C4NP technology: Manufacturability, yields and reliability

As a part of IBM movement from Pb-rich solders to Pb-free solder, a new low cost process has been developed to deposit the solder to a capture, or under bump metal (UBM) pad, with Suss MicroTech Inc as the equipment partner. The controlled collapsed chip connection new process (C4NP) has moved, over the last 2 years, from development into manufacturing for 300 mm wafers. During this transition, a great number of process improvements have resulted in high fabrication yields. Manufacturing robustness has been achieved by clearly identifying the processes which affect the C4 structural integrity. The solder composition has been optimized to improve its mechanical properties as well as low alpha emission rate requirement. Sector partitioning methodology was used to obtain root cause for various defects which then, through replication studies, were confirmed. Key process improvements in the capture pad build, mold fabrication, and mold fill tool have been accomplished as the process has matured. Thermal undercut was identified as a mechanism of Cu seed consumption when no top Cu was available on top of the Ni UBM. C4NP technology can produce yields comparable to that of electroplated C4 Bumps. Yield learning model shows a 15% defect reduction per month since the start of the C4NP program. Technology qualification for 300 mm wafers with 200um and 150 um pitch Pb-free C4 bumps has been successfully completed.