Printed Circuit Board Design Considerations

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     Printed Circuit Board
     Design Considerations                                                                                                                                                                                                    by PETER ALFKE x (

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                                                                                                                                                                             teady advances in IC technology have                 This means that the PCB plays an important
                                                                                                                                                                      fueled 30 years of rapid progress in digital             role in controlling the integrity of interconnect
                                                                                                                                                                      system speed and complexity. In the past,                signals. Trace width and trace spacing as low as
                                                                                                                                                                      system speed was determined by gate and                  4 mils (0.1 mm), controlled line impedance, and
                                                                                                                                                                      register performance, and you could easily               multi-layer boards with clean ground and Vcc
                                                                                                                                                                      take advantage of ever faster, bigger, and               planes, are all required to minimize reflections,
                                                                                                                                                                      cheaper integrated circuits.                             ground bounce, and crosstalk. Maintaining
                                                                                                                                                                           The printed circuit board (PCB) was just a          clock integrity is a big problem.
                                                                                                                                                                      means to hold ICs in
                                                                                                                                                                      place; PCB layout was an          Year     Max.System     PC-Board      PC-Board               IC Design
                                                                                                                                                                      exercise in topology and                   Clock          Complexity    Min.Trace Width        Rules
22                                                                                                                                                                    economics. Analog issues
                                                                                                                                                                      such as crosstalk, phase
                                                                                                                                                                                                        1965 1 MHz
                                                                                                                                                                                                        1980 10 MHz
                                                                                                                                                                                                                                1-2 layers
                                                                                                                                                                                                                                2-4 layers
                                                                                                                                                                                                                                              100 mil = 2.5 mm
                                                                                                                                                                                                                                              20 mil = 0.5 mm
                                                                                                                                                                      and amplitude distortion,         1995 100 MHz            4-8 layers    4 mil = 0.1 mm         0.5µ
                                                                                                                                                                      reflections, ground               2010 ? 1000 MHz         8-16 layers   1 mil = 25 µ           0.1µ
                                                                                                                                                                      bounce and so on could
                                                                                                                                                                      thus safely be ignored, or treated as minor
                                                                                                                                                                      irritants, because synchronous digital logic is          Time Domain vs Frequency Domain
                                                                                                                                                                      forgiving in amplitude and timing variations.                Digital designers usually express delays and
                                                                                                                                                                           Times have changed. At today’s circuit              rise times in the time domain, while analog
                                                                                                                                                                      speed, the PCB and its analog characteristics            designers often use the frequency domain to
                                                                                                                                                                      play a strong, if not dominating, role in deter-         describe circuit and component performance.
                                                                                                                                                                      mining digital system performance.                       The frequency domain is more meaningful for
                                                                                                                                                                           CMOS ICs are no longer the slow and forgiv-         analyzing many analog phenomena. For rough
                                                                                                                                                                      ing circuits of the past. They are now as fast as        estimates, we can easily convert rise and fall
                                                                                                                                                                      (if not faster than) the fastest TTL circuits; out-      times into a frequency spectrum as follows:
                                                                                                                                                                      puts ramp between 0 and 5 volts in 1 ns, clock                     There is a knee frequency
                                                                                                                                                                      rates approach 150 MHz, and ICs have up to                         F = 0.5•(1/ (Trise or Tfall))
                                                                                                                                                                      500 signal connections to accommodate mul-
                                                                                                                                                                                                                                  For analyzing circuit behavior, it is suffi-
                                                                                                                                                                      tiple 32-bit wide busses. The trend is moving
                                                                                                                                                                                                                               cient to evaluate the frequency response up to
                                                                                                                                                                      toward much higher speed and far more I/Os.
