What Is The Difference Between FR4 And Rogers Material in PCB Application?

FR4 Vs. Rogers Material

When designing printed circuit boards (PCBs), one of the most important choices is selecting the right substrate material. The substrate is the base material that forms the core of the PCB onto which conductive copper traces are patterned to form the circuit. The substrate material determines key electrical, mechanical, and thermal properties of the PCB that impact performance and reliability.

Two of the most common substrate materials used in PCBs today are:

• FR4 - A thermoset glass epoxy material that is relatively inexpensive
• Rogers - A thermoset ceramic-filled PTFE composite that offers superior high frequency performance

Understanding the key differences between FR4 and Rogers will help engineers select the right material for their particular PCB application and requirements.

Composition and Properties

FR4

FR4 is a thermoset glass fabric reinforced epoxy resin composite. The designation FR4 stands for Flame Resistant 4, indicating this material meets a flame resistance rating of V-0 per UL 94 standards.

The composition of FR4 includes:

• Epoxy resin - Typically brominated bisphenol A or bisphenol F based
• Glass fabric - Woven fiberglass cloth for reinforcement
• Fillers - Typically antimony trioxide for flame resistance

Key properties and characteristics of FR4:

• Low cost - FR4 is one of the most economical PCB substrate materials
• Moderate dielectric constant - Around 4.5 at 1 MHz
• Moderate loss tangent - Around 0.02 at 1 MHz
• Good mechanical strength - Glass fiber reinforcement provides rigidity
• Popular for consumer electronics PCBs due to cost/performance ratio

However, FR4 suffers from performance limitations at higher frequencies due to dielectric losses, making it unsuitable for many RF applications. The relatively high and inconsistent dielectric constant and loss tangent also make FR4 a poor choice for precision analog and high-speed digital circuits.

Rogers

Rogers is a family of PTFE composite substrates reinforced with ceramic filler particles. Some common Rogers materials include:

• Rogers RO4000 series - Woven glass reinforced
• Rogers RO3000 series - Non-woven glass reinforced
• Rogers TMM - Ceramic filled, no glass reinforcement

Some key properties of Rogers materials:

• Stable dielectric constant - Tightly controlled, varies by material in the 2.9 - 10.2 range
• Low loss tangent - Around 0.0009 - 0.002 at 10 GHz
• Excellent high frequency performance - Low signal loss up to mmWave frequencies
• Low thermal expansion - Matching the coefficient of copper for reliability
• High thermal conductivity - For good heat dissipation

The precise dielectric properties, close matching to copper, and low losses make Rogers an ideal choice for performance-driven microwave, telecom, aerospace and defense applications. However, the exotic material compositions result in Rogers costing 5-10x more compared to FR4 on a per area basis.

PCB Substrate Requirements

To determine whether FR4 or Rogers is best for a particular PCB application, it is important to consider the key electrical, mechanical, and thermal requirements:

Electrical

• Operating frequencies?- Rogers maintains stable dielectric properties up to mmWave frequencies. FR4 usable only to a few GHz before losses become problematic.
• Impedance control?- Rogers offers tight control of dielectric constant for controlled impedance traces. FR4 has less consistent dielectrics.
• Signal integrity?- Rogers has low loss for clean signal transmission. FR4 suffers high frequency attenuation.
• Precision analog?- Rogers low, stable dielectrics are ideal for analog precision. FR4 inconsistent properties limit analog performance.

Mechanical

• Structural rigidity?- Both FR4 and Rogers offer good rigidity from glass fabric reinforcement.
• Dimensional stability?- Rogers maintains stable dimensions over temperature due to low Z-axis expansion. FR4 can exhibit some warping.
• Flexibility?- If flexible PCBs are required, Rogers flex laminates are available. FR4 can also be used for rigid-flex.

Thermal

• Thermal conductivity?- Rogers has excellent thermal conductivity for heat dissipation. FR4 conducts heat poorly.
• Temperature tolerance?- Rogers maintains superior electrical and mechanical properties from -60°C to over 280°C. FR4 properites degrade above Tg (~140°C).

PCB Construction and Fabrication

There are some differences in how FR4 and Rogers PCBs are constructed that impact the fabrication process:

Layer Count

• FR4 - Typically used for simpler single, double, or multilayer boards up to about 12-16 layers.
• Rogers - Often used for advanced multilayer boards exceeding 16 layers required for complex designs.

