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In 2006, the European Union restricted lead (Pb) to > 0.1% (by weight) in electronics. This caused a renewal of interest in Sn whiskers, as the absence of Pb in Sn films had been known for years to promote Sn whisker growth. The early work on Sn whiskers had suggested that elements other than Pb may suppress whiskers. Bi was chosen as an attractive candidate, as it has similar phase formation behavior...
(Part A) Work by Hoffman (Savannah River National Laboratory) reported Sn whiskers which grew longer in nitrogen (sealed jar) environments compared to air environments. This is of concern in electronics systems which operate in nitrogen-backfilled environments, such as hybrid circuits and sensor systems. In this work we compare Sn whiskers grown in two different environments-pure N2 and ultrahigh...
(Part A) The structure zone (SZ) scheme allows for modification of the grain size of sputter deposited thin films by adjustment of the substrate temperature during deposition. Previous work by this group on whiskering has been done exclusively on room temperature sputter-deposited tin (Sn) films with a homologous temperature Ts/Tm = 0.6, where Ts is the substrate deposition temperature and Tm is the...
Studies of tin (Sn) whisker growth from Sn solder alloys shows that SAC deposits can be whisker prone. In fact, even SnPb solders may produce whisker growth. The Sn alloy specimens consisted of 2400 A SAC305 (∼3.0%Ag, 0.5%Cu and rest Sn), along with 750 and 1200 Å Sn-37Pb films, each sputtered under compressive stress conditions on electrochemically polished brass and incubated in ambient room temperature/humidity...
Some mechanistic models of tin (Sn) whisker growth presume that surface oxidation plays an important role in whisker formation. The notion is that Sn whiskers grow through weak spots in the Sn oxide, which is cracked during whisker growth and propagation. It is often implied that a surface oxide is necessary for whisker growth. However, the premise of a surface oxide requirement for whiskering is...
Application of conformal coatings has shown promise as a practical scheme for preventing whisker growth in electronic assemblies. Coatings have been proposed utilizing elastomeric, ceramic, and metallic materials. The ability to prevent whiskers is particularly important in assemblies containing Pb-free materials and finishes. Further, many otherwise desirable COTS (commercial off the shelf) electronic...
Studies of Sn whiskers under controlled, calibrated humidity conditions shows that the highest whisker densities occur for ~ 85% RH. The whisker specimens were 1500 Å Sn films sputtered under compressive stress conditions on silicon and electrochemically polished brass. Subsequently, the samples were exposed to a series of saturated aqueous salt solutions (which generated calibrated relative humidity...
Studies of tin (Sn) whiskers have emphasized the role of compressive stress as the driving force for whisker production, thought to be a necessary condition for Sn whisker growth. The goal of this work is to characterize Sn whisker growth in film systems having measurable net compressive, zero, and tensile intrinsic stress states. The stress states were generated for the case of ~ 0.2 µm sputtered...
Structural damage to BGA interconnects incurred during vibration testing has been monitored in the pre-failure space using resistance spectroscopy based state space vectors, rate of change of the state variable, and acceleration of the state variable. The technique is intended for condition monitoring in high reliability applications where the knowledge of impending failure is critical and the risks...
The transition from tin-lead to lead free soldering in the electronics manufacturing industry has been in progress for the past 10 years. In the interim period before lead free assemblies are uniformly accepted, mixed formulation solder joints are becoming commonplace in electronic assemblies. For example, area array components (BGA/CSP) are frequently available only with lead free Sn-Ag-Cu (SAC)...
Relative damage-index based on the leadfree interconnect transient strain history from digital image correlation, explicit finite-elements, cohesive-zone elements, and component's survivability envelope has been developed for life-prediction of two-leadfree electronic alloy systems. Life prediction of pristine and thermally-aged assemblies, have been investigated. Solder alloy system studied include...
The microstructure, mechanical response, and failure behavior of lead free solder joints in electronic assemblies are constantly evolving when exposed to isothermal aging and/or thermal cycling environments. In our prior work on aging effects, we have demonstrated that the observed material behavior variations of Sn-Ag-Cu (SAC) lead free solders during room temperature aging (25degC) and elevated...
Goldmann Constants and Norris-Landzberg acceleration factors for lead-free solders have been developed based on principal component regression models (PCR) for reliability prediction and part selection of area-array packaging architectures under thermo-mechanical loads. Models have been developed in conjunction with Stepwise Regression Methods for identification of the main effects. Package architectures...
Failures in electronics subjected to shock and vibration are typically diagnosed using the built-in self test (BIST) or using continuity monitoring of daisy-chained packages. The BIST which is extensively used for diagnostics or identification of failure, is focused on reactive failure detection and provides limited insight into reliability and residual life. In this paper, a new technique has been...
Leading indicators of failure have been developed based on high-frequency characteristics, and system-transfer function derived from resistance spectroscopy measurements during shock and vibration. The technique is intended for condition monitoring in high reliability applications where the knowledge of impending failure is critical and the risks in terms of loss-of-functionality are too high to bear...
Electronic assemblies deployed in harsh environments may be subjected to multiple thermal environments during the use-life of the equipment. Often the equipment may not have any macro-indicators of damage such as cracks or delamination. Quantification of thermal environments during use-life is often not feasible because of the data-capture and storage requirements, and the overhead on core-system...
Microelectronic encapsulants exhibit evolving properties that change significantly with environmental exposures such as isothermal aging and thermal cycling. Such aging effects are exacerbated at higher temperatures typical of thermal cycling qualification tests for harsh environment electronic packaging. In this work, the material behavior changes occurring in flip chip underfill encapsulants (silica...
Thermal cycling accelerated life testing is often used to qualify area array packages (e.g. Ball Grid Arrays and Flip Chip) for various applications. Finite element life predictions for thermal cycling configurations are challenging due to the complicated temperature/time dependent constitutive relations and failure criteria needed for solders and encapsulants and their interfaces, aging/evolving...
In this paper, a leading indicators based approach has been developed for prognostics and health monitoring of electronic systems. The approach focuses on the prefailure space and methodologies for quantification of damage progression and residual life in electronic equipment subjected to shock and vibration loads using the dynamic response of the electronic equipment. Traditional health monitoring...
Electronics may be subjected to shock, vibration, and drop-impact during shipping, handling and during normal usage. Measurement of transient dynamic deformation of the electronics assemblies during the shock and vibration can yield significant insights in understanding the occurrence of failure modes and the development of failure envelopes. Failure-modes include solder-joint failures, pad cratering,...
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