**1. Introduction**

The robustness of electronic assemblies is determined by the mechanical integrity of solder joints against various degradation influences. Thermal cycling is one of the environmental effects responsible for degrading the fatigue life of solder joints in real-life applications. Numerous switching on–off cycles for electronics systems as in power cycling and exterior sources of elevated temperature as the heat generated from engines are examples of thermal cycling sources. In thermal cycling, the behavior of solder joints is complicated due to their suffering from several damage mechanisms, including creep, fatigue, and the interaction of both mechanisms. Creep is dominant during dwelling periods, while fatigue is influential between ramps [1,2]. Mainly, joint failure in thermal cycling failure occurs due to the mismatch between the coefficient of thermal expansions between PCB and

**Citation:** Abueed, M.; Al Athamneh, R.; Tanash, M.; Hamasha, S. The Reliability of SAC305 Individual Solder Joints during Creep–Fatigue Conditions at Room Temperature. *Crystals* **2022**, *12*, 1306. https:// doi.org/10.3390/cryst12091306

Academic Editors: Ulrich Prahl, Sergey Guk and Faisal Qayyum

Received: 15 August 2022 Accepted: 12 September 2022 Published: 15 September 2022

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**Copyright:** © 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

components. This would generate cyclic shear stresses on joints as temperature alternating between extremes, named thermomechanical fatigue stress. Microstructure evaluation under thermal cycling includes stress concentration in the defected spots (such as voids) of the microstructure, then recrystallization of Sn grains, followed by intergranular crack growth among grain boundaries. In addition to thermal cycling, drop shock was found to be a vital cause in portable applications such as mobile phones and digital cameras. Researchers [3–7] found that crack initiation in drop tests started from the IMC layer, as it is brittle and considered to be the weakest. This would be enhanced if preceded with few thermal cycles and/or aging due to IMC layer growth caused by thermal aging with time.

Therefore, a joint's initial microstructure has grea<sup>t</sup> impact during thermal cycling, including precipitates (Ag3Sn and Cu6Sn5), Sn morphologies, and Sn grain orientations. This is due to the low number of grains in each solder joint: one to three large Sn grains, each with a number of dendrites with the same crystallographic orientation [8–13]. Generally, grain orientation correlates to a joint's reliability, as it plays a vital role in microstructure evolution [14–16] and other phenomena such as electromigration [17]. Moreover, after the reflow process, once the tin (Sn) in solder spheres comes in contact with the copper (Cu) on the solder pads, an intermetallic compound (IMC) is formed, which strengthens the mechanical bond. However, the growth of the IMC layer over time leads to weakening due to its brittle nature. A microstructure study revealed that the component side of the solder junction was where the majority of the cracks originated. There were primarily two sorts of cracks seen: those that go into the bulk solder and those that run along the IMC layer. It was discovered that the crack nearly always started next to the IMC layer, where thermal cycling causes the most stress, and eventually spread into the bulk or along the IMC layer. Other factors such as oxidation and phase change are of significant impact as well [18–22].

SnPb-based alloys show excellent performance in thermal cycling, but the industry switched to lead-free alloys after RoHS prohibited using lead in 2006. Many SAC-based alloys demonstrate competitive performance [23–25]; however, studies are still ongoing to investigate their properties comprehensively. Creep and fatigue are critical mechanical properties of actual solder joints that have been explored by many researchers. Hamasha et al. [26] studied the effect of various testing conditions of varying amplitude and strain rate on the fatigue life of actual joints of SAC105 and SAC305. Results show that SAC305 is more fatigue-resistant compared with SAC105 under varying amplitudes. Sinan et al. [27] investigated the fatigue properties of individual SAC-based doped spheres with various surface finishes. It is found that the higher the Ag content, the superior the fatigue and shear strength obtained. Xu et al. [28] explored the fatigue properties of SAC305 and 105 compared with SnPb alloys under isothermal fatigue conditions and wide strain ranges. SAC305 shows better fatigue properties than SAC105 and SnPb at specific ranges. The effect of surface finish under thermal cycling has been studied extensively. Francy et al. [29] investigated the effect of various surface finishes: OSP, ImAg, and ENIG and alloys under thermal cycling for BGAs and SMRs. ENIG outperformed other surface finishes in the case of BGAs, but with SMRs, OSP and ImAg were slightly better. Similar results were found by [30,31]. Generally, ENIC and ENIPIC were found to perform well in thermal cycling due to their Ni plating layer that blocks the growth of the IMC layer.

