Introduction
Printed circuit boards (PCBs) provide the foundation for mounting and interconnecting electronic components using conductive copper traces on an insulating base material. The properties of this insulating laminate substrate are critical in determining the performance limits of the PCB. Rogers RT/Duroid 5870 is a specialized microwave laminate designed for high frequency applications demanding tight electrical tolerances.
This article provides an in-depth look at Rogers 5870 covering its characteristics, properties, applications and usage considerations for PCB designers.
What is Rogers RT/duroid 5870?
Rogers RT/duroid 5870 is a glass microfiber reinforced fluoropolymer composite laminate material tailored for exacting radio frequency (RF) and microwave circuit designs. Some key features include:
- Extremely tight dielectric constant tolerance of ±0.40%
- Low dissipation factor (loss tangent) of 0.0019
- Z-axis coefficient of thermal expansion (CTE) of 16.5 ppm/°C
- Thermoset polymer system requiring no fusion during lamination
- Woven E-glass reinforcement for dimensional stability
- Halogen-free material for reduced environmental impact
- RoHS compliant and UL 94V-0 rated
These electrical and mechanical characteristics make 5870 well-suited for demanding high frequency applications up to mmWave bands.
Dielectric Properties
Rogers 5870 possesses very stable dielectric properties across a wide frequency range along with low loss, which are desirable traits for microwave PCBs:
Dielectric Constant (Dk)
- Dk of 2.33 ±0.0040 at 10 GHz
- Nearly constant Dk over 1-40 GHz
- Extremely tight tolerance of ±0.40%
Loss Tangent (Df)
- Low loss tangent of 0.0019 at 10 GHz
- Df less than 0.002 up to 20 GHz
- Enables higher Q factors for RF circuits
Temperature Coefficient
- Dielectric constant variation of -17 ppm/°C
- Excellent thermal stability of electrical properties
These attributes allow achieving target impedances accurately during RF circuit design and simulations using 5870 material.
Mechanical Properties
In addition to electrical performance, Rogers 5870 also possesses good mechanical characteristics:
Density
- Uniform density of 1.79 g/cm3
- Provides good thickness consistency
Glass Transition Temperature
- High Tg of 280°C
Decomposition Temperature
- Extremely high 405°C decomposition point
Moisture Absorption
- Low moisture absorption of 0.02% (immersion, 24 hours)
Z-axis CTE
- 16.5 ppm/°C coefficient of thermal expansion
The woven glass reinforcement results in lower Z-axis CTE for reliability under temperature changes. The high Tg and decomposition temperature enable soldering steps required in PCB fabrication.
Available Thicknesses
Rogers 5870 laminate can be obtained in a range of standard thicknesses from as low as 0.127mm to as high as 6.35mm.
Some commonly used thicknesses include:
- 0.127 mm (5 mils)
- 0.2 mm (8 mils)
- 0.25 mm (10 mils)
- 0.5 mm (20 mils)
- 1.0 mm (40 mils)
The thin flexible laminates are useful for flex PCB applications, while thicker sheets provide rigidity and dimensional stability required in rigid boards.
Common Designations
Rogers 5870 material is available under variants like:
Where:
- RT/duroid is the Rogers brand name
- 5870 denotes the dielectric constant value
- LM signifies low moisture absorption version
These all refer to the same base microwave laminate with minor modifications. The differing nomenclatures indicate process variations or compliance levels.
Lead-Free Compatibility
Rogers 5870 laminate is compatible with lead-free fabrication which is mandatory for RoHS compliance:
- Glass transition temperature exceeds Pb-free soldering temperatures
- Withstands multiple reflow cycles with peak temperatures up to 280°C
- Near zero Z-axis CTE minimizes stresses from temperature excursions
- Low moisture absorption reduces risk of delamination or blistering
PCBs made using 5870 can be reliably assembled using standard lead-free soldering processes.
