It’s undeniable: we are moving towards a more interconnected world. And ever-expanding communications ranges, such as 5G, offer proof of this trend. However, internet architecture is dynamic as computers are consistently connecting and disconnecting. The next stage of this evolution—that includes smart factories, cities, and infrastructures—consists of more stable connectivity between important systems and depends on the continued utilization of high-speed communications.
The ability to manage and control geographically dispersed systems is only possible due to communications hubs like satellites and base stations and the high-frequency boards and electronics that drive them. However, system designers must follow practices for high-frequency PCB traces that incorporate manufacturing considerations and operational objectives.
High-Frequency PCBA Challenges
With high-frequency boards, designers encounter considerations that may be less important in other applications. The major issue, operationally, is how to transmit/process/receive signals accurately to maximize signal integrity. This is especially critical for digital signals where misidentification (assuming a high for a low or vice versa) can have a significant impact on functionality. This and other operational concerns are described below.
- Misidentifying or misreading data
The ability to accurately identify data strings is critical. Misidentified bits can translate into erratic functionality, incorrect outputs, and other unacceptable operations. Accurately reading analog signals is also important since misreads can result in inaccurate quantization and similar operational errors.
- Signal distortion
In contrast to low-frequency boards, at high frequencies transmission errors such as reflection can occur. Moreover, impedance values have a more significant impact on signal transmission.
- Inaccurate or failed operations
As most ICs rely to some degree on setting or resetting control pins, operational integrity depends on accurate identification at the pins.
Controlling the amount of EMI or noise on your board is a primary concern. Interference can occur along and between traces and can have catastrophic results if not adequately mitigated.
EMC is often a problem in high-power environments; however, your board’s radiation should not interfere with nearby electronics or be susceptible to environmental radiation that can increase onboard EMI.
Solutions to the above concerns invariably result in design specifications that may cause PCB layout mistakes. Consequently, we should turn to a few important manufacturing considerations.
- ☐ Board material dielectric
- ☐ Mismatched copper trace sizes
- ☐ Pin trace alignments
- ☐ Adequate spacing between traces and surface elements
- ☐ Creepage and clearance distances
- ☐ Via placements and annular ring sizes
- ☐ Depanelization
As this abbreviated list shows, many aspects of your board’s construction process are impacted by high-frequency PCB designs.
DFM for High-Speed Digital PCBs
Designing High-Frequency PCB Traces for Manufacturing
As discussed above, considering your board build during design is critical, especially for high-frequency PCBAs. Before discussing how to best accomplish this, though, we should correct a miscategorization concerning high-frequency boards. For electrical signals, frequency and speed are often used interchangeably because the higher the frequency, the faster the signal transitions. However, when discussing the physical implications of signal frequency for a circuit board, the speed that is important is the ratio of the time it takes for the signal to propagate along the path to the rise time, as shown below.
Slow: Tflight : Trise < 0.25
Fast: 0.25 < Tflight : Trise < 1.0
High-speed: Tflight : Trise > 1.0
High speed actually depends on how fast the signal transitions to high as compared with the length of the transmission line. Typically, high-speed design considerations are not necessary at frequencies below 10 MHz. However, above 50 MHZ, high-frequency PCB trace considerations may be significant. We suggest following a few straightforward guidelines.
Design Guidelines for High-Frequency Traces
- ★ Select board materials where the dielectric constant, dk, is constant for high frequencies.
- ★ Use low-profile copper.
- ★ Adjust trace widths and use the shortest traces possible.
- ★ Ensure differential traces match in length and are equally spaced along the path if possible.
- ★ Utilize impedance matching to help control trace size.
By following the guidelines listed above, you can help your CM build boards to meet manufacturing requirements and DFM and satisfy your operational objectives.
|Tempo's Custom PCB Manufacturing Service
Tempo Automation leads the industry in building high-quality PCBAs fast and accurately for prototyping and on-demand manufacturing. We will assist you in making the best build choices for your high-speed boards.
And to help you get started on the best path, we furnish information for your DFM checks and enable you to easily view and download DRC files. If you’re an Altium Designer or Cadence Allegro user, you can simply add these files to your PCB design software. For Mentor Pads or other design packages, we furnish DRC information in other CAD formats and Excel.
If you are ready to have your design manufactured, try our quote tool to upload your CAD and BOM files. If you want more information on the special considerations for boards with high-frequency PCB traces, contact us.