Development of Wafer Scale Applied Reworkable Fluxing Underfill for Direct Chip Attach, Part II

摘要

Manufacturers of consumer electronic products are continuously striving tc confer greater functionality to smaller, lighter, and less expensive packages, and flip chip is an important enabling technology for these product trends. Underfill between the die and an organic substrate is necessary to compensate for the coefficient of thermal expansion mismatch. The underfill dispense step is not a typical process for an SMT factory, and demands additional capital equipment, floor space, cycle time and headcount. These additional investments, plus the inability to rework underfilled parts has limited wide scale implementation of flip chip technology. The National Institute of Standards and Technology Advanced Technology Program "Wafer Scale Applied Reworkable Fluxing Underfill for Direct Chip Attach" (NIST-ATP WARFU) was established to investigate the development of a flip chip underfill process that would be transparent to the SMT line. The program is supporting the successful development of fluxing, reworkable underfill materials and processes for direct application of the materials to die at the wafer level. While the application of underfill materials directly to the Wafer seems straight forward, many of the material requirements are incompatible with each other. For instance, the necessity of dicing the wafer using water is not compatible with the use of uncured epoxy materials. In addition, the incorperation of fluxing materials into the bulk underfill is known to degrade long-term stability at room temperature. This needs to be addressed, as the stated goal of the program is to provide at least 6 months of on part life prior to use. The methods and materials used to address these concerns are described in the paper.