Today most electronic devices, sometimes even very simple ones use integrated circuits that are capable of operating at very high speeds. These devices demand a carefully designed high-speed PCB to operate correctly.
When designing such a high-speed PCB, there are a large number of factors and parameters that have to be selected precisely to make sure that the design will work in the first go.
In this article, we will guide you through the process of designing a high-speed PCB and everything you need to consider in the process.
1、What is the High-Speed PCB Design
Whenever you have signals on a PCB that have extremely fast edge rates, they then become dependant on minute factors in the PCB layout. These signals can lose integrity and are affected by physical parameters such as board stack-up, impedance, loop area, general layout and package type.
So a PCB design process that is optimised to retain signal integrity for these fast-changing signals is referred to as a high-speed PCB design.
A high-speed design uses high-grade PCB materials with tight fibre interwinding and will also have tight tolerances and controlled impedance.
High-speed designs would also try to use small component packages such as BGA, LGA, MSOP, etc., instead of larger packages.
Image 1: A microcontroller with traces going to an external connector
2、High-Speed PCB Design Challenges
The following section will highlight the main design challenges when it comes to designing high-speed PCBs
Since even minimal changes in PCB characteristics can affect signal behaviour, it is essential for your manufacturer to have tight tolerances.
These are tolerances typically for things like controlled impedance traces, board Stackup and capacitance and also the overall length and width of marks.
2.2 Availability of high-speed material
Beyond certain speeds, you may start to require the use of unique PCB material if the signals get too fast.
Standard FR4 based fibreglass is not the best when it comes to designing high-speed boards. A few of the PCB materials used for high-speed designs are enhanced FR-4, Polyamide, PTFE.
2.3 Selecting the correct layer Stackup
Another critical factor, especially in multi-layer boards is selecting the right layer Stackup, were to have a thick prepreg layer, where to have your power and ground planes and also the separation between the sheets. All these factors, when chosen correctly, will improve signal integrity.
2.4 Board to Board Interconnections
When a design has multiple high-speed boards that need to be connected, it is essential to use a interconnect that can properly transmit high-speed signals, typically these will be shielded, and length matched cables, and the connectors would have their pins oriented in a way to minimise and signal losses.
Image 2: An extremely fine pitch QFN chip requires tight tolerances as shown
3、 High-Speed PCB Design Skills
3.1 Knowing a design software that is capable of advanced options
High-speed designs require many sophisticated features in your CAD software. Many programs intended for hobbyists may not have these. Web-based suites also don’t usually have advanced options.
So one needs to learn and be skilled at a power CAD tool.
3.2 High-Speed Routing
A designer needs to know the rules for necessary routing when it comes to high-speed traces. It would include things such as not cutting ground planes and keeping trails short. Keeping digital lines not too close to avoid crosstalk, and shielding any interference producing elements so that signal integrity is not compromised.
4.3 Routing traces with controlled impedance
Certain types of signals require impedance matching. These are usually in the order of 40-120 ohms. Characteristic impedance matched hints are Antennae and many differential pairs.
The designer must be familiar with how to calculate trace width and layer stack for required impedance values. If a trace does not have the correct impedance value, the signal can be severely degraded, and this would result in data corruption.
3.4 Length Matching Traces
High-speed memory busses and interface buses have many lines. These lines operate at very high frequency, and so it is essential that the signals arrive from the sending end to the receiving end at the same time. It requires a feature called length matching. Most common standards define tolerance values that must match length.
3.5 Minimizing Loop Area
High-speed PCB designers must know that high-frequency signals cause a lot of EMI, EMC issues. To minimise these issues, they must follow basic rules like having continuous ground planes and reducing loop areas by optimising current return paths for traces and putting in lots of stitching vias.
Image 3: Controlled impedance and differential pair traces coming out of a BGA chip
4、High-Speed PCB Design Considerations
The following are the most important design considerations when designing high-speed PCBs
4.1 Signal Reflections
When a signal is a high speed, the trace carrying it acts as a transmission line with its characteristic impedance, to minimise and reflections along the trail, it is required to match the line impedance with the signal source impedance.
4.2 Signal Ringing
Signal ringing is when you have some unwanted shifting in the voltage or current on the transmission line, which causes extra current to flow and causes delays in the arrival of your signal at its final destination. It can be an issue with messages such as a high-speed clock line. So these signals should be shielded from any interference.
4.3 Crosstalk Issues
When two traces with high-speed signals are close together, then they can induce voltages into each other. So it is essential not to place such marks too close as to prevent interference with each other.
4.4 Signal Timing
If two traces carry high-speed signals and their lengths are different, then the messages arrive at their destination, and slightly different times, this can be a big issue if the message is supposed to be synchronised to a standard clock line. In these cases, one needs to length match signals.
5、How to Know if You Need a High-Speed Design?
5.1 Does your PCB have high-speed interfaces?
A quick way to find out if you need to follow high-speed design guidelines is to check whether you have high-speed interfaces such as PCI-e, DDR or even video interfaces like DVI, HDMI etc.
All these interfaces require you to follow high-speed design rules. Provide the exact specifications of each data in its documentation.
5.2 The ratio of your trace length to the wavelength of signals
Generally speaking, if the wavelength of your messages is comparable to the trace length, then your board will definitely require a high-speed design. Some standards like DDR need traces that have to be length matched to minimal tolerances.
An excellent rough figure is that if your trace length and wavelength are within one order of each other. Then you should look at high-speed designs.
5.3 Boards with wireless interfaces
Any board that has an antenna, whether on the board or through the connector, it must design for high-speed signals. Onboard antennas also need close impedance matching and length tuning.
Boards with SMA connectors or similar would need the traces going to the connector to be of a specific impedance value.
A detailed video on high-speed design considerations can be found here: https://www.youtube.com/watch?v=6jrVZu7eqiw
Image 4: High speed interfaces like DVI and USB require high speed design
6、Why is OurPCB the Best Choice for You?
Image 5: A pick and place machine placing SMD 0201 components onto a pcb
It’s important to pick the right manufacturer for your high-speed designs. Someone who can make your designs with precision, OurPCB is one such manufacturer.
OurPCB is a multinational PCB Manufacturing and PCB Assembly company that provides Global service and support while utilising its Chinese manufacturing capabilities.
OurPCB’s factory is equipped with advanced machinery, including but not limited to three high-speed SMT lines, Siemens HS50 Pick and placers, Siemens F5 pick and placers, Heller 1809 Reflows, Speedline MPM printers, OK intelligent solder irons, and Otek Automatic Optical Inspection units to name a few. We assemble BGA, LGA, QFN, QFP, DIP, SIP etc. The smallest SMT footprint that can be mounted is 0201. We can also provide programming, wiring as well as injection and conformal coating services.
The high-speed PCB design is all about taking all the above points into account and choosing the right manufacturer for your high-speed design. These designs require accurate manufacturing processes and testing after production.
OurPCB handles the challenges of high-speed design and ensures you get a high-quality product.
For more information and to order, visit: https://www.ourpcb.com/.