High Productivity Thermo-Compression Flip Chip Bonding

Abstract

Advancements in electronic packaging performance and cost have historically been driven by higher integration primarily provided by fab shrinks that has followed the well-known Moore's law. However, due to the tremendous and continuously increasing cost of building new fabs, the performance/cost improvements achieved via node shrinks are negated. This leaves packaging innovation as the vehicle to achieve future cost-performance improvements. This has initiated a More-than-Moore idea that has led to vigorous R&D in packaging. Advanced packages which employ ultra-fine pitch flip chip technology for chip-to-substrate, chip-to-chip, or chip-to-interposer for the first level interconnect have been developed as an answer to obtaining higher performance. However, the costs are too high as compared to traditional wire bonding. The status today is that the fundamental technical hurdles of manufacturing the new advanced packages have been solved, but cost reduction and yield improvements have to be addressed for large-scale adoption into high volume manufacturing. In traditional flip chip assembly silicon chips are tacked onto a substrate and then the solder joints are melted and mass reflowed in an oven. This mass reflow technique is troublesome as the pitch of the solder bumps become finer. This is due to the large differences in the thermal expansion coefficient of the die and the substrate, which creates stress at the solder joints and warpage of the package when the die and substrate are heated and cooled together. To mitigate and resolve this issue, thermo-compression bonders have been developed which locally reflow the solder without subjecting the entire substrate to the heating and cooling cycle. This requires that the bondhead undergo heating past the melting point of solder and then cooling down to a low enough temperature to pick the next die from the wafer that is mounted to tape. Machines in the market today can accomplish this temperature cycle in 7 to 15 seconds. This is substantially slower than the standard flip chip process which leads to high cost and is delaying the introduction of these new packages. This paper shows a flip chip bonder with a new heating and cooling concept that will radically improve the productivity of thermo-compression bonding. Data and productivity cycles from this new bond head with heating rates of over 200°C/sec and cooling of faster than 100°C/sec are revealed. Experimental results are shown of exceptional temperature accuracy across the die of 5°C throughout the cycle and better than 3°C at the final heating stage. The high speed thermo-compression bonds are analyzed and the efficacy of the new concept is proven. Excellent temperature uniformity while heating rapidly is an absolute necessity for enabling good solder joints in a fast process. Without good temperature uniformity, additional dwell times need to be incorporated to allow heat to flow to all of the joints, negating any benefits from rapid heating. Whereas the current state-of-that-art is often to program temperature in steps, this bonder can be commanded and accurately follows more complex temperature profiles with great accuracy. Examples of how this profiling can be used to enhance the uniformity and integrity of the joints with non-conductive pastes, film, and without underfill along with the associated productivity improvements will be shown. Tests that show portability across platforms that will lead to set up time and yield improvements and are identified and quantified. Additionally new ideas for materials and equipment development to further enhance productivity and yield are explored.