Material characteristics such as film elasticity and surface roughness are key parameters in the film's adhesion to the encapsulant and its removal without damage. The film also protects the bump pad areas so that a final cleaning step is not required. A dual blade is used to singulate the wafer into separate units. The process is completed by aligning solder balls to each of the exposed bumps. To date, this package is more suitable for low-density lead counts.
Investigation is ongoing to increase to higher density lead count capability.
To do so, several areas must be examined to manage the mechanical stresses. One area is the UBM structure and its metallurgical effect on the die surface. Another area is the type of encapsulant used. It must have good adhesion with the die and contract at a rate that will not induce internal stresses. Optimization of solder joint integrity and fatigue life are critical for application reliability.