Tech Briefs

Creep Analysis of Lead-Free Solders Undergoing Thermal Loading

Finite element material models are used to evaluate the reliability of lead-free solders.

Lead and its compounds have been widely used for many years in the electronics industry. However, the global demand to reduce the use of hazardous materials has compelled electronics manufacturers to consider the use of lead-free materials in future products. This transition has heightened the necessity for new finite element material models that can be used to evaluate the reliability of lead-free solders.

Lead solders, such as the popular tinlead variety, have been used in the electronics industry for the past 50 years. Subsequently, their long-term reliability is well understood. However, lead and its compounds are highly toxic and the entry of these materials into the environment has become an issue of great importance. In conjunction with legislation requiring the eventual use of leadfree materials, the electronics industry is working towards reducing the amount of lead in end-user equipment. This transition requires the development of new analytical capabilities for estimating the reliability of components that use lead-free solder.

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Figure 1. Schematic Section View of the BGA model.
During their working lives, electronic components are subject to a large number of thermal cycles. Mismatch between the thermal expansion behaviors of the various materials may induce severe stresses that are high enough to cause plasticity and creep. In particular, solder balls are at risk because they are stressed at temperatures above half of the solder’s melting point. At such temperatures, the solder creeps; after a number of thermal cycles, the accumulation of large inelastic strains may lead to failure of the solder joints. Therefore, the creep analysis of solder balls under cyclic thermal loading becomes an important part of the design phase.

The Ball-Grid-Array (BGA) model used in this application was built using an ABAQUS/CAE custom application of ABAQUS Version 6.6 finite element analysis developed at Worley Parsons PTE Limited, Singapore. Scripting and GUI toolkit interfaces of ABAQUS software allowed the development of certain classes of models to be automated.

The process of building a BGA model is captured in several icons such as the Model icon, the Material icon, and the Load icon. For example, only the dimensions for a BGA model need to be specified in the Model dialog box, and the model is built and meshed automatically.

Figure 1 shows a section of the BGA model used in the present calculations. Thirty-six solder balls connect the silicon die and the substrate, with underfill material used in the space surrounding the solder balls. All components except the substrate are encapsulated in the mold.