As operating frequencies continue to increase, signal loss becomes one of the most important design challenges in high-frequency and millimeter-wave PCB applications. At frequencies such as 24 GHz, 60 GHz, and 77 GHz, even relatively small dielectric losses, conductor losses, impedance discontinuities, and material variations can significantly affect signal integrity.
This has made low-loss PCB materials and low-loss prepreg increasingly important for multilayer RF and millimeter-wave circuit boards used in 5G infrastructure, high-speed wireless communications, automotive radar, ADAS, RF modules, and other high-frequency electronics.
For many multilayer PCB designs, the circuit laminate is only part of the solution. The prepreg or bonding material between PCB layers can also make a significant contribution to overall insertion loss. Selecting a prepreg with a low dissipation factor (Df), suitable dielectric constant (Dk), and good compatibility with the surrounding laminate can help reduce high-frequency signal attenuation while maintaining reliable multilayer fabrication.
Why Prepreg Matters in a Millimeter-Wave PCB
In a multilayer PCB, prepreg performs two essential functions. First, it provides electrical insulation between copper layers. Second, it bonds the individual circuit layers together during lamination.
At low frequencies, the electrical performance of the bonding layer may not be a major concern. However, as the operating frequency moves into the microwave and millimeter-wave range, the dielectric properties of every material surrounding the transmission line become increasingly important.
A multilayer millimeter-wave PCB may use different materials for different functional layers. For example, a low-loss laminate can be used for RF transmission lines, while more cost-effective FR-4 may be used for power distribution, digital control circuits, or layers where high-frequency performance is less critical.
This type of hybrid multilayer PCB construction can provide a practical balance between electrical performance and manufacturing cost.
However, if the prepreg between the high-frequency layers has a relatively high Df, it can increase dielectric loss and reduce the benefit of using an ultra-low-loss laminate.
For this reason, prepreg selection for millimeter-wave PCB design should be considered together with laminate selection, transmission-line geometry, impedance control, copper roughness, and stackup design.
How Low-Loss Prepreg Reduces High-Frequency Signal Loss
One of the key parameters engineers use when evaluating high-frequency PCB materials is the dissipation factor (Df).
In simplified terms, a lower Df generally means that less RF energy is converted into heat as a signal travels through the dielectric material. As frequency increases, this becomes increasingly important.
A multilayer RF PCB may contain:
- High-frequency circuit laminates
- Prepreg or bonding layers
- Copper conductors
- Ground planes
- Vias and plated structures
- FR-4 or other supporting materials
When a transmission line is embedded between dielectric layers, part of its electromagnetic field interacts with the surrounding materials. Therefore, the electrical properties of the prepreg can directly influence the total insertion loss.
This is particularly important for stripline structures, where the electromagnetic field is substantially contained within the dielectric.
Consequently, a low-loss prepreg for millimeter-wave PCB applications can be an important factor in achieving lower overall transmission-line loss.
Comparing Low-Loss Prepreg Materials
Different applications require different levels of electrical performance. Manufacturing capability, material availability, lamination process, cost, and reliability must also be considered.
RO4450F Prepreg
Rogers RO4450F is a widely used bonding material for multilayer RF and microwave PCB fabrication.
Its major advantage is manufacturing compatibility. Compared with some specialized PTFE bonding processes, RO4450F can be integrated into conventional PCB manufacturing processes more easily.
Its typical dissipation factor is approximately 0.004 at 10 GHz.
For many RF and microwave applications, this represents a useful balance between electrical performance and manufacturability. For some applications operating at 24 GHz and above, RO4450F may still provide acceptable performance, particularly where the signal-loss requirement is not extremely aggressive.
Therefore, RO4450F can be considered when the project requires a practical combination of low-loss performance, multilayer fabrication compatibility, and production scalability.
Rogers 2929 Bonding Material
For designs requiring lower dielectric loss, Rogers 2929 bonding material provides another option.
Its dissipation factor is approximately 0.003 at 10 GHz, lower than the typical value of RO4450F.
When combined with a very-low-loss circuit laminate, Rogers 2929 can help reduce dielectric loss in multilayer high-frequency circuits.
This makes it relevant to RF and microwave applications where signal attenuation is more critical and where the PCB stackup requires a specialized bonding layer.
