Package-on-Package Paste Leveling (1/5)

This is the first of a series of entries dealing with tips for Package-on-Package assembly. A logical place to begin is material deposition. It is important to start with a level reservoir of paste (or flux) to prepare for component dipping. For manual/prototype equipment – begin by setting the blade height to 1/3 the sphere diameter of the component you are dipping. Next, apply about 1/3 of the paste or flux you need to fill the area between the reservoir and the blade. After applying the initial material, let the blade level the material, now you are ready to apply the next 1/3 of the paste or flux you will use. After leveling, add the remaining 1/3 paste/flux to the reservoir. Cycle the leveling blade a few times and observe the material in the flux tray. If a large amount of material accumulates along the perimeter you’ve applied too much flux. If the surface is wavy or has craters, just add some more paste or flux. Craters in the solder paste are a problem if left untreated, as they will create inconsistencies in solder transfer efficiency. A good dipping paste will look smooth after leveling. This is in part due to lower metal loading of Package-on-Package solder pastes, but can also be affected by the leveling blade design. Watch for the next installment – “Package-on-Package Component Dipping”