Investigation On Reliability Evaluation Of SnAgCu Based Lead Free Soldered Joints Under Drop And Impact Loading Conditions

Currently, the portability and the high-integrity, along with the layout design, are becoming the important issues in the development of electronic devices. Following these guidelines, there are many problems and challenges need to figure out. One of them is that, the risk of the vibration and dropping will be increased in the process of the transportation and daily usage, which require a higher reliability to the portable devices. It is well known that, as interconnects, the solder joints in the electronics products are subjected to a mixture of the electricity, heat and load. Usually, they are considered as a weakness in the packaging structures. With the shrinkage of the solder joints, the impact reliability is more susceptible to the issues of the high strain rate conditions (10-102/s). Meanwhile, since the in-depth investigations on the lead-free solders, the traditional tin-lead solders are gradually substituted by the tin-based lead-free ones. The SnAgCu-based lead-free solders have been regarded as the promising replacements. However, many limits of this kind are also found in service, especially under the high strain rate cases. The brittle fracture is a very common failure mode for the lead-free solder joints when the devices meet a sudden drop, which is responsible to hinder a wide application of lead-free solders. In present, there are only few testing standards for the solder joints in the high strain rate conditions, and the board level drop test standard (JESD22-B111) has been extensively adopted as a basic method to assess the drop reliability of the packaging structures for a long term. Based on a large number of experimental data, it is also summarized that there exist many influencing factors during the carryout of this standard, which does not work for the fundamental research in the lab but suit for the examination of the PCBs before putting into the market. Consequently, it is urgent to establish a certain testing methods and attempt to evaluate the impact performances of various solder joints. The ideal approaches should have the characteristics of an easy operation, a high repeatability, few influencing factors and low cost. Alternatively, by using these methods, it is desirable that the solder alloys with a relative high impact performance can be found and determined.Based on the above-mentioned, this research begins with the analysis of the board level drop test standard. According to the requirements of the packaging structures and actual testing conditions, such as the instantaneous impact acceleration and the out-plane flexibility at the center of PCB, the boundary conditions of board level drop tests are determined. By using the Input-G equilibrium method in the finite element analysis (FEA), the dangerous sites and stressing process of the BGA joints in the board level drop tests are analyzed. From the simulating results, it is found that, the dangerous sites are normally located at the outer corner of the solder joints on the PCB side. Furthermore, the BGA's joints mainly suffer a stress mixture of the high velocity tensile load, shear load during the board level drop tests. The most severe case for the solder joint is subject to the high velocity tensile loading conditions, which easily induce a catastrophic brittle failure after a certain intervals of drops. Therefore, considering the above analysis and the actual geometry of the BGA joint, the sub-size impact specimens and the corresponding soldering processes are designed, both of which give a support for the following researches.By mimicing the board level drop test standard, a mini-energy drop test apparatus is designed and fabricated, and then the joint level drop impact tests are carried out. Firstly, the comparison of the impact performance between the as-soldered eutectic SnPb and the lead-free Sn3.8Ag0.7Cu (SAC387) solder joints is done. From the results, the impact properties of two solder joints are distinguished well and the SnPb joints have a much better impact performance than the SAC387 ones. Based on the analysis of the scanning electron microscopy (SEM) observation, the distinctions of the drop reliability between two solder joints can be explained, the results of which demonstrate that it is feasible to assess the impact properties of various solder joints by using this approach. Meanwhile, the optimal drop height is determined by using the SAC387 solder joints. The empirical relationship between the drop impact height and number of drops is found. Eventually, the drop reliability evaluation of the as-reflowed lead-free solder joints with different Ag contents is carried out and the lead-free solder joints having higher impact properties are found. Besides, effects of the copper hardness and the specimen structure on the drop performance are discussed in this study. The conclusions address some suggestions to improve the joint's drop reliability. However, similar to the board level drop tests, there are many influencing factors on the results of the joint-level drop impact tests. The stress at the solder joint is not stable enough, which causes the constructive results have to be summarized by a large number of data and there are some uncertain cases affecting the reliability evaluation of solder joints.Depended on the results above, using the identical joint's pattern, the mini-energy multi-impact tests are conducted to evaluate the impact reliability of various solder joints. Firstly, acted as an important parameter, the impacting angle of the pendulum is optimized by using the as-reflowed SAC387 solder joints. The empirical relationship between the impacting angle and number of impact is also built by the power-law equation. Then, the impact properties of the as-reflowed lead-free solder joints with different Ag contents are measured and compared. The trend of the impact performance is similar to the results of joint level drop tests and the distinction among the impact properties is much more remarkable. By changing the impacting point on the solder joints, the impact reliability of various solder joints can be distinguished further. The effect of notch on the impact properties is discussed and the results are comparable to the ones of joint level drop tests. The variations on the fracture morphologies are observed and compared, which gives an explanation about the differences of impact properties among solder joints. Generally speaking, the mini-energy multi-impact tests (impacting point is away from the middle of joints) can be considered as an effective approach to distinguish the impact performances of the solder joints, which exhibit a high repeatability, the easy operation, the remarkable distinction and the low cost.In order to compare the results of multi-impact tests and simplify the test procedures, the Charpy impact tests are applied to differentiate the impact properties of various solder joints. From the results of the impact absorbed energy values, the trend of the impact properties is identical to that of multi-impact tests. Meanwhile, the effects of the notch, soldering process and thermal aging on the impact performance are discussed in this work. It is revealed that the impact performances of various solder joints are somewhat distinctive in several cases, which enrich the experimental methodologies and provide some recommendations to evaluate the impact properties of different solder joints.Furthermore, the effect of loading rate on the shear strengths of solder joints is considered in this study. The relationship between the shear strength and loading rate is found and the ductile-brittle mode transition on the SAC305 fracture surfaces is observed when the loading rate between 1mm/s and 10mm/s. The effect of ambient temperature on the shear strength is also discussed and finally the relationship between ambient temperature and loading rate on the ductile-brittle mode transition is uniform.Eventually, depended on the data fit of three impact tests, it is found that the general tendency of the impact performances on the solder joints can be described by the power-law equations and the fracture morphologies on the surfaces of three cases are similar, demonstrating a strong correspondence between the multi-impact and Charpy impact tests. Thus, the pendulum multi-impact test (impacting point is away from the middle of joints) and Charpy impact test are recommended as the efficient approaches to distinguish the impact properties of different solder joints. It is summarized from three impact tests that the joints with the 1 wt. % Ag content have a better impact performance among the SAC-based solders. Using the nanoindentation, the mechanical properties of the intermetallic compounds (IMCs) are measured. With the finite element simulation, the IMC layer is defined by the cohesive element and the major influencing factors of solder joints on the damage degradation rate are analyzed. It is revealed that the interfacial adhesive strength and the rate-dependent exponent of solders are the significant parameters to affect the impact performance of solder joints.