A high-frequency PCB takes on the role of more than just a circuit board. Instead, it develops into a system in which aerials are implemented as planar versions. Therefore, not only the materials used but also the production processes have to meet new challenges.
For such applications, PTFE-based base materials with a permittivity of around 3.00 and a loss factor of < 0.002 are generally used. Those materials may also contain inorganic fillers and/or woven glass. The grade and type of the fillers varies according to the type of product and manufacturer.
Radar performance can be further optimised by means of copper films with reduced treatment. Alternatively, there are epoxy-resin or PPE-based base materials which achieve equivalent characteristics.
The route to fully autonomous driving is determined by radar sensor systems. Adaptive cruise control systems for medium to long ranges are used for this purpose. To achieve cost and design benefits, those base materials can be combined with conventional FR4 systems to form hybrids, while external HF laminates are made use of for reliable radar function. The remaining part of the multi-layer board offers virtually unlimited design scope. The design rules for multi-layer circuit boards apply.
Despite the specific material properties of PTFE, standard technologies such as laser drilling, mechanical processing and the application of a solder resist mask can be implemented.
|Structure||Laminate technology, hybrid, 8 copper layers|
|Circuit board technology||High frequency, through connection, laser-cut blind holes on one side, mechanically drilled blind holes on one side, line/space: 100µm/100µm|
|Special features||hybrid structure, reduced etching tolerance ± 15µm|
Frequency range restricted? Are the HF performance demands lower in comparison with the 77GHz applications? In both the above cases, HF-optimised FR4 materials are mostly used. That results not only in substantial cost benefits in PCB production but also greater scope in layer stack-up design. Typically, such laminates on the top or underside or the multi-layer board are combined with FR4 materials in a hybrid system.
High-volume representative products include lane-change assistance and blind-spot warning systems. Since processing is based on conventional FR4 materials, a broad spectrum of standard technologies can be applied – including HDI/SBU technologies.
|Structure||Laminate technology, hybrid, 4 copper layers|
|PCB thickness||0,7 mm|
|Circuit board technology||High frequency, through connection, blind holes on one side, line/space: 100µm/120µm|
|Special features||Hybrid structure, reduced etching tolerance ± 25µm|
In the industrial environment, the diversity of applications for radar systems in the 24GHz frequency range has seen significant growth. Such HF circuit boards can be found, for instance, in electronic circuitry for automatic door systems, fill-level sensors, traffic monitoring systems, anti-collision systems in robotics, and alarm systems.
The great advantage of the circuit boards used in this area of application is that production of them requires neither HF-performance base materials nor adapted manufacturing processes.
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