PCB Type : Rigid-flex PCB
Application: Warship Radar
Material: Taconic RF-35A2
1. High frequency signal, low dielectric loss
2. Stepped groove structure, immersion gold + thick gold
Printed circuit boards (PCBs) used in radar systems for naval ships and submarines require high frequency, precision transmission line characteristics to maintain signal integrity. Rigid-flex PCBs enable integrating rigid board sections containing components with flexible circuits interconnecting modules.
This article provides an overview of design and manufacturing considerations for rigid flex PCBs used in shipborne radar systems operating in tough environmental conditions. We will focus on key electrical, mechanical and testing factors involved in ensuring the reliability and performance needed for these critical defense applications.
Overview of Shipborne Radar Systems
- Search radar – Detects surface contacts and provides situational awareness
- Fire control radar – Precisely tracks targets to enable weapon aiming
- Navigation radar – Identifies terrain, obstacles; supports safe passage
- Weather Radar – Monitors conditions like storms; helps plan navigation
These high power systems transmit and receive directional radio frequency signals that are processed to generate visual maps on displays. Operating parameters include:
- Frequency bands – L, S, C, X band (1-12 GHz typical)
- Peak transmission power – Megawatts
- Pulse widths – 0.1 to 250 microseconds
- Scan rates – 6 to 60 RPM continuous rotation
The radar PCBs must deliver RF consistency and reliability in harsh shipborne environments.
Rigid-Flex PCB Technology Overview
Rigid-flex PCBs contain interconnected rigid board areas and flexible circuits allowing three-dimensional configuration. Key elements include:
- Glass reinforced resin laminates for strength
- Copper layers with etched tracks and pads
- Components like ICs, passives are mounted here
- Polyimide films that can bend repeatedly
- Thin copper traces etched on films
- Interconnects between separate rigid sections
- Polyimide film bonded over traces for protection
- Openings in coverlayer allow connections
- Rigid and flex materials bonded using adhesive sheets
- Ensure adhesion reliability under shock, vibration
Rigid-flex integration enhances layout efficiency, ruggedness and serviceability.
PCB Substrate and Dielectric Materials
The PCB substrate materials for radar systems need to deliver:
- Stable dielectric constant over temperature
- Low thermal expansion for reliability
- Tight thickness and dielectric tolerances
- High thermal conductivity for heat dissipation
- Compatibility with lamination and drilling processes
Typical microwave materials used are:
- PTFE Composites – Lowest loss; costlier (Rogers 4000, Teflon)
- Hydrocarbon Ceramics – Balance cost, performance (Rogers 3003)
- WF CAF Free Laminates – Lead-free; improved stability (Isola 370HR)
- Quartz Loaded Glass Laminates – Improved Q values (Rogers TMM)
The materials determine loss characteristics, wave impedances and propagation speed.
Layer Stackup Configuration
A typical layer stackup for radar rigid flex PCBs comprises:
|Signal||High frequency traces routed matching impedance|
|Ground||Ground plane for return currents, controlled impedance|
|Power||Power distribution network|
|Dielectric||Prepreg bonding matrix between laminates|
|Soldermask||Protective coating over traces|
Key considerations for layer planning include:
- Number of routing layers required to fit all traces
- Minimizing overall PCB thickness for form factor
- Placement of ground and power planes to isolate RF and digital circuits
- Selectively thickening copper and dielectric to manage currents
- Balancing number of layers to facilitate lamination
The cross-section is optimized to achieve electrical performance within constraints.
PCB Design Factors
Here are key rigid flex PCB design guidelines for radar systems:
- Match transmission line impedance to system impedance, often 50Ω
- Consistent trace geometry for controlled inductance and capacitance
- Tight thickness tolerances of dielectric materials
- Minimize discontinuities on transmission path – vias, etc
- Isolate noisy digital routes using ground/power planes
- Model effects of connectors, trace width changes
- Use low loss dielectric materials to minimize insertion loss
- Minimize trace length variations between routes
Flex Bend Radius
- Allow sufficient flexible circuit bend radius ≥ 10X material thickness
- Avoid acute bends and tearing under motion
- Eliminate stubs; use teardrop pads at junctions
- Allow for thermal expansion and contraction stresses
- Model vibration, shock loads using FEA
Simulating the electrical performance using 3D EM analysis ensures the design meets radar signal integrity requirements prior to fabrication.
