Designing for System Scalability From 622 Mbps to 6.25 GbpsTeradyne Connection SystemsGautam Patel
RDC ConsultingRobert Cutler
2© 2005
AgendaRoutingMaintaining Differential ImpedanceAnti-Pad Shape & ResonanceBoard Material ConsiderationsUse of Back-DrillingConnector Selection
3© 2005
Implementation is the KeyMany Implementations That Function Acceptably at 622 Mbps Have Problems at 6.25 GbpsConsiderations Include:− Differential Trace Geometry
Stripline vs. MicrostripNeckdowns
− Maintaining Differential Impedance− Anti-Pad Shape & Resonance− Board Material Considerations− Use of Back-Drilling
4© 2005
Use of Microstrip Surface Layers
Microstrip Is Lossier Than Comparable StriplineMicrostrip Exhibits More Phase Dispersion Due to Differing Dielectrics− Board Material & Air
Microstrip Typically Requires ~10 w Spacing Between Pairs to Keep Coupled Noise <0.5%622 Mbps Signaling Could Tolerate More Noise, but at 6.25 Gbps, S/N Ratio Much More CriticalStripline Preferred at 6.25 Gbps
5© 2005
10W Spacing is Very InefficientA typical case involves 6 mil traces. Spacing to adjacent signals needs to be at least 50 mils.
FR4FR4
Not to scale!
> 50 6 > 50 6 > 50
Layer 2 ground planeLayer 2 ground plane
6.25G Diff Pair6.25G Diff Pair4
FR4 Surface FR4 Surface MicrostripMicrostrip StructureStructure
2
Adjacent signal
Adjacent signal
6© 2005
Typical Microstrip Implementation at 622 Mbps
31-Mil Spacing Between Pairs (6.5/10.5/6.5) Means Noise in Excess of 1.5%. (This is maximum backward crosstalk per a 2-D field solver).
7© 2005
Trace Neckdowns in BGA Pin Field Many CAD Designers Will Neck Down a Differential Pair in the BGA Pin Field to Accomplish Two-Track Routing
Significant Impedance Mismatch If Not Implemented Properly
8© 2005
Typical Neckdown at 622 Mbps Does Not Compensate for Impedance
3.75/7.25/3.75 Pair = 100Ω
3.75/4/3.75 Pair = 88Ω
3.75/4/3.75
3.75/7.25/3.75
9© 2005
“Closely Coupled” Pairs Must be Handled Properly
Closely-Coupled Differential Pair Must Maintain a Constant Space Between the P & N TracesIf Spacing Changes, So Does Differential ImpedanceAt 622 Mbps, Impedance Variation Can Typically Be Tolerated At 6.25 Gbps, Impedance Changes Will Degrade Signals Significantly
10© 2005
Typical Problem With Closely Coupled Pair
Trace Separates Around Pad
ZDIFF Increases
11© 2005
Pairs in Proximity to PWR/GND IslandsIn Many Designs, a Trace Pair Is Routed in Close Proximity to a Power or Ground IslandIt Is Critical That Spacing Between the Pair & Island Is Sufficient to Ensure That Differential Impedance Is Not Altered by the Effect of the Island
12© 2005
Typical Pair Routed Rear PWR/GND Island
Island Is Too Close to Trace Pair—Must Be at Least 20 Mils
13© 2005
Closely Coupled Pairs & SMAs
If a Closely Coupled Pair Splits to SMAs, Differential Impedance Will Increase SignificantlyTraces Should Widen After Split to Achieve a 50ω Single-Ended Impedance Thus, Differential Impedance in Both Sections Will Be 100ω
14© 2005
Closely Coupled Pair Splits to SMA Connectors
Traces Must Widen to 50ΩSE After the
Split to Maintain Impedance
15© 2005
Anti-Pad Shape at 6.25 Gbps is CriticalConnector Anti-Pads Are Not Critical at 622 Mbps At 6.25 Gbps, the Plated Through Hole Impedance, Return Loss & Resonance Will Significantly Impact Overall Signal IntegrityExpanded Anti-Pads Are Recommended for High-Performance Differential Connectors− Higher Resonant Frequency− Less Capacitance− Improved Return Loss
Removal of Non-Functional Pads Is Critical at 6.25 Gbps
16© 2005
Typical GbX Anti-Pad Design at 622 Mbps
Round Anti-Pads Around Each Pin Degrades Performance
17© 2005
Measured Resonance of Anti-Pad Diameters Need better legend
18© 2005
GbX Recommended Expanded Anti-Pad
Anti-Pad44 mils
Pad36 mils
TERADYNE GbX ANTI-PAD
35 mils
Covered by Teradyne Patent!
19© 2005
Simulated Resonance for Different Anti-Pads
Round Via With “Non-Functional”Pads in Place
“Ovalized”Anti-Pad & Non-Functional Pads RemovedRound Via With
“Non-Functional”Pads Removed
Highest resonance is preferred
20© 2005
Board Material Considerations FR4 May Or May Not Be Acceptable at 6.25 Gbps Factors to Consider Are:− Link Lengths− Trace Widths− Connector Interfaces− Board Thickness & Via Stub Issues− S/N Ratio Required− Device Parameters: Pre-Emphasis,
Equalization, Input Sensitivity, Etc.
