PCB design is always changing, and not for the easier. You’re constantly being asked for more; more tech, more productivity, more efficiency. Except for when you’re being asked for less; less cost, less time, less space. Managing all these design requirements at once can sometimes seem like an impossible task. Get help from the PCB design experts at EMA with the tips and tricks you need to achieve highspeed PCB design success.

1. 1
EMA Design Automation
Common High-Speed Design Issues and
How to Solve Them
10/16/2018
Real-Time Design:
Address Errors As They Occur
1
Jerry Long
Senior Applications Engineer

2. 2
Defining the High-Speed Problem
When designing “high-speed” circuits why can’t I…
Keep Projects
On Schedule?
Avoid Late
Stage Changes?
Keep Build Costs
Predictable?

3. 3
Defining the Problem
 What does “High-Speed” mean?
− When the “path delay” of a signal is long compared
to the rise-time of the conducted signal, it’s considered High-Speed
− Practically every design today has some aspect of
high-speed behavior involved
 What problems do High-Speed signals present?
− Signal “noise”
− Destroys signal quality
− Promotes EMI effects (electro-magnetic radiation)
− Signal Timing
− Signals aren’t arriving at their destinations when required
High-Speed Design Issues

4. 4
Defining the Problem
 Signal noise is considered to be anything that is undesired or
not intended
− Signals need to be interpreted as “ones” and “zeros”
− Noise tends to “muddy the waters”
Signal Noise
Want this…
…but get this…
…or worse!

5. 5
Defining the Problem
 Typically destroys signal quality
 Noise is also unwanted signal radiation (EMI)
Signal Noise
!

6. 6
Defining the Problem
 Every signal needs to arrive at it’s destination
within some specified time frame
 Typical timing requirements involve times for:
− Signal arrival
− Signal remaining stable
− Signal state to be recognized
Timing
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QD
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Cloud
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tpd
tsu/th

7. 7
Defining the Problem
 Today’s design requirements and time-to-market demands
don’t allow for open-ended build schedules
 Therefore it is EXTREMELY important to find problems as
early as possible to prevent:
What are the Costs?
Costly Re-spins
Missed Time to
Market Schedules
Increase MFG
Costs
Project Delay
Ripple Effect

8. 8
A Closer Look – Causes of High-Speed Problems

9. 9
Causes of High-Speed Problems
 Most of the problems from high-speed circuits can be
attributed to any combination of 3 subject areas
− Impedance mis-matches
− Coupling issues between adjacent “fast” signals
− Return path discontinuities
Impedance, Coupling, Return-Path

10. 10
Causes of High-Speed Problems
 Consistent impedance is VERY important for signal quality
 Each segmented entity within a signal trace can have drastic
changes in impedance values and cause a variety of
problems if not consistent
− Signal ringing
− Overshoot & Undershoot
(multiple threshold crossings)
− Reflections
Impedance Mismatches

11. 11
Causes of High-Speed Problems
 Impedance mismatching can also affect timing of your signals
− The time required for the signal to reach it’s destination is directly
related to the physical length of the trace as well as impedance
− Accumulated delay
caused by propagation,
reflections, and
distortions along the
signal path reduce
available margins
Impedance Mismatches
D
Clk
QD
Clk
Q D
Clk
QD
Clk
QLogic
Cloud
Logic
Cloudtco
tpd
tsu/th
Cl
k
Q
accumulated
delays
accumulated
delays
required
timing
required
timing
resulting
margin
(slack)

12. 12
Causes of High-Speed Problems
 Now that I know I have impedance mismatches how do I
resolve them?
− Locate and move problematic segments
− Resize trace widths
− Alter stack-up dimensions
 Typically a costly solution if the design has progressed to far!
Impedance Mismatch Resolutions

13. 13
Causes of High-Speed Problems
 Coupling issues are a function of signal “edge” speed and the
proximity of each signal trace to one another.
− Signal transitions make things “interesting”
− Fast signals are the aggressors and can “victimize” ANY adjacent trace
− Long parallel runs are the easiest victims
− Cause false switching points and unwanted noise
Coupling Issues

