Advanced Metallurgical Reliability Issues in Flip Chip Technology

在这项授权中，研究了倒装芯片焊点上的电动力、热机械力和化学力之间的相互作用。NSF之前的一个项目确定了倒装芯片焊点中的电迁移成为硅设备互连系统中薄弱环节的基本原因。当电迁移与热应力和固态老化(Kirkendall空洞形成)相结合时，电子公司遇到了许多新的和意想不到的倒装芯片焊点可靠性问题。这些独特的冶金可靠性问题在倒装芯片焊点中进行了研究。这项研究的科学理解不仅增强了焊点可靠性的基本知识，还使先进制造工艺和材料的优化成为可能。这并不是因为低熔点焊料中的原子扩散快于铝或铜中的原子扩散。实际上，在器件工作温度下，Al中的晶界扩散和铜上的表面扩散与焊料合金中的体扩散接近。相反，这是因为焊料合金的电迁移临界产物比铝和铜的临界产物小两到三个数量级。因此，电迁移可能会使焊点失效，电流密度比使铝或铜线失效所需的电流密度小两到三个数量级。此外，还有其他几个因素可以增强焊点中的电迁移。倒装芯片焊点独特的线对凸点结构在线与凸点之间的接触处产生了严重的电流拥挤现象。硅芯片拐角处的结点承受着很大的热应力，应力集中与当前的拥挤现象重叠。焊料与凸点下金属化的反应非常快，由于极性效应，反应速度和产物会受到电迁移的影响；阴极上的反应不同于阳极上的反应。先进电子消费产品的高密度系统级封装中对倒装芯片技术的需求正在迅速增长。具有焊料凸块面积阵列的倒装芯片技术是现有的唯一能够满足用于直接芯片连接的大量输入/输出互连的要求的技术。今天，工业上很少进行基础材料研究，尽管电子设备中的材料加工和可靠性问题相当微妙和复杂。过去，电子公司能够赶在国际半导体技术路线图(由半导体工业协会发布)中预测的时间表之前，获得市场提前期。今天，将低k材料集成到硅器件中的计划正在拉回！同样，由于缺乏对无铅焊料可靠性的了解，大型计算机制造商在2010年之前已经获得了允许使用高铅焊料的豁免。这两个案例揭示了材料研究在与电子制造相关的大学中的重要作用。在不久的将来，没有其他技术可以像倒装芯片技术那样提供大量的输入/输出片上互连。这项关于焊点在电力、机械和化学联合作用下的基本行为的研究将使美国的大型计算机和消费电子产品受益。

In this grant, interactions among the electrical force, thermo-mechanical force, and chemical force on flip chip solder joint are examined. A previous NSF project established the basis reasons why electromigration in flip chip solder joints becomes a weak link in the interconnect system on silicon devices. When electromigration is combined with thermal stress and solid-state aging (Kirkendall void formation), many new and unexpected reliability problems of flip chip solder joints are encountered by electronic companies. These unique metallurgical reliability issues in flip chip solder joints are studied in this grant. Scientific understanding from the study not only enhances basic knowledge of reliability of solder joints, but also enables optimization of advanced manufacturing processes and materials. It is not because atomic diffusion in the low melting point solder alloy is faster than that in Al or Cu. Actually, grain boundary diffusion in Al and surface diffusion on Cu is close to that of bulk diffusion in solder alloys at the device operation temperature. Rather it is because the.critical product of electromigration of solder alloys is two to three orders of magnitude smaller than that in Al and Cu. So electromigration can fail a solder joint by a current density that is two to three orders of magnitude less than that needed to fail Al or Cu lines. In addition, there are several other factors that enhance electromigration in solder joints. The unique configuration of line-to-bump of the flip chip solder joint produces a serious current crowding phenomenon at the contact between the line and the bump. The joints at the corners of a Si chip are under a very large thermal stress, and the stress concentration overlaps the current crowding. The solder reacts very fast with under-bump metallization and the reaction rate and products can be affected by electromigration due to the polarity effect; the reaction at the cathode is different from that the anode. The demand for flip chip technology in high-density system-in-packaging for advanced electronic consumer products is growing rapidly. Flip chip technology with an area array of solder bumps is the only existing technology that can meet the requirement of a very large number of input/output interconnects for direct chip attachment. Today, little basic materials research is conducted in industry, although materials processing and reliability problems in electronic devices are quite subtle and complicated. In the past, electronic companies were able to beat the timetable projected in the International Semiconductor Technology Roadmap (published by the Semiconductor Industry Association) and gained market lead-time. Today, the schedule for integrating low-k materials into Si devices is pulling back! Similarly, due to the lack of knowledge on reliability of Pb-free solders, mainframe computer makers have obtained exemptions for allowing the use of high-Pb solder until 2010. These two cases reveal the important role of materials research in universities related to electronics manufacturing. In the near future, there is no other technology that can deliver the large number of input/outputs on-chip interconnections as flip chip technology. This study of the basic behavior of solder joints under a combination of electrical, mechanical and chemical forces will benefit mainframe computers as well as consumer electronic products in the US.