Stackup

• FR4 - Requires careful stackup design to control impedance with highly variable dielectric constant.
• Rogers - Tight dielectric control enables easier stackup design to achieve target trace impedances.

Drilling

• FR4 - Standard drill bits can drill through glass/epoxy with ease.
• Rogers - Hard ceramic filler can prematurely wear drill bits. Diamond-coated bits needed.

Via Formation

• FR4 - Simple mechanical through-hole drilling followed by copper electroplating/filling
• Rogers - Laser drilling typically used to ablate small microvias due to ceramic hardness. Costs more.

Imaging

• FR4 - Standard lithographic processes apply.
• Rogers - May require special treatments like reduced baking temperatures.

Solder Mask

• FR4 - Wide range of solder masks adhere well to epoxy.
• Rogers - Some masks don't adhere to Rogers as well. May require plasma surface treatment.

Soldering

• FR4 - Easily soldered using standard soldering profile of under 300°C peak temperature.
• Rogers - Requires soldering around 260°C peak to avoid decomposing at higher temps.

Comparing FR4 vs. Rogers for Common Applications

Consumer Electronics PCBs

For cost-driven boards for TVs, computers, cell phones, appliances, automotive electronics, and IoT devices, FR4 is most often chosen due to its very low cost. High frequency performance is not critical for many consumer applications. FR4 provides adequate performance at low cost.

Communication Systems

For radios, antennas, wireless equipment, etc. operating up to several GHz, standard FR4 may still be usable, though performance will be limited. Rogers provides superior performance for these wireless applications without excessive material cost.

High Speed Digital

For server, telecom, and advanced computing applications with signals exceeding 5+ GHz, Rogers provides the electrical performance required. FR4 cannot maintain signal integrity at such frequencies.

RF Microwave Systems

For radar systems, satellite communications, medical imaging, etc. operating well into the mmWave region, FR4 losses would be prohibitive. Rogers or even ceramic substrates are required at such high frequencies.

Aerospace and Defense

Rogers materials are routinely used for high-performance, high-reliability aerospace applications. The stable electrical properties under wide temperature swings and mechanical robustness suit Rogers for extreme environment use.

Summary of Key Differences

ParameterFR4RogersDielectric Constant~4.52.9 - 10.2 depending on materialLoss Tangent~0.02 at 1 MHz~0.0009 - 0.002 at 10 GHzThermal Conductivity0.25 W/mK>0.5 W/mKFrequency RangeUp to ~5 GHzUp to mmWave regionsCostVery LowMedium to HighTypical ApplicationsConsumer electronicsCommunication systems, aerospace, defense

Conclusion

For cost-driven consumer PCBs where performance is not critical, FR4 remains a popular choice. When electrical performance and consistency is paramount for high frequency or precision circuits, Rogers microwave laminates are superior. Engineers must weight the cost vs performance trade-offs when selecting PCB substrate materials. Considering the application requirements, operating frequencies, fabrication factors, and cost constraints will guide the decision between FR4 or Rogers.

Frequently Asked Questions

What is the major difference between FR4 and Rogers material?

The key difference is that Rogers materials offer tightly controlled electrical properties with low dielectric loss that is stable across a wide frequency range. FR4 has less consistent electrical properties and higher losses that increase at higher frequencies.

Why is Rogers PCB material more expensive than FR4?

The exotic material compositions of Rogers with precisely engineered ceramic fillers result in higher materials cost compared to widely available FR4. The specialized dielectric properties also require more stringent processing controls adding cost.

Is it possible to use standard FR4 for a high frequency microwave PCB?

It is generally not recommended to use FR4 above about 5GHz. The dielectric losses will severely attenuate signals as frequency increases leading to poor performance. The inconsistent electrical properties also make maintaining impedance difficult.

What are some typical applications where Rogers material should be used over FR4?

Any application where performance is critical and cost is less of a concern would benefit from Rogers material. This includes radar systems, wireless infrastructure, satellite communication, defense electronics, and research instrumentation.

Does fabricating a Rogers PCB require different manufacturing equipment compared to FR4?

Some adjustments may be needed. Laser drilling is typically used for microvias. Certain bake and etch steps may need reduced temperatures. Diamond drill bits help improve hole walls. But same equipment can support both with process tuning.