress corrosion of aluminum alloys and development of techniques for its detection Annual summary report, 2 Jun. 1967 - 1 Jun. 1968

Stress corrosion cracking of high strength aluminum alloys investigated by electrochemical, mechanical, and electron microscopic technique

Influence of heat treatment condition on the stress corrosion cracking properties of low pressure turbine blade steel FV520B

Stress corrosion cracking (SCC) is a corrosion phenomenon which continues to plague the power generating industry especially in low pressure (LP) steam turbine blades operating in the phase transition zone. An investigation has therefore been conducted to examine the effect of heat treatment condition on the microstructure, mechanical properties and SCC properties of one such LP turbine blade material, FV520B, used in the steam turbines of coal-fired power stations in South Africa. The three stage heat treatment cycle of the FV520B turbine blades consists of homogenisation at 1020°C for 30 minutes, solution treatment at 790°C for two hours and precipitation hardening at 545°C for six hours. In this study, the precipitation hardening temperature was varied in the range 430-600°C to investigate how this variation would affect the material and SCC properties. Hardness and tensile testing were performed to obtain mechanical properties while the investigative techniques used to characterise the microstructures were light microscopy, dilatometry, X-Ray Diffraction (XRD), Scanning Electron Microscopy (SEM) and Transmission Electron Microscopy (TEM). Stress corrosion susceptibility for the different heat treatment conditions was quantified using U-bend specimens while crack growth rates and threshold stress intensities for SCC (KISCC) were measured using fatigue precracked wedge open loaded (WOL) specimens. Both SCC tests were conducted in a 3.5% NaCl environment maintained at 90°C. XRD results revealed the presence of reverted austenite in the higher tempered specimens due to the precipitation hardening temperature being close to the Ac1 temperature for the material. The presence of reverted austenite was shown to adversely affect mechanical strength and hardness which decreased with increasing precipitation hardening temperature. Light and electron microscopy (SEM and TEM) revealed the presence of Cr-rich precipitates along the prior austenite grain boundaries in all tested heat treatment conditions. The propensity, quantity and size of the Cr-rich precipitates increased as the specimen temper temperature increased. SCC susceptibility was shown to be dependent upon yield strength and decreased as precipitation hardening temperature increased with specimens in the overaged condition showing no cracking after more than 5000 hours in the test environment. WOL testing only produced cracking in the three highest strength specimens after 2000 hours. Crack growth rates and threshold stress intensities were found to be dependent on yield strength and decreased with increasing precipitation hardening temperature. Analysis of fracture surfaces revealed crack propagation along prior austenite grain boundaries in all test heat treatment conditions indicating intergranular stress corrosion cracking (IGSCC) as the dominant cracking mechanism