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A Wirebonding Instrument for Insulated and Coaxial Wires
Keywords: coax, wirebond, interconnect
Title: A Wirebonding Instrument for Insulated and Coaxial Wires Session: Flip Chip / 2.5D / 3D (Advanced Packaging) Track Topic Area: Materials/Process Keywords: coax, wirebond, interconnect Primary Author: Mitchell Meinhold | | phone: 617-258-4116 Affiliation: Draper | 555 Technology Square, Cambridge MA, 02139 Associated Authors: Caprice Gray, Jeffery B. DeLisio, Ernest Kim, Peter H. Lewis, Daniela A. Torres Abstract In the past decades, there has been growing demand for novel packaging and integration strategies in the electronics industry across the domains of logic, RF, analog, imaging and photonics. Examples of market drivers include applications that require heterogeneous integration such as communication, photonics integration (e.g. transceivers), high density imaging devices and mobile technologies [1][2]. Other market forces such as AI and big data drive high performance computing hardware where high bandwidth logic chips are placed on a shared interposer [3][4]. These developments are pushing the limits of packaging technologies in which conflicting demands pose challenges: low cost, small volume, high heat dissipation, high density interconnect, high bandwidth and high power. Across numerous packaging approaches, wirebonding remains a core technology due to its reliability, maturity and cost [5]. We posit that it will be highly enabling to utilize insulated wire and coaxial wire for system-in- package (SIP) interconnection in many current and future use cases [6][7]. Insulated wires have use in designs requiring a large number of interconnects that are likely to contain cross-over wires, as well as scenarios in which die are placed at varying heights or stacked in a heterogeneous assembly [8]. Coaxial wirebond interconnects are a new paradigm allowing the designer to interconnect high speed bus, shielded RF signals, low inductance power lines and tackle other unique design problems where traditional wire or ribbon fall short. This paper will present a novel system that can feed, strip and bond insulated wire and coax in a size regime of 2-4mil (50-100m) overall diameter. Supporting algorithms that drive the robotics and sequence the wire placement will be discussed. Details of the wires that are designed and fabricated specifically for this application will be presented followed by design details and characterization of the performance of the automated tooling that installs these wires. Generally, the wires designed for this tool are engineered to provide good electrical characteristics including DC isolation, low loss, low radiated emission, low cross-talk and low inductance for power distribution applications [9]. At the same time, wire designs are optimized to facilitate insulation and shield stripping by the tool. The system utilizes a thermal reflow stripping method to reflow and clear polymer insulation films. It uses an electric flame off (EFO) system to strip metallic shield films [10]. In the current design, the system integrates these two stripping mechanisms in a single machined ceramic block. Process control for the EFO step is accomplished with existing state-of-the-art negative EFO power supplies which control time and power. In our system, power is set to accommodate varying metal thicknesses while the EFO duration achieves variation of stripped distances. In order to control thermal stripping characteristics, the coax insulator is set in contact with the heater filament. The filament is driven to a set current for a set time which reflows the polymer dielectric forming a cleared location on the wire where the core metal can be bonded. A small inert gas manifold is used to flood the chamber in which thermal and EFO stripping takes place. It is found that gas flow is an important variable that strongly influences the power required for thermal reflow stripping as well as having an effect on oxidation and lifetime of the heater element. Polymers used in our coax include Parylene, PFA, polyurethane and polyethylene. Polymer thickness ranges from 1 to 40m. Wire stripping can occur at either a wire tip or along the length of the wire. Shield metal and insulation stripped at a location other than the end of the wire is termed “mid-wire stripping.” Mid-wire stripping facilitates a continuous feed wiring system because the wire does not have to be broken in order to be stripped. This continuous feed approach promotes a simplified automation design. A rotary wire feeding mechanism is used to advance the wire such that the stripped locations are presented to the bonding tool at the appropriate location. Computer control is required to synchronize wire feed with both the bonding and stripping locations. This is integral to how mid-wire stripping is leveraged to enable continuous wire feed. The bonding itself is performed by a capillary or wedge tool that is able to accommodate the portions of the wire having an enlarged diameter resulting from the reflow process. The exposed core metal, (typically copper ranging from 10 to 25 microns), is thermosonically bonded. It has also been shown that the shield of our wires can be thermosonically bonded without significant degradation to the insulation and core structure. This approach generally requires that each signal pad have a ground pad in its vicinity. Alternatively, once the core is bonded, wire installation can proceed to a separate tool, such as a solder jet system, for shield attach [11]. For complex and high-count wire installations, a software-based workflow is implemented. It begins with an electrical schematic and netlist followed by a CAD description containing component dimensions, positions, pad locations and the electrical net description of each pad. Several algorithms are then employed to not only specify positions of the wire but also the order in which they must be installed so as to prevent an installed wire from blocking pad access of a subsequent wire placement. Additionally, the electrical requirements of each connection are assessed such that the appropriate type of coax is used for each instance [12]. This is referred to as “net class partitioning”. For example, 50 coax is generally implemented for high speed signals. In contrast, wire optimized for low inductance and high current is used for distributing power. In order to quantify the performance of the system and assess next steps in development, characterization of bond strength, wire strip gap length repeatability and installed wire-length repeatability will be reviewed. While bond strength is important for reliability, wire strip gap control and wire-length precision are important for demanding applications requiring wire length matching such as LVDS systems or tuned RF transmission lines. References [1] H. Reiter. “Building an EcoSystem for User-friendly Design of Advanced System in Package (SiP) Solutions,” International Symposium on Microelectronics, Raleigh, NC, Fall 2017, Vol. 2017, No. 1, pp. 000083-000086. [2] J. Fahey. “The Evolution of Electronic Materials and The Information Age,” iMAPS Inaugural System in Package Conference, Sonoma, CA, June 2017. [3] Integration schemes and enabling technologies for three-dimensional integrated circuits Chen,K.N. et al. IET Computers & Digital Techniques(2011),5(3):160 [4] R.S. Patti, Three-Dimensional Integrated Circuits and the Future of System-on-Chip Designs, Proceedings of the IEEE, Volume: 94 , Issue: 6 , June 2006 [5] C. Palesko, E Vardaman, Cost Comparison for Flip Chip, Gold Wire Bond, and Copper Wire Bond Packaging, 2010 Electronic Components and Technology Conference, Pg 10-13 [6] C. Gray. “Interconnect Scheme for SiP Devices using Micro-Coaxial Cables”. iMAPS Inaugural System in Package Conference, Sonoma, CA, June 2017 [7] C. Gray, et al. “Wiring System.” U. S. Patent Application 15,592,694, International Application PCT/US17/32136, 11 May 2017. [8] Z.W. Zhong Wire bonding using insulated wire and new challenges in wire bonding Microelectronics International Vol 25 No 2 (2008) Pg 9–14 [9] D. A. Torres, A. Kopa, S. Barron, R. McCormick, R. D. White and C. Gray, “Characterization of Low Inductance Micro-coaxial Cables for Power Distribution,” iMAPS Journal of Microelectronics and Electronic Packaging, vol. 15, no. 4, 2018. [10] C. Wells, et. al., Micro-Coaxial Cable Stripping with Electronic Flame-Off Process, Poster, iMAPS New England 45th Symposium & Expo, May 1, 2018. [11] Z. Luo, Z. Li, X. Wang and W. Li,
Mitchell Meinhold,
Draper Laboratory
Cambridge, Massachusetts
United States

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