Reading time ( words)
The new 5G cellular infrastructure has many technological differences from previous infrastructures, which certainly impacts the PCBs and materials used to build these circuits. 5G applications are generally split up into two frequency bands; sub-6 GHz and millimeter-wave (mmWave). Most of the initial deployments for 5G technology will be based on the sub-6 GHz band of frequencies; however, there are already mmWave 5G systems, and in the future, there will be more.
A quick overview of the benefits of the 5G systems shows that 5G will have much higher digital rates and lower latency. The data rates will likely be in the hundreds of Mbits/second, and the latency will be better than 20 ms. The boosts in 5G technology will enable enhanced mobile broadband (eMBB), massive machine-type communications (MMTC) and ultra-reliable and low-latency communications (uRLLC). The PCBs associated with 5G applications will have a much higher level of integration and greater functionality, which translates to more demanding designs and a broader combination of circuit materials.
The enhancements of 5G can cause more PCB thermal management issues for certain circuit functions. The heat generated by the 5G PCB is often related to insertion loss. Basically, a circuit with higher insertion loss will generate more heat. Once the heat is generated, channeling the heat effectively to a heat sink structure is important. Some circuit material properties that are important to consider include dissipation factor and thermal conductivity.
A low-loss, high-frequency circuit material will have lower insertion loss and will, therefore, generate less heat. These high-frequency circuit materials usually have a low dissipation factor (Df) and often use a copper with a smoother surface. It is well-known that copper surface roughness will impact insertion loss, and a copper with a low profile, or smooth copper surface, will generate less insertion loss. More specifically, the copper surface roughness I’m referring to is the surface roughness at the substrate-copper interface of the high-frequency laminate.
Also, my comment about the laminate having a low Df is somewhat subjective and very dependent on the type of circuit being used in the 5G system. Typically, the high-frequency laminate should have a Df of 0.004 or less. This is a good general Df value to consider for the 5G sub-6 GHz applications; however, at mmWave frequencies, it is likely the Df value will need to be even lower.
Another circuit material property that can be important for some 5G circuitry is thermal conductivity. Using a high-frequency laminate that has a high thermal conductivity can be very beneficial to thermal management for 5G applications. As a general rule, a laminate with a thermal conductivity of 0.50 W/m/K is considered good, and there are some high-frequency, low-loss laminates with this property value. However, there are a few low-loss laminates with much higher thermal conductivity.
For example, a new A new Rogers laminate, TC350™ Plus, has an excellent combination of low-loss and high thermal conductivity. TC350 Plus laminate has a Df of 0.0017 when tested at 10 GHz and a thermal conductivity value of 1.24 W/m/K. The type of 5G circuit that is usually more sensitive to thermal management is the power amplifier. Also, feeding structures for the antenna elements and other circuits can have thermal management issues as well.
The antenna structures for 5G typically use low-loss materials, which generally have lower Dk values. As with all circuit functions, there are many tradeoffs, and antenna circuits usually have a lower Dk that will allow more efficient radiation. With the many tradeoffs considered, typically, antenna circuits will use a circuit material with a Dk value of about 3.
TCDk can be an important property for 5G antenna applications because these circuits will be exposed to a range of temperatures. TCDk is a material property that is the characteristic of the material to change Dk with a change in temperature, and all materials have this property. A good TCDk value is 50 ppm/°C for a laminate, and a value closer to zero is ideal. The RO4730G3 laminate has a TCDk value of 26 ppm/°C.
There are many things to consider when designing 5G PCB applications, such as material properties and possible materials interactions with PCB fabrication. I highly recommend that the designer work with the material supplier when considering a high-frequency material to be used in 5G applications.