PCB Design Guidelines for High-Frequency Circuits in Car Radar & 5G Applications

This morning I saw a very strange sight while walking down my street. A long, tangled rope of magnetic VHS tape was twisting down the road, carried by the wind. It took me back to a simpler time of Blockbusters and rewind machines. If you thought those rewind machines were fast, today’s electronic breakthroughs will make your head spin. One of the latest evolutions in PCB design is being driven by two emerging technologies, 5G networks and advanced driver assistance system (ADAS) enabled cars. Both of these technologies use something long feared by Printed Board designers, the extremely high-frequency (EHF) band. You’d best prepare your boards for a high-frequency future before they go the way of BetaMax and boomboxes.

Why Are We Using Millimeter Waves?

Weren’t RF and Microwave frequencies good enough, why are we moving up into the EHF Spectrum? There are two advancements forcing us into higher frequencies, 5G and ADAS radar.

• 5G - Telecommunication companies are looking to take us from today’s 4G/LTE speeds and latency to the faster, brighter tomorrow of 5G. Current cellular networks can give us download speeds of somewhere on the order of 10’s of megabits per second and latencies around 70 ms. 5G will be a big jump, up to 10 Gbps download with latencies under 10 ms. All of this is possible because 5G will operate in the EHF spectrum. Wider frequency bandwidths give us lower latencies, and faster frequencies give us better data rates. The industry expects 5G to start being implemented somewhere around 2018. At that time, you’ll need to be ready to deal with millimeter (mm) wavelength signals.

• ADAS Radar - One technology that’s already here is radar for ADAS enabled vehicles. Collision detection radars used to operate below 30 GHz, but recently the standard has moved up to 77 GHz. As manufacturers build more cars with ADAS capabilities, we can expect to see more radar systems driving around our streets. If you want to design PCB circuit materials that deal with any kind of car radar, you should be ready to work with EHF signals.

As both of these things grow, you’ll need to know more and more about how to deal with their operating frequencies. This is the part where I give you some material and design guidelines to help you cope with the rapidly changing PCB design ecosystem.

Material Guidelines

I actually just wrote about how to choose which material to use for your high-frequency boards. However, the frequencies we’re talking about are a bit higher than normal, so I’ll reiterate a few points.

• Very Low Dielectric Constant (Dk) - We engineers often find something that works and then stick with it. Maybe you moved your high-frequency boards one level up from FR4 and figure that will be ok for EHF. For mm waves you need to use materials with the absolute lowest Dks possible. Dk losses increase proportionally with frequency. That means a moderately low Dk is no longer acceptable.

• Very Little Soldermask - You may ask your supplier about the moisture absorption of your substrate, but I doubt you ask about the soldermask. Most soldermasks have high absorption, allowing them to gorge on water, which has a Dk of 70. A damp soldermask will introduce high losses into your mm wave circuit. You should use as little soldermask as possible on these PCB prototype designs.

• Very Smooth Copper - Your copper needs to be as smooth as possible on these boards. The skin depth for current at these frequencies is very shallow. So shallow, that it’s sometimes on the order of the mountains and valleys that make up a rough surface. Rough copper will give your current a longer path, increasing resistive losses. Use smooth copper.

ADAS car radar will require us to learn new design techniques.

Physical Guidelines

Along with material considerations, you need to think about geometry and other physical specifications. Two important things to think about are laminate thickness and transmission line characteristics.

• Laminate Thickness - You not only need to select the right kind of laminate, you need to select the right laminate thickness. Generally you want your laminate thickness to be somewhere between ¼ and ⅛ wavelength of the highest operating frequency. If your laminate is too thick, it can resonate and even propagate its own waves. Remember that laminate thickness can affect your conductor widths, so factor that into your decision.

• Transmission Line Characteristics - In regard to transmission lines, you’ll need to decide which type of conductor you want: microstrip, stripline, or grounded coplanar waveguide (GCPW). Microstrip is probably the most familiar but has problems with radiated losses and spurious mode propagation above 30 GHz. GCPW is a good choice but will suffer more conductor losses than microstrip or stripline. Stripline is great but can be difficult to manufacture, increasing costs. In addition, you’ll need to use microvias to connect the stripline to outer layers with minimal signal reflections. Microvias are difficult to fabricate, so if you choose this option work with your manufacturer to reduce potential defects.

All of us technology fanboys like to argue. First, it was Betamax vs VHS, then Blu-ray vs HD DVD, Firewire vs USB, etc. Unfortunately for us, we won’t be able to argue against high frequencies in our next gen PCB layers. Rising data rates and changing technologies like 5G and ADAS enabled cars are raising frequencies as well. Now we just have to learn how to deal with it. You’ll need to be careful about which Printed Circuit Board materials you use for these new high-frequency designs. You’ll have to keep an eye on the physical characteristics of your circuits as well.

The future can sometimes be a bit ominous, but luckily you don’t have to face it alone. Great PCB design software, like Altium Designer^®, can help you master the techniques of tomorrow. Altium Designer has a great range of advanced tools to make design easy for engineers like you.

Have questions about EHF circuit design? Call an expert at Altium.