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The Emergence of Local Reflow into High Volume Manufacturing
Keywords: Thermocompression, Equipment, Process
High volume use of thermocompression bonding in BEOL semiconductor assembly processes has been actively discussed for several years but has only recently achieved widespread adoption. The known benefits of the technology, such as reduced form factor, reduced power and high speed have not offset the higher cost associated with the process due to equipment UPH and yield loss. The wait and see status of TCB processes is changing now that TCB processes are ramping for production of stacked memory products. The TCB process is described as a Local Reflow (LR) process since only the die in process is heated to solder reflow temperature. It is differentiated from a Mass Reflow (MR) process where the entire assembly is heated to solder reflow temperature. The LR process is the only assembly process that enables TSV die with Cu pillars to achieve a 16 die stack with individual die thickness <50um. The process is able to maintain excellent control of both Bond Line thickness and Solder Line thickness throughout the entire stack. The LR aspect of the process also offers a compelling solution for assembly of stress sensitive single die onto laminate since the thermal expansion of the substrate is minimized with respect to the silicon. Slow adoption of LR assembly processes can be attributed to two significant factors. The first factor is Cost as compared to alternative technologies, wherein the known technical benefits of the technology do not offset the higher cost. In the case of Local Reflow, cost is primarily driven by the UPH of the equipment. A UPH of 1000 has long been considered as an industry threshold to make the process commercially viable. Current LR equipment is now able to achieve that for most processes and substantially exceed it in many processes. The second factor is Process Control. The complexity and difficulty of the process create an inherent risk for undetected yield or reliability issues in the assembly process. The equipment must have exceptional process control and programmable control limits on multiple factors to ensure any process deviation is detected quickly and product risk is controlled. The equipment must also employ full data logging capability including traceability for all die back to the silicon source wafer. Recent advances in LR equipment offer solutions for all these issues. The presentation will discuss the state of the art equipment and the methodologies that have enabled lower cost and reliable performance.
Thomas Strothmann, Director Advanced Packaing Business Line
Kulicke and Soffa
Fort Washington, PA

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