In modern electronic devices, high-speed digital and analog circuits are often found in close proximity to multiple RF modules on a single printed circuit board. When developing complex system projects, up to 75% of the time can be spent on the radio frequency part. That makes it necessary to find ways to increase the efficiency of this process. To streamlines the process, the knowledge of RF and Microwave PCB Design Guide is very important.
Another problem is the translation of circuits for the use of specialized microwave development programs. Since the diagram, topology and libraries are usually transferred separately, this process is not only lengthy, but also weakly protected from errors.
Integration of RF and System Design
With the Advanced RF Design option, microwave circuits can be designed directly in PADS Professional or imported from Key sight ADS or NI Microwave Office. You are no longer placing black boxes in a schematic, but working with full RF circuits as part of a system project (Fig. 2).
Microwave Component Design and Optimization
You can create microwave components or convert standard components to microwave without the need to import polygons, for example from DXF files. Schematic symbols are generated by an autogenerator for quick and easy use in topology.
You can also optimize microwave polygons with the HyperLynx® full wave EM solver. Powerful tuning and optimization combined with NI MWO and Keysight ADS (ADS Tuning expressions) tools based on a bi-directional interface with PADS Professional drastically reduce development time.
You have a huge number of functions available for developing RF projects. The PADS Professional Layout Layout Editor provides a set of all the necessary tools for the microwave designer. They are simple and straightforward to use – you need very little time to learn them.
Bulk installation of RF shielding vias can take a long time. With PADS Professional, you can do this in a matter of seconds (Figure 3). All holes are grouped so that they can be moved or deleted together. Transitions can be set based on custom rules.
RF projects typically have stringent clearance requirements. With PADS Professional, you have complete control over the clearances for all RF elements. Gaps can be controlled not only on the layout layer, but also on adjacent layers. Also, clearance rules can be set globally for the entire circuit or individually for each element.
The automatic layout allows you to place RF circuits in parts or in full. The RF polygons are connected correctly at the calculated angle. You can manage all your connections thanks to a well-thought-out user-friendly interface.
RF circuits can be grouped so that the circuit remains unchanged on the board within the overall system. The circuit can be divided into subgroups for modeling in parts or to protect individual parts of it. Non-RF objects such as high speed traces, polygons, or cutouts can be grouped together with RF objects for complete topology modeling. Additionally, project elements can be grouped under a custom name, for example, amplifier, filter, etc. The group is managed as if it were one object. Groups contain a hierarchy and can be transferred to ADS or MWO with one click.
Fig. 3. Meander can be added at any stage of the design
Checking and Finding Errors in RF Design
You can place the topology in ADS and MWO for 2D electromagnetic inspection, or transfer the design to the HyperLynx full wave solver for 3D structure inspection. Checking in a 3D simulator will eliminate all significant problems affecting the performance of the board, and at the stage of physical debugging, it will be enough to make only small adjustments. This will save you a huge amount of time and money. The costs of eliminating errors at the stage of physical prototyping, and especially production, are many times higher than the costs of testing a virtual model.
Schematic entry and RF topology design can start in PADS, ADS, or MWO for maximum flexibility. Simulations, tuning and optimization are also supported by RF simulators. RF elements are fully intelligently coupled to the simulator for accurate and reliable simulation. You do not need to broadcast the entire project, which always contains errors.
Modeling the RF portion of a real PCB can be a complex process, typically involving complex translation of intelligently unrelated topology objects, for example, via DXF, GDSII, or Gerber format. The technique offered in PADS Professional with the Advanced RF Design option allows complex polygons, cutouts, and outer joints to be transferred as ports. This smart link also supports simultaneous ADS or MWO connections. For large or complex RF circuits, simulations can be spread across multiple computing platforms to reduce time.
Integration with Third party RF Design and Simulation Software
For maximum flexibility, you can use the standard route for importing data from third-party systems, for example, complex polygons via DXF format and then converting them into RF components. PADS Professional will automatically generate a schematic symbol for quick and easy schematic use.
You can also export your design data to ODB ++ for transfer to production. PADS Professional guarantees the completeness and accuracy of data, including RF paths, to the manufacturer.
