**1. Introduction**

Solder alloys are widely used bonding materials in electronics industry. The reliability con‐ cerns for solder interconnections, which provide both mechanical and electronic connec‐ tions, are rising with the increasing use of highly integrated components in portable electronic products [1-6]. As shown in Fig. 1, a typical ball grid array (BGA) component board usually consists of a silicone die, molding compound, solder interconnections, and printed wiring board (PWB). In service, all products are subjected to thermal cycles as a re‐ sult of temperature changes due to component internal heat dissipation or ambient tempera‐ ture changes. The existence of coefficient of thermal expansion (CTE) mismatches between dissimilar materials (about 16 ppm/°C for PWB and 2.5 ppm/°C for Si die [7]) is the source of deformation and thermomechanical stress in the solder interconnection, which leads to the cracking of the interconnections and failures of the electronic devices.

© 2013 Li et al.; licensee InTech. This is an open access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2013 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Since failure of solder interconnections is a typical failure mode in many electronic devices, the reliability and life time prediction of solder interconnections become crucial. Various re‐ liability test and computer-aided simulations have been carried out to study the solder inter‐ connection reliability [8-17]. With the experimental and simulation results, a number of lifetime prediction models have been established and they can be classified into two main categories: strain-based and energy-based. For instance, the Engelmaier model is based on the total shear strain range, the Coffin-Manson model on the plastic strain, and the Dar‐ veaux model on the energy density [18-20]. However, microstructural changes in the bulk solder have not yet been included in any of the popular prediction models. Especially the microstructural changes associated with recrystallization and grain growth are of impor‐ tance because they can significantly affect the mechanical properties and can cause recrystal‐ lization induced failure of solder interconnections [21-26]. A new approach for lifetime prediction needs to be developed that takes into account the microstructural changes. In or‐ der to achieve this, the first step is to quantitatively study the recrystallization and grain growth occurring in solder interconnections.

In this chapter, the current understanding of the microstructural changes in solder intercon‐ nections is introduced, followed by a brief review of the Monte Carlo simulations of grain growth and recrystallization. Then, a new algorithm for predicting dynamic recrystallization in solder interconnections during thermal cycling tests is presented.

**Figure 2.** The as-solidified microstructure of a SnAgCu solder interconnection; (a) optical bright field image, (b) cross-

Simulation of Dynamic Recrystallization in Solder Interconnections during Thermal Cycling

http://dx.doi.org/10.5772/53820

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For decades, the industry has used recrystallization to control microstructures, and static re‐ crystallization of structural metals after deformation is probably the best understood recrys‐ tallization process [22]. On the other hand, dynamic recrystallization during cyclic deformation, which occurs in solder interconnections, has received much less attention and is still poorly understood. This is because the related microstructural events are highly com‐ plex from the microstructural point of view. The major understanding of this subject is brief‐

Thermal cycling tests with extreme temperatures in the range of about -40 °C to +125 °C are usually carried out to assess the reliability of electronic devices [35, 36]. During thermal cy‐ cling, the solder interconnections are under cyclic loading conditions. The induced thermo‐ mechanical stresses are often higher than the yield strength of the material, which leads to plastic deformation. A fraction of the energy associated with the plastic deformation of sol‐ der interconnections is stored in the metal, mainly in the form of dislocations. The stored en‐ ergy is subsequently released during restoration, which can be divided into three main processes: recovery, primary recrystallization and grain growth. Recovery and recrystalliza‐ tion are two competing processes, which are driven by the increased internal energy of the deformed solder. Recovery decreases the driving force for recrystallization and thus hinders the initiation of recrystallization. In high stacking fault energy metals such as Sn, the release of stored energy takes place so effectively by recovery that recrystallization will not practi‐ cally take place [22, 23]. Studies have shown that after a single deformation static recrystalli‐ zation rarely occurs in Sn-rich solders [37]. However, under dynamic loading conditions such as in thermal cycling tests, recrystallization often takes place in the high stress concen‐

Experimental observations indicate that the microstructure of solder interconnections may change significantly during thermal cycling tests. The as-solidified microstructure can trans‐

**2.2. Recovery and recrystallization of Sn-rich solder interconnections**

polarized light image.

ly summarized as follows.

tration regions of solder interconnections [28, 38, 39].
