5G mmWave antenna design requires trade-offs

5G New Radio FR2 (NR FR2) spectrum above 24 GHz, known as millimeter wave (mmWave), offers very high throughput speeds and can support a large number of devices , but the signals in this range are very different from the 6 GHz and below bands used by most mobile network developers.

Building an active phased array antenna for 5G mmWave requires a very compact design. Antenna elements need to be placed at half a wavelength (i.e. 5mm) apart. At the same time, each antenna element needs to have a transmit/receive channel connected to two polarization feeds. The corporate network is also included, and the entire design must provide high heat flow in a small area. Creating a stacked PCB that meets all the requirements is a challenge even for experienced engineers.

Slight changes in operating parameters can cause an antenna to not work as expected, requiring the redesign, remanufacturing and retesting of components, subsystems and even entire systems, resulting in longer development cycles and higher development costs. In addition, physical conditions need to be considered, as production, assembly and daily operating conditions can subject delicate electronic components to excessive stress, as well as damaging heat and temperature fluctuations. In addition to these challenges, most design teams are struggling to meet tight deadlines and strict delivery dates, and the learning curve is particularly steep for beginners in 5G mmWave technology.

Our team has just completed the development of a 64-element antenna demonstrator that operates at 24 GHz to 28 GHz frequencies for 5G mmWave, and the entire team has experienced the above difficulties during the development process.

As a semiconductor company, we help our customers with projects like this by producing system-level designs. Designing at the system level gives us in-house expertise to guide our customers through various design challenges, and more importantly, we create production-ready solutions that allow our customers to skip most of the learning curve. In other words, we go through the entire development process, making trade-offs, evaluating various options, and refining the design, without our customers having to reinvent the wheel.

Our Story

The image below shows an exploded view of the product created by the NXP team.

5G millimeter wave antenna demonstrator

A team of more than 20 subject matter experts completed the antenna design, calibration, and performance testing at NXP and key partner companies that produce and design the antennas, leveraging their expertise in beam pattern verification, thermodynamics, AC/DC and DC/DC converter design, LVDS control, FPGA design, and PCB manufacturing.

Core Components

At the heart of most 5G millimeter wave antenna designs are beamforming ICs, which focus high-frequency signals to specific receivers, making connections more direct, faster, higher quality, and more reliable. Multiple beamforming ICs are connected and arranged in a regular structure called a phased array. The phased array combines the signals to produce radiation patterns that cannot be achieved by a single antenna. Beamforming is used to change the amplitude and phase of the signal at each antenna element, making it easier to focus and steer.

NXP's 64-element antenna demonstrator design helps developers save time and effort. Watch the getting started process and get tips for calibrated measurements.

A good beamformer IC helps optimize the overall performance, power consumption, and cost of each radio component, so it should be prioritized when considering design options. In this example, we use the NXP MMW9014 beamforming IC, which is a highly integrated 5G 4-channel dual-polarization analog beamforming IC in a very small FO-WLPBGA package (6.5 mm x 6.1 mm x 0.56 mm) with 182 bumps.

After selecting the beamformer, the next step was to construct the antenna panel PCB and enclosure. This step proved to be particularly important and tricky.

View of the antenna panel PCB and housing

Preventing Warping

One of the biggest challenges we face is the trade-off between antenna warpage and thermal management. We need to get the right electromagnetic (EM) performance to create an antenna that works reliably at the target frequency while ensuring a stable thermal environment to protect the electronics from failure and prevent the antenna PCB from warping.

Our goal was to have a warpage of less than 0.22%, but we exceeded that, measuring between 0.132% and 0.175%. The very low warpage was achieved because of several important design decisions. Once the antenna element design was complete, we mapped this structure to our requirements for the antenna, control, enterprise network, power lines, and ground structures. A 12-layer PCB was created symmetrically around a central core. Any warpage is a cumulative stress caused by the different thermal properties of the metal and dielectric components of the PCB.

