Supporting the military is something RayMing is very proud of, and we were contracted to assemble a PCB for use in a military communications application. The RF prototype PCB assembly used Rogers 4350B material, and the board required surface mount technology for the double-sided assembly that featured 250 placements. Board specs included 3 layers, standard vias, and 1 panelization. Final measurements came to 8 x 5.75 in. We performed final testing using automated optical inspections and x-ray inspection before delivering product to the customer.
Introduction
Rogers RO4350B laminates are widely used for manufacturing high frequency PCBs for defense and aerospace applications like radars, electronic warfare systems, satellite communications, etc. This high performance PCB material enables circuits to operate reliably at microwave and mm-wave frequencies beyond 10 GHz.
However, assembling PCBs using RO4350B substrates requires special considerations during fabrication, component selection and assembly processes owing to its unique properties. This article provides guidelines on how to assemble RO4350B PCBs for mission-critical military applications.
RO4350B PCB Material Properties
RO4350B is a glass reinforced fluoropolymer composite circuit board material designed for exacting microwave and mm-wave circuit needs. Here are some of its key properties:
- Dielectric constant (Dk) of 3.48 at 10 GHz
- Low loss tangent of 0.0037 at 10 GHz
- Woven glass reinforced hydrocarbon/ceramic filled PTFE composite
- Good thermal conductivity of 0.69 W/m/K
- Low Z-axis CTE of 41 ppm/°C
- Glass transition temperature of 280°C
RO4350B’s electrical and mechanical attributes make it suitable for aerospace RADARs, satellite communication, electronic warfare systems and radio communications.
Design Considerations
The PCB layout for an RO4350B board destined for military usage must account for the following:
Controlled Impedance
- Maintain tight impedance control over traces and match to RF IC terminations
RF Shielding
- Incorporate adequate shielding enclosures for emission control
Minimal Passives
- Limit passive components to reduce losses at high frequencies
RF Transitions
- Carefully design components/connectors interfaces
Thermal Management
- Use thermal vias and/or metal backing to spread heat
Reliability Testing
- Subject prototype to thermal cycling, vibration and shock tests
PCB Fabrication Process
RO4350B PCBs require specialized fabrication to meet military specifications:
- Material – Use RO4350B laminates from Rogers or equivalent suppliers
- Layer Count – Typical stackups have 4 to 32 layers depending on complexity
- Dielectrics – Combination of RO4350 prepregs and bondplies
- Trace width – RF traces down to 3 mils line with 3 mil spacing
- Hole walls – Copper plated through holes with 1 oz copper
- Surface finish – Immersion silver or ENIG surface finish
- Testing – Full electrical testing and quality inspection per IPC standards
Component Selection
The active and passive components used on the PCB assembly must also conform to RF application needs:
RF Transistors
- Gallium arsenide (GaAs) RF power transistors with operating frequency > 10 GHz
Power Amplifiers
- GaN or GaAs MMIC power amplifiers from suppliers like Qorvo, Microchip, etc.
RF Connectors
- High frequency SMP, SMPM, SSMP connectors rated for microwave bands
Passive Components
- Chip resistors, capacitors and inductors with tight tolerances rated for high frequency use
EMI Suppression
- Multi-layer ceramic capacitors, ferrite chip beads for noise filtering
Assembly Process Steps
Assembling the RO4350B PCB requires precision SMT assembly and inspection:
Solder Paste Printing
- Use solder paste suitable for small RF components. Dispense accurate volumes.
Component Placement
- Use pick and place machine with capability to handle small chip components.
- Follow the thermal profile recommended by solder paste vendor. Use nitrogen if possible.
Conformal Coating
- Select coating suitable for high frequency. Apply selectively near components.
Testing
- Carry out RF measurements like insertion loss, VSWR etc.
Thermal Cycling
- Cycle over temperature range of product use environment.
Vibration and Shock
- Subject PCB assembly to vibration and shock pulses expected during deployment. Verify performance.
Conformal Coating
- Environmentally seal assembly with conformal coating compatible with operating temperatures.
Quality Inspection
Thorough inspection procedures must validate assembly integrity:
- Visual Inspection – Verify component placement, orientation, solder joints, coating coverage.
- Microscope Inspection – Check solder fillet wetting, bridging under high magnification.
- AXI Inspection – Confirm assembly defects, alignment issues are within limits.
- X-Ray – Employ x-ray imaging to inspect BGA/CSP hidden solder joints.
- RF Testing – Validate transmission fidelity at circuit operating frequencies.
- Environmental Testing – Subject samples to expected temperature, humidity, vibration levels.
FAQs
Q: What is the dielectric constant of RO4350B at 1 GHz?
A: The dielectric constant (Dk) of RO4350B at 1 GHz is 3.48.
Q: What finish is preferred for assembling RO4350B military PCBs?
A: Immersion silver and electroless nickel immersion gold (ENIG) offer the best finish for assembling RO4350B boards.
Q: What is the glass transition temperature of RO4350B material?
A: RO4350B has a high glass transition temperature of 280°C.
Q: What RF connectors work best with RO4350B boards?
A: SMP, SSMP and SMPM connectors rated for microwave frequencies beyond 10 GHz work well.
Q: What solder mask color is typically used on military PCBs?
A: Olive drab green colored solder mask is frequently used on military PCB assemblies.