For engineers searching for a low-loss bonding material for multilayer PCB, the combination of laminate Df and bonding-material Df should be evaluated rather than selecting the laminate alone.
Rogers SpeedWave 300P for Millimeter-Wave PCB Applications
For more demanding millimeter-wave applications, an even lower-loss prepreg may be required.
Rogers SpeedWave 300P is designed for multilayer circuits where low dielectric loss is particularly important. Its loss factor is approximately 0.002 at 10 GHz, making it a strong candidate for advanced high-frequency and millimeter-wave PCB applications.
SpeedWave 300P is available in different thicknesses and glass-cloth constructions, including spread-glass options designed to improve electrical uniformity.
Depending on the selected construction, its Df at 10 GHz is approximately 0.0019 to 0.0022.
Its typical Z-axis dielectric constant at 10 GHz is approximately 3.0 to 3.3, depending on construction.
For millimeter-wave PCB designers, material selection is not simply about choosing the lowest Df. The dielectric constant, glass-cloth architecture, thickness, resin distribution, and interaction with the adjacent laminate should also be considered.
Why Spread Glass May Matter
At millimeter-wave frequencies, the electromagnetic field can interact with the glass fibers within a composite dielectric material. This can contribute to local variations in the effective dielectric environment and potentially affect signal propagation.
For this reason, spread-glass or low-profile glass-cloth constructions can be attractive for high-frequency applications.
For SpeedWave 300P, the 1035 series with approximately 0.0020 or 0.0025 inch thickness and the 1078 series with approximately 0.0030 or 0.0035 inch thickness can be considered for designs where minimizing the influence of glass reinforcement is important.
The exact material selection should ultimately be determined according to the PCB stackup, transmission-line structure, target impedance, operating frequency, and fabrication capability.
CLTE-MW and SpeedWave 300P: A Low-Loss Multilayer Combination
A practical way to evaluate a bonding material is to test it together with the actual circuit laminate.
Rogers CLTE-MW is a glass-reinforced PTFE circuit laminate designed for high-frequency applications. Its loss factor is approximately 0.0015 at 10 GHz.
When CLTE-MW is combined with SpeedWave 300P in a multilayer stripline structure, the resulting transmission-line performance can demonstrate the importance of the bonding layer in overall RF loss.
Testing of this material combination has demonstrated an insertion loss of approximately 2.25 dB/in at 77 GHz.
The 77 GHz frequency is particularly relevant to modern automotive radar PCB and ADAS PCB applications, where high-resolution radar systems require stable RF performance and low signal attenuation.
This example also demonstrates an important engineering principle:
The electrical performance of a multilayer millimeter-wave PCB depends on the complete material system, not only on the primary circuit laminate.
The prepreg, laminate, copper, transmission-line geometry, and stackup must work together as one RF system.
Why 77 GHz Radar PCB Design Requires Low-Loss Materials
Automotive radar systems commonly operate in the 77 GHz frequency range. At this frequency, available signal power can be limited, while transmission-line loss and impedance discontinuities can have a significant effect on system performance.
For 77 GHz radar PCB design, engineers need to consider:
- Dielectric loss
- Conductor loss
- Copper surface roughness
- Dk stability
- Df stability
- Transmission-line geometry
- Impedance matching
- Layer-to-layer registration
- Via transitions
- Material thickness tolerance
- Glass-cloth effects
- Lamination consistency
A low-loss laminate alone cannot guarantee a low-loss multilayer RF PCB.
If the bonding layer has significantly different dielectric properties, the effective impedance and electromagnetic field distribution may change. This can introduce additional loss or impedance discontinuity.
Therefore, prepreg selection for 77 GHz radar PCB should be treated as part of the RF stackup design process rather than as a purely mechanical material decision.
Matching Dk Between Prepreg and Circuit Laminate
Loss is not the only consideration.
Dielectric constant matching between the bonding material and circuit laminate can help maintain more consistent impedance throughout the multilayer structure.
For example, SpeedWave 300P has a typical Z-axis dielectric constant of approximately 3.16 at 10 GHz, while CLTE-MW has a typical Z-axis dielectric constant in the range of approximately 2.94 to 3.02.
These values provide relatively good compatibility for the material combination.
When the dielectric properties of adjacent layers are reasonably matched, designers can better control the electromagnetic environment around the transmission line.