Basic Types of RF / Microwave PCB Base Materials
RF and microwave devices require printed circuit boards that operate efficiently and reliably at these frequencies. To manufacture such boards, materials with specific characteristics are needed. The article describes what materials are used in RF / microwave boards, what their characteristics are and what material is optimal for a particular application.
Laminates are widely used made of epoxy resins, glass fiber reinforced with Tg = 185-220 ° C and used in the range up to 10-20 GHz. This is due to the good compatibility of this material with standard PCB manufacturing processes, the ability to withstand several lamination cycles, resistance to lead-free soldering temperatures, etc. different types of reinforcement.
Various Mixed Polymer Systems are also Used:
Bismaleimide triazine / epoxy resin (BT / epoxy), polyphenylene oxide / epoxy resin (Epoxy / PPO). For example, the BT / epoxy combination is advisable for boards for housing with a Ball Grid Array (BGA). Fluoroplast-4 (also known under the names polytetrafluoroethylene (PTFE), Teflon, FAF-4D) provides good electrical characteristics – constant permeability, low insertion loss.
However, the production of printed circuit boards from this material requires special preparation for drilling and metalizing holes. And its main drawback is the too high value of the coefficient of thermal expansion along the Z axis (129 and above), which leads to unreliability of plated through holes in multilayer printed circuit boards. In addition, for high-quality metallization of the holes, a thorough preliminary preparation is required to ensure the wettability of the PTFE-4 surface. Therefore, PTFE is used only for the production of printed circuit boards with no more than two layers.
In the 1950s – 1960s, fluoroplastic-4, reinforced with glass cloth or glass fiber, with εr in the range 2.2–3.2 and tan δ = 0.0009–0.003 was used as the base material . Applications for these materials are in the defense and aerospace industries, where reliability of performance rather than price is the determining factor. The advantages of these materials are the accuracy of the εr and tan δ values. But their inherent high TKεr value can cause a frequency shift in resonant elements, for example, in bandpass filters. The main problem of such materials (as in the previous case) is that the Z-CTE is too high, which does not allow the use of this dielectric in multilayer printed circuit boards with a thickness of more than 0.762 mm (0.03 ″).
The search for a composite material suitable for a wide range of applications has led to the emergence of PTFE-4 with ceramic fillers with / without glass fiber reinforcement. The use of laminates of this type implies an increase in the range of values of the relative permittivity, as well as an increase in resistance to mechanical and thermal influences.
The εr value varies depending on the type of filler and ranges from 1.96–10.2. It should be noted that to achieve a lower permeability value, hollow glass beads are used as filler, for example, in RT / Duroid 5880 LZ laminate. Due to this, it has a mass of 30% less than other materials with similar properties. Another distinctive feature of this material is the impossibility of laser drilling due to the risk of damage to the structure of the balls.
In the early 1990s, a new generation of materials appeared
The manufacturing processes for PCBs using thermosetting polymers and the widely used FR-4 are similar. The range of εr values for them is from 3 to 12.78, tg δ is less than 0.003. Also, these materials have low TKεr and Z-KTP values.
Separately, it is worth noting prepregs – materials obtained by impregnating a reinforcing base with a uniformly distributed binder (glass cloth impregnated with underpolymerized epoxy resin, hydrocarbon ceramics, etc.). Prepregs can be used for DIY laminates and can be used as an adhesive cushioning material for multilayer PCB fabrication. In the latter case, adhesive films composed of a fluoroplastic with ceramic filler, a hydrocarbon polymer, and a liquid crystal polymer can also be used.
In terms of the percentage of laminate and prepreg when creating a multilayer PCB, a large number of options are possible. The prevalence of prepreg in a multilayer printed circuit board leads to a decrease in its cost, and foil laminate – to an improvement in electrical and mechanical properties. True, in the latter case, the cost of manufacturing the product increases.
If necessary, in microstrip transmission lines (LP) with stringent performance requirements, the upper and lower layers are made using a laminate, and prepregs or lower-frequency laminates with less accurate characteristics are used for the inner layers of strip LP. If the characteristics of the strip line are most important, then the reverse approach is used