As shown in the figure, the lower 6 layers of the PCB create the antenna, and the upper 6 layers manage the feed, power, and analog and digital distribution.

Cross section of antenna panel including material grade, layer allocation, dimensions of realized panel and photo

The PCB stackup is symmetric about the z-axis. Since copper can interfere with the operation of the antenna element, we distribute all the copper in the system to the sides of the PCB, away from the antenna array.

To further improve the antenna’s reliability and make the PCB more resilient to failures due to thermal cycling, we limit the number of stacked vias to three and use staggering if more vias are needed. Staggering vias counteracts the damaging effects of different thermal expansion coefficients of PCB materials such as copper and dielectrics. High temperatures are encountered during the manufacturing phase, especially when soldering is performed, and this approach reduces warpage even after soldering.

To prevent the heat sink from damaging the tiny MMW9014K package, the clamping force is kept to less than 1g per ball, preventing solder ball creep during the antenna’s operating life from causing shorts.

To increase the protection of the PCB and maintain its shape, the PCB is placed in an adjustable frame made of nylon on one side to reduce antenna interference and metal on the other side to mount the PCB without adding physical stress to the delicate circuitry.

Panel Demonstrator Kit

Keep Cool

To guarantee the longevity of the IC, heat flow needs to be managed. This is another means of minimizing warpage, as it means that very thin thermal interface materials (TIMs) can be used. The TIM is typically the item with the highest thermal resistance in the thermal chain, so the goal is to make it as thin as possible. To simplify assembly, the TIM is used integrally on the heat sink silicon interposer that is the basis of the demonstrator’s mechanical design. The demonstrator’s physical housing is removable, allowing for easy management of the internal connectors. This is a huge advantage when working in a small space antenna measurement chamber where space is constrained.

This mmWave splitting network is used as an enterprise beam splitter. To improve the isolation between the beam splitter and the antenna feed, we put the transmission line on the inner layer. The results show that there is still no oscillation characteristics with a beamforming gain of 30dB. We also designed the transmission line to work with the TIM and heat sink to meet the heat dissipation requirements of the design. The scanning range of the antenna is ±45°.

Finally, our Vcc distribution decisions simplified the design and improved efficiency. We used a 19V power supply to generate the required 2.8V operating voltage, which allowed the use of a single power supply through a standard AC/DC converter and reduced the amount of wiring required in the antenna measurement room. The current routing of all power lines placed both the forward and return paths in designated locations.

Ready to run

We leveraged the combined expertise of our teams and NXP’s long history of success in antenna arrays and volume production to create a standalone solution that can be used by anyone building 5G mmWave antenna arrays.

The panel demonstrator is included in a kit that contains everything needed to analyze antenna parameters, including the temperature of each antenna in the array. The demonstrator is fully calibrated with antenna patterns, beam pattern verified, and follows mass manufacturing guidelines, so design teams can quickly move from prototyping to mass manufacturing, with strict design rules for trace widths, blind vias, and layer thicknesses applied to the system.

A separate evaluation kit for use with Matlab comes with an AC/DC converter and sample code in .dll format, providing additional analysis options. The demonstrator also features an FPGA interface board that converts the USB connection from the PC into the LVDS main and control signals used by the antenna array.

Adjustable frame protects delicate circuitry and helps minimize antenna interference

Thanks to the entire team that contributed to the project:

 NXP:

 Mustafa Acar

 Konstantinos Giannakidis

 Harm Voss

 Nick Spence

 Arthur van de Kerkhof

 Ashutosh Dwivedi

 Dick van de Broeke

 Evert van Capelleveen

 Jan Willem Bergman

 Jeroen Zaal

 Arjan van den Berg

 John Janssen

 Rajesh Mandamparambil

 Ramon Groot Wesseldijk

 Paul Mattheijssen

 TNO

 Roland Bolt

 Erwin Suijker

 Stefania Monni

 Philips

 Yizhe Yin

 Arthur van Es