PCB (Printed Circuit Board) Design and Routing Tips and Tactics


1. Introduction

A PCB is a printed circuit board. PCBs are part of our every day lives; Computers, Cellphones, Calculators, Wrist-watches and every electric component we interact with on an everyday basis.

This guide is targeted at professionals who are familiar with Hardware design and possess PCB design background.

2. Shaping the PCB

Even the most common shape for pcb manufacturing usa is rectangle. Lots of people also prefer to have the corners rounded, as this decreases the chance of edge-cracking. The shape of PCB tremendously depends upon the place you’re going to put the plank, and what your mechanical requirements are (the final box where the merchandise is set ).

Usually, you will find 4 significant holes in the board, each hole in 1 corner. These holes have been used to grip the board instead working with a patch or even a PCB holder. The diameter is more than two millimeters, also it’s plated.

3. How many layers touse?

Now we get to the following thing, how many layers should we use? This highly depends upon the most frequency employed in the design, the amount of components you might have, whether you have Ball-Grid-Array components or perhaps not, & most important of all, how dense your design is.

For systems running around 80 MHz, often it’s ok to use 2 Layers, should it be possible to route the plank doing this. Take C.E. Certification and FCC regulations into consideration. Most of the days, the need a maximum of -130dBm emission on public radio group (FM 80-108MHz). This could be problematic if you work with a more high-current clock operating between 40 to 80 MHz (The second harmonic could be between 80 to 160 MHz, which could readily violate these rules).

For systems functioning above 80MHz, it is essential to consider using more layers, (4 is very good example).

You will find 2 tactics in 4 layers:

  1. Upper and bottom layers could be Ground and Power planes. The middle layers used for routing.
  2. Top and bottom layers Useful for signal, Middle layers employed for airplanes

The first method has an excellent signal quality, since signals are sandwiched between 2 power airplanes, and consequently, you’ll have minimum emission.

The next procedure can make routing easy, since you won’t need a via (vertical interconnect access) for each pin, as the pin resides on exactly the exact same signalling stratum. In addition, the inner airplanes can have multiple islands, to pay all of your power needs, reducing the via swipe farther. However, This method Can Be Quite tricky, and it

Is very essential not to break power planes under high performance signal, since this can result to a return path loop, so making unwanted emission more likely that occurs .

Using more layers consistently results into better quality of product, however it will make it more expensive to grow, especially in the design stage. (The difference between 2 layers prototype and 4-6 layers, can be as large as many hundred dollars).

The six-layer+ procedure is nearly ideal. Utilizing top and bottom layer as power-planes and internal layers for routing can avert burnout, boost immunity to noise and radically reduce design campaigns, since there are more layers to use for routing. Impedance matching can be done readily, and we are going to cover this section for highspeed signals.

4. Organizing layers for Impedance-matching

Now I suppose you working with a highspeed system which includes SSTL, HSTL, LVDS, RSDS, GTL+, highspeed TTL and other high speed inter connections (USB HS, 2.5Gbps PCIExpress, etc.). These routings need special considerations. The lines require Impedance matching. For all beginners, this is sometimes considered a questioning term. The gap between Impedance and also Resistance is great. If you require resistance matching, you can readily make use of a resistor and be finished with it.

Impedance fitting, however, offers nothing to do with resistors. It depends upon the Length of the track, the bottom power-plane, if or not it Strip-Line (Surrounded between two power airplanes ) or even uStrip (this means features an electrical plane below, but the flip hand is free, like in top-layer or even BottomLayer).

To achieve a certain impedance on a track, you ought to carefully pick these parameters.

Be informed a miss-matched impedance connection (especially on RF, High-Speed USB, SATA or PCIExpress, and memory lines such as SSTL or HSTL), and also make the plank neglect without any obvious explanations. This will make you go to your next model, without finding what induced that the first prototype to neglect.

5. Power-planing.

Power-islands are just one the most important aspects in a high-speed digital structure. An FPGA or highspeed chip board with in-accurate power-planing may be extremely unstable. In early days, you could route power tracks somewhat wider than signal-trackstreated and treated like normal connections. Today, the story differs.

