**1. Introduction**

With the integration and miniaturization of electronic products, electronic packaging and assembly technologies are progressing toward achieving high density and small size. Therefore, the size of solder joints has dropped sharply from hundreds of microns to tens of microns, or even a few microns [1]. Intermetallic compounds (IMCs) are formed between the solder joint and pad during soldering because of interfacial reactions. At present, the most commonly used solders in electronic products are the Sn–Ag–Cu (SAC) and Sn–Cu (SC) lead-free solders [2,3], and the primary composites of IMCs are Cu6Sn5 and Cu3Sn [3]. IMCs are a prerequisite for reliable connection for electronic products [4]. Therefore, the mechanical properties of IMCs significantly affect those of the entire solder joints [3].

As the thickness of the IMC is only a few microns, it is difficult to obtain its mechanical properties using conventional methods. There are usually two methods to study the mechanical properties of IMC: experiments and simulation. In the experimental method, specific samples are usually required, and nanoindentations are adopted to test the hardness and Young's modulus of the IMC [5–11]. However, the results tested by nanoindentation fluctuate within a certain range because it is difficult to obtain a pure IMC to ensure the consistency of samples. In the simulation method, first-principles and MD methods are usually used. First-principles based on quantum mechanics can be used to obtain relatively accurate physical parameters of a material [12,13]. However, the first-principles method requires massive computing resources; therefore, only a system with just a few atoms can be calculated. Molecular dynamics (MDs) based on classical mechanic theories can ease this problem. With the development of computer technology, MD simulation can be used to simulate atomic systems at the micro and nanoscales. IMC properties

**Citation:** Huang, W.; Pan, K.; Zhang, J.; Gong, Y. Strain Rate and Temperature Effects on Tensile Properties of Polycrystalline Cu6Sn5 by Molecular Dynamic Simulation. *Crystals* **2021**, *11*, 1415. https:// doi.org/10.3390/cryst11111415

Academic Editors: Wojciech Polkowski and Pavel Lukácˇ

Received: 26 October 2021 Accepted: 17 November 2021 Published: 19 November 2021

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such as thermodynamic, mechanical, and diffusion properties can be obtained by MD simulation [14–19]. The crystal size and strain rate can influence the tensile properties of a single crystal Ni3Sn4 via MD simulation [16,19]. Studies have shown that the strain rate affects the tensile properties of the materials, and the UTS of the material increases with the increase in the strain rate [20–25]. The results of previous studies were primarily based on experiments. However, because the thickness of IMCs is only a few microns, it is difficult to study their tensile properties by traditional experimental methods.

The previous studies on the mechanical properties of diffusion properties of IMCs were all based on monocrystals. However, most of the IMCs in solder joints are polycrystals, and their mechanical properties differ from those of monocrystals. In this study, the mechanical properties of a polycrystalline Cu6Sn5 were studied using the MD method, considering the effects of strain rate and temperature on the tensile properties. The temperature range in this study is within 250 to 500 K (melting point of Cu6Sn5 is 688 K [5]), and the strain rate is 0.00001 to 100 ps<sup>−</sup>1. In this study, the strain rate exceeding 0.001 ps<sup>−</sup><sup>1</sup> represents a high strain rate, and from 0.00001 to 0.001 ps−<sup>1</sup> represents a low strain rate. The tensile properties in both situations are discussed separately in Section 3.

## **2. Methodology**