This becomes increasingly important when designing:
- 77 GHz automotive radar PCB
- 5G millimeter-wave PCB
- RF front-end modules
- High-frequency antenna modules
- High-speed wireless communication boards
- Advanced ADAS electronics
The objective is not simply to minimize Df. The objective is to create a stable and predictable RF transmission environment.
Microstrip vs. Stripline in Millimeter-Wave PCB Design
Transmission-line selection also influences total insertion loss.
In a microstrip structure, part of the electromagnetic field exists in air. This can reduce the effective dielectric interaction and may provide favorable loss characteristics under certain designs.
However, multilayer PCB architectures often require stripline structures for routing, shielding, isolation, or integration with other circuit layers.
In a stripline, the signal conductor is surrounded by dielectric material. Therefore, the dielectric properties of the laminate and prepreg have a stronger influence on transmission loss.
This explains why low-loss prepreg becomes especially important in multilayer stripline millimeter-wave PCB designs.
For a 77 GHz radar board, for example, the engineer must evaluate the complete transmission path rather than considering only the RF laminate specification.
How to Select Prepreg for a Multilayer Millimeter-Wave PCB
There is no single prepreg that is optimal for every RF application. Material selection should be based on electrical requirements and manufacturing constraints.
A practical selection process should consider:
Operating frequency:
Higher frequencies generally require more aggressive loss control.
Target insertion loss:
If the system has a strict RF link budget, a lower-Df bonding material may be necessary.
Transmission-line structure:
Stripline designs are particularly sensitive to dielectric properties because more of the electromagnetic field is contained within the dielectric.
Dk compatibility:
The bonding material should be evaluated together with the surrounding laminate to maintain impedance consistency.
Glass-cloth construction:
Spread-glass or suitable glass styles may help reduce the influence of glass-fiber geometry at very high frequencies.
PCB manufacturing capability:
Theoretical electrical performance is not enough. The material must be compatible with the PCB manufacturer’s lamination, drilling, plating, registration, and inspection processes.
Production volume:
A material that performs extremely well in laboratory testing may not always be the most practical solution for high-volume production.
Engineering Trade-Off: Electrical Performance vs. Manufacturability
For high-frequency PCB manufacturing, material selection is always a balance between electrical performance, mechanical reliability, manufacturing complexity, and cost.
PTFE-based prepreg systems can provide extremely low loss, but multilayer PTFE bonding may require specialized processing such as fusion bonding. Not every PCB manufacturer has sufficient experience with these processes.
For production-oriented applications, a material such as RO4450F may therefore be more attractive even if its Df is higher than that of a specialized ultra-low-loss material.
For demanding millimeter-wave applications, however, the additional electrical performance provided by materials such as Rogers 2929 or SpeedWave 300P can justify the additional material and manufacturing requirements.
The best solution depends on the actual RF link budget and manufacturing objectives.
Recommended Material Strategy for RF and Millimeter-Wave PCB Projects
For a multilayer RF PCB, engineers can consider the following general strategy:
| Application Requirement | Possible Material Direction |
|---|---|
| Cost-sensitive RF PCB | Conventional RF laminate + suitable bonding material |
| Microwave PCB | Low-loss laminate + RO4450F |
| Low-loss multilayer RF PCB | Low-loss laminate + Rogers 2929 |
| 24 GHz radar PCB | Low-loss RF laminate + optimized prepreg |
| 77 GHz automotive radar PCB | Ultra-low-loss laminate + SpeedWave 300P |
| High-resolution millimeter-wave radar | Ultra-low-loss material system with controlled stackup |
| 5G mmWave PCB | Low-Df laminate + compatible low-loss bonding layer |
These are engineering directions rather than universal rules. The final stackup should be validated according to the target frequency, impedance, line width, copper thickness, dielectric thickness, and PCB fabrication process.
How PCB Manufacturers Can Control Millimeter-Wave PCB Loss
Material selection is only one part of the process. A qualified high-frequency PCB manufacturer should also control:
- Lamination pressure and temperature
- Resin distribution
- Dielectric thickness
- Copper roughness
- Etching accuracy
- Line width and spacing
- Via geometry
- Registration accuracy
- Impedance tolerance
- Surface finish
- Material lot consistency
For high-frequency projects, the PCB supplier should ideally be involved before mass production.
