204 PI3HDMIxxx 4Layer PCB Layout Guideline for HDMI Products Introduction The differential trace impedance of HDMI is specified at 10015 in Test ID 88 in HDMI Compliance Test Specification Rev12a and 13a Rev 13a has a more relaxed specification which allows an occurrence of a single excursion out to a maxmin of 10025 and of a duration less than 250ps Generating PCB layout with care can compensate or prevent from significant deviation from desired impedance General PCB knowledge PCB Pr......
204
PI3HDMIxxx
4-Layer PCB Layout Guideline for HDMI Products
Introduction
The differential trace impedance of HDMI is specified at 100Ω±15% in Test ID 8-8 in HDMI Compliance Test
Specification Rev.1.2a and 1.3a. Rev. 1.3a has a more relaxed specification which allows an occurrence of a single
excursion out to a max/min of 100Ω±25% and of a duration less than 250ps. Generating PCB layout with care can
compensate or prevent from significant deviation from desired impedance. General PCB knowledge (PCB Prepreg
Selection, Critical Signal Trace Length and Microstrip Stub Effect) and impedance control on 4-layer PCB of
1080+2116 Prepreg will be discussed in this layout guideline.
PCB Prepreg Selection
Although 4-layer FR4 PCB with 2116 (high-density) Prepreg is popular for PC motherboard, it will be costly if
impedance control is required and its insertion loss is not acceptable. Therefore, it is not recommended in HDMI
applications. Instead, Prepreg of 1080+2116 (mid-density) or 2116+7628 (low-density) is recommended. With
different Prepregs, trace width and spacing have to be adjusted accordingly. Recommended dimensions are stated
below.
For 1080+2116 Prepreg:
(a) Stack Up
Signal
Prepreg
1080+2116
Gnd
Core
Power
Prepreg
1080+2116
Signal
1.9 mil
7.3 mil
1.2 mil
44 mil
1.2 mil
7.3 mil
1.9 mil
For 2116+7628 Prepreg:
(a) Stack Up
Signal
Prepreg
2116+7628
Gnd
Core
Power
Prepreg
2116+7628
Signal
1.9 mil
11.6 mil
1.2 mil
31 mil
1.2 mil
11.6 mil
1.9 mil
(b) Isolation Spacing
30 mil
(c) 100Ω Differential Trace Width/Spacing
W/S = 8.0 / 9.0 mil
or 9.0 / 11 mil for long traces
(d) 50Ω SE Trace Width
W = 12 mil
(b) Isolation Spacing
45 mil
(c) 100Ω Differential Trace Width/Spacing
W/S = 8.5 / 6.5 mil
or W/S = 12 / 7.0 mil for long traces
(d) 50Ω SE Trace Width
W = 20 mil
Table 1: PCB Prepreg Recommendation
Normally, PCB manufactories control width and spacing of traces at
±1-mil
tolerance. At the locations of HDMI
connector, components and IC devices, the tolerance is best adjusted to
±0.5mil
so as to reduce deviation. The trade
off is cost.
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204
Critical Signal Trace Length
To prevent from signal reflection, signal trace length cannot be longer than the following two critical length limitations:
(a) 1/16 wavelength of Signal,
λ;
the relationship between signal wavelength and signal frequency is defined as
where
ε
R
= 4.3 ~ 4.7 and
μ
R
~ 1 for FR4 material.
For a 1.25GHz signal running on FR4 material, critical length limitation is determined as
Critical Length < (1/16)λ
≅
280 mil;
and
(b) 1/3 length of rising edge of signal; the length, l, is defined as
where l = length of rising edge of signal in inch
Tr = rise time of signal in ps
D = delay in ps/inch
The delay of FR4 material is 180 ps/inch. As Tr for HDMI signal is 200ps, signal speed cannot exceed 370 mil
which is derived from
1
Critical Length <
3
×
200
ps
×
1,000
mil
/
in
= 370 mil.
180
ps
/
in
If the signal traces are long, it is recommended to use wider differential trace width and spacing since the impedance
is more consistent and easier to be controlled if the width is increased. Please refer to Table 1 for detailed width and
spacing selection.
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204
Microstrip Stub Effect
As stub adds reactance to PCB trace and lowers the trace impedance, it is not preferable to create any stub on HDMI
layout. If an open stub is 1/2 wavelength long, it seems to have a shunt capacitor shorted to Ground on the trace.
Likewise, if a short stub is 1/2 wavelength long, it seems to add an inductor or an open on the trace.
Figure 1: ½λ Open Stub
Figure 2: ½λ Short Stub
If it is unavoidable, stub must be kept shorter than 1/6 length of rising edge of signal. In practice, for a 200-ps HDMI
signal, the length of stub cannot be longer than 1/6
×
200ps = 33ps.
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AN204 06/27/07
Pericom Semiconductor Corp.
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6/27/2007
204
Compensation of Solder Pad / Via
Solder pad and via create discontinuities of traces. As a result, the trace impedance will be lowered. Creating holes on
ground plane of PCB under these components can minimize the capacitance between pad/via and ground plane so
as to compensate the impact on trace impedance. Dimensions of holes are recommended in (i) – (iv) in Section (A).
The HDMI connector pads are designed to interact with connector pins so as to produce desired trace impedance and
create robust signal paths. In order to achieve these, the reference plane, which is the ground plane for HDMI
connector in the example in (v) in Section (A), should be removed immediately under the solder pads.
A case study of restricted PCB area is provided in Section (B) for reference.
Section (A): Recommended Dimensions of Holes on Ground Plane
A PCB of 1080+2116 Prepreg is used as an example below. Differential trace width and spacing of 8.0 and 9.0 mil are
employed. The dimensions of holes on the ground layer are drawn.
(i) Under 0603 ESD / Pull-up Resistor Pads
Figure 3: Layout Recommendation of ESD / Pull-up Resistors in 0603 Package
(ii) Under 0402 ESD / Pull-up Resistor Pads
Figure 4: Layout Recommendation of ESD / Pull-up Resistors in 0402 Package
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204
(iii) Under Solder Pads of HDMI Switch, e.g. TQFN-56 below
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