21© 2005
6.25-Gbps Eye-Pattern Over 12" of FR4 & N4000-13SI & 2 GbX Connectors
FR4 N4000-13SIMeasurements Taken With a Min Stub
22© 2005
S-Parameter Measurements of GbX Over Various Lengths W/ Min Stub Over N4000-13SI
GbX Min stub over N4000-13SI
-70
-60
-50
-40
-30
-20
-10
050 500
950
1400
1850
2300
2750
3200
3650
4100
4550
5000
5450
5900
6350
6800
7250
7700
8150
8600
9050
9500
9950
1040
0
1085
0
1130
0
1175
0
1220
0
1265
0
1310
0
1355
0
1400
0
1445
0
1490
0
Frequency(MHz)
SDD
12 (d
B) 12in
24in30in36in
23© 2005
S-Parameter Measurements of GbX Over Various Lengths W/ Min Stub Over FR4
SDD12 GbX min stub over FR4
-140
-120
-100
-80
-60
-40
-20
0
50 500
950
1400
1850
2300
2750
3200
3650
4100
4550
5000
5450
5900
6350
6800
7250
7700
8150
8600
9050
9500
9950
1040
0
1085
0
1130
0
1175
0
1220
0
1265
0
1310
0
1355
0
1400
0
1445
0
1490
0
Frequency (MHz)
SDD
12 (d
B) 12in
24in30in36in
24© 2005
Loss Summary of FR4 & N4000-13SI
Length FR4 N4000-13SI Length FR4 N4000-13SI12in(305mm) min stub -4.9 -4.11 12in(305mm) min stub -8.58 -7.3812in(305mm) max stub -4.63 -4.73 12in(305mm) max stub -22.43 -17.0124in(610mm) min stub -8.07 -5.76 24in(610mm) min stub -13.49 -9.8424in(610mm) max stub -8.84 -5.77 24in(610mm) max stub -25.64 -20.0430in(761mm) min stub -10.1 -6.83 30in(761mm) min stub -16.04 -11.330in(761mm) max stub -10.8 -6.92 30in(761mm) max stub -29.3 -20.9136in(914mm) min stub -11.39 -8.09 36in(914mm) min stub -18.39 -12.6336in(914mm) max stub -12.36 -8.15 36in(914mm) max stub -30.62 -21.29
Length FR4 N4000-13SI12in(305mm) min stub -14.84 -13.1612in(305mm) max stub -21.66 -24.7424in(610mm) min stub -24.65 -17.3524in(610mm) max stub -33.97 -32.6730in(761mm) min stub -28.92 -20.1730in(761mm) max stub -39.09 -34.5736in(914mm) min stub -33.94 -22.6636in(914mm) max stub -43.32 -39.02
Freq. = 3.125GHzFreq. = 1.625GHz
Freq. = 6.25GHz
25© 2005
Laminate Cost Factor By Material Type
0123456
FR4FR4 H
i Tg
Turbo
Megtro
nN40
00-13
Getek
IS 620
N4000-
13 SI
Polyimide
N6000
LD 621
Tacpreg
N6000-
SIGete
k 2 TLX
R4350
PWB Price Ratio by Material Type
0.81.01.21.41.61.82.0
FR4
FR4 Hi T
gTu
rboMeg
tron
N4000-
13Gete
kIS 62
0N40
00-13
SIPolyim
ideN60
00LD 62
1Tacp
regN60
00-SI
Getek 2 TL
X R43
50
Material & PWB Costs by Material Type
26© 2005
Back-Drilling Not Required at 622 Mbps
Back-Drilling the Backplane Is Not Required at 622 Mbps Via Effects at 322 Mbps Are Typically Not SevereAt 6.25 Gbps, Back-Drilling Will Significantly Improve Eye Integrity
27© 2005
Stubs Add Delay & Rise-Time Degradation
62ps
77ps
99ps
80%
20%
50%
PTH Stub Tr (ps) (20-80%) Delay (ps)Trace Only 123 Reference15 mil Stub 124 6265 mil Stub 131 77
Full 225 mil Stub 169 99
28© 2005
Frequency Domain Analysis of a Plated Through Hole (Resonant Structure)
Bottom Signal Layer
Top Signal Layer
Top Signal Layer
Bottom Signal Layer
29© 2005
3.125 Gbps Example
Ff
.22" (5.6 mm)Stub
Reference Trace
.090" (2.3mm) Stub
7.0 dB1.2dB
1.2 dB0.5 dB
3.0 GHz1.5 GHz
Delta From Reference
Stub Depth0.090" (2.3 mm)0.220" (5.6 mm)
30© 2005
Backdrilling, A Solution to the Stub Effect
31© 2005
S21 of Back-Drilled BackplaneVia Stub About 72-Mils Long in 300-Mil Thick BackplaneResonant “Notch”Increases in Frequency to>8 GHz—Well Beyond Frequencies of Interest
32© 2005
Frequency Domain Analysis of Back-Drill Depths at 0.5mm Intervals
4.5mm0.180"
2.5mm0.100"
Stub Depth
33© 2005
Max Stub (4.5 mm) Min Stub (0.5 mm)
Stub-Effect Eye Pattern Analysis 2.5 Gbps FR-4 Over 12" (305 mm), NO Connector
34© 2005
Stub Effect Eye Pattern Analysis 5.0 Gbps FR-4 over 12" (305 mm), NO Connector
Max Stub (4.5 mm) Min Stub (0.5 mm)
35© 2005