A manufacturability review can identify potential problems in the stackup and transmission-line structure before tooling and production begin.
This is particularly valuable for 77 GHz radar PCB, 5G mmWave PCB, RF module PCB, and high-speed communication PCB projects, where small manufacturing variations can affect electrical performance.
FAQ: Low-Loss Prepreg for Millimeter-Wave PCB
What is the best prepreg for a millimeter-wave PCB?
The best prepreg depends on operating frequency, insertion-loss requirements, stackup, transmission-line structure, and manufacturing capability. For demanding millimeter-wave applications, ultra-low-loss materials such as SpeedWave 300P may be considered.
Why does prepreg affect millimeter-wave PCB loss?
Prepreg surrounds and bonds PCB layers. Its dielectric loss and dielectric constant influence the electromagnetic field around the transmission line, especially in stripline structures.
Is RO4450F suitable for 77 GHz PCB applications?
RO4450F can be used in some high-frequency applications, but whether it is suitable for a specific 77 GHz design depends on the total insertion-loss budget and transmission-line structure. More demanding designs may require a lower-loss bonding material.
What is the difference between RO4450F and Rogers 2929?
A key difference is dielectric loss. Their typical Df values at 10 GHz are approximately 0.004 for RO4450F and 0.003 for Rogers 2929. The lower-loss material may provide better performance in demanding RF applications.
Why is SpeedWave 300P suitable for millimeter-wave PCB designs?
SpeedWave 300P provides very low dielectric loss, with a typical Df around 0.002 at 10 GHz depending on construction. It is therefore a candidate for multilayer RF and millimeter-wave applications requiring low transmission loss.
What prepreg is suitable for 77 GHz automotive radar PCB?
A low-loss prepreg with compatible dielectric properties should be considered together with the RF laminate. SpeedWave 300P is one candidate for demanding 77 GHz radar applications, but the final choice should be validated against the complete PCB stackup and RF requirements.
Does lower Df always mean a better PCB?
No. Df is important, but engineers must also consider Dk, copper roughness, dielectric thickness, glass-cloth construction, impedance control, thermal reliability, manufacturability, availability, and cost.
Why is Dk matching important in multilayer RF PCB design?
Matching the dielectric properties of adjacent materials can help reduce impedance discontinuities and maintain more predictable signal propagation across multilayer structures.
Conclusion: Build the Complete Low-Loss Material System
As PCB operating frequencies move into the millimeter-wave range, reducing signal loss requires more than selecting a low-loss circuit laminate.
The prepreg and bonding material are integral parts of the RF transmission environment. Materials such as RO4450F, Rogers 2929, and SpeedWave 300P provide different combinations of dielectric performance and manufacturing practicality.
For demanding applications such as 77 GHz automotive radar, ADAS, 5G mmWave, RF modules, and high-speed wireless communication, engineers should evaluate the laminate and bonding material as a complete system.
The most effective approach is to optimize Df, Dk, transmission-line structure, copper roughness, dielectric thickness, stackup configuration, and manufacturing process together.
For high-frequency PCB projects, early cooperation between the PCB designer, material supplier, and PCB manufacturer can significantly improve the probability of achieving the required RF performance in mass production.
A 77 GHz automotive radar PCB requires more than a low-Df laminate. The complete RF structure—including the circuit laminate, bonding material, copper, transmission line, vias, and multilayer stackup—must be optimized for low insertion loss and stable impedance.
For a representative 77 GHz automotive radar PCB project, KKPCB can evaluate the RF material system before production to determine whether the selected laminate and prepreg are suitable for the target frequency and transmission-line structure.
The engineering objective is to achieve a stable, low-loss RF transmission path while maintaining manufacturability for multilayer PCB production.
Project Engineering Focus
The key engineering considerations for a 77 GHz multilayer radar PCB include:
- Ultra-low-loss RF laminate selection
- Low-loss prepreg selection
- Dk compatibility between laminate and bonding material
- Controlled dielectric thickness
- 50-ohm impedance control
- Low copper surface roughness
- High-precision RF trace fabrication
- Controlled via geometry
- Multilayer registration
- Reliable lamination
- Production-level DFM validation
For a stripline-based RF structure, the prepreg becomes particularly important because a larger portion of the electromagnetic field is concentrated inside the dielectric.
