Microwave RF and mmWave PCB
Introduction
This section focuses on PCBs for analog signal transmission, reception and processing, particularly at millimeter-wave (mmWave) frequencies.
5G cellular communications are pushing up towards 100 GHz and expected to go beyond 100 GHz for 6G. One of the main frequency bands for advanced driver-assistance systems (ADAS) is 77GHz. Bandwidths are hundreds of MHz to multiple GHz. PCBs are expected to support this (mostly analog) processing: antennas, filters, amplifiers, filters, frequency conversion, phase shifters, splitters and combiners.
Operations at these frequencies are demanding in terms of maintaining signal integrity and keeping losses below maximum acceptable levels. These, in turn, result in extreme challenges in controlling feature and surface dimensions, particularly with the small skin depths at high frequencies. Furthermore, stability of these and of material parameters across temperature ranges and stability across the product lifetime are critical. Increasing functionality (i.e., signal processing) results in more multi-layer PCBs in this space. In parallel, the use of key materials like Teflon may become restricted due to concerns over the sustainability of per- and polyfluoroalkyl substances (PFAS) materials.
Technical Needs, Gaps and Solutions
The technology issues surrounding Microwave RF and mmWave PCBs, the associated needs, technology status of those needs, as well as gaps and challenges to overcome, are summarized below. The time period considered is from 2025 to 2035.
Technology Status Legend
For each need, the status of today’s technology is indicated by label and color as follows:
In-table color + label key | Description of Technology Status |
|---|---|
Solutions not known | Solutions not known at this time |
Solutions need optimization | Current solutions need optimization |
Solutions deployed or known | Solutions deployed or known today |
Not determined | TBD |
Table 1. Microwave RF and mmWave PCBs Gaps, and Today’s Technology Status with Respect to Current and Future Needs
| ROADMAP TIMEFRAME | |||
TECHNOLOGY ISSUE | TODAY (2025) | 3 YEARS (2028) | 5 YEARS (2030) | 10 YEARS (2035) |
Issue #1 Frequency Drivers | ||||
NEED | ADAS: 25+77 GHz | ADAS: 25+77 GHz | ADAS: 25+77 GHz | ADAS: 25+77 GHz |
CURRENT TECHNOLOGY STATUS | ADAS: Solutions deployed | |||
CURRENT TECHNOLOGY STATUS | Wireless: Solutions deployed | Wireless: Solutions need optimization | ||
GAP | Improvement from today’s hybrid stacks | Material development + process (including manufacturing equipment) development needed | New materials + process research needed | |
CHALLENGE | ADAS: Low-cost materials with high reliability | Wireless: Small form factors; multiple materials; high power circuits on lossy materials; impact of conductor surface roughness with small skin depths (see Appendix). | ||
Issue #2 Styles of Circuits (antennae, ADAS, base stations, sensors) | ||||
NEED | Multi-layer antenna/ADAS board | Cavity-based solutions on PCB; | ||
CURRENT TECHNOLOGY STATUS | Hybrid-layer only or in-lays. Development needed for multi-layer | Solutions not known | ||
GAP | Lack of conventional multi-layer solution | Process issues; | ||
CHALLENGE | Availability of process equipment at the right curing temperature | Low-losses, reproducibility | ||
NEED | Multi-layer microwave/mmWave boards for signal processing | Mixed-signal boards for millimeter frequencies | ||
CURRENT TECHNOLOGY STATUS | Coin or in-lays. Development needed for multi-layer, but hybrid solutions exist | Solutions need optimization | ||
Issue #3 Thermal | ||||
NEED | Capability for higher maximum operational temps for dielectric materials above 120°C | Capability for higher operational temperatures for dielectric materials above 150°C | Capability for higher operational temperatures for dielectric materials above ~160°C | |
CURRENT TECHNOLOGY STATUS | Materials are available | |||
GAP | Cost of processing and cost for high-speed materials and availability | |||
CHALLENGE | Alignment of high-speed materials for higher operational temperatures (goes hand-in-hand with development for high-speed) | |||
CHALLENGE | Transition to new materials (design, fabricators, approval/validation processes (UL) time) consuming and costly | |||
NEED | Better heat transmission for dielectric materials >1W/m.K | Material >3W/m.K | Material >10W/m.K | |
CURRENT TECHNOLOGY STATUS | Solutions need optimization | |||
GAP | Solutions today, lower costs for heavier Cu layers | Stability of electrical properties require static operating temperatures and heat transmission of dielectric material | ||
CHALLENGE | Thermally conductive and low Dk needed simultaneously | |||
CHALLENGE | Lack of agreed measures for validation | |||
NEED | Better solutions for heat extraction from power amplifier boards | Better solutions for heat extraction from power amplifier boards in smaller form factor due to frequency increases and smaller systems. | ||
CURRENT TECHNOLOGY STATUS | Solutions need optimization | Solutions not known | ||
GAP | Through-hole solutions (e.g., via forms) not adequate | New materials and processes needed | ||
CHALLENGE | Limited number of suppliers for the available solutions–cost is an issue | Higher thermal flux density | ||
NEED | PCB design feasible and optimal for manufacturing and delivers the necessary thermal performance | |||
CURRENT TECHNOLOGY STATUS | Solutions needs optimization | |||
GAP | Materials/ structure thermal characterization (e.g., thermal resistance of contacts) could be more consistent/ standardized | |||