14. 14
Causes of High-Speed Problems
 Stack-up can also affect coupling issues
− Typical stack-up with minimal impact on coupling
− But what if stack-up looks something
like this…
− This situation is not so obvious
to diagnose
Coupling Issues
5mil 5mil15mil
3.5mil
5mil 5mil15mil
3.5mil
35mil

15. 15
Causes of High-Speed Problems
 Resolving incidents of coupling can be costly as well!
 Usually involve:
− Trace-to-trace spacing too narrow – have to move one of the traces
− Too long of a parallel routing length – disrupt the parallelism
− Stack-up characteristics that promote it – alter the stack-up
− Signal edges too fast for board design techniques – slow down the
signal edges if possible
Coupling Resolutions

16. 16
Causes of High-Speed Problems
 Return paths are very important to signal quality
− Can affect trace impedance (reference for signal path)
− Signal currents will use path of least resistance to return to the source
− Split planes create gaps and return-path discontinuities
− Any return path discontinuity will prevent current flow and effectively
lengthen the return-path.
Return-Path Discontinuities

17. 17
Causes of High-Speed Problems
 Return paths are very importance to signal quality
− Discontinuous return paths can also generate unwanted radiated signal
emissions from your board (EMI)
Return-Path Discontinuities
GND12V
3.3v
5v
Impedance mismatch 1
Impedance mismatch 2
Impedance mismatch 3
Impedance mismatch 4
Radiation Radiation
5 milTOP Bottom
Radiation

18. 18
Causes of High-Speed Problems
 Solving return-path issues usually involve:
− Adding more reference planes (simple cases)
− Moving power/ground planes around to avoid breaks under signal
paths
− Altering signal paths to avoid discontinuity
Return-path Discontinuity Resolutions

19. 19
Causes of High-Speed Problems
 Prevalent timing
issues due to:
− Phase mismatching
(diff-pairs)
− Propagation delays
(total & relative)
− Impedance
mismatching
− Rise & fall time
distortions
− Threshold crossing
distortions
Impact to Signal Timing Requirements

20. 20
Causes of High-Speed Problems
 Must solve impedance related problems
− Board alterations (see previous impedance discussion)
 Make sure complete delay tracking on the path
− Accounting for pin delays (die to package pin)
− Accounting for any z-axis delay (via delays)
 Resolve any phase issues with differential clocking signals
− Lead to severe timing issues by distorting reference clocks
Timing Specification Resolutions

21. 21
Solutions for High-Speed Problems

22. 22
Solutions for High-Speed Problems
 Most designers employ a set of “Rule-of-Thumb” guidelines
when dealing with PCB design
− Usually a good starting point
− But are no guarantee to find related problems
− Typically have no ‘vision’ or feedback with regard to
− Impedance
− Coupling
− Return-path reference
− Signal Timing
 Need more conclusive analysis during design cycle
Is Rule-of-Thumb good enough?

23. 23
Solutions for High-Speed Problems
 Your Interconnect Is Interconnected
− Designs are based on a compounding set of decisions
 When one decision (especially a critical one) needs to be
changed late in the process this can have a ripple effect
throughout the design
The cumulative effect of design errors
Impedance issue
identified late in
design cycle
…
High Speed Route
IC Placement
based on route
channel
Via added to
accommodate
route changes to
placement
Backdrill required
(cost adder)
PI Issue due to vias
Decaps added to
compensate
Other routes
shoved
Timing issue
created
Timing issue fixed
by moving another
component
DFA issue created
Escape via pattern
needs to be
changed
Decap needed to
be moved
PI problem created
PI problem fixed
with more decaps
Re-evaluate?

24. 24
Wouldn’t it be better to know what potential problems
you’re dealing with early in the design cycle ??

25. 26
Solving High-Speed Problems in Real-Time
With Real-Time Analysis you can…
See & Fix Violations
In Real-Time
Eliminate Multiple
Validation Iterations
Streamline Design
Release Process
Improve Time
To Market

26. 27 10/16/2018
Real-Time Design:
Address Errors As They Occur
27
Demo...

27. 28 10/16/2018
Real-Time Design:
Address Errors As They Occur
28
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info@ema-eda.com
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Hi-Speed Design