Therefore, KKPCB evaluates the RF laminate + prepreg + copper + stackup as an integrated material system rather than selecting the circuit laminate independently.
Representative Material Configuration
For a demanding millimeter-wave application, an ultra-low-loss material combination such as Rogers CLTE-MW with SpeedWave 300P bonding material can be considered.
CLTE-MW provides a very low dielectric loss characteristic, while SpeedWave 300P provides a low-loss bonding layer for multilayer construction.
A representative material evaluation can include:
| Engineering Parameter | Representative Value / Direction |
|---|---|
| Application | 77 GHz Automotive Radar PCB |
| PCB Type | Multilayer RF / Millimeter-Wave PCB |
| RF Frequency | 77 GHz |
| Circuit Laminate | Rogers CLTE-MW |
| Bonding Material | Rogers SpeedWave 300P |
| Laminate Df @ 10 GHz | Approx. 0.0015 |
| Prepreg Df @ 10 GHz | Approx. 0.0019–0.0022 |
| Prepreg Dk, Z-axis @ 10 GHz | Approx. 3.0–3.3 |
| CLTE-MW Dk, Z-axis @ 10 GHz | Approx. 2.94–3.02 |
| Example Prepreg | 1035 / 1078 constructions |
| Transmission Line | Microstrip or Stripline |
| Target Impedance | Typically 50 Ω for RF signal paths |
| Primary Loss Concern | Dielectric + conductor + transition loss |
| Key Application | Automotive Radar / ADAS |
The values above should be treated as representative engineering reference data rather than universal production specifications. The final PCB stackup must be calculated according to the actual material datasheet, resin content, finished dielectric thickness, copper thickness, trace geometry, and fabrication process.
RF Stackup Optimization
For a multilayer millimeter-wave PCB, KKPCB’s engineering review should focus on the complete RF signal path.
The RF layer should be positioned so that the transmission line has a predictable dielectric environment. The bonding layer should also be selected according to its Dk and Df rather than only its mechanical bonding capability.
For example, if a stripline is surrounded by significantly different dielectric materials, the effective dielectric constant can change and cause impedance discontinuities.
The engineering team therefore evaluates:
Material → Stackup → Transmission Line → Impedance → Fabrication → Electrical Verification
This approach helps reduce the risk of selecting an excellent RF laminate but achieving disappointing final-board performance because of the bonding material or manufacturing variation.
Engineering Parameter Table for Millimeter-Wave PCB Manufacturing
For customers developing 77 GHz radar PCB, 5G mmWave PCB, RF module PCB, and high-frequency communication PCB, the following parameters should be reviewed before fabrication.
| Parameter | Engineering Consideration | Why It Matters |
|---|---|---|
| Operating Frequency | 24 / 60 / 77 GHz or higher | Determines loss sensitivity |
| Df | Select low-loss dielectric | Reduces dielectric loss |
| Dk | Stable and well characterized | Supports impedance control |
| Prepreg Dk | Match adjacent RF material where practical | Reduces impedance discontinuity |
| Dielectric Thickness | Controlled after lamination | Directly affects impedance |
| Copper Roughness | Low-profile copper preferred for demanding RF | Reduces conductor loss |
| Trace Width | Calculated from actual stackup | Controls characteristic impedance |
| RF Impedance | Commonly 50 Ω | Prevents signal reflection |
| Via Geometry | Minimize unnecessary RF discontinuities | Improves transition performance |
| Glass Construction | Consider spread-glass options | Reduces local dielectric variation |
| Layer Registration | High precision required | Prevents stackup misalignment |
| Lamination | Controlled temperature and pressure | Maintains dielectric thickness |
| Surface Finish | Selected for RF and assembly requirements | Supports signal and soldering performance |
| Testing | Impedance and electrical testing | Confirms production consistency |
The actual tolerance should always be defined according to the PCB design, material system, manufacturing capability, and customer’s electrical specification.
DFM Guidelines for 77 GHz and Millimeter-Wave PCB
A millimeter-wave PCB should undergo DFM review before fabrication. At 77 GHz, manufacturing variations that may be relatively insignificant on a conventional FR-4 board can become important RF variables.
Control the Finished Dielectric Thickness
Do not design the RF stackup based only on nominal prepreg thickness.
