The Opportunities and Challenges to 5G PCBs

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As a known PCB (Printed Circuit Board) might be the heart of each electronic device, is very important not only because it allows the electrical connections amongst the various components, but also because it carries digital and analog signals, high-frequency data transmitting signals, and power supply lines. With the introduction of 5G technology, what will be the challenge to 5g PCB design?

Mixed Signal Acceptance 5G PCB Designs

Most devices now are taking on old generation PCBs. This means components are transmitting and receiving frequencies that will range from 600 MHz up to 5.925 GHz and bandwidth channels of 20MHz, or 200kHz for the Internet of things. When creating a 5g PCB design, the components will require mm-wave frequencies of 28GHz, 30GHz, and even 77GHz in line with the application. For bandwidth channels, 5G systems should be dealing with 100MHz right below and 400 MHz right above 6GHz frequencies.

These higher speeds and higher frequencies will demand the appropriate materials inside the PCB to recapture and transmit both lower and higher signals on top of that without experiencing signal loss and EMI. In addition, an added problem is the fact that devices will become lighter, portable, and smaller. With strict weight, size, and space limitations, the PCB materials should be flexible and light while accommodating all of the microelectronics over the board.

Thinner lines and more strict impedance control will have to be followed when it comes to PCB copper lines. The standard subtractive etching utilized for 3G and 4G high-speed PCBs could be switched out for modified semi-additive processes. These improved semi-additive processes will provide you with more precise trace lines and straighter walls.

Material substrates are also being newly designed. Printed PCB companies will be looking at materials that offer a dielectric constant as low as 3, as standard materials for lower-speed PCBs are usually 3.5 to 5.5. More restrictive fiberglass weaves, lower heat dissipation factor loss materials, and low-profile copper can also be choices for 5g PCB employed for digital signals to prevent signal losses and to promote greater signal reliability.

EMI Shield Concerns

EMI, cross talk, and parasitical capacitance are major issues with 5g circuit boards. To manage cross talk and EMI that will be present due to the analog and digital frequencies over the board, separating the traces is highly recommended. Using a multilayer board will offer greater versatility to select how exactly to position the high-speed traces therefore the routes for both analog and digital return signals will undoubtedly be kept far from each other, while also setting the AC/DC circuits apart. Adding shielding and filtering when scheduling components also needs to lower the quantity of natural EMI that’ll be present regarding the PCB.

To make sure that there are no defects on top associated with the copper in addition to critical shorts or opens, higher-level robotic AOI systems and 2D metrology with greater features are going to be used more regularly to inspect the conductor’s traces along with to measure them. These technologies will help 5g circuit boards fabricators seek out possible signal degradation risks.

Thermal Management Challenges

Higher signal speeds will result in higher generated heat from the electricity passing through the PCB. PCB materials utilized for the dielectric materials, as well as the core substrate layers, will have to adequately handle the high speeds required for 5G technology. In the event that materials are insufficient, copper trace peeling, delamination, reducing, and warping can result given that problems could cause deterioration to the 5g PCB.

To manage this higher environment, fabricators will have to give attention to material selections that target thermal conductivity and thermal coefficients. A material offering higher heat dissipation, excellent heat conversion, and steady dielectric constants will undoubtedly be necessary to create a beneficial PCB that may provide all of the 5G functions which are required for the application.

5g PCBs Design Hints

The style of a printed circuit board for 5G applications is entirely centered on the handling of mixed high speed and high-frequency signals. In addition to the standard rules regarding the design of 5g PCBs with high-frequency signals, it is necessary to pick the materials appropriately so that you can prevent power losses and guarantee the integrity of the signal. In addition, EMI which could arise between your parts of the board that manages analog signals and those that handle digital signals should be prevented, thus meeting the FCC EMC requirements. The 2 parameters that guide the option of the material are thermal conductivity and thermal coefficient of dielectric constant, which describes changes in the dielectric constant (typically in ppm/°C). A substrate with high thermal conductivity is actually preferable since it is able to easily dissipate the warmth made by the components. The thermal coefficient of the dielectric constant is an equally important parameter, as variations within the dielectric constant can induce dispersions, which in turn can stretch digital pulses, replace the signal propagation speed, and in some cases also produce signal reflections along a transmission line.

PCB geometry also plays a simple role, where geometry means laminate thickness and transmission line characteristics. Pertaining to the initial point, it’s important to choose a laminate thickness that can be typically between 1/4 and 1/8 of the highest operating frequency’s wavelength. If the laminate is just too thin, there is a risk of it resonating, and on occasion even propagating the waves through the conductors. Pertaining to transmission lines, it’s important to decide which type of conductor you intend to use: microstrip, stripline, or grounded coplanar waveguide (GCPW). Microstrips are one of the most familiar ones, but they have problems with radiated losses and spurious mode propagation above 30 GHz. Strip lines are a valid solution, too, but they are tough to manufacture and for that reason more costly. In addition, micro vias are employed to connect the strip lines to your outermost layers. GCPWs are a fantastic choice but they offer higher conduction losses than microstrips and straplines

After selecting the substrate material, designers shall proceed with the common rules applicable to high frequency 5g circuit board design: utilize the shortest possible tracks and check both the width together with the distance between your tracks in order to keep consitently the impedance constant along with all the interconnections. Here are some recommendations, or hints, useful for designing a PCB for 5G applications:

Choose materials with a low dielectric constant (Dk)

since Dk losses increase proportionally with the frequency, it is crucial to select materials utilizing the lowest possible dielectric constants;

Use little solder mask

most solder mask inks have a top moisture absorption capacity. In the event this happens, high losses can happen when you look at the circuit;

Use perfectly smooth copper traces and plans

the existing skin depth, in fact, is inversely proportional towards the frequency and so, on a 5g PCB with high-frequency signals, it is very shallow. An irregular copper surface will offer you the existing an irregular path, increasing the resistive losses;

Signal integrity

High frequencies represent the most difficult challenges for the integrated circuit designer. To be able to maximize I/O, high-density interconnections (HDI) require thinner tracks, a factor that will cause signal degradation leading to further losses. These losses adversely affect the transmission of this RF signal, which can be delayed for a couple of milliseconds, in turn causing problems in the signal transmission chain. In the high-frequency domain, signal integrity is almost entirely based on checking impedance. Traditional PCB manufacturing processes, for instance, the subtractive process, have the disadvantage of making tracks with a trapezoidal cross-section (the angle, when compared to vertical perpendicular to the track, is normally between 25 and 45 degrees). These cross-sections modify the impedance for the tracks themselves, placing serious limits on 5G applications. However, the difficulty could be solved using the mSAP (Semi-Additive fabrication Process) technique, which allows generating traces with greater precision, allowing trace geometries to be defined via photolithography. In the following figure, we are able to see an assessment regarding the two manufacturing processes.

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Automatic inspection

5g PCB for high-frequency applications require to be subjected to automatic inspection procedures, both optical (AOI) or performed through ATE. These procedures allow to enormously increase the quality associated with the product, highlighting possible errors or inefficiencies associated with the circuit. The recent progress manufactured in the field of automatic inspection and testing of PCBs has resulted in significant time savings and reduced costs connected with manual verification and testing. The employment of new automated inspection techniques may help overcome the difficulties imposed by 5G, including global impedance control in high-frequency systems. Increased adoption of automated inspection methods also allows for consistent performance with high production rates.

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