PCB Design Flow Connectors, backplanes, and other interconnects – potential of unwanted noise, delays, and reliability problems. Slowest logic family can adequately meet worst-case performance and timing goals Minimize layers should be considered Equivalent Integrated Circuit Count (EIC) technique (??? – correlation factors for total layer count) Computer Aided Design (CAD) Rule violations should be avoided PCB process documented Fabricate prototype
Mechanical Design and PCBMaterials Printed circuit board sizes Board thickness Panel sizes (more than one PCB size) Mechanical partitioning (ex. space for components and cable routing) Mechanical support Thermal analysis Mechanical Interface Control Drawing (relates the mechanical chassis constraints to the board connector constraints) PCB Materials (considerations – mechanical, electrical, chemical, thermal)
Electrical Considerations Impedance – capacitance, resistance, and inductance Cross talk; a phenomenon where a signal on one line/trace is capacitively coupled to an adjacent line/trace. It can be a form or noise. Heat; another form of noise – 1.5*kT Switching (on-off-on-off) noise; increases while impedance decreases. Higher impedance MAY solve problem.
More Electrical Considerations1 voltsignal Rise time - tr Fall time - tf Propagation delay - tpd
Capacitors - advantages f f What is the use for a 2 capacitor?V = 4kTR Coth ∆ f ≈ 4kTR ∆ f R f f 1) traps noise z z 2) stores charge for memory ex. DRAM R Voltage noise equation C What is noise? from Dr. John Choma’s lectures at USC
Capacitors - disadvantages Tradeoff – capacitors slow propagation timeR R R R R C C C C C Propagation delay - tpd tpd = 15 RC for 5 capacitors and 5 resistors
Capacitors have some noise too.dielectric parasitic resistor
What is noise margin? VOH = 1 VLogic 1 VIH = 0.8 V Noise spike outside noise margins VIL = 0.2 VLogic 0 VOL= 0 V Noise tolerance is the degree to which a gate is impervious to noise spikes at its input.
Microstrip configurations w t h Dielectric Reference plant1. Lines on surface – easy to 1. Subject to physical damage cut and jumpered. 2. Radiated emissions of 15dB2. Higher impedance by changing h. 3. Crosstalking problems.3. Easily fabricated4. Solder mask have negligible effect on impedance
Embedded microstrip1. Allows inclusion of second 1. Radiated emissions of signal layer in addition to 15dB surface microstrip. 2. Signal propagation slower2. Helps when signal layers than surface are required in a constrained board thickness.
Strip-line1. Exhibit less cross-talk 1. Difficult to repair or rework2. Better impedance control 2. Slower propagation time
Dual strip line Reference plane Reference plane1. Maximizes interconnection 1. Results in very high layer to density layer coupling.2. Routed perpendicularly to 2. Impedance will be lower for reduce cross talk. dual strip line signal trace closer to reference plane compared to strip line signal trace in the center to the reference plane.
Cross-talk Mutual capacitive and inductive b/t signal lines produces cross-talk. Moving ground plane closer to signal lines reduces cross-talk. Forward cross-talk (IL - IC) – normally opposite of signal source Backward cross-talk (IC + IL) – same as signal course, twice the line delay (worst case)
Cross-talk control Full or partial termination reduce cross-talk amplitudes. Signals must be routed orthogonally. Void parallel of high speed logic parallel to low level analog circuits. Keep all buses physically separate from clocks and strobes. Distance separation between critical traces must be three times the width of the traces measured from centerline to centerline. Thin traces reduce cross-talk. Separate traces at 2 mils/inch of trace length. Keep lines short!
Clock and strobe distribution Use point to point clock configurations. Centrally locate the clock drivers. Avoid branching. Make clocks, strobes, and control signals first priority. Do not use the same buffer to drive on and off the card simultaneously. Do not assign clocks of different frequencies to the same clock driver package.
