After more than five years of IBM research and development, Injection Molded Solder (IMS) bumping has been commercially introduced by IBM/SUSS in the C4NP system. The essence of IMS is separating the bump creation in a mold from the bump transfer to the wafer in an oven. Tutorial 55 gives an overview of the C4NP system, the process, and the potential advantages. This tutorial gives more details, including a video, on the injection molding portion of the process.
Bumps are formed by injecting molten solder into etched cavities in a glass mold plate. Figure 1 shows a portion of a mold plate. The etched cavities match the pattern of bumps required on the wafer. The thermal expansion of the glass mold matches that of the silicon wafer. Molds specific to each wafer may be cleaned and re-used over and over.
Figure 1. Bump cavities etched in glass mold plate.
Figure 2 shows a simplified graphic of solder injection. Solder is injected into the mold cavities by passing an injector across the wafer. The mold plate, in contact with the dispenser, is heated to just below melting point of the solder. The injector includes a slightly pressurized reservoir of molten solder of any composition. The injector automatically detects and fills all cavities in the mold.
Figure 2. Schematic of solder injector and reservoir.
Figure 3 is a video of injection molding in operation on IBM’s development tool, not on the final system. Solder may be seen filling the cavities as the injector progresses on its merry way across the wafer. VIDEO (781k 10 second avi)
Figure 4 shows solder in the filled cavities of a cooled mold.The filled mold may be stored indefinitely at room temperature in dry nitrogen until needed.
Figure 4. Solder filled mold cavities.
Eventually, the filled mold and the wafer are aligned and brought together in a furnace, where the bumps depart from the mold to their new home on the wafer. Figure 5 shows a portion of a bumped wafer with lead-free bumps after transfer.
Figure 5. Portion of bumped wafer after transfer.
FOR MORE INFORMATION
All photos and the video appear with permission of IBM Research. More information and graphics for IMS are on the IBM Research web site at IBM Research
A paper detailing the development of IMS by IBM Research is at IBM Journal
Information about the capabilities and performance of the commercially available C4PN system may be found at SUSS MicroTec