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How 5G wireless is impacting the PCB design and manufacturing

Article By : Ken G

New process methodologies are required along with advanced testing and inspection facilities in assembly houses serving the 5G industry.

The 5G wireless technology is characterized by very high speed, greater connectivity reach, and low latency. Compared to the 4G network, 5G wireless can offer 10-20 times higher transmission rates, around 100 times higher data capacity, and less than 1-millisecond latency. Frequency spectrums extend up to the millimeter-wave band (mmWave), and this extremely high-frequency chunk is one of the toughest challenges for the PCB manufacturing industry.

So, 5G has impacted various design and manufacturing aspects, including PCB assembly. There are several challenges in building a PCB that can fulfill the advantages offered by a 5G device. Hence, new process methodologies are required along with advanced testing and inspection facilities in the assembly house.

The 5G-related PCB design challenges

Below are some of the major issues engineers confront while designing and manufacturing PCBs for 5G applications.

With the growing design complexity, 5G devices will likely use high-density interconnect (HDI) PCBs with thinner traces and higher connection pad density. But these thinner traces will cause signal integrity issues at the high-speed operations.
Impedance irregularities occur in HDI PCBs due to various factors like trace dimension, width, and cross-section. If the trace cross-section is created using a conventional subtractive etching process, the chances are that impedance anomalies cause signal loss.
PCB manufacturers have to incorporate sophisticated techniques like multiple input multiple output (MIMO) to integrate several antenna array units (AAUs). Moreover, 5G designs will require more base stations and antenna arrays to function effectively at a very high operating frequency. Not surprisingly, therefore, EMI, crosstalk, and parasitic capacitance become key issues when designing a PCB for 5G wireless.
Thermal management is of critical concern in 5G PCB designs. As high-speed signals generate enormous heat, the chosen substrate and dielectric constant should adequately handle the heat dissipation. Otherwise, problems such as copper track peeling, delamination, and board warping can degrade the PCB performance.

Figure 1 PCB designs require sophisticated new methodologies to serve 5G applications. Source: Technotronix

These 5G-related challenges have greatly impacted the PCB assembly process and have pushed the limits of traditional PCB manufacturing methods.

Technologies implemented in 5G-based PCB designs

New technologies are being implemented in PCB designs serving 5G applications; below are two techniques adopted by PCB designers to cater to the emerging 5G technology demands.

Modified semi-additive process (MSAP): To achieve high circuit density with minimal signal degradation, PCB manufacturers are using the MSAP process instead of the usual subtractive etching method. In this process, a thin copper layer is coated on the laminate where photo-resist is absent. The copper present between the conductors is further etched out. Here, photolithography is used to ensure high-precision etching with the least signal loss.

Automated optical inspection (AOI): For 5G designs, advanced AOI systems are used during PCB manufacturing to identify potential faults by measuring top and bottom signal-line conductors either in through-hole or SMT assembly. The improved accuracy of AOI fault detection reduces false alarms as well as production-line delays. New approaches using artificial intelligence (AI) focus on the actual errors that can be fixed using automatic optical shaping (AOS) systems. A consolidated AOI system can provide required data to analyze the production line efficiency.

Figure 2 High-frequency 5G networks mandate greater circuit density and lower signal degradation in PCB designs. Source: Technotronix

The 5G applications are based on high-frequency signals, and hence the mixed-signal PCB design is quite complex. Apart from using the new technologies discussed above for fabrication and testing, there are several best practices followed for an efficient 5G PCB design. Here is a sneak peek of these best practices implemented in PCBs serving 5G designs.

Guidelines to build 5G-ready PCBs

Select substrate material with the lowest possible dielectric constant (Dk) as the Dk losses increase proportionally with higher frequency.
The hygroscopic value of most solder marks is quite high and they may absorb moisture if applied in large quantities. Hence, using a little solder mask is recommended to avoid any faults due to moisture.
The skin effect of the traces increases at a higher frequency, which can restrict the current flow. So, proper design of traces with a uniform copper surface is highly recommended in the 5G-ready PCBs. Even copper planes should be smooth and symmetrical throughout the PCB stack-up to avoid any resistive losses.
Selecting the right laminate thickness is essential in the PCB for 5G designs. If thickness is higher than recommended value, laminate can resonate and propagate its waves causing interference.
The transmitter and receiver components used in the 5G design are highly sensitive. Mounting the parts accurately is essential for the proper operation of these components. Therefore, following strict footprint tolerance is crucial.
Antennas used in 5G circuits should be impedance matched with the traces. The recommended keep-out areas should be maintained to avoid EMI issues.
Choosing the appropriate transmission line is critical in high-frequency PCB designs. Among microstrip, strip line, and grounded coplanar waveguide (GCPW), using strip line may be a better choice. But it’s difficult to manufacture and may increase the production cost. Also, micro vias are required to connect the strip lines to outer layers with the least signal reflections. It needs expertise and a discussion in advance with contract manufacturer to avoid possible issues during the PCB fabrication.

5G: A growing opportunity for PCB designers

The 5G technology is continuously growing with features being added every day. So, PCB manufacturers must understand the requirements both in terms of raw material and equipment upgradation. Furthermore, investing in R&D of PCB assembly process to meet 5G technological requirements can prove to be a tremendous growth opportunity.

The consumer industry is rapidly adapting to the 5G network as it provides innovative features with robust performance. And the PCB industry will expand with the growing demand for powerful and flexible 5G devices. For that, however, PCB assembly providers must offer tools and processes that support the rapidly evolving 5G technology.

In the final analysis, it’s essential to understand the 5G’s design requirements and growing trends to keep an edge in the PCB manufacturing industry.

This article was originally published on EDN.

Ken G is a sales engineer at Technotronix.