Novel test methodology for the most consistent and accurate characterization of solder materials in electronics engineering

  • Reinikainen T
  • Suhir E
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Predictive modeling tools and techniques, whether computer-aided, such as, e.g., finite-element analysis (FEA), or analytical ("mathematical"), are currently widely used in physical design and reliability evaluations in electronics engineering. The implementation of these tools and techniques requires accurate input data for obtaining trustworthy output information that is intended to be used in the subsequent physical design and reliability evaluations efforts. Confidence in the consistency and accuracy of this information depends on the consistency and accuracy of the input data. One should always be mindful of the possibility of a "garbage in - garbage out" situation, no matter how good the model itself might be. It is equally important that one possesses a clear understanding of the physics and mechanics of the material behavior. With the continuing trend for miniaturization of packaging technologies, solder joint interconnections remain the weakest link, as far as the thermal-mechanical reliability is concerned. This is due, to a great extent, to the complexity of the mechanical behavior of the solder material, and particularly to its inelastic and time-dependent performance: various plasticity and creep mechanisms in the presence of significant stress concentration affect considerably the usage of the products containing solder. The behavior of solder materials depends strongly on their microstructure, as well as on the amount and the type of the alloying elements. In addition, the composition of the solder material in a solder joint structure may be different than that in the bulk solder material, primarily because of the dissolution of alloying elements from the joint interfaces. These effects should be considered when determining the constitutive properties of solder materials in the reliability modeling and physical design efforts. In the analysis that follows we present a novel methodology for material characterization. Although this methodology is developed in application to solders employed in electronic packaging, we believe that it might have an impact on the materials engineering in general. The methodology takes into account various major requirements for testing solder deformation properties important for the subsequent physical design and reliability analyses, including modeling and experimental efforts, whether carried out on the joint level, board level or product level. The specimens are essentially single-lap shear joints (LSJs) with deliberately introduced transverse grooves. These grooves "separate" the solder joint area ("structure") from the outside portions of the "adherends" (test pins). It has been demonstrated, first by the finite-element analysis (FEA) and then by analytical stress modeling, that deep enough transverse grooves in small size LSJs can lead to a nearly uniform shear stress distribution and, as the consequence of that, to very low "peeling" stresses in the joint, thereby facilitating significantly the assessment of the stress-strain relationships for the materials.

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