Issue link: https://iconnect007.uberflip.com/i/699765
July 2016 • SMT Magazine 105 spheres (SAC305, 0.64 mm in diameter), sol- dered on the pads (0.50 mm in diameter, two kinds of surface finish, OSP and ENIG) of PCBs. Prior to soldering, the pads were covered with a thin layer of additional solder paste of SP1 to ensure good solder joint formation. Laser Soldering Process Traditionally arc lamp pumped semiconduc- tor diode laser (Likuan LLU-450101) has been used in laser soldering. Semiconductor diode la- ser has advantages over CO 2 and Nd:YAC lasers to the facts that the wavelength of energy pro- duced is highly absorbed by the metals used in solder but less absorbed by PCB materials [10]. In order to provide uniform heating along the BGA spheres and solder paste, the diameter of the laser beam has to be compatible with the size of the objects to be soldered. Laser beam delivery may be by X-Y-Z stage motion of the laser head. In addition, an arrangement facil- ity of solder paste dispensing can be applied. The process can be monitored using a micro- scope and a CCD set on the optical axis, thus it is possible for direct observation during laser soldering. The schematic experimental setup of the laser system, and the temperature profile of heating with power control for both parts of the research were illustrated as Figures 1 and 2. Analysis and Observation Evaluation of rheological properties of sol- der pastes, such as viscosity against shear rate and visco-elastic behavior, on the applicabil- ity for dispensing was determined via mea- surements obtained by a rheometer (Physica MCR101). The correlations between flux formu- las in solder paste and soldering performance using laser were also investigated on the study of thermogravimetric analysis of solder paste by TGA (PerkinElmer Diamond TG/DTA). Solder- ing performance of SP1 and SP2 was observed by digital microscope (Keyence VHX-2000). In the second part of the research, voiding check Figure 1: The schematic experimental setup of the laser system. Figure 2: Applicable power profile for (a) solder paste only, and (b) 0.64 mm BGA spheres on pads with a thin layer of solder paste. Table 2. The description of two lead-free solder pastes. A NEW DISPENSING SOLDER PASTE FOR LASER SOLDERING TECHNOLOGY