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2.6 Rework & Repair

ARework dedicatedand sectionrepair 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 "Theautomated Art of Undoing." How to replace a BGA (Ball Grid Array) using hot airprecision, rework stations.relies Howon to repair a lifted pad. This requires highoperator skill and strict processthermal controlmanagement. 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 avoidrestore damagingfunctionality without compromising the PCB.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

Parameter

Limit / Mandate

Preheat

100 C – 120 C (Multilayer PCBs)

Thermal Cycles

Max 3 per location

Desolder Tip

350 C (Target)

Vacuum Dwell

Max 2 Seconds

Hot Air Ramp

< 3 C/sec

Shielding

Required within 10 mm

Pad Repair Glue

Epoxy Only (No Super Glue)

Jumper Routing

X/Y Axis Only + Tack Down

Standard

IPC-7711 / 7721


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