CHALLENGE | Impedance control versus thermal conductivity | |||
CHALLENGE | Thermal design is unknown to the PCB manufacturer | |||
NEED | Low coefficient of thermal expansion (CTE) needed, as we move to higher 6G frequencies | |||
CURRENT TECHNOLOGY STATUS | Solutions need optimization | |||
GAP | Material solutions today are used only in highly specialized, low-volume applications | |||
CHALLENGES | Low tolerance for increases in cost | |||
Issue #4 Materials | ||||
NEED | Design-out of PFAS1 with polytetrafluoroethylene (PTFE)-based materials and constituents elements in processing | R&D for PFAS: PTFE replacements by maximizing alternatives and new designs | PFAS: PTFE replacement for short-life products/high volume manufacturing (HVM), and management of low use, critical exemptions | PFAS: PTFE replacement in all product segments |
CURRENT TECHNOLOGY STATUS | No known equivalent solutions but Teflon alternatives underway | Solutions need optimization | ||
GAP | Materials with required properties | |||
CHALLENGE | Teflon is stable across wide temp ranges and hard to replicate | Cost and process impact of new alternative materials, e.g., quartz. | ||
NEED | Loss, Df < 0.002 at frequencies of interest and for standard operational conditions (temperature, humidity) | Loss, Df < 0.0015 at frequencies of interest and for standard operational conditions | Loss, Df < 0.001 at frequencies of interest and for standard operational conditions | |
CURRENT TECHNOLOGY STATUS | Solutions need optimization | Solutions need development (Aerogels under research) | ||
GAP | Not commercial yet (except for PTFE) | Combination of base material building layers not at the low-loss levels needed | ||
CHALLENGE | Right cost point for large volume manufacturing | All components need to be low loss: Cu, resin systems, reinforcement. | ||
CHALLENGE | Design has to work in hand with materials, e.g., hybrid stack-ups to limit high frequency to some layers. Adhesion becomes a challenge. | |||
NEED | Parameter stability (including CTE/dimensional stability) target over temp. for Df and Dk: <5% degradation between room temp and operating temp. Also need stability over humidity range and in condensation conditions. | 5% parameter stability becomes a hard requirement | ||
CURRENT TECHNOLOGY STATUS | Solutions need optimization | New solutions needed | ||
GAP | Right cost point for large volume manufacturing | Meeting requirements without design compromises | ||
CHALLENGE | For higher frequencies and data rates, the absolute tolerances decrease | |||
NEED | Linearity: Parameter stability over frequency for Df and Dk (<5% over operational bandwidth) | |||
CURRENT TECHNOLOGY STATUS | Solutions need optimization | New solutions needed | ||
GAP | Available today at 5G at a cost. | Tighter requirements for 6G not met | ||
CHALLENGE | Df is the challenge—Dk linearity is usually acceptable | |||
CHALLENGE | Higher bandwidth with 6G | |||
NEED | Design and stack-up flexibility for more complex, mixed material and multi-layer boards with development of stable materials | |||
CURRENT TECHNOLOGY STATUS | Single material solutions under development (glass), smooth copper, foils, limited oxides, extremely low-loss | Development of high-speed materials in use but mixed materials require further R&D and validation | New product types require new materials and processes | |
GAP | Current low-loss material (PTFE) performs poorly here | |||
CHALLENGE | Difficult to machine PTFE | |||
CHALLENGE | Asymmetric stack ups (in general) | |||
Issue #5 Process Accuracy and Variations | ||||
NEED | Deliver high signal integrity through tight control of lines, features and copper roughness. 5% for feature size; Rz < 1 um | Move away from PTFE (due to PFAS restrictions) towards mainstream high-speed solutions. | Move away from PTFE and towards high-speed solutions where feature sizes are reducing as a result of higher frequencies | |
CURRENT TECHNOLOGY STATUS | Solutions need optimization | Solutions not known | ||
GAP | High-volume manufacture with in-process measurement and control | No support by mainstream solutions | ||
CHALLENGE | Supply limited to a few specialists | Lack of volume to make mainstream solutions viable | Manage trade-off between high-volume mainstream solutions and specialist solutions | |
CHALLENGE | Manual inspection today. Audio-video interleave (AVI) accuracy not good enough to inspect down to 5% of feature size (100% coverage needed) | Antenna design needs to be incorporated into rest of solution | Supply chain too slow to implement testing and qualification. | |
NEED | Dielectric/electrical consistency and stability across deployment temperature and over lifetime at 5G frequencies (up to 100 GHz). | Dielectric/electrical consistency and stability across deployment temperature and over lifetime at 6G frequencies (100 GHz+). | ||
CURRENT TECHNOLOGY STATUS | Solutions need to optimized | Solutions not known | ||
GAP | PTFE good, but needs to be replaced. Suitable for antenna applications, but not multilayer | New materials needed, adapt from other applications? | ||
CHALLENGE | PTFE replacement (See Issue #4) | More demands on process control, due to reduced feature size and needs regarding copper roughness. | ||
Approaches to address Needs, Gaps and Challenges
Table 2 considers approaches to address the above needs and challenges. The evolution of these is projected out over a 10-year timeframe using technology readiness levels (TRLs).
