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New Column: PCB 101

A very smart woman once told me that when she wants to get a quick, clear understanding of a subject, she buys a children's book on that subject. It makes a lot of sense. This new series by Bob Tarzwell, PCB 101, will be based on that theory.

Starting today, Bob will simplify some of the more confusing, yet vitally important technology terms used in the printed circuit board industry. His goal is to explain things in a manner that is so simple that even a fifth grader will understand it. Read on and find out if you're as smart as a fifth grader.

--Dan Beaulieu

We all know the buzz words: impedance, 50 ohms, 10%, balanced lines, CTE, dielectric constant and loss. In this new series, PCB 101, I will break down these buzz words in layman's terms in the hopes that more people will understand what we're talking about and what's happening inside the circuit. 

Impedance is a great place to start. In PCB design and fabrication, I would say the most used term--yet the least understood--is impedance.

Impedance can be thought of as an energy level, and the traces and dielectric cores as devices that must carry these energy levels. Whenever we send out a signal, it has a specific energy level. Every bit of the signal must be accounted for, because whatever signal you put in, you will get back out in various ways. Some of the signal will be lost in magnetic radiation; some will be reflected back from changes in the impedance along the trace.  Some of the original signal will be absorbed by the dielectric. What's left after all the losses is the signal at the other end of the trace.

What is a dielectric? The term dielectric refers to a material that is basically an insulator, in that it does not directly conduct electricity. But a dielectric can absorb a small amount of electricity. The term dielectric constant, or Dk, is a number which relates to the ability of a laminate to hold a charge of electricity, just like a battery. This is how a capacitor operates.

The typical FR-4 laminate has a Dk of 4.2 to 4.4. Compare this to Teflon laminate, which has a lower Dk of 2.3. The higher the Dk, the greater the capacitance and the more electricity the laminate can store. Also, the worse it is for our signals, especially at higher frequencies. The Teflon PTFE laminate's lower Dk means the signal energy has a smaller capacitor to charge. The less energy absorbed along the trace also means we'll have a better signal at the end of the trace.

Along with dielectric constant, we have dielectric loss. The dielectric loss describes how much of the signal is absorbed or lost into the dielectric material. Dissipation factor (Df) is a measurement of dielectric loss. Df is usually expressed as a very small number; typical FR-4 has a Df of .01, while PTFE has a Df of .002. The lower the laminate's Df, the less it absorbs or affects the signal energy level and the more signal we get out at the end of the track. To add to the confusion, Dk and Df can change widely with different signal frequencies.

A frequency is the number of times a second that a signal changes from positive to negative.  Alternating signals switch from negative to positive. A 1 KHz signal switches 1,000 times per second, while 1 MHz means 1 million cycles per second.  We now work in the GHz range, which is an astounding 1 billion cycles per second.  As the frequency goes up, strange things start to happen. The matching of dielectric and impedance needs to be perfect.  Also, as the frequency goes up, the electricity no longer travels inside the copper traces, but instead travels along the outer skin. This is called the skin effect.

The effect that trace impedance has on signal strength and integrity has a lot to do with energy. Think of the board trace as a hose. If we try to pump too much water into the hose, all of it won't fit. But it has to go somewhere, so the extra water pressure will reflect back creating a spray at the input of the hose.

The same happens with mismatched impedance. The trace has a certain amount of capacitance; the higher the impedance, the lower the capacitance and the less energy can be stored. This capacitance can only be charged so fast and with only so much energy.  If we send an electronic signal into a higher impedance trace with its lower capacitance, we will not be able to absorb and charge the capacitor. The extra energy will bounce around and end up as reflections and interfering signals that we do not want. Just like throwing a rock into a small pond, the waves will flow outward hitting the shore and reflecting back and forth many times, getting smaller each time. The less signal energy the trace will accept, the less comes out the other end.  At 6 GHz, only about 15% of the original signal makes it to the end of the trace.You would think that the solution is to feed the signal into a low-impedance trace to increase the amount of signal we can carry. But if we put energy into a trace with impedance that is too low, then the signal will lose energy as it tries to charge the bigger capacitor with too little energy. The signal voltage will drop even farther.

Taking our hose idea further, if we try to use a garden hose to fill a big pipe, the small amount of water from the hose will not fill the big pipe. To maintain the same flow and speed of water out of the garden hose, it must be connected to a similar sized hose; it is exactly the same with impedance.

Signal levels and signal integrity are the reason we need to control impedance on a trace-by-trace basis. 

I'll be back soon with another installment of PCB 101. Don't be late for class.

Bob Tarzwell is CEO and founder of DMR Ltd. He can be reached at rtarzwell@megadawn.com.