Quasi-ambient bonding to enable cost-effective high temperature Pb-free solder interconnects

对能够在超过200摄氏度的温度下工作的电子产品的需求不断增加，远高于传统硅微电子的最高工作温度。主要应用领域是电力、汽车、航空航天和国防工业。能够在这样的高温下工作的电子设备现在是可用的。然而，还需要新的方法将这些器件集成到电路和系统中，特别是将它们机械地和电地连接到电路板和散热器上。目前，高温设备通常是通过使用高熔点、含铅丰富的焊料进行焊接。然而，在所有电子产品中减少铅的使用是一个强烈的环境要求，所以这不能被接受为一个长期的解决方案。可以使用富金焊料或烧结纳米银浆料的替代解决方案，但这些解决方案价格昂贵，而且存在可靠性问题。低成本，无铅高温钎料合金也可以；然而，这些往往需要更高的焊接温度和更长的加工时间，导致生产速度变慢，并且在焊接期间器件上的热负荷更高。该项目将探索使用反应性纳米箔的准环境键合（QAB）作为降低高温电子器件封装过程中的工艺时间和热负荷的途径。反应性纳米箔是由两种元素（通常是镍和铝）交替层组成的多层材料，它们进行放热反应，即释放热量。一旦反应被触发，它就会自我传播并扩散到整个箔片。如果将箔夹在预先涂有焊料的两个部件之间，则产生的热量可用于暂时熔化相邻的焊料层并形成永久粘合。加热是强烈的，但发生在很短的时间尺度上，所以当局部温度可以达到1500摄氏度时，加热仅限于箔周围的一个狭窄区域，其他地方发生的温度上升可以忽略不计。到目前为止，准环境键合应用一直使用传统的低温焊料。在这个项目中，我们将把QAB的应用扩展到一系列低成本、无铅的高温合金。主要目标将是开发适合高温电力电子和光电子应用的键合工艺。我们还将探索QAB在密封封装中的应用，这是另一个低成本和低热负荷优势的关键领域。开发的工艺将根据结合强度和使用可靠性进行评估，并与基于铅基和金基焊料的替代工艺进行基准测试。在过程开发和评估的同时，我们将进行广泛的建模和特征描述，以提高对QAB过程的理解。迄今为止的发展主要是经验性的，对这一进程的基本方面仍然知之甚少。QAB从根本上不同于传统的焊接，因为过程发生的时间非常短。为了使其在主流电子制造中建立起来，残余应力和微结构缺陷纳入QAB键的潜在有害影响需要充分了解。提出的研究有可能为电子制造提供低成本，可持续的连接技术，可以在未来许多年内继续满足高温电子产品的工作温度要求。同时，它将对复杂合金的快速凝固过程产生新的基本见解，这将引起材料科学和制造研究界的广泛兴趣。

There is an increasing demand for electronics that can operate at temperatures in excess of 200 degrees C, well above the maximum operating temperature of traditional silicon microelectronics. Key application areas are in the power, automotive, aerospace and defence industries. Electronic devices capable of operating at such high temperatures are now available. However, new methods are also needed for integrating these devices into circuits and systems, and in particular for attaching them, both mechanically and electrically, to circuit boards and heatsinks. At present high-temperature devices are typically attached by soldering using high-melting-point, lead-rich solders. However, there is a strong environmental imperative to reduce the use of lead in all electronics, so this cannot be accepted as a long-term solution. Alternative solutions employing gold-rich solders or sintered nano-silver pastes can be used, but these are expensive and can suffer from reliability issues. Low-cost, lead-free high-temperature solder alloys are also available; however, these tend to require significantly higher soldering temperatures and longer processing times, leading to slower production and higher thermal load on the devices during soldering.This project will explore the use of quasi-ambient bonding (QAB) with reactive nanofoils as a route to lowering the process time and thermal load during packaging of high-temperature electronic devices. Reactive nanofoils are multilayer materials comprising alternating layers of two elements (typically nickel and aluminium) that react exothermically i.e. with the release of heat. Once the reaction is triggered, it is self-propagating and spreads throughout the foil. If the foil is sandwiched between two parts that are pre-coated with solder, the heat generated can be used to melt the adjacent solder layers momentarily and form a permanent bond. The heating is intense, but occurs over a short timescale, so that while the local temperature can reach up to 1500 degrees C, heating is confined to a narrow region around the foil, with negligible temperature rise occurring elsewhere. Up to now, quasi-ambient bonding applications have used traditional lower-temperature solders. In this project we will extend the application of QAB to a range of low-cost, lead-free high-temperature alloys. The primary aim will be to develop bonding processes tailored for applications in high-temperature power electronics and optoelectronics. We will also explore the use of QAB for sealing of hermetic packages which is another key area where low cost and low thermal load can be an advantage. The processes developed will be evaluated in terms of bonding strength and in-service reliability, and benchmarked against alternative processes based on lead- and gold-based solders. Alongside the process development and evaluation, we will carry out extensive modelling and characterisation aimed at gaining an improved understanding of the QAB process. Developments to date have been mainly empirical, and fundamental aspects of the process remain poorly understood. QAB is fundamentally different from traditional soldering because of the very short timescale over which the process takes place. In order for it to become established in mainstream electronics manufacturing, the potential detrimental effects of residual stresses and microstructural defects incorporated into QAB bonds need to be fully understood. The proposed research has the potential to provide a low-cost, sustainable joining technology for electronics manufacturing that can continue to meet the operating temperature requirements of high-temperature electronics for many years to come. At the same time it will yield new fundamental insights into processes involving rapid solidification of complex alloys that will be of wide interest to the materials science and manufacturing research communities.