In-table color key | Range of Technology Readiness Levels | Description |
|---|---|---|
2 | TRL: 1 to 4 | Levels involving research |
6 | TRL: 5 to 7 | Levels involving development |
9 | TRL: 8 to 9 | Levels involving deployment |
Table 2. Microwave RF and mmWave PCBs Potential Solutions
|
| EXPECTED TRL LEVEL* | |||
TECHNOLOGY ISSUE | POTENTIAL SOLUTIONS |
TODAY (2025) | 3 (2028) | 5 (2030) | 10 |
Frequency Drivers | [See solutions in Materials and Process Accuracy and Variations below] | ||||
Styles of mmWave Circuits | Hybrid stack-ups to limit high frequency challenges to limited layers | 6 | 6 | 7 | 8 |
Additive manufacturing of structures, both dielectrics and metals | 5 | 7 | 8 | 9 | |
3D-printed structures, both dielectrics and metals | 4 | 6 | 7 | 8 | |
Thermal | Resin-coated Cu with HDI layers | 7 | 8 | 9 | 9 |
Conductive pre-pregs (e.g., boron nitride) | 9 | 9 | 9 | 9 | |
Carbon-fiber thermal spreaders in x,y-direction | 6 | 7 | 8 | 9 | |
Aligned carbon fibers in z-axis | 7 | 8 | 9 | 9 | |
Sinter paste for build-up material or connecting boards | 7 | 8 | 9 | 9 | |
Heavier Cu inner layers + connections for heat transmission | 8 | 8 | 8 | 8 | |
Embedding of the die of high-power components (e.g., MOSFET) directly into the board | 7 | 7 | 8 | 8 | |
Embedded Cu coins for better heat extraction | 8 | 8 | 8 | 8 | |
Materials | Lower Dk glass | 6 | 6 | 7 | 8 |
Quartz | 4 | 5 | 6 | 7 | |
Hydro-carbon resin systems | 3 | 4 | 5 | 6 | |
Aerogels | 4 | 5 | 6 | 7 | |
Process accuracy and variations | Controlled chemical surface etching/conversion coatings | 8 | 9 | 9 | 9 |
Improved chemical bonding approaches (polymer to metal) to give controlled Cu roughness balanced with high adhesion | 6 | 7 | 7 | 8 | |
Joint design of improved chemistries and process equipment to give improved control of uniformity in core build-up and contour/volume fill. | 7 | 8 | 8 | 9 | |
Conclusions and Recommendations
R&D is needed on new materials, including flexible materials, to support mmWave applications, with Dk and Df as key parameters
Material options for high-speed digital are increasing rapidly to address concerns around z-axis thermal expansion. In addition, a range of materials are emerging that allow better CTE matching in the x- and y-directions. Adjusting resin modulus properties is also a focus of development efforts to address thermomechanical reliability.
Circuit design in general is a 3D design challenge at mmWave frequencies, requiring extra flexibility in manufacturing in the third dimension. Within the next 10 years, the industry needs to master new manufacturing techniques, such as additive manufacturing.
·Increased pressure to reduce process variation, right across the various parameters—particularly as mmWave applications move from low to high-volume manufacturing. Quality control will have to be extreme and support for industry initiatives2 will be important.
Industry initiatives focused on incremental reductions in feature size. They will have to address the significant step-change reduction in feature sizes expected with higher frequencies in 5G/6G applications.
PCB Acronyms
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References
FluoroProducts and PFAS for Europe (FPP4EU), “FPP4EU outlines a 6-point plan to reach a workable restriction proposal on PFAS,” https://www.fpp4eu.eu/news/fpp4eu-outlines-a-6-point-plan-to-reach-a-workable-restriction-proposal-on-pfas/, April 21, 2023.
IPC, “New IPC Initiative Focuses on E-mobility Quality & Reliability,” https://www.ipc.org/news-release/new-ipc-initiative-focuses-e-mobility-quality-reliability, May 24, 2022.