In an increasingly digitalized and fast-moving world Speed trump card. This applies to both everyday and industrial applications, be it wireless transmission systems, sensors or radar applications.
High-quality and reliable PCBs are an important building block for these high-frequency applications. But what exactly are the success factors for high-frequency PCBs and what needs to be considered during planning and production? KSG Group experts provide the answers.
Mr. Hackl, what is meant by high frequency and what role does the printed circuit board play in this?
The boundary between low and high frequency is assumed to be at 10 MHz, as the typical thermal effects of high-frequency fields appear from this point onwards. In the electromagnetic spectrum, the high-frequency range extends to around 300 GHz. The typical high-frequency range on the PCB is between 10 and 100 GHz.
The PCB acts here as a transmission element with deliberately trimmed conductor paths for the transmission of high-frequency signals. The desired impedance of such lines is mainly dependent on the track width and the defined distances to the plane reference layers. The etched high-frequency components on the PCB include, for example, finger couplers and filters, l/4 elements, patch antennas or ring resonators.
Interview: Johann Hackl
Johann Hackl from Technical Support at KSG supports companies from the very first minute of the product idea with the efficient and economical development of PCBs suitable for series production. The expert knows what is important for high-frequency applications and discusses five important aspects of HF PCBs below.
What basic requirements must an HF printed circuit board fulfill?
Smooth and precise signal transmission is essential for high-frequency applications. Any irritation on the PCB leads to a reflection or change in the high-frequency signal, which is why a straight contour of the conductor routing is particularly important here. To illustrate this, I like to use the image of a rock in the surf, which we usually perceive as positive. However, for high-frequency applications, we don't want to have a rock like this in the conductor path. which the waves break. This means that irritations, indentations or impressions must be avoided.
As high-frequency signals are pressed against the outer skin according to the skin effect, all four sides of a conductor should also be as smooth as possible. The critical side is the underside of the copper foil, which is artificially pre-treated by oxidation. There are different levels of roughness here, which should be as low as possible for high-frequency applications. At the same time, however, the minimum adhesive strength must also be met.
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What role does the choice of material play in high-frequency printed circuit boards?
Selecting the right material is an important success factor for HF PCBs. The most important parameters for material selection include
- the dielectric constant (Dk),
- the loss factor (Df),
- the stability of the HF parameters over the relevant temperature range,
- reliability according to temperature-humidity load,
- the copper adhesion strength,
- dimensional stability,
- the water absorption,
- and the costs.
The last point in particular should not be underestimated, as the cost of high-quality high-frequency materials is enormous. It is therefore advisable to consult experienced project engineers at an early stage in order to find the most suitable and at the same time minimum necessary material in terms of cost.
What must be taken into account when processing the materials?
The drilling and milling of PTFE materials are further success factors for HF PCBs. The respective drilling and milling parameters should be optimized depending on the material used. In the case of drilling, this would include the tool type, cutting speed, feed rate and return stroke. Mixed setups are often used for cost reasons and their properties must be taken into account in order to find the right parameters and tools. After all, in addition to low tool wear, we naturally want to achieve good cutting quality without fraying or smearing.
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What is important when structuring the conductor pattern of HF layouts?
When implementing HF layouts within narrow tolerance limits it may well be necessary that individual conductor elements in the film templates must be adapted. Particular attention should be paid to the following Aspects:
- Minimization of corner rounding of individual
- Layout element
- Compliance with the required edge steepness
- Avoidance of excessive edge roughness due to
- Remaining treatment tips
The structural fidelity can be checked by means of an automatic optical inspection of relevant HF layers, ideally during the first production panel. Here, the individual layout elements are measured and critical elements are displayed so that the rest of the batch can be etched with the same or, if necessary, with adjusted parameters.
Tilo Leistner provides a more detailed insight into the etching process and the final inspection in the following videos:
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Even more HF PCB knowledge
Johann Hackl and Tilo Leistner provided a comprehensive and intensive insight into the world of high-frequency printed circuit boards at the XPERTS webinar "Need for Speed" in October 2022. The recording of the webinar is available on-demand - register now.
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