Series termination Series termination does not increase power dissipation. Low state reflections are more critical compared to high state reflections. Must be lumped very close to the end of the lines
Parallel termination Purpose: 1. Reduces reflections from ends of the bus. 2. Provides high state pull-up for open collector drivers. 3. Provides discharge path for the line when 3-state devices are disabled. 4. Allows it to put a larger initial voltage step intoSignal line the line on positive transitions. Guidelines: 1. 0.1µF capacitors should be placed as close as possible to Vcc pin. 2. Choose types that give access to the individual resistor lead since they afford better grounding.
Parallel termination Signals travel relatively undistorted. Loading delay is about half that of series termination. Helps to offset the driver versus line voltage divider delay Higher power consumption. Higher cross-talk levels. Decreases the low level noise margins.
Differential signal topologies Strip-line structure Dual strip-line structure Odd-mode coupling Even-mode coupling Even-mode coupling Odd-mode couplingCoplanar differential structure Broadside differential structure Even-mode coupling is for digital applications.
Coplanar vs. Broadside Odd mode coupling is easier to control with the strip line approach than the dual-strip line. Less dielectric to achieve desired impedance for coplaner. Dielectric loss is higher for broadside. Common-mode noise is rejected in the coplanar approach while it is inherently introduced in the broadside configuration. Vias are required for broadside to control common mode noise.
Net layout guidelines Make critical signals first priority. Keep signal lines short. Use slowest logic family meeting timing requirements. Terminate transmission lines. Keep loading along distributed nets equal and well balanced. Avoid branching. Avoid driving on and off card simultaneously. Keep stubs off of transmission lines <2 inch. Proper grounding practices mandatory. When logic devices have risetimes of 1 ns or less, use 45° instead of right anges in circuit paths. Avoid long test point circuit traces. Testability should also be considered.
Definitions Ground shifts; difference in ground current Backplane; A circuit board containing sockets into which other circuit boards can be plugged in. EMI; Any electromagnetic disturbance that interrupts, obstructs, or otherwise degrades or limits the effective performance of electronics/electrical equipment. R R R R R Ig Ig Ig Ig Ig
Reducing ground shifts Do not use thermal relief “butterflies.” Use the appropriate connectors. Used as many ground pins per connector. Logic cards that represent the heaviest current drain should be located nearest the end where the ground enters the backplane. Cards with the most single-ended logic interconnects between them should be assigned to slots as close together as possible. Consider differential driving.
Multilayer power distribution Keep plane spacing as close as possible. Multiple 1-oz planes arranged in couplets are recommended for distributing backplane power. Shorter pins will have less resistance. Stagger power/ground inputs on the backplane so as to distribute power more evenly into the plane. Keep power supply as close as possible to the backplane to minimize voltage drops in connecting planes. 2% maximum voltage drop from supplies to any circuit device is recommended. Use additional planes to distribute auxiliary power in the backplane.
Better design Poor Backpanel Better Backpanel +5 +5 G G G G +5 G +5 GHigh speed RF currents exist on the edges of power planesdue to magnetic flux. Interplane coupling is called fringing.Need to implement the “20-H” rule suggested by W. MichaelKing. Assuming a separation of 6 mils between planes, 20XH= 120 mils. Make the power plane 120 mils smaller than theground plane.
Types of capacitors NPO – “temperature compensating” materials. Capacitance change +/- 0.3% from -55 to +125°C. X7R – “temperature stable” Capacitance change +/- 15% from -55 to +125°C. Z5U – “general purpose” provide highest capacitance – very unstable under environmental changes – small size, low ESL, low ESR, and excellent frequency response.
EMI layout guidelines Use multilayer boards with dedicated power and ground planes. Strip-lines reduced radiated emissions up to 15 dB. Auxiliary power or ground traces are required – keep them as wide as possible to lower inductance and DC resistance. Adequate decoupling reduces current inductive traces and thus lowers emissions. Keep highest speed logic closer to the card connector. Keep signal traces as short as possible. If a noisy card is anticipated, use a strip-line approach.