                                                                                                                                                                                                                               the knee frequency, but there is no need to go
                                                                                                                                                                           However, signals still travel along PCB
                                                                                                                                                                      traces at only half the speed of light, and
                                                                                                                                                                                                                                  For a typical rise time of 2 ns, the knee
                                                                                                                                                                      sharp signal edges get reflected at every trace
                                                                                                                                                                                                                               frequency is 250 MHz. When low-pass circuits
                                                                                                                                                                      discontinuity. You must now not only control
                                                                                                                                                                                                                               are cascaded, the resulting transition time is
                                                                                                                                                                      the source-to-destination path, but you must
                                                                                                                                                                                                                               the square root of the sum of the squared
                                                                                                                                                                      also pay attention to the complete circuit loop
                                                                                                                                                                                                                               transition times. For example:
                                                                                                                                                                      and its inductance, from the positive power
                                                                                                                                                                                                                               ® A 250 MHz scope with a 250 MHz probe
                                                                                                                                                                      supply terminal back to the negative terminal,
                                                                                                                                                                                                                                  displays a 2 ns rise time as a 3.5 ns rise
                                                                                                                                                                      then through the decoupling capacitors back
                                                                                                                                                                                                                                  time (√22 + 22 + 22). This is a +75% error.
                                                                                                                                                                      to the positive supply terminal.
® A 500 MHz scope with a 500 MHz scope            Transmission Lines                                            Figure 1.
  probe displays a 2 ns rise time as a 2.4 ns         Short connections can be treated
  rise time (√22 + 12 + 12). This is only a       as lumped capacitors, but any line
  +20% error.                                     with a propagation delay of more
Beware of slow scopes and slow scope probes.      than 30% of the signal transition
                                                  time must be analyzed as a trans-
PCB Characteristics
                                                  mission line. With a 3-ns transition
® Min. trace width: typically 10 mil
                                                  time, any line longer than six
  (0.25 mm), down to 4 mil (0.1 mm)
                                                  inches (15 cm) is a transmission
® Min. trace pitch: twice trace width
                                                  line. With a 1-ns transition time,
® Hole diameter: 20 mil, down to 8 mil
                                                  any line longer than two inches (5
  (0.5 mm to 0.2 mm). Thin traces and small
                                                  cm) is a transmission line.
  holes make the board more expensive.
® Layer thickness: typically 8 mil (0.2 mm),      Characteristic Impedance
  down to 4 mil (0.1 mm). The dielectric          of a PCB Trace
  constant ε is 4.5 for typical FR4 PCB           ® 50 Ω for outer trace, width = 2x PCB layer
                                                     thickness (microstrip).                                    Figure 2.
                                                  ® 75 Ω for outer trace, width = 1x
Beware of Trace Inductance
                                                     PCB layer thickness (microstrip).
    A 0.5-inch long, 10-mil wide trace, over an
                                                  ® 50 Ω for inner trace, width =
8-mil thick PCB layer, connected to the under-
                                                     0.6x PCB layer thickness
lying ground plane through a 14-mil via at the
end, has an inductance of 9 nH. To a 2-ns rise
                                                  ® 75 Ω for inner trace, width =
time, this is an impedance of 15 Ω.
                                                     0.25x PCB layer thickness
Typical Lumped Parameters                            (stripline).
® Capacitance - A narrow trace has a capaci-         Higher characteristic impedances
  tance of 2 pF per inch (0.8 pF per cm).         are unrealistic on modern PCBs.
  Copper area has 130 pF per square inch (20
                                                  How to Prevent Signal Degradation
  pF per cm2).                                    Due to Reflections (Ringing)
  ® Ceramic decoupling capacitors have a          ® Keep the trace short (best solution, but
      series resistance of several ohms, and an     often impossible).
      inductance of 3 to 30 nH. Use several                                                                     Figure 3.
                                                  ® Terminate at the destination
      capacitors in parallel to lower the high-     (end termination). (See Figure1.)
      frequency impedance.                          Problem: High sink or source
  ® Via capacitance is 0.5 pF (insignificant)       current, or both. High power
      but the inductance of >1 nH can be            consumption.
      significant for very fast transitions         Solution: Capacitive coupling to
      (3 Ω for a 1-ns transition).                  the terminating resistor. Time
® Inductance - A 1-inch long wire (25 mm)           constant should be longer than
  has an inductance of 80 nH, which at 100          the transition, but much shorter
  MHz means 50 Ω (definitely not a short            than the flat time. For example:
  circuit).                                         200 pF x 50 Ω = 10 ns.
® Resistance - 1/4 W axial resistors have a         See Figure 2.
  series inductance of 2.5 nH and a parallel      ® Terminate at the source
  capacitance of ~2 pF. 1/8 W axial resistors       (series termination).
  have 1 nH and <2 pF.                              Advantage: No static power consumption.
