B a c kg r o u n d
Electronics
How to design a high-speed PCB NCAB Group’s Randy Wessels gives a couple of design pointers. Randy Wessels
N
ot so long ago, the words “high speed” didn’t exist in the vocabulary of PCB designers. In those days, their work was all about putting the pieces together and strategizing their way through a physical board layout. Today, they’re dealing with data rates of 10 or even 25 Gb/s. At these speeds, there’s a bunch of invisible forces to worry about, like electromagnetic interference (EMI), crosstalk, signal reflection, material weave and the list goes on and on. At NCAB Group, we see a lot of customers in need of guidance at the beginning of a project. We help them find the best starting point for a high-speed PCB that’s not only manufacturable but also takes into account the cost and quality drivers. Based on our experience, here are a couple of design tips.
IPC 4103 specifies materials for highspeed/high-frequency applications. FR-4, classified as low-speed, is great when you’re working with clock speeds under 5 Gb/s. It has a decent ability to control impedance and is also known for its low cost – depending on the characteristics. In the realm of high-speed design, you’ll likely be working with Nelco, SI or Megtron. Each is suited for 5-25 Gb/s clock speeds. The price and lead times are also relatively good. If your first high-speed design is pushing 56 Gb/s, then you’ll likely end up using a Rogers laminate. This is a high-frequency, high-temperature material known for its good impedance consistency, but it’s also expensive to produce and has long lead times.
Materials
Impedance
Start your high-speed design process with a plan. Without a plan and a strategy for your project, you’ll likely encounter setbacks and unexpected issues. So before even laying down a symbol or connecting a net, you need some kind of a checklist at hand of what you can expect and what you want as an end product. Document every detail of your board stackup for manufacturing. Take enough time to thoroughly define the stackup requirements. This is a perfect moment to get together with your manufacturer and determine which materials or IPC spec you should use for your board and which specific design rules you should follow.
Another important aspect is impedance matching. When energy is transmitted, the load impedance must be equal to the characteristic impedance of the transmission line. In that case, there’s no reflection in the transmission, indicating that all energy is absorbed by the load. Otherwise, there’s energy loss in transmission. In high-speed PCB design, impedance matching is related to signal quality. Rather than looking at the frequency, the key is to look at the steepness of the signal edge, ie the rise/fall time. We’re talking high-speed if the rise/fall time is less than six times the wire delay – which is typically 150 ps/inch. In that case, impedance matching is called for.
If there’s a consistent signal propagation speed everywhere on the transmission line and the capacitance per unit length is the same, then the signal always sees a completely consistent instantaneous impedance during propagation. This is called the characteristic impedance of the transmission line. It’s related to the board layer on which the PCB conductors are located, the material (dielectric constant) used by the PCB, the trace width and the distance between the conductor and the plane – it has nothing to do with the trace length. The characteristic impedance can be calculated using software like Speedstack and Si9000. In high-speed PCB layout, the trace impedance of a digital signal is generally designed to be 50 ohms. This is an approximate number. Generally, the coaxial cable baseband is 50 ohms, the frequency band is 75 ohms and the twisted pair (differential) is 100 ohms.
Help
There are many other things to consider when designing a high-speed PCB. I recommend consulting your PCB supplier when you have questions or need help to achieve a good design for manufacturing. Randy Wessels is an account manager at NCAB Group. For more information on RF PCBs, see www.ncabgroup.com/rf-radiofrequency-pcb/. Edited by Nieke Roos
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