From cells to laminate: probing and modeling residual stress evolution in thin silicon photovoltaic modules using synchrotron X‐ray micro‐diffraction experiments and finite element simulations

Fracture of silicon crystalline solar cells has recently been observed in increasing percentages especially in solar photovoltaic (PV) modules involving thinner silicon solar cells (<200 μm). Many failures due to fracture have been reported from the field because of environmental loading (snow, wind, etc.) as well as mishandling of the solar PV modules (during installation, maintenance, etc.). However, a significantly higher number of failures have also been reported during module encapsulation (lamination) indicating high residual stress in the modules and thus more prone to cell cracking. We report here, through the use of synchrotron X‐ray submicron diffraction coupled with physics‐based finite element modeling, the complete residual stress evolution in mono‐crystalline silicon solar cells during PV module integration process. For the first time, we unravel the reason for the high stress and cracking of silicon cells near soldered inter‐connects. Our experiments revealed a significant increase of residual stress in the silicon cell near the solder joint after lamination. Moreover, our finite element simulations show that this increase of stress during lamination is a result of highly localized bending of the cell near the soldered inter‐connects. Further, the synchrotron X‐ray submicron diffraction has proven to be a very effective way to quantitatively probe mechanical stress in encapsulated silicon solar cells. Thus, this technique has ultimately enabled these findings leading to the enlightening of the role of soldering and encapsulation processes on the cell residual stress. This model can be further used to suggest methodologies that could lead to lower stress in encapsulated silicon solar cells, which are the subjects of our continued investigations. Copyright © 2017 John Wiley & Sons, Ltd.     

Constraining Effects of Lead-Free Solder Joints During Stress Relaxation

Reliability and quality control of microelectronics depend on a detailed understanding of the complex thermomechanical properties of miniaturized lead-free solder joints. With the continuous reduction in size of modern electronic devices, including also the size of the solder joints themselves, mechanical constraint effects may become of importance for the reliability of the joints. In the present study stress relaxation tests in tensile mode were performed on model solder joints consisting of eutectic Sn-3.5Ag solder between Cu substrates. The gap size of the joints was varied between 750 μm and 150 μm in order to investigate the variation of the mechanical properties as a function of the gap size. As it turned out, stress relaxation was dramatically reduced when the solder gaps became smaller due to constraint effects already well known from earlier measurements of the tensile strength. By employing a traditional creep model, the stress exponents and the activation energies were derived and compared with available data in the literature. The consequences of these constraint effects for the case of thermomechanical fatigue are discussed.     

再流焊工艺的统计过程控制

统计过程控制着重于生产过程的控制，是保证产品质量，预防废品产生的一种有效工具。本文介绍了再流焊工艺的统计过程控制。     

波峰焊常见问题及解决对策

对波峰焊常见的焊接不良问题给予分析并提出解决对策。希望能对电子制造业者的生产起到一定的参考作用。     

SMT再流焊接工艺预测与仿真技术研究现状

综述了电子电路表面组装技术（ＳＭＴ）再流焊焊接工艺仿真与预测研究的必要性、重要意义及其研究现状，并对其应用现状及其发展趋势进行了评述。     

Characterization of a laser-soldered avionic component using lead-free paste

Electrical cable-connector sets used in airplanes have been laser soldered using a lead-free paste. These connections are usually crimped using a hand tool, which necessarily leads to variable mechanical and electrical properties in the connection. Calorimetric studies have shown that paste melting occurs within tenths of a second at laser intensities of 400 or 500 W cm⁻². However, when such laser intensities strike the connector surface, some boiling occurs. In order to avoid paste losses, a stepped heating curve is applied, which allows good weldability. It was verified that a reaction layer of 2 μm length exists between the wires and the solidified solder paste, and also that there is some diffusion of copper from the wires to the paste. The soldered joints are 38% more electrically conductible and have 113% more tensile strength resistance than crimped joints.     

The adhesion strength of A lead-free solder hot-dipped on copper substrate

Eutectic Sn-Zn-Al solder alloy was used [composition: 91Sn-9(5Al-Zn)] to investigate the effects of dipping parameters such as the temperature, rate and time dipping on the adhesion strength between solder and substrate using dimethylammonium chloride (DMAHCl) flux. The optimum conditions for the highest adhesion strength (about 8 MPa) were determined as dipping at 350°C, and a rate of 10.8∼11.8 mm/s for 5∼7.5 min. A poor solder coating was obtained as dipped at 250°C. Some defects by non-wetting were found as dipped at a slow rate (slower than 8.2 mm/s). Quite different from the most tin-based solders for copper substrate, γ-Cu₅Zn₈ intermetallic compound particles were found by x-ray diffraction (XRD) analysis at the interface of solder and substrate as dipped at 300°C after pull-off test by etching out the unreacted solder layer. The morphology of the intermetallic compound formed was observed by scanning electron microscopy (SEM). The elements of Al (near Cu), Zn (near Sn) are enriched at the interface of solder and copper substrate as determined by the line scanning and mapping analysis.     

SMT设备的最新发展趋势

表面安装技术自1980年代以来已在电子工业中得到了越来越广泛的应用和发展。本文就SMT设备未来的发展趋势作了初步分析。从中可以看出SMT设备与电子技术相互促进、协调发展密不可分的关系。     

通孔再流焊技术

介绍混装印制板装联技术的工艺流程和各自特点,介绍了通孔再流焊技术的原理和工艺参数设计.通孔再流焊技术可以有效地适应表面组装技术的发展,科学地选用通孔再流焊技术是一种有效而可靠的焊接方法.     

片式元件焊点的热循环应力应变模拟技术研究

采用ANSYS软件,以0402片式元件焊点为对象,系统探讨了焊点热应变损伤的有限元仿真方法,分析了焊点在热循环过程中的应力应变响应,并基于修正的Coffin-Manson方程,预测了焊点的热疲劳寿命。结果显示:焊点应力集中区域和应变最大区域均位于焊点与PCB焊盘的交界面,基于应变失效原则,推断焊点裂纹将在此界面萌生和扩展,直至失效。指出了焊点有限元热应变损伤模拟技术的不足及未来的研究方向。