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54 SMT Magazine • April 2015 pOSiTiON aCCuraCy MaCHiNES FOr SElECTivE SOlDEriNG OF FiNE-piTCH COMpONENTS continues FeAture gerjan diepstraten is advanced technology manager at Vitronics Soltec. 3. Z-direction is guaranteed by stand-off pins on the nozzle plate. Drag soldering over select wave: The require- ments for a consistent drag process are: 1. Position of the PCB in the gripper has to be defined via mechanical fix or fiducial recognition. 2. Machine point of the nozzle should be verified via fiducial x, and y position or z- height verification with a sensor. 3. Z-position of the PCB is critical during soldering. Special attention is needed to make sure that there is no excessive warpage. Special tooling in the gripper or warpage compensation (measured with laser) by software are methods to compen- sate for a lack of flatness. Experience and machine capability data show that despite the temperature changes and board warpage, THT connectors can be soldered in a very accurate way. The knowledge of the pro- cess and small, but very efficient, tooling devices help improve accuracy. Special designed nozzles have non-wettable screens mounted on top to avoid bridging. This option requires a place ac- curacy of 0.10 mm. The robot showed to have a consistent placement accuracy that can meet the tough placement requirements with the help of mechanical fixtures or fiducials. SMT references: 1. PCB 101: Coefficient of Thermal Expan- sion, Bob Tarzwell, DMR Ltd. 2. "Tools to increase the process window of a selective soldering process," EPP Europe, November 2013, Gerjan Diepstraten, Vitronics Soltec. 3. "Selective soldering in an optimized ni- trogen atmosphere," OnBoard Magazine, April 2013, Gerjan Diepstraten, Vitronics Soltec. figure 6: The variation of wave height and robot in z-direction. Most critical for this process is the flatness of the board.