Mechanical Loading Conditions

For large boards, excessive board flexure, during PWB manufacturing, testing, assembly, and use, often can cause solder interconnect failures. Establishment of the strain rate-strain limit for lead-free solder interconnection on different PWB surface finishes can greatly help safeguard the reliability of the products (Ref 23). 

Reliability of lead-free solder PWB assemblies under vibration is another area for further study (including failure characterization and reliability modeling). 

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The above problems of fabrication and performance present a challenging task of identification of the governing material mechanisms. Use of nonlinear finite element analysis enables close simulation of actual thermal and mechanical loading conditions when combined with measurable geometrical and material parameters. As we continue to investigate real phenomena, we need to incorporate non-linearities in behavior into carefully refined models in order to achieve useful descriptions of structural responses. 

The previous sections provided general guidance on materials for hydrogen gas service and emphasized the metallurgical variables that influence hydrogen embrittlement. This section describes additional factors that impact hydrogen embrittlement, primarily environmental and mechanical-loading conditions. 

In some environments and under certain conditions, a microscopically brittle fracture of materials can occur at levels of mechanical stress that may be far below the level required for general yielding or those that cause significant damage in the absence of an environment. The susceptibility also depends on the chemical composition and microstructure of the alloy. This form of corrosion requires an interaction between the electrochemical dissolution of the metal, hydrogen absorption, and the mechanical loading conditions (stress, strain, and stain rate) (73). The nature of these fracture modes varies from one class of material to another. However, all fracture modes are largely similar to one another. 

The corrosion properties and other funetional properties of materials depend on several external factors such as geometry, manufacture, surface conditions, environmental factors and mechanical load conditions. For each functional property, these factors have to be evaluated. The final materials selection is often a result of compromises between various properties and their dependence on external factors. 

Such systems can be solved by program packages such as SANDYS, SABER, DYMOLA etc.. In that approach the mechanical boundary conditions are defined by the mechanical load conditions. The rod ends can be clamped, free or attached to some load defined by a network of passive mechanical components. In a more general case dehvered forces and/or displacements can be prescribed explicitly. 

Then, it was shown that EAC arises from complex interactions between the mechanical loading conditions (stress, strain, strain rate), electrochemical behavior (partieular-ly anodic dissolution), and hydrogen absorption. At least during the propagation stage, these interactions occur under local conditions which are usually very different fi om those that prevail on free surfaces. 

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