Superconducting Technology Assessment - nitrd

Lead Inductance (pH)
1,000
100
10
TRW
Bonded
Interconnect
Pin (BIP)
UNIAX
Solder Bump
(Flip-Chip)
Wirebond
Figure 3. Lead inductance vs. bond length or height for different attach techniques.
Use of RSFQ logic circuits offers the advantage of ultra-low power, but the signal levels are low, so chip-to-chip
bandwidth is limited unless on-chip driver circuits are used to boost the signal level. Such off-chip drivers need to
match the impedance of the driver circuits and the transmission line interconnects. To date, 10 Gbps asynchronous
off-chip drivers have been demonstrated. Alternatively, simple synchronous latches may be used. With the planned
reduction in feature size and increase in J c to 20 kA/cm 2 , it is not difficult to envision that SCE chips may communicate
across an MCM at 50 Gbps in the timeframe required. Further work must be done to address the area and power
overhead of these driver circuits. Direct SFQ-to-SFQ signal transmission, which would greatly simplify chip floor
planning and signal routing needs development
2) MCM-to-PCB Connection: Three methods of high data rate motherboard-daughterboard connections have
already been developed:
■ A connector developed for the IBM superconductor computer project.
■ Another connector developed by IBM for commercial applications using
a dendritic interposer technology.
■ A “beam-on-pad” approach developed by Siemens.
Multilayer
TAB
Single
Metal
Layer TAB
HTMT
Requirement
1
.1 1 10 100 1000
Bond Length or Height (mils)
Although these three techniques are not immediately applicable due to either low interconnect density or low
bandwidth, they serve as conceptual springboards for new designs. The main issue with the PCB to MCM
interface is that approximately >500 Multi-Gbps lines must connect in the cross-sectional area of 50 mm 2 , or 0.1
mm 2 per pin. For reference, a typical commercial “high-density,” low-speed connector has pins spaced on a 1.25
mm pitch for a density of 1.5 mm 2 per pin; a factor of 15 too large.
3) MCM-to-MCM Connection: The method of routing is to use short flexible ribbon cables, similar to those being
used for the I/O cables. The density requirement of 2,000 per edge is considerably less than either for the external
I/O cables or for the MCM-to-PCB. Given the length along the MCM bottom and top edges of 20 cm, the signal
line pitch would be 4 mils, a density that is available today from flex circuit vendors.
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