【作者】 韩宗杰；

【导师】 薛松柏；

【作者基本信息】 南京航空航天大学 ， 材料加工工程， 2009， 博士

【摘要】 微电子元器件微、小型化以及绿色环保无铅钎料的应用,给传统的电子组装工艺带来了很大的挑战。研发新型钎焊技术,以适应微、小型电子元器件“无铅”组装的需要,显得尤为重要。本文着重研究采用短波长、高效率的半导体激光进行电子元器件的无铅钎焊连接的技术,选用两种最具有代表性的表面组装电子元器件——矩形片式电阻元件和QFP器件进行了较为深入、细致的研究。激光软钎焊方法具有其它再流焊方法不可比拟的优点,诸如局部加热、快速加热、快速冷却等,局部加热使得在高密度基板上钎焊热敏感和吸热元器件成为可能,并可以减少焊点间的桥连;而快速加热、快速冷却可在钎焊时产生良好的显微组织从而提高焊点的抗疲劳性能。由于半导体激光比CO₂激光、Nd:YAG激光的波长更短、电光转换效率更高、结构更紧凑、维护更方便,更适合于微、小型电子元器件的“无铅”组装,具有广阔的应用前景。通过研究半导体激光钎焊无铅钎料的钎焊性能,发现激光钎焊时间固定（如0.5s）时,随着激光输出功率的增加,Sn-Ag-Cu无铅钎料的钎焊性能明显改善,并在激光输出功率为17.5W左右时达到最佳,此后随激光输出功率的继续增大,无铅钎料的钎焊性能开始变差。激光输出功率不同,获得最佳钎焊性能所对应的最佳激光钎焊时间不尽相同:激光输出功率越高,最佳激光钎焊时间越短。当激光输出功率过低（P≤13W）或过高（P≥19W）时,无论怎样改变激光钎焊时间,无铅钎料在Cu基体上的润湿铺展效果均很差。激光软钎焊可以获得比红外再流焊更加优异的无铅钎料的钎焊性能,这是因为激光软钎焊具有快速加热的优点,增大了液态金属的表面张力,从而显著改善了钎料的钎焊性能。研究了片式电阻元件的半导体激光无铅软钎焊工艺,对片式电阻元件采用半导体激光软钎焊系统在PCB基板上进行组装,得到了表面光亮、无氧化、成型良好的焊点。研究结果表明,采用半导体激光软钎焊的方法钎焊片式电阻元件所得钎焊焊点接头成型好,能够获得比红外再流焊方法更加优异的焊点力学性能。其中半导体激光钎焊片式电阻元件Sn-Ag-Cu无铅焊点强度比红外再流焊提高达18.13%,而半导体激光钎焊片式电阻元件Sn-Pb焊点强度比红外再流焊提高了38.81%。激光软钎焊可以获得比红外再流焊更加优异的钎料/基体的显微组织。而采用半导体激光软钎焊的方法钎焊片式电阻元件,钎料在焊盘金属Cu和片式电阻金属化端的润湿性更好,钎焊接头焊点成型美观,焊点强度较其它方法显著提高。对焊点的断口显微组织的观察、分析发现,断口现典型的剪切伸长韧窝形貌,表明焊点的塑性变形能力强。采用半导体激光软钎焊工艺在PCB基板上进行QFP器件的组装,通过对QFP器件半导体激光无铅软钎焊工艺的研究,得到了无桥连、无钎料球等外观缺陷的优良焊点。焊点力学性能测试结果表明,半导体激光钎焊QFP32器件Sn-Ag-Cu无铅焊点的强度比红外再流焊提高了10.39%,而半导体激光钎焊QFP100器件Sn-Ag-Cu无铅焊点的强度比红外再流焊提高了12.61%。试验研究表明,这主要与半导体激光能够显著改善钎料的钎焊性能、优化焊点的显微组织有关。通过对半导体激光钎焊QFP器件焊点宏观、微观组织分析,除了发现QFP器件焊点内部组织细微、均匀外,特别是观察到了钎料/焊盘和钎料/引线之间均出现了细小、平缓的金属间化合物层,正是这一“金属间化合物层”,确保了焊点具有良好冶金结合,从而使焊点的力学性能得到了改善与提升。这一“金属间化合物层”属于焊点中的强化区,从而使半导体激光软钎焊焊点的拉伸断裂方式表现为韧性断裂。通过研究发现,与Nd:YAG激光和CO2激光相比,波长更短的半导体激光更容易被钎料金属吸收,钎料合金快速加热、快速冷却的效果更加明显。由于钎料合金是Sn基有色金属合金,快速的加热、冷却过程能够更显著地实现固溶强化的效果并细化合金的显微组织,不仅大幅度提高了微电子元器件焊点的强度,而且显著改善了焊点的塑性。半导体激光钎焊提高电子元器件无铅焊点力学性能的机制主要是细晶强化和第二相弥散强化。由于快速加热、快速冷却使得在焊点内部产生了均匀细小的晶粒和细小弥散的第二相金属间化合物,给位错运动带来了很大的阻力,这是半导体激光钎焊改善无铅焊点力学性能的内因。对无铅焊点进行了热循环试验研究,结果表明激光钎焊无铅焊点的可靠性优于红外再流焊焊点。对焊点显微组织的系统分析表明,由于半导体激光加热快速加热、快速冷却的特点,从而在焊点内部获得均匀细小的钎料组织和薄而平缓的界面金属间化合物组织,形成了良好的冶金结合。对焊点显微组织的系统研究表明在相同的热循环次数条件下,半导体激光软钎焊无铅焊点中钎料/Cu焊盘界面上的金属间化合物厚度小于红外再流焊无铅焊点;同时半导体激光软钎焊无铅焊点中体钎料内部的金属间化合物颗粒要比红外再流焊无铅焊点组织更细小、均匀。本文研究结果不仅从理论上阐明了微电子元器件半导体激光软钎焊技术的微观机理,而且对于在研课题“××××组件批量制造技术”和“××××多芯片系统组装”采用半导体激光软钎焊技术提高产品合格率、延长其使用寿命提供了理论依据、技术储备和数据支持。更多还原