PCB Fabrication Process Considerations
Here are key aspects of the fabrication process for high reliability radar rigid flex PCBs:
- Quality laminate materials with controlled thicknesses
- Careful process control for bond reliability without voids
- Minimize resin starvation around thick copper shapes
- Excellent hit-to-hole registration accuracy
- Precise depth control on multilayers
- Minimal smear generation – no via wall pulls
- High quality electroless copper uniform coverage
- Dry film resist patterning for fine features
- Etching process control to hit 5 mil lines
- No undercuts or opens violating spacing rules
- Excellent throw power on high aspect ratio holes
- Uniform plating distribution inside barrels
- No measling or pullaway from hole walls
- LPI for registration precision, 5 mil openings
- Cure profile matched to dielectric materials used
- Adhesion processes ensure robustness
A rigorous process focus enables fabrication of high performance radar boards.
PCB Testing and Inspection
Testing rigid flex boards for radar systems involves:
- Cross-section microsectioning to validate fabrication quality
- Microscope inspection of holes for plating defects
- Scanning acoustic microscopy to identify delamination issues
- Verify electrical connectivity and isolate shorts
- Impedance testing of critical traces and transmission lines
- Detect ANY layer registration problems using flying probe
Automated Optical Inspection
- Scan soldermask and legend print quality
- Detect spacing violations from fabrication process
- Radar module functional testing
- Power-on system-level test of populated board
- Burn-in stress testing for early life failures
This combination of fabrication quality checks and post-assembly validation ensures reliability.
Design for Manufacturing Guidelines
Here are some key design-for-manufacturing guidelines for radar rigid flex PCBs:
- Allow adequate spacing around fine pitch traces for fabrication tolerance
- Minimize use of acute angles; use tear drops when unavoidable
- Keep rigid section thickness uniform; avoid sudden changes
- Eliminate copper stubs; provide radius at conductor junctions
- Check minimum hole size with stackup thickness
- Verify registration capabilities for dense microvia transitions
- Take vibration into account for flexible circuit bend radius
- Budget for connector spacing and rotations
- Simulate thermal expansion mismatches in layout
- Partner with manufacturer early to review design
DFM practices tailored to rigid flex boards prevent issues during fabrication and assembly.
Rigid flex PCB technology provides an optimal solution enabling advanced radar systems for naval platforms that require high frequency circuitry combined with ruggedness. Careful design practices including matching transmission line impedances, managing discontinuities, allowance for thermal stresses, and design for manufacturability verification are vital for manufacturing complex boards that perform reliably in harsh conditions over long system lifetimes. Partnering with manufacturers that have demonstrated expertise building high-reliability rigid flex circuits for defense applications helps mitigate risks for these critical programs. With rigorous quality processes that verify fabrication and assembly quality prior to population, smooth integration into radar modules with confidence is achieved.
Frequently Asked Questions
Q: Why are rigid flex PCBs suited for shipborne radars vs. rigid boards?
Rigid flex integration enables optimal electrical performance through flat rigid sections for components while providing flexible interconnections that withstand shock/vibation better without solder joint cracks.
Q: What are some key dielectric materials used for radar PCBs?
Low loss thermoplastic hydrocarbon ceramics like Rogers 3003TM and glass reinforced hydrocarbon/ceramics such as Rogers 4000® series are commonly used for radar boards.
Q: How do standard impedances vary for different PCB stackups?
A 50 ohm transmission line could have width variations from ~9 mils on 5 mil dielectric to ~4.4 mils on 3 mil dielectric depending on stackup.
Q: Why is impedance control important for radar PCBs?
A consistent impedance match to radar components is vital to minimize signal reflections and insertion loss which can reduce operating range and resolution.
Q: What tests validate radar module performance?
Testing focuses on verifying directionality, sidelobe suppression, gain uniformity, noise figure, and mapping radiation patterns in azimuth and elevation.