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Signal-Integrity Measurements Support the Candidacy of PTFE at High Data Rates
A growing need is developing in the high speed digital arena (backpanels, motherboard, line cards etc) for high
performance laminate, prepreg materials, connectors, intelligent routing of differential pairs, and other strategies
to improve signal integrity at the 10 gbps range. Polytetrafluoroethylene (PTFE) has a long history of meeting the
needs in radio frequency applications up to 77 GHz. PTFE based materials reinforced with woven glass fabric have
traditionally offered the advantage of very low loss (<0.003 @ 10 GHz). The laminate and prepreg material that will
be used as a baseline for signal integrity measurements (SI) is a PTFE/fiberglass/BT-epoxy composite (TacPreg TM )
1 that offers a dissipation factor from 0.004 to 0.005, depending on BT-epoxy resin content, at 14.5 GHz.
Described in this paper are SI measurements carried out using standard practices that rely on eye openings and
jitter using a 20 layer 170 mil thick backplane test vehicle. The signal integrity of the test vehicle is dependent
on the total system, the FR4 daughtercards, the SMA connectors, and Teradyne's GBX TM connectors 2.
The future of design is taking some tips from microwave engineers where insertion loss, (loss in signal in dB/inch)
is routinely characterized. The PTFE/fiberglass/BT-epoxy laminate was evaluated in a separate test vehicle with no
daughter cards to best characterize the laminate under test. The loss in power (dB) for different trace lengths was
then investigated at different frequencies. All the data suggests that the combination of a high performance laminate
in a properly designed system with the optimal connectors will offer superior performance at 10 gbps and higher.
Preliminary data suggests that the laminate material will cost 4.0x FR4 and finished printed circuit boards will
cost OEM's 1.6-1.9x FR4, depending on volume. Preliminary processing information further suggests that although
there are a few steps such as plasma treatment of drilled holes before electrolysis, the PTFE/fiberglass/BT-epoxy
processes similar to BT-epoxy with a few changes in fabrication that must be made for the high PTFE content.
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