PCB Design and Routing Tips and Tactics


Printed Circuit Board Design and Routing Tips and Tactics

1. Introduction
A PCB may be a computer circuit board. PCBs ar a locality of our everyday lives; Computers, Cellphones, Calculators, Wrist-watches and each electrical part we have a tendency to move with on a routine.

This article is targeted at professionals UN agency ar aware of Hardware style and have PCB style background.

2. Shaping the PCB

The most common form for PCB is parallelogram. many of us additionally like better to have the corners rounded, as this decreases the chance of edge-cracking. the form of PCB extremely depends on wherever you're about to place the board, and what your mechanical needs ar (the final box wherever the merchandise is placed).

Usually, there ar four huge holes within the board, every hole in one corner. These holes ar wont to hold the board in situ employing a patch or a PCB holder. The diameter is quite two millimeters, and it's plated.

3. what number layers to use?

Now we have a tendency to get to following step, what number layers ought to we have a tendency to use? This extremely depends on the most frequency employed in the planning, what number elements you have got, whether or not you have got Ball-Grid-Array elements or not, and most vital of all, however dense your style is.

For systems running up to eighty MHz, sometimes it's alright to use two Layers, ought to or not it's doable to route the board doing therefore. Take C.E. Certification and FCC laws in thought. Most of the days, the need a most of -130dBm emission on public radio band (FM 80-108MHz). this may be problematic if you employ a high-current clock operative between forty to eighty MHz (The second harmonic would be between eighty to a hundred and sixty MHz, which may simply violate these rules).

For systems running on top of 80MHz, it's vital to contemplate victimisation additional layers, (4 is nice example).

There ar two ways in four layers:

Top and bottom layers may be Ground and Power planes. the center layers used for routing.

Top and bottom layers used for signal, Middle layers used for planes

The first methodology contains a superb signal quality, since signals ar sandwiched between 2 power planes, and as a result, you'll have minimum emission.

The second methodology will create routing straightforward, since you'll not want a via (vertical interconnect access) for every pin, because the pin resides on a similar signalling layer. more additional, the inner planes will have multiple islands, to hide all of your power desires, reducing the via count even more. however this methodology may be terribly difficult, and it is extraordinarily necessary to not break power planes underneath high-speed signal, as this may result into a come back path loop, creating unwanted emission additional doubtless to occur.

Using additional layers forever results into higher quality of product, however it'll create it dearer to develop, particularly within the prototyping stage. (The distinction between two layers image and 4-6 layers, may be as high as few hundred dollars).

The six-layer+ methodology is sort of ideal. victimisation prime and bottom layer as power-planes and internal layers for routing will forestall emission, increase resistance to noise and dramatically cut back style efforts, as there ar additional layers to use for routing. Impedance-matching may be done simply, and that we can cowl this section for high-speed signals.

4. composition layers for Impedance-matching

At this time I assume you handling a high-speed system that has SSTL, HSTL, LVDS, RSDS, GTL+, High-Speed TTL and alternative high-speed interconnections (USB HS, 2.5Gbps PCI-Express, etc.). These routings need special issues. The lines need impedance-matching. for several beginners, this may be a questioning term. The distinction between resistance and Resistance is nice. If you would like resistance matching, you'll simply use a electrical device and be through with it.

Impedance matching, on the opposite hand, possesses nothing to try to to with resistors. It depends on the dimension of the track, the face power-plane, whether or not is it Strip-Line (Surrounded between 2 power planes) or uStrip (which suggests that contains a power plane under that, however the opposite facet is free, as in TopLayer or BottomLayer).

To achieve a definite resistance on a track, you must fastidiously choose these parameters. Use associate resistance calculator (search google) to seek out the proper values for dimension, height on top of the power-plane, and thickness of the gold layer, to attain the specified resistance (usually fifty or seventy five ohms).

Be suggested that a miss-matched resistance affiliation (especially on RF, High-Speed USB, SATA or PCI-Express, and memory lines like SSTL or HSTL), and create the board fail with none obvious reasons. this can force you to travel for following image, while not ever finding what caused the primary image to fail.

5. Power-planing.

Power-islands ar one the foremost necessary factors in an exceedingly high-speed digital style. associate FPGA or high-speed processor board with in-accurate power-planing may be terribly unstable. In youth, you'll route power tracks alittle wider than signal-tracks, and treated them like traditional connections. Today, the story is totally different.

If you employ and FPGAs or High-Speed processors, you must recognize that an excellent variety of flip-flops ar shift at any given moment in your system. Their shift causes an enormous quantity of current going back-and-forward through their power and ground pins. The ground-pins during this case will produce ground-bounce if the quantity of current (and particularly the slew-rate) is high. i have to prompt you of the notable V=L. di/dt (Delta-Voltage equals inductance x current-rate). If you employ a track (for instance) to route ground signal, you'll have totally different voltages on both sides of the track. it'll be terribly funny to possess +0.5V on one facet of your ground, and -1V on the opposite facet.

This will cause COMPLETE SYSTEM FAILURE. I bear in mind experiencing this issue in youth, that forced Pine Tree State to question even the terribly basic physique rules I knew. Discovering this bug may be tough, and albeit discovered, you'll don't have any alternative however to form another image.

The same rule applies for power-plane 2. you'll simply have drops in bound tracks if you are doing not use a plane, or an outsized power-islands, to support your power voltage. employing a bigger variety of decoupling capacitors is extremely suggested for high-speed and high-powered processors/FPGAs, close to their power lines.

The RF section, and therefore the power-supply shift sections desires special take care of their ground-planes. Their islands ought to be isolated from the system ground-plane, and should have tracks connecting your shift island to system ground (the tracks ought to be giant enough to possess near-zero DC resistance, however not more). this is often as a result of shift and RF section, will produce waves on ground-plane, which may produce ground-bounce on your systems ground. you'll search google on this subject if you would like additional rationalization.

6. High-Speed differential Signals

Todays styles forever have a high-speed differential affiliation. Examples ar PCI-Express, High-Speed USB and SATA. For these lines, bound rules apply:

  • There mustn't be any ground-plane split underneath these connections.
  • Their resistance ought to be fastidiously matched.
  • There mustn't be quite two millimeters distinction long for every affiliation.
  • Connections ought to maintain a similar distance between one another till they reach destination.
  • There mustn't be any sharp corners. Avoid forty five degrees or ninety degrees. this might cause unwanted electrical phenomenon coupling, or it will cause the ar act atiny low antennas.
  • Keep all alternative signals off from these lines. i like to recommend minimum five millimeters separation. this can cut back cross-talk.
  • I recommend victimisation Strip-lines for these connections. But again, several Micro-Strip can do fine furthermore.


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