After lamination, the final dielectric thickness depends on:
- Copper pattern density
- Resin content
- Lamination pressure
- Temperature
- Glass-cloth construction
- Copper thickness
- Adjacent layer structures
Therefore, impedance calculations should use the actual expected finished dielectric thickness.
Consider Copper Surface Roughness
At high frequencies, conductor loss becomes increasingly sensitive to copper surface roughness.
For demanding 77 GHz applications, the PCB designer and manufacturer should confirm the copper foil type and roughness requirements before production.
Using a low-loss dielectric while overlooking copper roughness can undermine the overall loss budget.
Avoid Unnecessary RF Vias
Vias can introduce parasitic capacitance, inductance, and impedance discontinuities.
For high-frequency signal transitions, via geometry should be optimized according to:
- Via diameter
- Pad diameter
- Anti-pad
- Via stub
- Ground-via arrangement
- Layer transition
- Signal frequency
Where practical, unnecessary via transitions should be avoided in critical RF paths.
Maintain Consistent RF Geometry
Small variations in trace width or spacing can affect impedance.
For this reason, RF traces should be designed with sufficient manufacturing margin rather than being pushed to the minimum capability of the PCB process.
A robust design is generally more suitable for volume production than a design that depends on extremely tight process limits.
Review Glass-Cloth Effects
At millimeter-wave frequencies, the electromagnetic field can interact with the glass reinforcement inside the dielectric.
For sensitive RF transmission lines, engineers should consider appropriate glass-cloth constructions and routing strategies.
Spread-glass constructions may be considered where dielectric uniformity is particularly important.
Control Layer-to-Layer Registration
Multilayer RF structures require accurate registration between signal, ground, and reference planes.
Registration errors can alter the effective transmission-line geometry and consequently affect impedance and RF performance.
This is especially important for fine-pitch RF structures and high-density multilayer radar boards.
Validate the Stackup Before Production
The final stackup should be reviewed before tooling.
A proper RF DFM review should verify:
- Material availability
- Laminate thickness
- Prepreg selection
- Finished dielectric thickness
- Copper thickness
- Trace width
- Trace spacing
- Impedance
- Via structure
- Registration
- Manufacturing tolerance
- Testing requirements
This prevents avoidable engineering changes after production has started.
KKPCB Engineering Support for High-Frequency PCB Projects
Selecting a low-loss material is only the beginning of a successful millimeter-wave PCB project.
KKPCB can support customers through the engineering evaluation process, including:
Material Selection
Evaluate suitable high-frequency laminates and bonding materials according to frequency, loss budget, dielectric requirements, and application.
Stackup Review
Review multilayer structures, dielectric thickness, copper configuration, RF layers, ground planes, and impedance requirements.
DFM Review
Identify manufacturing risks before fabrication, including trace geometry, vias, registration, lamination, and material compatibility.
Prototype Fabrication
Build engineering samples for customers to evaluate RF performance before moving to volume production.
Impedance Control
Control critical RF transmission-line geometry according to the approved stackup and impedance requirements.
Production Support
After prototype validation, the engineering parameters can be transferred into repeatable production processes for small-batch or volume manufacturing.
From RF Material Selection to Production
For 77 GHz automotive radar PCB and 5G millimeter-wave PCB projects, the most important question is not simply:
“Which PCB material has the lowest Df?”
A better engineering question is:
“Which complete material and manufacturing system can achieve the required RF performance consistently in production?”
The answer depends on the interaction between the laminate, prepreg, copper, stackup, transmission line, vias, lamination process, and manufacturing tolerances.
That is why early engineering communication between the PCB designer and manufacturer can reduce development risk and prevent expensive redesigns.
Request a Low-Loss Millimeter-Wave PCB Review from KKPCB
If you are developing a 77 GHz radar PCB, 5G mmWave PCB, RF module, automotive radar board, or other high-frequency multilayer PCB, KKPCB can review your design requirements before fabrication.
Send us your Gerber files, stackup requirements, material preference, operating frequency, impedance requirements, and annual volume.
Our engineering team can help evaluate:
- Low-loss laminate selection
- Prepreg / bonding material selection
- RF stackup
- 50 Ω impedance
- Transmission-line structure
- Via transitions
- DFM risks
- Prototype and production requirements
Have a millimeter-wave PCB project under development? Contact KKPCB for an engineering review and quotation.