                                                                                                 Continued on
                                                                                                 next page
     PCB Design                Problem: Use only for single-destination.       only matter at setup time before the next active
        Continued from         50% amplitude along the trace!                  clock edge. However, beware of all asynchronous
       the previous page       See Figure 3.                                   inputs and clocks — they are susceptible to mis-
                               Signals propagate at the speed of light di-     interpretation due to ground bounce. Make sure
                           vided by the square root of the effective dielec-   these signals are parked far away from the input
                           tric constant. The speed is therefore higher on     thresholds.
                           the outer traces that have air on one side, than    Crosstalk
                           on the inner traces that are completely sur-            Crosstalk is especially strong when many lines
                           rounded by epoxy.                                   run closely parallel, as in data busses. Inductive
                           ® Propagation delay on an outer trace:              crosstalk is usually bigger than capacitive.
                               140 ps per inch ( 55 ps per cm ).               Crosstalk can be minimized by using an unob-
                           ® Propagation delay on an inner trace:              structed ground plane. Synchronous busses can
                               180 ps per inch ( 70 ps per cm ).               tolerate a lot of crosstalk if it occurs only after the
                           Ground Plane                                        synchronous data transitions.
                               Always analyze the whole current loop: Vcc
                                                                                  Problem: Asynchronous control lines, strobes,
                           terminal to Vcc distribution, to Vcc-pin, to
                                                                                  interrupts that run parallel to the data or ad-
                           device output, to PCB trace, to ground distribu-
                                                                                  dress busses.
                           tion, to ground terminal, through the
                                                                                  Solution: Increase the spacing.
                           decoupling capacitors, and back to Vcc. Keep
24                         this loop inductance as small as possible by        Vcc Decoupling Capacitors
                           reducing the area of the loop. The return cur-          Supply decoupling is not a luxury. For fast
                           rent tries to follow the signal trace; it follows   internal and external transitions, these capacitors
                           the path of least inductance (least energy).        are the only instant source of current. The power
                           ® Make it easy for the signal and return paths      supply with its big electrolytic capacitor is too far
                               to stay close together.                         away and has too much inductance. (See XCell
                           ® Avoid obstacles in the ground plane.              #20, pages 42-43). Low-impedance ceramic
                           ® Investigate the reason for any detour of the      decoupling capacitors are required to supply
                               current.                                        dynamically changing Icc inside the chip, and to
                           ® Watch out for slots in the ground plane           provide a return path for external current
                               causing detours in the return path, leading     changes. In CMOS systems, all power is dynamic.
                               to crosstalk and ringing.                       The instantaneous current peaks are much higher
                                                                               than the average dc current, which is between
                           Ground Bounce
                                                                               100 mA and 2 A for the larger Xilinx devices.
                               Ground bounce is caused by a voltage drop
                                                                                   Decoupling capacitors must have low induc-
                           on the inductance between chip internal
                                                                               tance and low series resistance. The capacitance
                           ground (bonding wire + leadframe, especially
                                                                               value is irrelevant, as long as it is sufficient. Use
                           in ceramic PGAs) and PCB ground. The pulse
                                                                               0.01 to 0.1 µF capacitors, mounted very close to
                           width increases with capacitive loading, but the
                                                                               each Vcc pin and directly connected to the
                           pulse amplitude is independent of capacitive
                                                                               ground plane. Keep the lines very short. A nar-
                           loading. Ground bounce can cause wrong
                                                                               row, 0.25-inch (6 mm) long trace represents 20
                           output levels on adjacent outputs and can
                                                                               nH; a current change of 100 mA/ns causes a
                           cause inputs to be misinterpreted. For a High-
                                                                               voltage drop of 2 volts across this inductance.
                           to-Low transition, the internal ground jumps
                           first to a positive voltage, then undershoots.      Credits
                           (See the 1998 Xilinx Data Book, pages 13-16)           For an excellent, in-depth treatment of these
                               Synchronous designs with one common             subjects, read “High-Speed Digital Design” by
                           clock are surprisingly resilient. They tolerate     Johnson and Graham, Prentice Hall, 1993, or attend
                           ground bounce, because it occurs directly after     a class given by the author, Howard W. Johnson,
                           the triggering clock edge, whereas input levels     Redmond WA. ( x

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