Preparing a Flip Chip Mask Set
Converting a "conventional" wirebond die into a flip chip die generally requires that a series of back end steps are performed on a full sized wafer. This normally requires a set of 4 masks on large glass plates (10 inch for 200 mm wafers and 14-15 inches for 300 mm wafers.)
These masks include the following:
VIA-1 - used to expose the original die pads after a polyamide layer has been spun over the wafer.
RDL - (redistribution layer) used to define metal traces from the original die pads to the new locations for the bumps. Note that the RDL metal is applied initially by sputtering and then is plated up to the desired thickness.
VIA-2 used to create openings in the dielectric over the RDL metal
UBM - (under bump metal) used to define and plate up metal connecting the RDL layer to the bump.
SOLDER - used to produce a screen or stencil through which solder is applied to the under bump metal. The round solder bumps will adhere to this solder. You don't need an expensive glass mask for this -- a film mask will be sufficient.
The Unit Cell Design
The unit cell can be designed with various software tools: IC Layout tools such as Cadence Virtuoso or Tanner's LEDIT, Package Design tools such as Cadence APD/SIP or even AutoCAD. The designs are relatively straightforward - normally starting with the original chip's passivation layer to identify the die pad openings. Shown below are the 4 layers (UBM and SOLDER MASK are identical) that I drew for this example.
If you want to examine the GDSII file for the unit cell you can download it from the link below:
unit_die.gds 84 Kb GDSII Stream
Next - The wafer and how the unit cell will be arrayed ....