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Discontinuity Cancellation to Boost Package Bandwidth up to Material's Limitations - 40-Gbps Package Design using Wire-Bonded PBGA
Keywords: 40-Gbps package design, discontinuity cancellation, signal integrity
Processors’ increasing computational capability is driving a need for high-speed links to communicate the processed information. Today’s internal circuits can run at tens of Gbps, but the channel bandwidth limits link performance. Wire-bonded plastic ball grid array (WB-PBGA) is the most popular package for cost-effective conventional mid-speed applications. Previous work has studied the use of WB-PBGA packages for up to 10-Gbps data rates. As data rates continue to increase, transitioning to flip-chip interconnects or low-loss substrate materials results in excessive cost. This paper presents a 40-Gbps packaging solution that uses low-cost WB-PBGA technology. Since such a high speed is beyond the reach of conventional package design, new design methodology is proposed – discontinuity cancellation in signal-current paths and return-current paths. The design methods of bonding wires, vias, ball pads and power distribution networks are suggested, and the effect of each design method is examined. Two versions of four-layer WB-PBGA packages are designed, one according to the proposed methodology, and the other conventionally. The proposed package design is successfully verified with both simulation and measurement. The insertion loss of the conventional design is measured above 10 dB at 30 GHz, and even a resonance occurs at around at 23 GHz. However, the proposed design’s insertion loss is below 3.5 dB up to 30 GHz. Furthermore, there is no resonance, meaning that the signal does not feel any sever discontinuity when passing through the package. Measured time-domain reflectometry (TDR) waveforms also clarify the proposed design’s remarkably enhanced performance over the conventional design. Above all, measured 40-Gbps eye diagrams present a decisive demonstration that the proposed design methodology achieves 40-Gbps data rate. By avoiding the use of low-loss dielectric material and/or advanced packaging technology, the result of this research can provide a low-cost packaging solution for future high-speed serial links.
Dong Gun Kam, Postdoctoral Researcher
IBM T.J. Watson Research Center
Yorktown Heights, NY

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