RF HDI PCB

RF Meets HDI PCB Design in mmWave Sensors

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Modern sensor systems are all the rage in areas like healthcare, military, and increasingly automotive. The newer sensor systems seen in these areas integrate a digital processor with multiple sensor blocks such that data from the sensors can be used to create a more complete vision of the surrounding world. The systems also involve the inclusion of RF circuits and subsystems, and increasingly the inclusion of mmWave sensors and interfaces. The PCBs for these systems can be very complex in terms of construction, design, and layout.

To fit all of this functionality on one board, today's mmWave sensors are using HDI design techniques. These are necessary both for the digital portion of the PCB and the RF portion, with the RF section relying on HDI for routing and to incorporate specialized RF ASICs.

Post designers will look at the systems and have something to say about the loss on copper interconnects, as HDI designs typically use thin copper, which then has high skin effect loss for RF signals. However, it is possible to balance the HDI needs for component placement with the loss requirements of RF signals in these systems.

System-Level View of Modern mmWave Sensors

The block diagram below shows a general system view of many mmWave sensors with onboard digital processing. The digital section is dominated by a large processor, typically an FPGA, as well as supporting components like memory.

 

RF PCB HDI

 

The architecture above is typical for many digital systems, but with the inclusion of sensor interfaces alongside a large digital processor. There is also the RF section, which involves its own set of layout and routing challenges.

HDI design techniques become apparent when we start looking at some of the components used in these systems, both for the digital processor and the RF subsystem. At mmWave frequencies, RF ASICs must use fine-pitch packaging for purposes of ensuring TEM mode propagation into copper interconnects on the PCB. This is something I have discussed in another article at this link. Let's dive deeper and look at the digital section and RF section in detail to see how to approach these designs.

Digital Section

With these systems mostly running with large FPGAs as the main system processor and application host, the packaging on these components typically involves a moderate pitch BGA. When form factor is not heavily constrained, 1 mm pitch BGAs are common, but finer form factors will demand a finer pitch BGA package for the system processor. Depending on the ball pitch in the BGA package, the design may be forced to use a mix of the following design approaches:

  • Stacked blind and buried vias
  • Via-in-pad fan-out
  • Laser drilling of small microvias

All three features are solidly in the realm of HDI design. The greatest reason to use such an approach is to accommodate very high layer counts needed to reach all of the IOs in the design while allowing room for fan-out below the BGA footprint. In a larger BGA footprint, through-hole vias could be used for dog bone fan-out. It is once the package gets smaller that HDI techniques must be used.

 

RF PCB HDI

 

In this example, we have multiple impedance-controlled traces that must route into a BGA, and the ball size and pitch both limit the available space for routing. When given a fine-pitch package with impedance-controlled routing and small via size, the board quickly becomes an HDI build. The thought process goes as follows:

  1. The BGA pitch determines the ball size, and thus the space available for routing.
  2. The available routing space also sets an upper limit on the via size.
  3. To keep the aspect ratio low and ensure impedance-controlled traces on the surface layer are thin enough to enter the BGA, a thin layer may be required.
  4. The choice of layer thickness is checked against the aspect ratio constraint such that the via aspect ratio under the BGA is at or below one.

This effectively completes the digital routing plan for the main processor in an advanced mmWave sensor. Due to the various applications for mmWave sensors requiring small size and weight, this pushes the BGA package size for main processors to much finer pitches. The same applies for RF ASICs, but there is an additional reason for the fine pitch and small layer thickness.

RF Section

RF interfaces in some mmWave sensors can operate as high as 60 GHz. In radar-based mmWave sensors, automotive bands are sometimes used and operate at up to 77 GHz. The data collected by the systems then has to be extracted at a high data rate, so these components will have multiple high-speed interfaces on them which must then route to the main processor.

The first instance where we see HDI design techniques in mmWave systems is in connector or ASIC ball-out arrangements. Take a look at the image below, which I frequently show in my live courses. This image shows the signal output along the edge of a package for an RF ASIC. Note that the ground wall arrangement surrounds the signal ball in a nearly coaxial pattern.

 

RF PCB HDI

 

This nearly coaxial arrangement of ground balls around a signal ball ensures the impedance and TEM bandwidth for an incoming microstrip line can be maintained on the transition into the ASIC package. Some simulation studies have shown that 1 mm ball pitch on RF ASICs causes a TEM mode cutoff of approximately 50 GHz. To ensure sufficient margin and TEM bandwidth, the ball-out needs to be much finer pitch; a 60 GHz system could operate near the TEM bandwidth limit in a 0.8 mm ball pitch package, while 77 GHz needs smaller ball pitch. The example above is for AWR2243, a radar transceiver from Texas Instruments, which has a 0.65 mm ball pitch.

Indeed, it is well known that the same effect occurs with connector footprints, particularly RF coaxial connectors with SMD mounting. The image below shows an approximation for the TEM cutoff and transition region impedance for a coaxial connector footprint; image credit goes to Samtec. We essentially have the same structure in the RF ASIC as we do in the connector footprint shown below.

 

RF PCB HDI

Image source: Boesing, D. "Maximizing RF Launch Performance: The Impact Of GND Vias." Samtec.

 

Routing in mmWave sensor systems is normally done as grounded coplanar waveguides in microstrip configuration. In simpler RF modules, we normally do not use HDI techniques, which means the vias along the grounded coplanar waveguide are through holes. In a high-density mmWave sensor with a digital section, it makes much more sense to use blind vias along the grounded coplanar waveguide. The following image shows why this is the case.

The image below shows blind vias used along a grounded coplanar waveguide on the top layer in an mmWave sensor. The digital routing on inner layers is shown as highlighted traces in the image. It should be quite clear why vias on the grounded coplanar waveguide need to be blind vias. If these vias were through-hole vias, there would not be any room at all for the digital routing. When the RF routing is parallel to multiple waveguides, such as you would have in a phased array, the use of through-hole vias would totally eliminate digital routing in a large section of the PCB.

 

RF PCB HDI

 

TEM Cutoff in an HDI RF Waveguide

Finally, the size of the trace and the distance between vias in the waveguide will determine the TEM mode bandwidth in an mmWave waveguide. The TEM bandwidth for the above example can be calculated with the method shown in this linked article, which involves an approximation of the waveguide as a closed rectangular waveguide that is grounded on three surfaces. When we calculate the eigenmodes of this waveguide and their frequencies, we can see that even modest-density HDI techniques push the TEM mode cutoff above 100 GHz.

 

RF PCB HDI

 

The example shown above could still be fabricated with mechanically drilled blind vias on the top layer. Laser-drilled microvias could be used to provide finer via pitch and thus increase the shielding effectiveness of the via fence along the waveguide.

Taken together, HDI design and manufacturing processes are instrumental in building compact sensors that can integrate digital and mmWave features into the same board. Normally, mixed-signal design at lower frequencies would make control over noise propagation and crosstalk quite difficult, and one would have to rely on creative placements to prevent mixed-signal crosstalk. This can be less of a challenge with an mmWave sensor system that uses HDI design techniques for layout and routing.

 

Whether you're designing high-speed PCBs for mil-aero embedded systems or a complex RF product, you should work with a design and development firm that can ensure your product will be reliable and manufacturable at scale. NWES helps aerospace OEMs, defense primes, and private companies in multiple industries design modern PCBs and create cutting-edge embedded technology, including power systems for high reliability applications and precision control systems. We've also partnered directly with EDA companies and advanced ITAR-compliant PCB manufacturers, and we'll make sure your design is fully manufacturable at scale. Contact NWES for a consultation.

 



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