Study On Diode-laser Soldering Technology Of QFP Devices

It becomes more and more difficult to soldering high density QFP devices by traditional infrared reflow soldering method. The diode-laser soldering system, which is a new one, makes it possible to improve strength of QFP soldered joints, while also reducing bridging between the solder pads. Its applied field is getting larger and larger.The effect of diode-laser soldering process parameters on mechanical properties of QFP soldered joints were studied in this paper. Using Sn-Ag-Cu lead-free solder, soldering experiments of QFP100 were carried out by diode-laser soldering system. The results show that: using Sn-Ag-Cu lead-free solder, when the laser soldering time is 2mm/s and the laser output power is 38.3W, appearances and mechanical properties of soldered joints are the best, cold solder connection and bridging defects do not exist. Competitive experiments results show the intermetallic compound layer with laser soldering system is smoother than that with the infrared reflow soldering method, and excellent mechanical properties are gained for the soldered joints. In contrast with Sn-Pb soldered joints, The tensile force, data stabilization and reliability of Sn-Ag-Cu soldered joints is much better.With the changing of the laser output power, the change law of the microstructures of soldered joint was also analyzed by optical microscope and SEM. The results show that: no excessive grow of Cu6Sn5 intermetallic compound in the microstructures of soldered joint, appropriate thickness of Cu6Sn5 layer on solder/substrate interface is also got. There are some homogeneous and fine dimples on fracture, which is called toughness fracture. Intermetallic compound layer in microstructures of soldered joints increases with the increasing of laser output power, which lead to variety of dimples and existing of ligule pattern in some place of the fracture, and the fracture mechanism changed from toughness fracture to brittle fracture. Temperature cycling test of QFP100 devices soldered by laser soldering method was also done in this study, and the tensile forces of QFP joints were tested by means of STR-1000 micro-joints tester, then mechanical properties changes of QFP100 micro-joints were researched detailly. The results show that: mechanical properties of micro-joints decrease with the increasing of temperature cycling numbers, the fracture micrograph of joints was also analyzed by means of SEM. With the increasing of temperature cycling times, fracture microstructures of QFP micro-joints exhibit ligule pattern, and the fracture mechanism changed from tough fracture to brittle fracture.As a matter of the experimental data, all the results about diode-laser lead-free soldering technology of QFP devices have provided effective supporting theoretically and practically for the application of. diode-laser lead-free soldering technology.