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46 SMT Magazine • April 2015 pOSiTiON aCCuraCy MaCHiNES FOr SElECTivE SOlDEriNG OF FiNE-piTCH COMpONENTS continues FeAture ture differences and expansion differences of the different parts' fiducials can be installed in the machine. With a fiducial camera mounted on the gripper it is possible to make corrections if required. The camera on the gripper measures the offset of the solder pot and the software will guide the print in the correct position. Dip sol- dering is an application that is common used in high-volume lines where cycle time is critical so the camera should respond fast to minimize time loss. Another method that is used for most as- semblies is to have a position pin on the gripper that slides into a position hole on the nozzle plate. Thus the position of the print toward the nozzle plate is mechanically secured. Also, this point has to be learned with a machine at oper- ation temperatures otherwise, due to difference in expansion, the robot might have an offset of 1 mm or more. On the nozzle plate there are spacers in- stalled with a defined height. In this way the z-position of the print is always guaranteed and board is kept in a flat position. Drag Soldering Station The more flexible soldering method is to have a small nozzle and drag the assembly over the wave. This single-point soldering is a robust method, but it is time-consuming and creates longer cycle times. There are four methods for drag soldering. The first selection is the nozzle material. This can be wettable or non-wettable. Second, one can solder horizontal or under an angle. For a non-wettable nozzle the solder-flow direction is defined and the board or the noz- zle should have a U-rotation option to drag the print in the right direction over the wave. For a wettable nozzle the solder overflows in all direc- tions, which make it possible to drag the print from all directions. Nozzles have different shapes and even may have a different height. Therefore the machine should have a dedicated machine point for each nozzle. After changing a nozzle this machine point may have been changed in x, y-position or even in height (z-position). A general rule is that after maintenance or removing parts, ma- chine points must be verified to avoid small offsets. The nozzle has been taught and the po- sition of the print in the robot gripper is me- chanically secured or defined by fiducials, so all required information is there for the system to drag the board properly over the small nozzle without touching the surrounded SMD compo- nents. The board warpage should be compen- sated by to robot z-position to avoid that the board is immersed too deep in the solder. Experiment The next experiments are done to verify the accuracy of the robots used to transport the print to the proper position. These measure- ments are also part of the quality program that is run for every new machine before production starts. For many automotive lines this capabil- ity analyses are repeated before an audit or after the machine has been moved. Each machine has at least two robots. The drop-jet nozzle of the fluxer is installed on a x,y robot. This moves the drop-jet to spray at those spots that require flux. A very sensitive liquid flow measurement guarantees a controlled flux amount per con- nector. The low flux flow can be measured using a device that contains a heating element that registers every temperature change which is di- rectly related to the flow rate of the flux. A fork laser sensor can be used to define if the sprayed figure 3: The normal distributed placement accuracy of the fluxer robot in the x-direction.