Process Flow
Solder Paste Printing – 4–6 mil stencil, laser-cut, electro-polished. Snap-off 0.5–1.0 mm. Verify paste height ±10% via SPI.
Component Placement – Pick-and-place accuracy ±25 µm. Validate nozzle condition daily. BGA/CSP requires vision alignment.
Reflow Soldering – Profile per alloy: SAC305 peak 245±5°C, soak 60–90 s, ramp 1–2°C/s. Nitrogen atmosphere for QFN/BGA.
Through-Hole Insertion – Wave solder: preheat 100–120°C, pot temp 255–265°C. Selective solder for mixed SMT/THT.
Cleaning & Coating – DI water wash for no-clean residues if reliability critical. Conformal coating after AOI/ICT.
Critical Process Controls
Stencil Management: Clean every 5 prints. Replace at 50k cycles.
Placement Pressure: 20–30 g for 0201/0402; 50–70 g for QFP/QFN. Adjust for pad finish (ENIG vs. HASL).
Reflow Atmosphere: O₂ <500 ppm for lead-free BGAs to prevent voids.
THT Solderability: Pre-bake moisture-sensitive parts >24 h @ 125°C.
Quality Gates
SPI: Reject paste volume <80% or >150% of nominal.
AOI: Inspect BGA via X-ray. Accept voids <15% ball area.
ICT/Flying Probe: Test nets with <2 Ω resistance variation.
Functional Test: Burn-in @ 55°C/85% RH for high-reliability builds.
Common Defects & Fixes
| Defect |
Root Cause |
Correction |
| Tombstoning |
Uneven heating/pad design |
Balance thermal mass; revise land pattern |
| BGA Voids |
Moisture/contaminated paste |
Bake components; verify nitrogen purity |
| Head-on-Pillow |
Warpage/misalignment |
Optimize reflow profile; check placement pressure |
| Cold Joints |
Insufficient wetting |
Increase soak time; verify flux activity |
Optimization Levers
DFM First: Pad geometry per IPC-7351B. Keep component spacing ≥3× body width for rework access.
Stencil Design: Step-down apertures for fine-pitch QFNs. Home plate apertures for BGAs.
Profile Tuning: Use K-type thermocouples on test coupons. Lock profile after 3 consecutive good runs.
Material Control: Store paste @ 2–8°C. Thaw 4 h before use. Track lot numbers for traceability.
Field Notes
Reject boards with >0.75% moisture content (IPC-1601).
For flex PCBs, use polyimide stiffeners under components.
Never bypass AOI/X-ray for Class 3 assemblies.
Log all process deviations. Trend data weekly.
Email: all@poe-pcba.com
Whatsapp: 85292069596
Tel: 0755-25312250/ +8613798543496
Factory Address: Floor 3, Jinyuan Industrial Park, No. 56, Tangtou Avenue, Shiyan Town, Baoan District, Shenzhen China