If you employ and FPGAs or High-Speed processors, you need to be aware that a great number of flip flops are shifting at any given moment in your system. Their switching causes a large amount of current moving back-and-forward through their power and ground pins. The ground-pins in cases like this could create ground bounce if the quantity of current (and notably the slew-rate) is high. I must remind one about the famous V=L. di/dt (Delta-Voltage equals inductance x ray current-rate). If you take advantage of a track (for instance) to route ground signal, you’ll have different voltages on either side of this track. It’s going to be quite funny to possess +0.5V using a single side of one’s own ground, and -1elbows on the other hand.

This may create COMPLETE SYSTEM FAILURE. I remember experiencing this issue in ancient days, which induced me to question even the very basic body rules I knew. Detecting this bug can be challenging, and also though detected, you will have no choice but to generate yet another prototype.

The same principle applies to power-plane two. You may readily get drops in certain tracks if you do not make use of a plane, or perhaps a sizable power-islands, to support your power voltage. Using a greater amount of decoupling capacitors is highly recommended for highspeed and high profile processors/FPGAs, close their power lines.

The RF section, and the powersupply switching sections needs special care for his or her ground-planes. Their oceans ought to be dispersed from the system ground-plane, and has to possess monitors connecting your shifting island to system ground (the paths should be large enough to own near-zero DC immunity, but perhaps not more). This is because switching and RF section, can create waves ground-plane, that may create ground-bounce in your own systems earth. It’s possible to search google on this subject in the event that you’ll need more explanation.

6. High Speed differential Signals

Todays designs consistently have a high-speed relay link. Examples are PCI-Express, High-Speed USB and SATA. For these lines, particular rules apply:

  1. There should not be any ground plane divide under these links.
  2. There must be no a lot more than two millimeters difference in LENGTH for each connection.
  3. Connections should maintain the exact same space between each other till they hit destination.
  4. There shouldn’t be any sharp corners. Avoid 45 degrees or 90 degrees. This can cause undesired Capacitive coupling, or it can bring about the are act a little antennas.
  5. Maintain all other signs much from such lines. I recommend minimum 5 millimeters separation. This will decrease crosstalk.

I suggest using Strip-lines for these links. But again, many Micro-Strip will do fine too. High Speed single-ended connections

Working with high-speed single-ended relations can be challenging. Since they are not differential lines, some other noise on those lines will influence their state, and will lead to system failure. HSTL, SSTL and also GTL+ are illustrations. LVTTL ought to be treated as well.

When calculating these traces, consider these tips into consideration:

  1. Impedance-matching Is Crucial for all these links.
  2. No more ground-splits under these links.
  3. Crosstalk ought to be minimized. This highly depends upon the kind of the text. LVTTL is most likely to cross talk, as they do not need terminating resistors. I recommend using SSTL or even HSTL where potential.
  4. Quite traces should be kept away from connections that are active. These lines are control-lines and any cross-talk can be catastrophic (Envision a cross-talk on chip-select connection!) .
  5. Sharp-corners are OK with these signals, since they mostly operate under 800MHz.
  6. Diminishing the amount of vias used for these links. Maximum of two is advised.

8. High Speed Memory routing strategies

Memory routing is a different narrative. When dealing with DDR2+, QDR, RDRAM, XDR and other high-speed processors, certain Crucial rules apply:

The clock signal should appear after than every one of these signs, otherwise there’ll be synchronization problems. Usually high speed memory control have a’Return-Clock’ that can be the clock trace came back into the control, and so the controller can tell when precisely the clock signal was intercepted by the chip.

  • Data lines should never cross any plane-splits, because these lines more active than any other connection from the system.
  • DDR systems have especial termination requirements (Normally Voltage-Termination). This voltage that’s half of the memories supply voltage, so must be quite STABLE, because this sections provides the termination resistors at each line-end. This supply voltage should possess proper power-planing and a great deal of capacitor decoupling (10nF for each four traces I would urge ).

Again, ask your manufacturers datasheet for more considerations.

We’re done for today, and I hope this article helped make things simpler for you personally in high-speed PCB routing techniques. This article will remain in PCB Routing recommendations and Tactics 2.

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