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