FlipChip utilizes several dielectric materials that offer various advantages such as improved electical performance and reliability. In addition the dielectric will planarize the die surface and add a stress buffer layer improving the solder joint reliability. Because of this WLCSP die typically do not require underfill during assembly.
Bump on I/O WLCSP
WLCSP's use pre-formed solder spheres of 200μm to 500μm in diameter to routinely bump device pitches ranging from 0.35 to 0.8 mm pitch and reflowed for final bump heights of 160μm to 400μm. In this process, the bumps can be placed directly on the device I/O's or the bump can be redistributed to a more desirable die location. Redistribution of the I/O can be achieved by utilizing a sputtered metal or copper plated RDL trace.