Diode Laser Soldering To Electronic Mounting Components/Devices With Lead-free Solder

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.