7 PCB design techniques

From signal integrity to copper roughness and laminate design, you can use the following PCB design techniques:

• Insertion loss and attenuation
• Signal integrity of automotive electronics and IoT
• Thin line
• Copper on ground and power plane
• Copper roughness
• Stack design
• Working with manufacturers

1. Insertion loss and attenuation

Insertion loss and attenuation are very important to designers, especially at very high speeds. For lower clock frequencies, if designers can slow down the rising and falling edges, they won’t worry so much. But those operating in the gigabyte field must pay attention, especially attenuation and loss. I took a lesson at PCB West on that topic. Many good materials can help quell the effects of loss, such as I-speed materials. I did not use those exact materials but used other materials with similar loss characteristics, so I know the advantages they provide.

In this regard, the design community has some good news. I currently being designed and manufactured usually have pre-emphasis and equalization built into the driver and receiver stages. The result is that even if the speed is increasing, high-speed materials are now not needed. You can purchase an IC that handles loss by changing the shape of the waveform before or at the receiving end. This is a good thing for the design community. This does not mean that designers do not need high-speed materials, but the demand is not so serious. When they reach 10 gigabytes, they will need both, equalization and/or pre-emphasis, and low-loss PCB materials.

2. Signal integrity of automotive electronics and IoT

People in the automotive and Internet of Things fields usually want to use very low-layer circuit boards, if they can. This is very important because low layer count means low cost. The key is that people need to learn to design 1, 2, and 4 layer circuit boards while maintaining high-quality signal integrity and controlling EMI. This is not always an easy task. Even in a single-layer circuit board, there are ways to control impedance and contain fields, but you must typically do everything. Each trace must have a return path as if it were a high-speed board with high-level numbers.

The key element, the spacing between the wiring trace and the ground plane or ground wiring must be very close. As our friend Dan Beeker always said, “Everything is about space.” This is how the world of IoT, cars, and home appliances must start thinking. Besides, as IC sizes decrease, they will be plagued by the fine line problems faced by many other industries. This will affect everyone, and we all need to consider the impact.

3. Fine line

Thin lines are not necessarily required for low levels. As long as the density is low, you can use standard line widths and standard copper weights without any problems. However, designers still must understand the meaning of designing transmission lines. They must know that every signal from these devices is fast. IC rise time is getting faster and faster. It is not about the clock frequency, but about the rise and fall times of the energy in the transmission line.

4. Copper for ground and power plane

In averaging circuits, the power output has a relatively fixed voltage (ie 3.3v), so many people think that they are dealing with DC energy, which is not the case. The current associated with the magnetic field is almost always very high frequency, so powering the IC driving the transmission line is a high-frequency event.

This is very rare when you deal with real low-frequency currents, and designers may need thicker aircraft. What some people overlook is that when it comes to power transmission, most of the energy is not at low frequencies, but at very high frequencies. The frequency of the energy must match the switching speed of the IC output. The power bus must provide all the harmonic frequencies in the square wave, from the clock to .5 divided by the rise time, to the output driver of the IC. This energy is a very high frequency. The result is that the skin effect takes over during power supply.

When the IC output edge rate is less than 500 picoseconds, you are now dealing with one, two, and three gigahertz frequencies. In terms of how much current copper can handle, the skin effect will become the dominant factor. In this field, one ounce of copper is no better than half an ounce of copper. Once a certain frequency is exceeded, the entire copper thickness is not used.

Some people deceive themselves to believe that they need a two-ounce copper plane. The key is to check the current at high frequencies, and then calculate the skin effect and determine the thickness of the copper really needed. In many cases, they often find that even a quarter ounce of copper is enough to meet the requirements of the aircraft. In most cases, you do not need to use an ounce or two ounces of copper on a flat surface.

5. Copper roughness

Unfortunately, the roughness of copper does have an impact. People tend to look at the voltage and current when analyzing problems. The skin effect is about the field. Everything is about the field, but the skin effect is definitely one of the problems. The roughness of copper makes it more difficult for the magnetic field to establish a uniform current to the trace copper and plane because the current is caused by the field. Since the magnetic field builds the current on the copper wire, the rougher the copper wire, the more difficult it is even if the current flows through the copper wire. As a result, the rougher the copper, the greater the loss.

At high frequencies, the skin effect loss almost requires the use of thin copper on the PC board. When manufacturing low and very thin ED copper, the process only takes longer, so the cost is higher. If we expect to obtain insertion loss in PCB transmission lines, thin copper is almost necessary.

6. Stack design

Since the energy in the circuit exists in the field, if we want our circuit board to operate according to our requirements and pass the EMI test, we must know that the field is on the PC board. Stacks must be designed to contain fields, so they will not expand and will not cause interference problems or EMI problems. Once again, to quote Dan Beeker, “Everything is about space.” The best advice I can give designers is to focus on the field. Must contain fields. This requires proper circuit board stacking and proper circuit partitioning. Correct routing, layer changes, etc.

Ralph Morrison recently released a book called’Fast Circuit Boards/Energy Management’. This book will tell designers what they really need to know about energy management in circuits to help control interference, EMI, and signal integrity issues. Very important publication.

7. Cooperation with manufacturers

One thing I encourage people to do in class is to understand the circuit boards made by manufacturers. In other words, if you want to get a 6-layer, 8-layer, or 10-layer circuit board, find out the natural 6, 8-layer, or 10-layer circuit board that the manufacturer wants to build. What is the dielectric? What is the copper weight? Find a way to design around the board. Design around these dielectrics. Designed around the weight of copper. Even on different layers, different line widths are needed to reach a certain target impedance of 50 or 60 ohms, or whatever.

Design around the manufacturer’s building. Let them know your factory drawings, “This is your 6th layer, this is your 8th layer or this is your 10th layer.” If you do, they will get better throughput. We have done this in the past, and our manufacturers have even achieved 90% high throughput on 12-layer, 14-layer, and 16-layer circuit boards with very strict impedance control.