An HF circuit that looks perfect in the simulation tool is not yet a PCB that will behave the same way in the field. Between the digital model and the real component lie manufacturing tolerances, material behavior and physical effects that carry far more weight at high frequencies than they do on conventional PCBs. Anyone developing an HF circuit therefore faces a very concrete question: how do I transfer my simulated performance into series production reproducibly and with low loss?
This is exactly where the collaboration between developer and PCB manufacturer begins. Varioprint outlines below what really matters technically.
Simulation is only the beginning
Many projects start with a simulation based on ideal conditions. And that is precisely where the actual problem begins. Manufacturing tolerances differ considerably depending on the production method, whether subtractive processing is used or additive processes such as mSAP or SAP. These tolerances have to be known before the simulation can deliver meaningful results at all.
The reliable route from simulation to PCB therefore runs in three steps:
First, the realistic manufacturing tolerances of the chosen production method are discussed and documented.
These tolerances are then fed into the simulation, ideally as an assessment of minimum and maximum values, so that the design still works at the edge of the tolerance band.
Only in the third step do the materials follow, meaning prepreg or core, copper and surface finish, because this choice of material has a direct effect on the S-parameters of the circuit.
Only when these three steps are completed in this order can electromagnetic performance also be transferred stably into series production.
There is no right material, only the suitable one
One of the most frequent questions concerns the choice of material. Does it need PTFE, is a low-loss HF laminate sufficient, or will even an optimised FR4 system do? The honest answer is: there is no fundamentally right or wrong material. There is only the compromise that best suits the respective application.
It is therefore worth prioritizing your own requirements first. Which parameters absolutely must be met, and which would be desirable but not decisive? The must-have parameters define the minimum technical solution. The optional parameters mainly influence cost. And some points, such as the coefficient of thermal expansion (CTE) or availability, must be aligned with the manufacturer in any case, because they play a role of their own in production.
The most important criteria include the frequency band, tolerable losses, the temperature range of the application, required tests, shrinkage and expansion behavior, processability, moisture behavior and compatibility with standard FR4 material. One-point matters here: the most expensive material is not automatically the best solution. What counts is the material system that is an optimal fit for the specific application.
How tightly can an impedance really be held
For 50 ohm, 90 ohm, 100 ohm or customer-specific impedances, a tolerance of plus/minus 10 per cent applies in practice. Exactly how strongly copper thickness, etching process, dielectric, conductor geometry and lamination process influence the result in detail depends on the individual design and production method. With targeted counter-compensation this tolerance can be held and, if necessary, narrowed further.
Precision you cannot see, but can measure
At Varioprint, reproducible HF properties do not arise by chance, but through precision at several points in the manufacturing process at the same time. This includes an accurate etching process, metallization that is as homogeneous in layer thickness as possible, and a high aspect ratio for vias. Equally important are accurate mechanical machining, precise deviation measurement with corresponding compensation methods, and high-resolution coating and exposure processes for solder resist and photoresist. Only the interplay of these factors makes an HF PCB truly reproducible in series production.
When gigahertz turns into millimeter waves
In applications involving radar, sensor technology, telecommunications, aerospace or demanding measurement technology, developers often operate in the range of 24, 60 or 77 to 79 gigahertz and above. From around 30 gigahertz, the first specific processes and auxiliary materials already come into play. From about 80 gigahertz, additional factors become relevant in material selection. Here, too, the same applies: the must-have and optional criteria from the material selection are the yardstick for finding the right combination of material and process.
Copper, roughness and subtle losses
Skin effect and copper roughness are among the factors that significantly influence HF losses. The frequency determines which metal layer is lostotalt in the first place and what share the individual components have in the total loss. If ohmic, dielectric and inductive losses are considered separately, the appropriate countermeasure in material selection can be found for each of these loss types.
When FR4 and high-frequency material meet
High-frequency signal layers mostly use HF materials with PTFE or ceramic. The lower the frequency, the more likely it is that low-loss FR4 derivatives can also be used, which combine more easily with standard FR4 material. In practice, stack-ups are mostly core-prepreg combinations or prepreg-copper-foil constructions in sequential builds. A special case is fusion bonding, in which PTFE cores are bonded directly to one another under defined pressure and defined temperature.
Vias: small features with a big effect
For an electromagnetic wave, a via always means a transition between two different structures. The trace itself is a planar structure, whereas the via is a quasi-coaxial structure. Both structures must be optimized in their own right, as do the two transitions between them. For this reason, so-called guard vias are frequently placed in HF designs, bringing the via closer to a true coaxial structure.
It is also worth knowing that not only metallic structures work as a via. Waveguides can take on this function as well, although as a very low-loss special variant that is only an option for selected applications.
When the geometry is right, but the electrical behavior is not
Even a PCB that is geometrically exactly within specification may still not be electrically optimal. Such deviations generate losses that have to be compensated for in the overall system, for example through more power from the HF amplifier. In the less favorable case, they upset the balance of the overall system, so that the circuit no longer performs its function properly.
Where most performance is lost in practice
High-frequency signals can basically be viewed in two ways: digitally or analogously. In the digital view, talk is more of signal integrity, and behavior is assessed statistically in the time domain. In the analogue view, the signal is examined in the frequency domain and assessed based on its S-parameters as the result of a four-port network. In practice, this distinction is often not drawn clearly enough.
Another frequent weak point is that the approach described, from tolerance assessment through simulation to material selection, is too rarely applied consistently. This results in designs that later must be corrected at considerable effort instead of working reliably from the outset.
How the HF properties of a PCB can be verified
So that the electrical properties of a manufactured PCB are not merely claimed but can be proven, Varioprint uses various coupons. These include impedance coupons, IST coupons, D coupons, A/B/C coupons, E coupons for testing insulation, cross-sections with optical evaluation, solderability coupons, CAF coupons for electrochemical migration, adhesion coupons for solder resists and specific HF coupons for measurements with the vector network analyser. This makes it possible to assess and document impedances, metallisation quality, and material and via quality by measurement. Trust is thus created not by a promise, but by traceable measured values.
The decisive point: collaborate early instead of correcting late
The best HF PCB is not created by correcting as much as possible at the end of manufacturing. It is created by developers and the Varioprint team working together from the very start. When the framework conditions of the application are communicated early and clearly and engineering is involved from the outset, the best compromise between HF performance, miniaturisation, thermal management, reliability and cost can be found together. It is precisely this early co-engineering that ultimately makes the difference between a PCB that looks good on paper and one that works reliably in series production.
