In the context of the PCB industry, C4 has multiple meanings depending on the specific context. One interpretation of C4 is as standard IBM solder balls used in flip-chip bonding. These C4s come in various sizes and pitches, with examples being “3 on 6” (mil) and “4 on 8”. They serve as on-chip electrodes that are compatible with the standard CMOS process, requiring minimal post-processing and offering low cost and high yield. C4s are crucial for flip-chip bonding, enabling the integration of penetrating electrodes with the presented IC for single unit recording.