Others examined fatigue properties of SAC alloys under various conditions of aging durations and temperatures [28,29], stress amplitudes [30,31], strain ranges [32,33], and shear rates [34]. Many researchers utilized bulk [35,36] and dog-bone [37,38] samples in their work to examine the microstructure evolution [39,40] and fatigue properties of different alloys [40,41] under various conditions. Even though valuable results were obtained, they were not as much as utilizing actual joints due to missing the IMC layer, surface finish, and different microstructure, which impact reliability. Several researchers studied the creep effect under various testing conditions. Fahim et al. [41] examined the evolution of microstructures, including IMC of SAC-based alloys and several surface finishes at various temperatures using nanoindentation testing after aging. Significant degradation in reliability is observed after long-term aging with increasing testing temperature. The

long-term aging of solder junctions at high temperatures causes considerable changes in their microstructure, including a thickening of the intermetallic compound layer (IMC) and coarsening of the precipitates. The mechanical and fatigue characteristics of solder junctions are significantly influenced by their microstructure. Different conditions of aging durations and temperatures were studied [42–46], and similar results were obtained for dominating aging temperature over duration for degradation. Others [28,47,48] explored creep properties and microstructure evolution after several thermal cycles. It is concluded that creep rate and deformation were higher when testing joints after more cycles. Doping is introduced to enhance the properties of the materials by adding small (micro) amounts of certain elements such as Ni, Bi, Cr, etc., to SAC-based alloys. The properties of doped alloys under thermal cycling and iso-thermal cycling have been studied by different researchers [34,42–52]. Doped alloys showed mechanical property improvements depending on the doping element. Various mechanical properties were investigated such as hardness [53], tensile strength [54], and modules of elasticity [55].

There are many models established to predict the fatigue life of several solder materials under thermal cycling with various testing conditions and geometries. Norris and Landzberg [56] were the ones who started early development of the reliability models of SnPb in thermal cycling, considering various temperature and frequency levels. Their model was a modified version of the Coffin–Manson equation. Engelmaier W. [57] studied the fatigue life of SnPb-based CCC joints during power cycling and generated models to predict their life based on the Coffin–Manson model. Most models in the literature were generated based on [51–53] models. Vayman S. et al. [58] created an empirical model for low-tin alloys (SnPb based) under isothermal fatigue conditions including extensive temperature range, hold times, strain ranges, frequencies, and environmental conditions. Salmela O. [59] investigated several acceleration factors for lead-free material under different thermal cycling conditions. He modified the Norris–Landzberg model with a correction factor to compromise for material and geometry. Others established empirical models for lead-free material under thermal cycling [60,61] and isothermal fatigue with aging [62–66].

According to the literature, there is extensive research that investigated the fatigue and creep properties for SnPb-based and lead-free materials. However, limited research has been performed under the combination of both effects in a systematic way. Moreover, comprehensive studies generated models for SnPb and lead-free materials under thermal cycling tests. Nevertheless, there is no study that created a model for actual SAC-based solder joint material under a combination of creep and fatigue. In this study, expanded research of our previous work related to the combined effects of fatigue and creep explored at room temperature [65], and various iso-thermal temperatures [8], is implemented. Moreover, reliability models as a function of dwell time for fatigue life based on Morrow Energy and Coffin–Manson models are generated for SAC305 actual solder joints. These mathematical/empirical models seem to be sophisticated enough to predict life as it was generated according to defined procedures, including identifying the failure mechanism and modes and developing/modifying models based on physics or pervious models considering many factors such as temperature, dwell time, etc.
