PCB Design and Footprint Specifications
Shielding cover soldering quality first depends on careful planning during PCB design.
Pad (Footprint) Design Specifications
- Frame Pads: Must cover all solder edges of the shield can frame, minimum width 0.5mm, recommended 1.0-1.5mm
- Pad Spacing: Gap between pads ≥ 1.0mm to prevent solder bridging
- Pad Thickness: Copper thickness ≥ 35 µm (1 oz) to ensure soldering strength
- Via Design: Place sufficient vias around pads (spacing ≤ 2mm) to provide thermal relief paths
- Solder Mask: Remove solder mask at pads to ensure solder paste contacts bare copper
Design Verification Checklist
- ✓ Frame pad width ≥ 0.5mm
- ✓ Pad area ≥ 95% of can frame solder area
- ✓ Pad spacing without solder mask interference
- ✓ Adequate thermal relief vias to prevent solder float
Solder Paste Selection and Application
Solder Paste Specifications
- Composition: Recommended SAC305 (Sn96.5/Ag3/Cu0.5) or SAC387, RoHS compliant
- Particle Size: Type 4 or Type 3, powder diameter 25-45 µm
- Viscosity: Recommended 800-900 Pa·s @ 25°C for printability without excessive flow
- Flux Activity: Medium activity (ROL0) to prevent over-activation and corrosion
- Storage Conditions: Cold chain storage 2-8°C, use within 24 hours of opening
Solder Paste Printing Parameters
| Parameter | Specification | Remarks |
|---|---|---|
| Stencil Thickness | 125-150 µm | Recommended 125 µm for precise control |
| Print Speed | 20-50 mm/s | Fast printing (40+ mm/s) ensures uniform paste |
| Squeegee Angle | 45° | Standard angle ensures complete filling |
| Paste Thickness | 100-150 µm | For shield cans recommend 120-150 µm for full solder joints |
| Paste Volume | 90-110% of pad area | Ensures complete solder, prevents voids or splashing |
Reflow Soldering (Reflow) Parameter Optimization
Shielding cover soldering requires careful reflow temperature control, especially for large cans.
Standard Reflow Temperature Profile
- Preheat Phase (Preheat): Temperature 150-200°C, duration 60-120 seconds. Purpose is to preheat solder paste and PCB to prevent splashing
- Soak Phase (Soak): Temperature 200-250°C, duration 60-180 seconds. Solvents in solder paste evaporate, flux activates
- Reflow Phase (Reflow): Peak temperature 245-260°C, duration 10-30 seconds. Solder paste melts and forms metal bond with PCB and can
- Cooling Phase (Cooling): Temperature drop < 5°C/second (prevent rapid cooling causing joint cracking), cool to room temperature
Large Shielding Can Soldering Tips
For cans > 50mm, large metal mass and high thermal capacity require special attention:
- Extend preheat time to 120 seconds
- Peak temperature can increase to 255-260°C, not exceeding 260°C
- Use infrared thermometer or thermal imaging to verify can bottom reaches 245°C or higher
- Consider using dual-stage or zone reflow techniques
Post-Assembly Inspection Methods
Visual Inspection (AOI)
- Solder Joint Integrity: Joints should be bright silvery, no black oxidation or dull appearance
- Solder Coverage: Solder covers ≥ 95% of pad area, no visible gaps
- Solder Splashing: No solder balls > 0.5mm around joints
- Cold Joint Detection: Joint surfaces should have no indentations or irregular shapes
X-Ray Inspection (for High-Reliability Applications)
- Check internal voids in solder joints (< 25% void ratio acceptable)
- Verify complete solder paste filling, especially at can edges
- Detect hidden cold spots or cracks
Electrical Testing
- Ground Impedance Test: Impedance between can and PCB ground < 50 mΩ (test conditions: 25°C, DC, 1A)
- Insulation Resistance: Between can and adjacent signal lines > 10 MΩ (DC 500V)
- Shielding Performance: Optional Faraday cage test to verify shielding effectiveness ≥ 95% of design value
Rework and Removal Techniques
Rework Process
Step 1: Fault Diagnosis
Before removal, confirm the can actually has a defect (soldering fault, damage, etc.). Electrical testing confirms poor grounding or isolation failure.
Step 2: Heating and Desoldering
- Heat the area around the shielding can with hot air gun (temperature 350-400°C) for 30-60 seconds
- Gently push the can to feel solder softening signal
- Use solder wick or vacuum solder sucker to remove solder
- Perform multi-stage heating and desoldering to avoid PCB overheating
Step 3: Can Removal
- Once solder is sufficiently soft, gently pry up one corner of the can with tweezers
- Gently rock and shake (not pulling hard), gradually separating solder
- Maintain temperature during removal to prevent cooling
Step 4: PCB Cleaning
- After removal, use solder sucker to remove excess solder
- Use lead-free solder wick to clean pads
- Wash with isopropyl alcohol to remove paste residue and flux
- Important: Inspect pads per IPC-A-610 standard to check for damage
Step 5: Re-soldering
- Inspect pads; if copper is intact, can apply fresh solder paste
- If pads are damaged, use conductive silver adhesive or solder wire for repair
- Re-reflow using normal parameters
Common Defects and Solutions
| Defect Symptom | Root Cause Analysis | Solution |
|---|---|---|
| Cold Joint | Insufficient solder paste, low reflow temperature, poor PCB preheating | Increase paste thickness to 150 µm; raise peak temperature to 255°C; extend preheat time |
| Solder Balls/Splashing | Excessive paste thickness, fast reflow ramp, overactive flux | Reduce paste thickness; lower peak ramp rate; switch to moderate-activity paste |
| Poor Grounding | Pad oxidation, old solder paste, insufficient soldering temperature | Check paste storage period; clean pad surfaces; raise reflow temperature 5-10°C |
| Can Warping | Uneven paste volume, PCB warping, asymmetric pad design | Improve paste printing; check PCB flatness; optimize pad distribution |
| Cold Solder | Insufficient temperature, fast cooling, contamination | Raise peak temperature to 255°C; reduce cooling rate; strengthen cleaning |
Best Practices Checklist
Design Phase
- ☐ Frame pad width ≥ 1.0mm
- ☐ Via spacing ≤ 2mm
- ☐ Solder mask clarity verification
Soldering Preparation
- ☐ Solder paste cold-chain storage, warm to 23°C 30 minutes before opening
- ☐ Stencil cleanliness check for residual paste or contaminants
- ☐ Solder paste viscosity test within 800-900 Pa·s range
Soldering Process
- ☐ Reflow oven temperature profile verification (after first use or parameter changes)
- ☐ Solder paste thickness 120-150 µm
- ☐ Baking pan preheat (optional but recommended)
Inspection Phase
- ☐ AOI pass rate ≥ 99%
- ☐ Ground impedance < 50 mΩ
- ☐ Sampling of shielding performance or X-ray inspection (high-reliability applications)
Rework Standards
- ☐ Rework not exceeding 2 times
- ☐ Post-rework pad quality not lower than first-soldering standard
- ☐ 100% electrical testing after rework
