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50 SMT Magazine • April 2015 pOSiTiON aCCuraCy MaCHiNES FOr SElECTivE SOlDEriNG OF FiNE-piTCH COMpONENTS continues FeAture Table 1: The average wave height at different pump settings. The number of samples is 120 for each setting. ing the nozzle rim, which would result in solder webbing/oxides on the solder mask, the nozzle plate has stand-off pins to support and level the board. The robot gripper itself is also accurate in z-direction. The standard deviation is equal to x and y-direction, 0.008 mm. Select Wave Nozzle Multi-wave nozzle plates have stand-off pins which provide a mechanical means to fix the z- position. The small select wave nozzle has no mechanical tooling to secure the height which makes this soldering more challenging. Three parameters are critical and all have variation: board flatness (warpage), z-position accuracy of the robot gripper, and the stability of the wave height. How Stable is a Small Wave? In order to address the wave height stability of a small nozzle a special test with a camera measuring the wave height was designed. For a given design of nozzle there are three fac- tors that define the wave height and its stability: 1. Solder alloy 2. Solder temperature 3. Pump speed For the experiment, a 5 mm (inside diam- eter) wettable nozzle was used. The solder was lead-free SACX and the solder temperature var- ied from 280–320°C. The wave height of the select wave can be controlled by the pump speed. As shown in Figure 5, there is a small process window. The height can only be changed with a few rpms, otherwise it becomes more unstable. In this ex- ample the wave height is most stable between 2.4 and 2.9 mm (460–475 rpm). Special Tooling and Component Accuracy: In a robot gripper, dedicated tooling can be in- stalled for increased accuracy or for special re- quirements. Examples of these tools are: