Investigation On Laser Soldering Technology Of QFP And SOP Devices

Electronic products are rapidly developed towards high power、high density、highreliability and green packaging (lead-free) especially miniaturization, as well asincreasingly demanding connection properties. Surface mount technology (SMT) isthe main method for electronic components assembling. All the conventional reflowsoldering such as infrared、gas phase and hot air are whole heating method. It is easyto produce defects such as bridge, bubble and so on. For some high reliabilitythermo-sensitive elements, it needs to look for other methods. In addition the use oflead-free solder brings some new challenges to traditional reflow soldering.Laser soldering has the advantages of localized, non-contact, rapid rise and fallin temperature, as well as the feature of flexibility, which is commendable toovercome the shortcoming of whole heating. Soldering technology of quad flat packdevices (QFP) and small outline packaging (SOP) components are studied using afiber laser based on scanning galvanometer with SnAg3.0Cu0.5solder. The relationsbetween tensile strength of micro-joints and laser soldering parameters are obtained.Excellent lead-free solder joints are achieved at the laser soldering parameters11W,20mm/s or8W,10mm/s. The speed of fiber laser soldering is improved dramatically.The mechanical properties of micro-joints are compared through different solderingmethods, which are CW fiber laser soldering, pulsed fiber laser soldering and infraredreflow soldering. The results indicate that CW fiber laser soldering can obviouslyimprove the tensile strength of the joints with SnAg3.0Cu0.5solder. In addition, thetensile strengths of SnAg3.0Cu0.5solder and Sn63Pb37solder soldered by CW fiberlaser are compared. The mechanical property of SnAg3.0Cu0.5is observed to beoutstanding. The microstructure of fracture surfaces and metallographic structure ofthe solder joints are both investigated.Compared with Sn-Pb solder, lead-free solder has higher melting point andpoorer wetability and becomes deterioration easily in the air. Laser welding QFP andSOP components without solder is investigated and it is demonstrated that this methodcan meet the SMT requirements through testing the micro-joints’ electric andmechanical properties as well as X-ray inspection. The tensile strength of the micro-joints welded by laser without solder is directly related with the laser weldingparameters. The joints’ qualities on a FR4-PCB consisted of Cu/Sn or Cu/Ni/Au pads are compared. The result verifies the PCB with tin-plating pads is suitable for laserwelding without solder. The parameters of the component’s lead foot also affect thelaser welding parameters. The pin size is smaller and thinner, the required laserwelding power is smaller, and the smaller adjacent pin interval is, the faster weldingspeed reaches. The mechanical properties are compared between CW fiber laserwelding and pulsed fiber laser welding without solder. The result of pulsed fiberwelding is better than CW laser welding. In addition, the microstructure of fracturesurfaces and metallographic structure of the micro-joints by laser welding withoutsolder are both researched.According to the experimental parameters of laser welding QFP device withoutsolder, the finite element method is employed to simulate the temperature of thewelding process. The relations of joints’ temperature and laser power as well aswelding speed are achieved. The simulation results at parameters of20W,12mm/sabout the welding spot’s temperature distribution on the space and time are analyzed,indicating that the pad and lead’s temperature distributions are uniform. Thetemperature rises linearly with the laser heating time, whereas the increase of centertemperature is much faster than other locations. When the laser heat source iscompletely loaded, the temperature on the contact-surface of the pin and pad is646K.The actual measured highest temperature is613K at welding parameter of20W,12mm/s. Though the measured temperature is slightly smaller than the simulation result,it can verify the established model is correct and the simulation results are reliable,which is of significant importance to guide the experiment.