1 PCB DESIGN Dr. P. C. Pandey EE Dept, IIT Bombay Rev. Jan 16 2 Topics 1.General Considerations in Layout Design 2.Layout Design for Analog Circuits 3.Layout Design for Digital Circuits 4. Artwork Considerations References W.C. Bosshart, Printed Circuit Boards: Design and Technology, TMH, 1992 C.F. Coombs : Printed Circuits Handbook, McGraw-Hill, 2001 R.S. Khandpur : Printed Circuit Boards : Design, Fabrication, and Assembly, McGraw-Hill, 2005. 1
3 1. GENERAL CONSIDERATIONS IN LAYOUT DESIGN Main issues Component interconnections Physical accessibility of components Effects of parasitics Power dissipation Subtopics 1.1 Parasitic effects 1.2 Supply conductors 1.3 Component placement 4 1.1 Parasitic Effects R & L of conductor tracks C between conductor tracks Resistance Resistance of 35 µm thickness, 1 mm wide conductor = 5 mω/cm Change in Cu resistance with temperature = 0.4% / C Current carrying capacity of 35 µm thickness Cu conductor (for 10 C temperature rise): Width (mm) 1 4 10 Ic (A) 2 4 11 2
5 Capacitance Tracks opposite each other - Run supply lines above each other - Don t let signal line tracks overlap for any significant distance Tracks next to each other - Increase spacing between critical conductors - Run ground between signal lines Inductance To be considered in High frequency analog circuits Fast switching logic circuits 6 1.2 Supply Conductors Unstable supply & ground due to Voltage drop due to track R & L (particularly for high freq. current) Current spikes during logic switching local change in Gnd & Vcc potentials (i) Ripples in supply voltage (Vcc Gnd), (ii) Errors introduced in input voltage with reference to Gnd Digital ckts: Possibility of false logic triggering; Analog ckts: Degradation of SNR. Solutions Conductor widths : W (ground) > W (supply) > W(signal) Ground plane Track configuration for distributed C between Vcc & ground Analog & digital ground (&supply) connected at the most stable point 3
7 1.3 Component Placement Minimize critical conductor lengths & overall conductor length Component grouping according to connectivity Same direction & orientation for similar components Space around heat sinks Packing density Uniform Accessibility for adjustments component replacement test points Separation of heat sensitive and heat producing components Mechanical fixing of heavy components 8 2. LAYOUT DESIGN FOR ANALOG CIRCUITS Supply and ground conductors Signal conductors for reducing the inductive and capacitive coupling Special considerations for Power output stage circuits High gain direct coupled circuits HF oscillator /amplifier Low level signal circuits 4
9 2.1 Ground & Supply Lines Separate GND (& Vcc) lines for analog & digital circuits Independent ground for reference voltage circuits Connect different ground conductors at most stable reference point Supply lines with sufficient width and high capacitive coupling to GND (use decoupling capacitors) Supply line should first connect to high current drain ckt blocks Supply line independent for voltage references 10 2.2 HF Oscillator / Amplifier Decoupling capacitor between Vcc & GND Capacitive load on o/p Reduce capacitive coupling between output & input lines Vcc decoupling for large BW ckts. (even for LF operation) Separation between signal & GND to reduce capacitive loading 5
11 2.3 Circuits with High Power O/P Stage Resistance due to track length & solder joints modulation of Vcc & GND and low freq. oscillations Large decoupling capacitors Separate Vcc & GND for power & pre- amp stages 12 2.4 High Gain DC Amplifier Solder joints thermocouple jn Temp gradients diff. noisy voltages Temp.gradients to be avoided Enclosure for stopping free movement of surrounding air 6
13 2.5 Low Level Signal Circuits A) High impedance circuits - Capacitive coupling B) Low impedance circuits - Inductive coupling 14 High -Z circuits If R» 1 jw(cxy+cy) then coupled Vy = Va [Cxy/(Cy+Cxy)] Increase separation between low level high Z line and high level line (decrease Cxy) Put a ground line between the two (guard line) Example: Guard for signal leakage from FET output to input 7
15 Low Z Circuits Voltage induced in ground loops due to external magnetic fields Current caused in the low- Z circuit loop due to strong AC currents in nearby circuits Vm= - (d/dt) B da Avoid ground loops Keep high current ac lines away from low level,low Z circuit loops Keep circuit loop areas small 16 3. LAYOUT DESIGN FOR DIGITAL CIRCUITS Main problems Ground & supply line noise Cross-talk between neighboring signal lines Reflections : signal delays, double pulsing 8
17 3.1 Ground & Supply Line Noise Noise generated due to current spikes during logic level switching, drawn from Vcc and returned to ground Internal spike: charging & discharging of transistor junction capacitances in IC ( 20 ma, 5ns in TTL) External spike: charging & discharging of output load capacitance Ground potential increases, Vcc decreases: improper logic triggering. Problem more severe for synchronous circuits. Severity of problem (increasing): CMOS, TTL. 18 Solution for ground & supply noise Decoupling C between Vcc & ground for every 2 to 3 IC s : ceramic, low L cap. of 10 nf for TTL & 0.5 nf for ECL & CMOS Stabilizes Vcc-GND (helps against internal spikes Not much help for external spikes Low wave impedance between supply lines (20 ohms): 5 to 10 mm wide lines opposite each other as power tracks Ground plane : large Cu area for ground to stabilize it against external spikes Closely knit grid of ground conductors (will form ground loops, not to be used for analog circuits) Twist Vcc & GND line between PCBs 9
19 3.2 Cross-talk Occurs due to parallel running signal lines (ECL: 10cm,TTL: 20 cm, CMOS: 50 cm) Problem more severe for logic signals flowing in opposite directions Solutions Reduce long parallel paths Increase separation betw. signal lines Decrease impedance betw. signal & ground lines Run a ground track between signal lines 3.3 Reflections 20 Caused by mismatch between the logic output impedance & the wave impedance of signal tracks. Signal delay (low wave imp.) Double pulses (high wave imp.) TTL (Z: 100-150 Ω) 0.5 mm signal line with GND plane, 1 mm without GND plane. Signal lines between PCBs twisted with GND lines. CMOS (Z: 150 300 Ω) 0.5 mm signal line without GND plane. Gnd not close to signal lines. 10
21 Summary of Layout Design Considerations (for 1.6 mm thickness, double sided boards) Family: TTL CMOS Signal GND Zw (Ω) 100-150 150-300 0.5 with Gnd 0.5, no gnd Signal line width (mm) 1 without Gnd Vcc -GND Zw (Ω) < 5 < 20 Vcc line (mm) 5 2 GND line (mm) Very broad 5 (plane /grid) 22 4. ARTWORK RULES Conductor orientation Orientation for shortest interconnection length. Conductor tracks on opposite sides in x-direction & y- direction to minimize via holes. 45 or 30 / 60 orientation for turns. Conductor Routing Begin and end at solder pads, join conductors for reducing interconnection length. Avoid interconnections with internal angle <60. Distribute spacing between conductors. 11
23 Conductor routing examples 24 Solder Pads Hole dia Reduce the number of different sizes. 0.2-0.5 mm clearance for lead dia. Solder pad Annular ring width 0.5 mm with PTH 3 hole dia without PTH Uniformity of ring around the hole. Conductor width d > w > d/3. 12
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