【作者】 杨雪霞；

【导师】 树学峰；

【作者基本信息】 太原理工大学 ， 固体力学， 2013， 博士

【摘要】 随着电子封装的无铅化和微型化,焊料与衬底金属间生成的界面金属间化合物(intermetallic compound简称IMC)对焊点的可靠性产生了不可忽视的影响。本文工作对IMC层的力学性能进行了理论与实验研究,分析了IMC层在热循环和跌落冲击载荷下对焊点可靠性的影响。1、采用纳米压痕技术对焊点连接各层材料的力学性能进行研究,并对不同工况制备的IMC层进行性能测试。通过对Sn3.OAgO.5Cu焊料、IMC层和Cu焊盘的力学性能进行分析对比,发现IMC层的性能与无铅焊料和Cu的性能存在较大差异,焊点连接层在承受外荷载作用时,IMC周围将产生较大的应力集中,此处为焊点失效的关键位置；不同工艺制备条件(无铅焊料成分、回流焊接次数和焊接曲线的加热因子值)对IMC层的力学性能均存在一定的影响,研究结果为电子封装制备工艺的进一步优化提供了研究基础。2、根据实验结果确定电子封装中IMC层的弹塑性本构关系。采用ANSYS有限元分析软件,结合量纲分析方法和反演分析技术,建立载荷位移曲线与材料弹塑性本构参数之间的联系。根据特征应力和特征应变的概念,建立了无铅焊料Sn3.0Ag0.5Cu内生成Cu3Sn和Cu6Sn5、Sn3.5Ag内生成的Cu6Sn5以及Sn0.7Cu内生成的Cu6Sn5的弹塑性本构关系。3、针对不同时效时间下的IMC厚度测量值,分析时效对焊点抵抗热疲劳能力的影响,对无铅焊点Sn3.OAg0.5Cu、Sn3.5Ag和Sn0.7Cu的热疲劳可靠性进行评估。采用ANSYS有限元分析软件,对焊点在热循环载荷下的力学行为进行分析,并采用修正的Coffin-Manson经验方程对关键焊点进行热疲劳寿命预测。可见PBGA中的关键焊点位于芯片右下方；关键焊点的等效应力最大值随着IMC层厚度值的增大而减小,等效塑性应变最大值随着IMC层厚度值增大而增大；模型中IMC层的厚度对关键焊点的疲劳寿命具有重要的影响,其寿命周期随着IMC层厚度的增大而减小。IMC层厚度为19μm的关键焊点寿命周期比厚度为2μm时下降了21.46%；无铅焊料Sn3.5Ag的热疲劳寿命最大,分别为Sn0.7Cu和Sn3.OAgO.5Cu的2.97倍和1.33倍。4、分析跌落冲击载荷下IMC层厚度对焊点可靠性的影响,讨论了Sn3.5Ag、Sn3.OAgO.5Cu和Sn0.7Cu三种无铅焊点在跌落冲击载荷下的可靠性。按照电子产品板级跌落测试标准(JEDEC Standard JESD22-B111)采用ANSYS/LS_DYNA有限元分析软件和Input-G方法对PBGA在板级跌落条件下的力学行为进行计算。分析表明,IMC层为2μm-19μm与不考虑IMC层(即IMC层厚度为0)时焊点最大剥离应力的差值范围为0.0314～0.1032GPa,相应的增大比例为10.9%～36%；随着IMC层厚度的增大,关键焊点的最大剥离应力值增大,增大的速率从0.0157GPa/μm逐渐减小到0.0009GPa/μm;同时,Sn3.5Ag、Sn3.0Ag0.5Cu和Sn0.7Cu的最大剥离应力值依次增大,分别为0.326GPa,0.391GPa和0.421GPa,表明Sn3.5Ag无铅焊点抵抗跌落冲击载荷的性能更强。更多还原

【Abstract】 With the lead-free and miniaturization of solder joints in electronic packaging, the intermetallic compound (IMC) which formed at the interface between the solder ball and under bump metallization(UBM) have considerable effects on the reliability of solder joints. In this paper, the mechanical properties of IMC were studied using nanoindentation. Based on which the effects of IMC on solder joint reliability under thermal cycling load and drop impact load conditions were researched by finite element simulation respectively.1. The nanoindentation was used to study the mechanical properties of every layer of lead-free solder and to test the IMC layers in the different working conditions. Compare the mechanical properties of the lead-free solder joints, the interfacial compound layer of IMC and the Cu pad, it showed that there are obvious differences between the IMC layer and the lead-free solder joint:the stress concentration occured around the IMC layers under the external loads, so the IMC layers become the key position of solder joint failure. Moreover, the different processing conditions (the composition of lead-free solder, the heating factor value and the number of reflow soldering) also have certain effects on the mechanical properties of IMC layers and it provides a research foundation for the further study of electronic products reliability.2. The elastoplastic constitutive equation of the IMC in electronic packaging can be obtained based on the nanoindentation test results. The nanoindentation process was simulated by the finite element analysis software ANSYS, and combining the dimensional analysis method and the inversion analysis technology, the link between the nanoindentation load-displacement curve and the material constitutive parameters was established. According to the concept of representative strain and representative stress, the elastoplastic constitutive models of IMC were obtained.3. Based on the thicknesses of IMC under different aging time, the effects of IMC thickness on the thermal fatigue reliability of solder joints were analyzed. The mechanical behavior of solder joints under thermal cycle loading were analyzed using finite element analysis software ANSYS. The finite element analysis software ANSYS was used to analyze the mechanical behavior of solder joints under the thermal cycle loading, and the Coffin-Manson’s empirical modified formula was used to predict the thermal fatigue life of solder joints. The main conclusions were as follows:the key solder joints were at the lower right of the chip where gets the maximum equivalent plastic strain values, and it is more likely to become invalid. Both the equivalent stress and the thermal fatigue life of critical solder joints decrease with the increasing of the IMC layers thickness. The thermal fatigue life of PBGA critical solder joint with19μm IMC thickness declines by21.46%than that with2μm IMC thickness. Lead-free solder Sn3.5Ag has the longest thermal fatigue life which is2.97times and1.33times of Sn0.7Cu and Sn3.0Ag0.5Cu respectively.4. The paper studied the influences of IMC layer thickness on the solder joint reliability under the drop impact loading and discussed the reliability of three different types of lead-free solder joints (Sn3.5Ag, Sn3.OAgO.5Cu and Sn0.7Cu) under drop impact loading. According to the JEDEC Standard JESD22-B111, ANSYS/LS_DYNA finite element analysis software and Input-G method were used to calculate the mechanical behavior of PBGA packaging under the conditions of board-level drop. The main conclusions were as follows: compared with the maximum peeling stress when the IMC layers thickness is0, the difference range of that is0.0314-0.1032GPa when the IMC layers thickness in the range of2-19μm, the corresponding increasing proportion is10.9%-36%. The maximum peel stress value of critical solder joint increases with the increasing of IMC layer thickness, and the increasing velocity declines gradually from0.0157GPa/μm to0.0009GPa/μm; meantime, the maximum peeling stress of Sn3.5Ag、Sn3.0Ag0.5Cu and Sn0.7Cu orderly increases from0.391GPa,0.326GPa to0.421GPa, which drawn a conclusion that Sn3.0Ag0.5Cu shows a better ability to resist deformation in the drop test.更多还原