【作者】 陈霞；

【导师】 树学峰；

【作者基本信息】 太原理工大学 ， 力学， 2014， 硕士

【摘要】 由于人们对便携式电子产品日益增加的需求以及无铅焊料的强制使用,焊点跌落碰击可靠性已经成为支撑便携式电子产品行业的重要挑战之一。研究无铅焊点在跌落冲击载荷下的动态响应具有重要意义。通过板级跌落碰撞实验对比分析跌落冲击下无铅焊点和锡铅焊点的破坏行为。设计跌落实验所用的试件和实验方案,监测跌落过程中PCB中心的应变信号和焊点的动态电压。采用染色法标记焊点的破坏形貌。实验结果表明锡铅和无铅BGA封装均为最外围拐角焊点靠近PCB侧最先发生失效；锡铅焊点的破坏为韧性断裂,无铅焊料发生脆性破坏；从染色图中可以看出锡铅焊点的破坏模式为焊盘的断裂,而无铅焊点失效模式为IMC层的脆性破坏。焊点的失效机理为封装和PCB板的弯曲刚度存在差异,当PCB朝下发生弯曲时,最外围焊点承受拉应力；PCB朝上发生弯曲时,最外围焊点承受压应力。跌落过程中,随着PCB反复弯曲,最外围焊点拉压应力交错产生,从而使焊点产生裂纹最终完全失效。采用ABAQUS隐式动态分析法计算无铅BGA封装跌落碰撞动态响应。建立BGA封装三维有限元模型,载荷选取JEDEC标准工况B,载荷施加采用Input-G法,边界条件为宽度方向水平位移约束。对BGA封装进行有限元模态分析,研究其各阶模态的振型和频率。模拟结果表明,PCB中心长度方向应变的模拟曲线与实验曲线基本重合,应力最大值出现在最外围拐角靠近PCB侧,与实验结果相符。说明所建模型的正确性。PCB挠度曲线为正弦曲线,表明PCB板在跌落碰撞时发生上下弯曲。跌落冲击载荷下BGA封装振动变形,第一阶模态是主要的,高阶模态影响较小。最外围拐角焊点的各应力中,垂直于PCB板方向正应力,即剥离应力,远大于其他应力,是导致焊点产生裂纹的主要原因。有限元模型建立正确的基础上,对外围尺寸相同的三种不同的焊点分布进行分析。结果表明：三种不同的分布,Mises应力最大焊点均为最外围拐角焊点,最外围横向焊点的应力明显小于对应纵向焊点的应力；不同焊点分布对PCB挠度影响较小；对于全阵列的焊点分布,在相同跌落冲击载荷下,密度小的焊点分布危险焊点剥离应力明显大于焊点密度大的分布；除去中心若干焊点对危险焊点剥离应力影响极其微小。说明外围焊点分布密度对危险焊点的剥离应力具有显著影响。外围焊点密度减小,危险焊点剥离应力增大,容易发生失效。研究结果为有效改善焊点分布,提高封装可靠性提供了理论参考。更多还原

【Abstract】 As growing demand for portable electronic products as well as the compulsory use of lead-free solder, for the industry which support the portable electronics, solder joint drop impact reliability has become one of the important challenge. Study on dynamic response of lead-free solder joints under drop impact load is very necessary.The damage of lead-free solder joints and tin-lead solder joint under drop impact was analyzed through drop test. The specimens and experiment scheme of drop test were designed, and strain signal on PCB center and dynamic strain of solder joint were monitored. Dyeing method was used to mark the damage of the solder joint morphology. The experimental results showed that the outermost corner solder joint near PCB side failed first both for lead-free package and tin-lead package; The damage of tin-lead solder joint was ductile fracture, and lead-free solder joint was brittle failure; As can be seen from the dyeing figure of tin-lead solder joint, the failure mode was welding pad fracture, but for lead-free solder joint, the failure mode was brittle failure of the IMC layer. The failure mechanism of solder joints is the bending rigidity difference between package and PCB, when PCB is bending downwards, the outmost solder joints are under tensile stress, and when PCB is bending upwards, the outmost solder joints are under compressive stress. As the repeated bending of PCB, tensile and compressive stress was generated alternately in the outmost solder joints which results in crack and then eventually complete failure.ABAQUS implicit dynamic analysis method was used to calculate dynamic response of lead-free BGA package under drop impact. BGA package three-dimensional finite element model was built. The load was according to JEDEC standard condition B and was applied with Input-G method. The boundary conditions were set as the horizontal displacement constraints on width direction. The modal analysis of BGA package was conducted to study vibration mode and frequency of each order modal. The simulation results showed that the simulation strain curve in length direction of PCB center was essentially coincident with the experimental strain curve. The maximum stress was at the outmost corner solder joint near PCB side which consisted with experimental results. The conclusion illustrated the correctness of the model. PCB deflection curve was a sine curve, which indicated that PCB bent up and down under drop impact. The first order modal is the main vibration mode of package under drop impact, and the high orders have little effect. Peeling stress of the outmost corner solder joint that obviously greater than other stress is the main cause of the solder joint crack.Based on the correctness of modal established with ABAQUS, three different kinds of solder joint distribution with the same peripheral size was analyzed. The results showed that biggest Mises stress were all at the outermost corner solder joints in three different kinds of distribution. Among the outmost solder joint, the stress of horizontal solder joints significantly less than the longitudinal ones; Different solder joint distribution has little impact on PCB deflection. For the full array distribution, the peeling stress of dangerous solder joint for lower density of solder joint distribution obviously bigger than higher density distribution under the same drop impact load. Removing some of central ones has extremely small influence on peeling stress of dangerous solder joint. It indicates that peripheral solder joint distribution density has significant effect on peeling stress of dangerous solder joint. When peripheral solder joint density is small, peeling stress of dangerous solder joint is big, and it will fail easily. The results provide theoretical references to the effective improvement of solder joint distribution and package reliability.更多还原