Before even thinking of dipping a component, you had better verify that your paste or flux is 1) the proper thickness and 2) uniform across the surface of the flux reservoir. You can check this by using a film depth gauge. After picking up a Package-on-Package component from its tray, center it over the paste or flux it will be dipped into. Verify again that the surface of the paste or flux is smooth and even. Dip the component into the reservoir and let it bottom out in the reservoir. You can now pick the component out of the reservoir and observe the imprint left by the components solder spheres. If the imprint looks distorted or incomplete, there may be a problem with your material depth or co-planarity. If the pattern looks defined and regular, continue by placing the component. Be sure to level out the flux reservoir each time by advancing the blade (linear tray) or rotating the reservoir (rotary tray).
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