【Abstract】 The increasing miniaturization of electronic components/devices and the use of green environmental protection lead-free solders have resulted in many challenges in conventional electronic assembly processes, and it is particularly important to develop new soldering processes to meet the demand of miniaturization and lead-free mounting and packaging. In this thesis, a lead-free soldering technology of electronic components/devices has been emphasized, which is diode laser soldering with short wavelength and great efficiency. In order to meet the needs of mounting high density packaging components/devices, selected two kinds of typical, i.e. rectangle chip resistor component and quad flad pack (QFP) device, a new soldering method was developed with diode laser system.The advantages of laser soldering arise from its properties of localized heating and the rapid rise and fall in temperature of the soldered joints. Localized heating makes it possible to solder heat sensitive devices and heat sink assemblies in densely populated boards, while also reducing bridging between the solder pads. A rapid rise and fall in temperature creates a fine microstructure in the solder giving improved fatigue properties. Diode lasers have been playing more and more important role in the fields of electronic packaging as a result of its shorter wavelength than CO2 and Nd:YAG lasers, higher electro-optical efficiency, compactness and long operational lifetime.Solderability of lead-free solder was investigated using diode laser soldering method, it is found that when the laser soldering time is fixed (for instance, 0.5s), as the laser output power increases, the solderability of Sn-Ag-Cu lead-free solder gets significant improvement, the optimal solderability is obtained while the laser output power increase to about 17.5W. Different laser output power is corresponding to different optimal soldering time, the larger the laser output power is, and the smaller the optimal soldering time is. While the laser output power is too low (P≤13W) or too high (P≥19W), the solderability of Sn-Ag-Cu lead-free solder on Cu substrate is always poor whatever the laser soldering time is short or long. Laser soldering Sn-Ag-Cu lead-free solder acquires more excellent solderability than IR reflow soldering, as a result of the rapid temperature rise of soldered joint using laser soldering method which increaces the surface tension of liquid solder alloys.Diode laser soldering process of rectangular chip resistors was studied using lead-free solders, and good soldered joints with bright surface, non-oxidation and good forming were obtained using diode laser soldering system. The results show that laser soldering chip resistor soldered joints gain better mechanical properties than IR reflow soldering process, and the shear strength of chip resistor Sn-Ag-Cu soldered joints soldered by diode laser soldering system is 18.13% higher than that soldered with IR reflow method, while the shear strength of chip resistor Sn-Pb soldered joints soldered by diode laser soldering system is 38.81% higher than that soldered with IR reflow method. Better microstructures of solder/substrate metal are gained soldered by diode laser system than these soldered by IR reflow soldering method, excellent wettability of solders on Cu pads and chip resistor metallized end is gained, and the soldered joints show good shape and better mechanical properties. Fracture microstructure observations indicate that fracture morphologies of laser soldered joints show typical shear elongation dimples, which indicates that intence plastic deformation appears before fracture and excellent plastic property.Diode laser soldering process of QFP was studied using lead-free solders, and excellent soldered joints without appearance defects such as solder bridging or solder balls were obtained using diode laser soldering method. Mechanical properties tests show that the tensile strength of QFP32 Sn-Ag-Cu soldered joints soldered by diode laser soldering system is 10.39% higher than that soldered with IR reflow method, while the tensile strength of QFP100 Sn-Ag-Cu soldered joints soldered by diode laser soldering system is 12.61% higher than that soldered with IR reflow method. The theoretical analysis and test results indicate that laser soldering QFP soldered joints can gain better mechanical properties than IR reflow soldering process, as a result of the improvement solderability and optimized microstructures obtained by diode laser soldering system. Fine and homogeneous microstructures are gained in QFP micro-joints soldered by diode laser soldering system, and fine flat intermetallic compound layer also exist between solder and Cu pad, between solder and lead, which insures good metallurgical bonding of QFP soldered joints. The fracture type of diode soldered QFP micro-joints is toughness fracture.Diode laser with shorter wavelength than Nd:YAG and CO2 lasers is easy to be absorbed by solder alloys, and the rapid rise and fall in temperature is more obvious. The solder alloy is Sn-based nonferrous metals, the rapid rise and fall in temperature is helpful to solid solution strengthening and fine-grain strengthening, which not only improves mechanical properties of soldered joints greatly, but also improves plasticity of soldered joints significantly. The main strengthening mechanism of soldered joints mechanical properties is fine-grain strengthening and second phase dispersion strengthening. The rapid rise and fall in temperature results in fine homogeneous grains and fine dispersed second phase IMC that are attributed to the hindrance of dislocation movement, which is why mechanical properties are improved after diode laser soldering.The thermal cycling reliability of lead-free micro-joints soldered by diode laser soldering system is superior to micro-joints soldered by IR reflow soldering methods, that is because the rapid rise and fall in temperature results in fine homogeneous grains and thin mild interfacial IMC, then good metallurgical bonding is gained. Systematic study of microstrutures indicates that with the same thermal cycling times, IMC thickness of diode laser soldered joints is smaller than that of IR reflow soldered joints, at the same time smaller and more homogeneous IMC particles also exist in diode laser soldered joints.All the results not only theoretically illustrates the microscopic mechanism of diode laser soldering process, but also gives theoretical basis, technical reserve and data support to practical application of diode laser soldering technology in batch manufacture of××××modules and assembly technology of××××multi-chip-module (MCM) systems.更多还原