In-depth article | Trade-offs in 5G millimeter wave antenna design

Building active phased array antennas for 5G millimeter wave requires a very compact design. Antenna elements need to be placed at half-wavelength (i.e. 5mm) intervals. At the same time, each antenna element needs to have a transmit/receive channel connected to two polarized feeders. Corporate networks were also included and the entire design had to provide high heat flow within a small area. Even for experienced engineers, creating a stacked PCB that meets all requirements can be a challenge.

Small changes in operating parameters may cause the antenna to not work as expected, requiring components, subsystems, or even the entire system to be redesigned, remanufactured, and retested, 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 precision electronic components to excessive stresses, as well as damaging heat and temperature fluctuations. On top of these challenges, most design teams are struggling to meet tight deadlines and tight delivery dates, and the learning curve is particularly steep for newcomers to 5G mmWave technology.

Our team has just completed the development of a 64-element antenna demonstrator that operates at the 24GHz to 28GHz frequency of 5G millimeter wave. The entire team experienced the above hardships during the development process.

As a semiconductor company, we help our customers through these types of projects by producing system-level designs. Designing at the system level, we gain in-house expertise to guide customers through a variety of design challenges and, more importantly, we create production-ready solutions that enable our customers to skip much of the learning curve. In other words, we go through the entire development process, making trade-offs, evaluating options, and improving the design without our customers having to repeat the work.

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

More than 20 teams of subject matter experts completed antenna design, calibration, and performance testing at NXP and important partner companies that produce and design antennas. They gave full play to their respective expertise in beam pattern verification, thermodynamics, AC/DC and DC/DC conversion. Expertise in the fields of device design, LVDS control, FPGA design and PCB manufacturing.

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 into a regular structure called a phased array. Phased arrays combine signals to create radiation patterns not possible with a single antenna. Beamforming is used to change the amplitude and phase of the signal at each antenna element, allowing for easy focusing and steering.

A good beamformer IC helps optimize the overall performance, power consumption, and cost of each wireless component and should therefore 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 is to build the antenna panel PCB and enclosure. It turns out that this step is particularly important and particularly tricky.

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

Our goal was for warpage to be less than 0.22%, but in fact we exceeded this goal, measuring between 0.132% and 0.175%. The ability to achieve very low warpage is the result of several important design decisions. After completing the antenna unit design, we mapped this structure into our requirements for the antenna, control, enterprise network, power lines, and ground structure. Create a 12-layer PCB symmetrically around the central core. Any warpage results from accumulated stress caused by the different thermal properties of the PCB's metal and dielectric components.

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.

The PCB stackup is symmetrical on the z-axis. Because copper can interfere with the operation of the antenna elements, we distributed all of the system's copper on the sides of the PCB, away from the antenna array.

To further improve the reliability of the antenna and make the PCB more resilient to failures due to thermal cycling, we limit the number of stacked vias to 3, using staggering if more vias are required. Staggered vias counteract the damaging effects of different thermal expansion coefficients of PCB materials such as copper and dielectrics. High temperatures occur during the manufacturing phase, especially when welding is performed, and this method reduces warping even after welding.

To prevent the heat sink from damaging the compact MMW9014K package, we keep the clamping force to less than 1g per ball to prevent the solder balls from creeping during the antenna's operating life and causing short circuits.

To add protection to the PCB and maintain its shape, we place the PCB in an adjustable frame. The adjustable frame is made of nylon on one side to reduce antenna interference, and metal on the other side to enable PCB mounting without adding physical stress to the delicate circuitry.

To ensure the life 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 (TIM) can be used. The TIM is usually 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 in the heat sink silicon interposer, which is the basis for the demonstrator's mechanical design. The demonstrator's physical housing is removable for easy management of internal connectors. This is very advantageous when working in small space-constrained antenna measurement rooms.

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 decision simplifies the design and improves efficiency. We use a 19V power supply to generate the required 2.8V operating voltage, enabling the use of a single power supply via a standard AC/DC converter and reducing the amount of wiring required in the antenna measurement chamber. The current path for all power lines places both the forward and return paths at the designated locations.

We leveraged the team’s combined expertise and leveraged NXP’s longstanding 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 the kit, which contains everything needed to analyze antenna parameters, including the temperature of each antenna in the array. The demonstrator is fully calibrated with antenna patterns, verified with beam patterns, and adheres to volume manufacturing guidelines so design teams can quickly move from prototyping to high-volume manufacturing, using strict design rules such as trace widths, blind vias, and layer thicknesses.

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

Thanks to the entire team who contributed to the project!