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Abstract
Signal-Integrity Measurements Support
the Candidacy of PTFE at High Data
Authors:
Meagan Morrell, Amphenol TCS
Thomas F. McCarthy, Taconic
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
(TacPregTM)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 Amphenol's
GbX connectors2. 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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