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April 2015 • SMT Magazine 45 pOSiTiON aCCuraCy MaCHiNES FOr SElECTivE SOlDEriNG OF FiNE-piTCH COMpONENTS continues FeAture • Mechanical fix: The majority of PCBs has tooling holes for positioning the print in fix- tures and machines to process the board. Pins in the gripper will lock the board in the defined position. The machine software knows where the ro- bot is and once the print is locked or defined by the fiducial camera, the position of the print is defined in the software. Dip Soldering Station The robot will move the print to the solder unit. There are two soldering options: dipping on a nozzle plate or dragging over a small nozzle. Dipping on a Nozzle plate For high volume assemblies the dip sol- dering on a nozzle plate that is dedicated for each print design is the most efficient method for a short cycle time. In one single dip, all sol- der connections are made. It is a challenge to dip a preheated printed circuit board onto a nozzle plate that is installed in liquid solder of 280°C–320°C. The nozzle plate is made with the Gerber/CAD data of the assembly. The tem- perature difference and expansion of the stain- less steel is compensated. The CAD data has the dimensions defined at ambient temperature, but in the selective dip process the robot (alu- minum) may have a temperature of 50°C, the print (FR-4 material) 120°C and the nozzle plate (stainless steel) 320°C. Different temperatures, CTEs and materials should result in a robust process where joints are soldered up to 1 mm from surrounded SMD, metallized pads or other components. In order to match the hot nozzle plate with the preheated PCB, the following equation can be used: L Nozzle (Ta) /L PCB (Ta) = (1 + Ec PCB * (Tp – Ta))/(1 + Ec Nozzle * (Ts - Ta)) The factor L Nozzle (Ta) /L PCB (Ta) expresses the mul- tiplication factor for the nozzle plate dimen- sions at ambient temperature related to the PCB dimensions at ambient temperature, so that both will fit at soldering conditions. L PCB = PCB length [mm] L Nozzle = Nozzle plate length [mm] Ta = Ambient temperature [°C] Tp = Preheat temperature [°C] Ts = Solder temperature [°C] Ec PCB = Thermal Expansion Coefficient of the PCB [ppm/°C] Ec Nozzle = Thermal Expansion Coefficient of the nozzle plate [ppm/°C] All the programming needed to solder the board can be done off-line on a separate computer. However, teaching of the machine points can only be done in a machine that has achieved its operation temperature. The robot has to learn where the machine points are. Ma- chine points are the reference of a part in the process, like teach camera, solder pot, pick up, and fluxer position. To compensate, tempera- figure 2: The camera mounted on the gripper verifies the position of the solder pot by checking the location of the fiducials.