Typical Applications
Rogers 5870 is used in demanding RF and high speed digital designs including:
- Satellite communication systems
- 5G equipment – MIMO antennas, splitters
- Military and aerospace avionics
- Automotive driver assistance radar
- High speed data acquisition
- Low noise amplifiers and power amplifiers
- Broadband transformers
- Medical imaging
- Test and measurement equipment
- Microwave components
The tight and stable electrical tolerances provided by 5870 laminate are critical for achieving optimized impedance control and signal integrity in such applications.
Benefits of Using Rogers 5870
Here are some of the key benefits offered by Rogers 5870 laminates for PCB fabrication:
- Extremely consistent dielectric constant for impedance control
- Low loss tangent for high frequency and Q factor
- Tight thickness tolerance for etch consistency
- Excellent dimensional stability under temperature fluctuations
- High Tg and decomposition rating for lead-free assembly compatibility
- Reduced EMI/RFI interference due to woven glass reinforcement
- Halogen-free and RoHS compliant for environmental needs
- Cost effective compared to PTFE substrates
- Good availability due to widespread usage in RF industry
For designs requiring precision electrical performance under 10+ GHz along with robust mechanical characteristics, Rogers 5870 provides an optimal microwave PCB substrate solution.
Design and Fabrication Guidelines
Here are some guidelines to follow when designing PCBs using Rogers 5870 material:
- Allow ±0.2mm thickness tolerance in layouts
- Use teardrop pads at junctions to reduce potential cracks
- Limit unsupported copper spans to under 0.25mm to prevent lifting
- Watch for acid entrapment under traces causing potential leakage
- Remove residues after wet processing steps
- Allow for 0.1% shrinkage during lamination stage
- Use two-stage lamination for boards over 3mm thick
- Select compatible FR4 and prepreg materials if designing hybrid multilayer boards
Rogers provides detailed design manuals that cover additional fabrication and assembly recommendations when working with their microwave laminates.
Comparing Rogers 5870 to Other Materials
Rogers 5870 has some advantages compared to other common PCB substrate materials:
| Parameter | Rogers 5870 | FR4 | Polyimide | PTFE |
|---|---|---|---|---|
| Dielectric Constant | Stable | Fair | Good | Excellent |
| Loss Tangent | Excellent | Fair | Good | Excellent |
| Lead-free Process Compatibility | Good | Good | Fair | Poor |
| Moisture Resistance | Good | Fair | Excellent | Excellent |
| Thermal Conductivity | Fair | Fair | Poor | Good |
| Cost | Moderate | Low | High | Very high |
So Rogers 5870 provides a good balance of electrical, thermal, mechanical and cost characteristics for optimal RF PCB performance.
Conclusion
Rogers RT/duroid 5870 is a specialized glass microfiber reinforced fluoropolymer laminate engineered specifically to meet the tight tolerances and low loss required in high frequency PCBs. It enables accurate impedance control and signal integrity up to mmWave bands. The stable dielectric properties, lead-free soldering compatibility, low moisture absorption and moderate cost make 5870 a popular choice for fabricating microwave circuits used in radar, communication, aerospace and other demanding RF applications.
Frequently Asked Questions
Can Rogers 5870 be used for common FR4 processing?
Yes, Rogers 5870 can be processed using standard FR4 PCB fabrication methods. It is compatible with typical PCB manufacturing processes involving etching, drilling, plating, lamination etc.
Does Rogers 5870 require special handling or storage?
No special storage conditions are needed. It has an indefinite shelf life at room temperature before lamination. Low moisture absorption also avoids special drying requirements before processing.
Can I use 5870 with different dielectric constant materials in one PCB?
Rogers 5870 can be combined with properly selected FR4 prepregs to produce hybrid multilayer boards. But significant Dk mismatches could cause issues and is not recommended.
What is the typical copper thickness used with Rogers 5870?
1/2 oz to 3 oz copper foils are commonly used. 1 oz provides a good balance for moderate thickness and current capacity for most RF designs. 2 oz may be preferred for high power applications.
Is Rogers 5870 material expensive?
5870 is costlier than FR4 but less expensive than PTFE or ceramic substrates. The tight tolerances and specialization for RF applications lead to higher cost than standard glass epoxy PCB laminates.
