2.6 Rework & Repair

Rework and repair are invasive procedures that carry a high risk of Collateral Damage (CD) to the printed circuit board assembly (PCBA). Unlike primary assembly, which relies on automated precision, rework relies on operator skill and strict thermal management. This chapter defines the mandatory protocols for component removal, pad restoration, and jumper wire installation in accordance with IPC-7711/7721. The objective is to restore functionality without compromising the long-term reliability of the laminate or adjacent components.

Thermal Management Protocols

The primary failure mode in rework is thermal shock, leading to pad cratering, barrel separation, or delamination.

• Preheating: Mandatory for all multilayer boards (>4 layers) or boards with large ground planes.
  • Bottom-Side Preheat: The PCB assembly must be soaked at 100 C – 120 C for a minimum of 2 minutes before applying localized top-side heat. This reduces the thermal delta ($\Delta T$) and prevents warping.
• Reflow Cycles: A specific location on the PCB is limited to 3 thermal excursions (Original Reflow + 2 Rework Cycles). Exceeding this limit degrades the epoxy resin matrix of FR-4.
• Cool Down: Forced air cooling immediately after rework is prohibited. The assembly must cool naturally to ambient temperature to prevent mismatched Coefficient of Thermal Expansion (CTE) stress.

Through-Hole Component Removal

Removing multi-lead through-hole components (e.g., connectors, relays) requires vacuum desoldering to clear the plated through-hole (PTH).

• Vacuum Desoldering:
  • Temperature: Set iron tip to 350 C (max 380 C for heavy copper).
  • Technique:
    1. Add fresh solder to the joint to improve thermal bridge.
    2. Place tip over lead/pad interface.
    3. Wait for complete melt (observe solder slump).
    4. Activate vacuum trigger for 2 seconds while oscillating the tip in a circular motion (orbital).
    5. Lift-Off: Remove tip before releasing vacuum to prevent solder suck-back.
• Barrel Inspection: After removal, inspect the hole against a light source. The barrel must be 100% clear of solder obstruction. Do not use a drill bit to clear blockage; use a heated clearing tool or wick.

SMT Rework (Hot Air)

Hot Air Rework Systems are required for BGA, QFN, and fine-pitch IC removal. Hand soldering irons are prohibited for removal of components with hidden joints.

• Profiling: The nozzle airflow and temperature must mimic a standard reflow profile.
  • Ramp Rate: < 3 C/sec.
  • Peak Temp: 235 C – 245 C (SAC305).
• Shielding: Heat-sensitive components (e.g., plastic connectors, electrolytic capacitors, crystals) within 10 mm of the rework site must be shielded with Kapton tape or metal baffles.
• Zero-Force Removal: Use a vacuum pickup tool to lift the component only when the solder is fully liquidus. Prying or pulling a component with tweezers before full reflow will rip pads (Pad Cratering).

Pad and Trace Repair

If a copper pad is lifted or a trace is severed, it must be restored using epoxy-backed circuit frames (IPC-7721 Method 4.0).

• Adhesive: Use distinct two-part epoxy thermally rated for the operating environment. Cyanoacrylate (Super Glue) is strictly prohibited.
• Curing: Repair epoxy must be cured under clamping pressure at elevated temperature (e.g., 60 C for 60 mins) per manufacturer spec.
• Trace Patching:
  • Overlap: The replacement conductor must overlap the existing trace by > 3 mm.
  • Soldering: Lap joints must be soldered with a minimum fillet. Butt joints are not permitted.

Jumper Wire Installation

Jumper wires ("mod wires") modify logic or bypass broken traces.

• Wire Type: Kynar-insulated, silver-plated copper wire (30 AWG typical).
• Routing:
  • Wires must run parallel to X or Y axes. Diagonal routing is prohibited.
  • Wires must be tack-soldered or adhered with epoxy/UV glue every 25 mm and at every bend to prevent vibration fatigue.
  • Strain Relief: The wire must have a stress-relief loop near the solder joint. It must not be pulled tight (violin string).
• Termination: Do not solder wires directly to component leads (Leg-to-Leg). Wires must terminate at existing pads or vias.

Final Checklist