Stub Effect Eye Pattern Analysis 12.0 Gbps FR-4 Over 12" (305 mm), NO Connector
Max Stub (4.5 mm) Min Stub (0.5 mm)
36© 2005
S-Parameter Measurements of GbX Over Various Lengths W/ Max Stub Over N4000-13SI
SDD12 of GbX Max Stub N4000-13SI
-90
-80
-70
-60
-50
-40
-30
-20
-10
050 500
950
1400
1850
2300
2750
3200
3650
4100
4550
5000
5450
5900
6350
6800
7250
7700
8150
8600
9050
9500
9950
1040
0
1085
0
1130
0
1175
0
1220
0
1265
0
1310
0
1355
0
1400
0
1445
0
1490
0
Frequency (MHz)
SDD
12 (d
B) 12in
24in30in36in
37© 2005
6.25 Gbps Eye-Pattern Over 24" of N4000-13SI & 2 GbX Connectors
Min Stub ~0.22" stub
38© 2005
Teradyne Back Drilling Experience
Reliability− Extensive Telcordia Testing for 20 Year
Service Life Indicated No Adverse Effect on Compliant Pin Gas Tight Interface
− Additional Testing Beyond Telcordia Could Not Make It Fail
Negligible Effect on Yield
39© 2005
7.5
8
8.5
9
9.5
10
10.5
11
11.5
12
12.5
First-I Second-I Third-I First-R Second-R Third-R
Forc
e, lb
s
Telcordia Group 1 Results: Compliant Pin Insertion-RetentionCompliant Pin Performance Testing Shows No Adverse Effects on the Back Drilled Hole After 3 Insertions & Retentions
88 Insertion Data Points66 Retention Data Points
40© 2005
Assumes All Boards Are FR-4 Material
Back-Drilling Costing Examples
9%7%6%Back-Drilling Cost Increase %
$30$500
00
50020
0.250Example 1
$140$50Back-Drilling Charge$1500$750Rough PWB Price10000Depth 3 / Hole Count
Example 3Example 2
2000500Depth 2 / Hole Count30001000Depth 1 / Hole Count4030Layer Count
0.3500.300PWB ThicknessAttribute
41© 2005
Connector SelectionConnector Performance Requirements for 622 Mbps Are Vastly Different Than for 6.25 GbpsAt 622 Mbps, Risetime of the Signal Is Around
1 ns, Which Typically Exceeds the Electrical Length of the ConnectorConnector Crosstalk & Reflections Become a
Much Larger Issue at 6.25 Gbps Than at 622 Mbps—This Needs to Be Considered When Making a Connector Selection
42© 2005
Connector Selection
43© 2005
Connector Selection• Crosstalk Will Also Become An Issue at 6.25-Gbps Data Rates• Unfortunately, a Connector That Had Acceptable Performance at 622 Mbps May Be Inadequate at 6.25 Gbps
Differential Backward X-talkRow 100ps 1nsAB 4.40% 0.70%CD 4.50% 0.90%EF 4.20% 1.00%GH 4.10% 0.80%
44© 2005
Connector Selection• Ideally, Choose a Connector Family That Is Flexible, Cost Effective & Has the Performance Desired at Both High & Low Speeds• A Connector Family That Offers Density, Performance & the Ability to Mix & Match Low- & High-Speed Signals Is Preferred
45© 2005
Hybrid Connector Solution
Design Any Combination of the VHDM® Product Family on One Stiffener to Create a Customized Board Slot− VHDM L-Series: Optimized
Solution for Low-Speed Connections
− VHDM: For Single-Ended, High-Density Connections
− VHDM H-Series: High-Density, High-Speed With Greater Signal Integrity
− VHDM-HSD™: For High-Speed Differential Signals
Optimize Cost & Performance Within a Single Connector
VHDM L-Series
VHDM
VHDM H-Series
VHDM-HSD
46© 2005
SummaryImplementation Is the Key to System ScalabilityBack-Drilling Provides Best Cost/Performance Balance & Has Become Accepted Practice Due to Its High ReliabilityFlexibility Within a Connector Family to Allow for Both High- & Low-Speed Signals
47© 2005
Teradyne Connection SystemsResources
Complete Product Libraries on Our Website at www.teradyne.com/tcs− High-Speed, High-Density Connectors− High-Performance Circuits− Backplane Systems
White Papers− Practical Guidelines for the Implementation of Back
Drilling Plated Through Hole Vias in Multi-Gigabit Board Applicationsby Tom Cohen, Teradyne Connection Systems