PCB Design

RF PCB Design: A Practical Guide for Engineers

RF PCB

What is RF PCB Design?

We all know PCB design includes schematic design, PCB layout design and Gerber file generation. RF PCB is called radio frequency PCB, it is a high-speed PCB board. As for RF PCB design, it is a kind of PCB design which will operate at a high frequency typically above 100MHz. So all above 100MHz will call it high-frequency signals.

This article is a practical guide to RF PCB design engineers, which would be helpful to designers who are not very experienced in radio frequency or RF PCB design.

Important Rules for Schematic Design and PCB Layout Design

Some important rules should be considered on the schematic side before going into the layout side.

1. Schematic Design: When it comes to high-speed design, we have to take care of making the proper schematic that is

  • Making the block of critical components,
  • Specifying the high-speed signals,
  • Specifying the differential pair signal,
  • Considering the routing information (impedance matching, length matching, power integrity and signal integrity).

These all things need to be taken care of for designing the high-speed schematic design.

2. PCB Layout Design: There are some high-speed guidelines we need to follow while designing the high-speed boards:

  • Make sure that the length matching, impedance control, space, width, and differential pair routing are defined properly
  • The proper return path for all high-speed signals
  • Proper ground reference
  • The high-speed signal should be routed in the same layer
  • Try to route track as short as possible
  • Try to minimize the via

RF PCBs

In this high-speed design, we will see a lot of issues, including crosstalk, EMI and EMC, if we don’t route correctly. Let’s see first what crosstalk, EMI, and EMC are.

Crosstalk: The coupling between an unwanted signal or the adjacent layer on a circuit board will be called a crosstalk. There are two different kinds of crosstalk:

  • forward crosstalk
  • backward crosstalk

To avoid the crosstalk, we need to follow the following things that are:

  • Route the signals in the same layer
  • Try to route the signal with the respective ground
  • Proper stack-up design
  • Proper ground reference

EMI and EMC: EMI is short for electromagnetic interference, while EMC represents electromagnetic compatibility. To reduce EMI and EMC issues, take care of the below considerations:

  • Arranging PCB layers properly
  • Placement and segregating the sensitive components
  • Ground plane
  • Shielding
  • Using the decoupling capacitors
  • Termination resistor in the transmission line

When high-speed signals travel from source to destination, some signals will not reach the destination because of long distances, causing parts of signals to reflect back. So this kind of signal is called signal distortion.

This problem occurs when the signal is low and the destination is so far. In this condition, we need to use the termination resistor to control the impedance.

The other side is the decoupling capacitor. What is a Decoupling Capacitor? This is nothing but to protect the low frequency /low noise signals. Mainly the decoupling capacitor of one pin will connect to the power and another pin is ground.

Also read: The Basics About PCB Design and Layout

There are lots of things we need to take care of the PCB layout design. For high-speed design, a PCB designer should know how to build the stack-up. If the stack-up is properly designed, then we can easily route the signal. Many high-speed boards contain multi-layer boards because the design only contains different kinds of high-speed signals (Ethernet, USB, PCI, etc.).

For example, if you start to design the DDR4, you should take care of the data signal, address signal, control and command signals. In this way, you can’t route the data signal in the same layer (D0 to D7 and D8 to D15). You have to route each data group signal in different layers.

There is one example of how to design the 12 layer stack-up shown in the image below.

12 layer stack-up

Other Important Things You Should Know in RF PCB Design

Once you start routing, you should verify the stack-up which will be provided by the fabrication person. Most PCB designers will get the stack-up from manufacturers.

The high-speed design which contains different kinds of vias that are blind, buried and micro vias. Even you should have the knowledge about how to route the signals. Other important things you should know in RF PCB Design:

  1. Ground bouncing
  2. Back drilling
  3. Deferential pair routing
  4. Signal integrity
  5. Power integrity
  6. Material used for high-speed board
  7. Stitching via
  8. 8. Via-in-pad
  9. 9. Impedance calculation
  10. 10. DFM, DFT and DFA

Let’s discuss the above topics one by one thoroughly.

  • Ground Bouncing: It occurs when the proper ground path is not given or the ground path is poor. To reduce the ground bouncing, we should use the decoupling capacitor.
  • Back Drilling: Removing unwanted stub to create the via so that the signal can travel easily from one layer to another layer. This is also called the controlled depth drilling (CCD).

PCB Back Drilling

  • Deferential Pair Routing: It is nothing but the signal can route side by side with equal magnitude and opposite polarity. While routing the differential pair, the space and width of the track should be same.
  • Signal Integrity: When signals transmit through the transmission line from transmitter to receiver, it will measure the signals.
    Power Integrity
  • PCB Materials: There are different kinds of materials used for PCB manufacturing. The application of those materials depends on their properties and characteristics. Normally for high-speed design, PCB boards will use resin impregnated woven glass fabric (FR-4).
  • Stitching Via: Stitching via is used to tie a via where large copper areas on different layers are used to create strong connections between different layers. For this, we can maintain low impedance, thermal analysis, heat dissipation, etc.
  • Via-in-pad: The via is placed in the electronic component to make electrical connections between layers. Using the via-in-pad gets extra place and also lowers impedance.

Via in Pad

  • Impedance Calculation: Strip line and microstrip line are two factors that we take into consideration. The Strip line connects only the inner layers but not the outer layers. Likewise, the microstrip line is used to route the outer layers.

DFT, DFM and DFA: DFT, DFM, and DFA are important in high-speed design.

DFA:
Try to minimize the number of parts in the PCB design

  • Place the maximum components on the single side
  • Take care of the component’s orientation
  • Provide the minimum clearance from via to components or pad to pad, etc.

DFM:
Try to avoid tight component placement

  • Maintain PCB spacing, track width, and thickness
  • Make the PCB penalization to reduce the cost
  • Try to avoid the sharp edge of pad
  • put proper solder mask

DFT:
Maintain the spacing between the test point and PCB edge (3.175mm)

  • The test point should be on a single side
  • The test point should have proper clearance
  • Build the fixture for automatic testing
Conclusion

Nowadays, high-speed design is more popular and different software is used to design the PCB. With high-speed board design working with this kind of design, we will get lot.

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