All versions of TWO COOL® and PRO COOL® enforce the assumption that equipment and defect-limited yields at a process operation are 100% or less. Some tools and processes for sub-micron wafer fabrication and small outline packaging can improve yield by recovering losses from prior operations. For example, the following tools can remove particles or defects deposited by prior operations:
Laser Assisted Clean
Similarly, lead conditioning tools can recover yield lost to lead deformation at package handling operations in assembly and test.
For such operations, equipment and defect-limited yields may be 100% or greater. However, the yield improvement cannot be greater than the yield loss caused by defects deposited by prior operations. Thus, for a PRO COOL® sequence involving both yield loss and recovery, the sequence composite yield cannot be greater than 100%.
WWK proposes that future versions of TWO COOL® and PRO COOL® be modified to allow equipment and defect-limited yields greater than 100%. Parametric yield loss will not be modified and will remain bounded between 0% and 100%. Procedures to recover parametric yield loss will involve rework and should be modeled accordingly. (See Technical Alert #12.)
WWK would appreciate your comments on the proposed modification.