基于载荷与失效双解耦的无铅焊点温度-疲劳耦合破坏评价新方法研究

Concerning the acceleration test of electronic packages, most previous studies were focused only on the relations between the applied loads and the so produced lives. Unfortunately, the failure mechanisms before and after such acceleration might be changed, the so obtained lives may not reflect the real basis due to the enclosed tolerance and uncertainties. The study on the mechanisms and evaluation of thermo-mechanical failure of SnAgCu/Cu joints, which are widely employed in BGA packages, is proposed in this proposal. The thermo-mechanical loads will be decoupled into temperature parameter and mechanical fatigue parameter, the thermo-mechanical failure will be decoupled into creep fraction and fatigue fraction. Based on the failure mechanisms, the correlation between the applied temperature load parameter and the resulted creep failure fraction will be investigated. The correlation between the applied fatigue load parameter and the resulted fatigue failure fraction will be investigated as well. Consequently, the damage of each temperature-fatigue coupling cycle could be evaluated by the above obtained creep failure fraction and fatigue failure fraction. By the accumulation of the damage produced by each cycles, the failure behavior and life of the SnAgCu/Cu joints, in the temperature-fatigue coupling cycles, could be evaluated and predicted. Finally, the failure mechanism based acceleration life equation and the corresponding acceleration factor could be established. The proposed study will not only highlights the accelerated failure nature of SnAgCu/Cu soldered joints, but also provides applicable new methodology for life prediction and reliability assessment. The expected original features of this proposal could be summarized as follows: on one hand, by applying double-decoupling both on the applied temperature-fatigue coupled loading and the creep-fatigue coupled failure, the individual contributions originated from temperature load parameter and mechanical fatigue load parameter, to the corresponding creep fraction and fatigue fraction in the coupled failure, could be considered and calculated on the failure mechanisms basis. On the other hand, the temperature-fatigue coupled acceleration life and the corresponding acceleration factor of SnAgCu/Cu joints could be evaluated by applying damage accumulation.

焊点加速失效的研究以往大多仅关注加速载荷与寿命的表观关系，加速中失效机理的一致性并无保证，误差和不确定性较大。本项目针对BGA封装中常用的SnAgCu/Cu焊点及热力耦合服役条件，首先将服役荷载解耦为温度分量和疲劳分量，将热力耦合失效解耦为蠕变失效分数和疲劳失效分数；据此，分别研究基于蠕变机制的温度加速载荷与蠕变失效分数的关系、研究基于疲劳机制的疲劳加速载荷与疲劳失效分数的关系；进而以载荷与相应的失效机理为基础，通过对温度-疲劳耦合加速每一循环中蠕变与疲劳失效分数的累积来表达加速寿命和加速因子。在理论上掌握焊点热力耦合加速失效的本质，在实际上为焊点寿命和可靠性评价提供新的科学方法。预计创新是：通过对载荷与失效的双解耦，确保焊点温度-疲劳耦合加速前后失效机理的一致性，有望克服以往研究对机理的模糊性;以基于机理的损伤累积所表达的加速寿命和加速因子，物理和力学意义明确，有望克服以往研究的唯象性。

现有的电子封装结构中焊点加速失效的研究大多关注加速载荷与寿命的表观关系，加速中失效机理的一致性并无保证，基于此背景，本研究将焊点承受的温度-疲劳耦合加载，解耦为温度加载和疲劳加载，对应的失效解耦为蠕变失效和疲劳失效，克服了以往研究对机理的模糊性，并建立了基于载荷及其相应失效模式双解耦的寿命模型，物理和力学意义明确，克服了以往研究的唯象性。.主要研究内容有：.1）温度载荷对SnAgCu/Cu焊点蠕变加速失效机理的影响及蠕变寿命表达方法的研究。.2）疲劳载荷对SnAgCu/Cu焊点疲劳加速失效机理的影响及疲劳寿命表达方法的研究.3）温度-疲劳耦合对SnAgCu/Cu焊点加速失效机理的影响及加速寿命表达方法的研究。.本项目主要研究成果有：.1）焊点在蠕变加载过程中，β-Sn枝晶与共晶组织界面是焊点的薄弱地带，易萌生空洞，得到了焊点蠕变寿命与稳态蠕变速率之间的关系，焊点的蠕变断裂延性与稳态蠕变速率之间的关系。其中，焊点的蠕变断裂延性随着稳态蠕变速率的增加而增大。.2）疲劳加载条件下，相邻晶粒变形不一致，在晶界处易产生裂纹。弥散分布金属间化合物与钎料基体间易产生裂纹，但并不会进一步扩展，得到了塑性应变范围与疲劳寿命的关系。.3）蠕变疲劳耦合加载下，随着温度增加，沿晶失效比例增加，断裂路径从近金属间化合物层界面处向钎料内偏移。随着驻留时间的延长，焊点的疲劳寿命不断下降。驻留时间诱发钎料体内萌生蠕变空洞，枝晶界为空洞形核点，蠕变疲劳的交互作用，表现为空洞促进疲劳裂纹的扩展。.4）对于温度-疲劳耦合加载方式，蠕变损伤的累积主要在驻留阶段，疲劳损伤的累积主要在加载阶段，得到了基于解耦的无铅焊点温度-疲劳耦合寿命模型。.本项目研究成果对于理解无铅电子封装结构失效的机理有重要的理论意义，对建立无铅电子封装结构加速破坏评价的合理方法提供了理论依据，对无铅电子封装结构寿命和可靠性保证提供了技术支撑。这些研究成果的应用，对于我国现代电子制造产业的发展、产品技术含量的提高及产品质量的保证具有重要的意义。