2.6 Rework & repair techniques

Manual rework and repair are invasive procedures that induce localized thermal and mechanical stress to the printed circuit board assembly (PCBA). Unlike automated SMT assembly, manual rework bypasses controlled process windows, establishing a critical dependency on operator skill and strict adherence to thermal management protocols. This chapter outlines mandatory guidelines for component removal, pad restoration, and jumper wire installation, governed by the IPC-7711/7721 standard. The primary objective is to restore functionality without compromising the long-term reliability of the FR-4 laminate and adjacent components.

Thermal management protocols

Improper thermal management is the leading cause of rework-induced failures, including pad cratering, barrel separation, and PCB delamination.

• Preheating (Mandatory): A bottom-side preheater is required for all multilayer boards (>4 layers) and assemblies containing significant copper ground planes.
  • Thermal Soak: The PCBA must be preheated to 100 °C to 120 °C for 2 minutes prior to applying localized top-side heat (via a soldering iron or hot air tool). This reduces the thermal differential (ΔT) across the Z-axis, preventing warpage and localized thermal shock.
• Thermal Cycle Limits: A specific physical coordinate on a PCB is strictly limited to a maximum of 3 total thermal excursions (1 primary reflow cycle + a maximum of 2 rework cycles). Exceeding this limit degrades the epoxy resin matrix of the PCB material.
• Cooling Protocol: Warning: The use of forced air (fans or compressed air) to accelerate cooling immediately after rework is strictly prohibited. The assembly must cool naturally to ambient temperature to prevent stress fractures induced by mismatched Coefficients of Thermal Expansion (CTE).

Through-hole component removal

Removing multi-lead through-hole components (e.g., connectors, relays) mandates controlled vacuum desoldering to evacuate the plated through-hole (PTH) without stressing the copper barrel or the annular ring.

• Vacuum Desoldering Parameters:
  • Temperature: The desoldering iron tip must be set to 350 °C (increases to 380 °C for heavy copper planes require explicit engineering authorization).
  • Procedure: 1. A minimal amount of fresh flux-cored solder must be applied to the existing joint to establish a rapid thermal bridge. 2. The desoldering tip must be positioned concentrically over the lead and pad. 3. Complete solder melt (slump) must be visually verified. 4. The vacuum must be activated for 2 seconds while inducing a minimal orbital motion to free the component lead from the barrel wall. 5. Lift-Off: The tip must be removed entirely from the pad prior to releasing the vacuum trigger to prevent liquid solder from drawing back into the hole.
• PTH Inspection: Following removal, the hole must be inspected using backlighting. The barrel must be 100% free of solder. Mechanical clearing (utilizing drill bits or metal picks) is strictly prohibited; blockages must be cleared using a heated clearing tool or fluxed desoldering braid.

SMT rework (hot air systems)

Programmable Hot Air Rework Systems are mandatory for the removal and replacement of BGAs, QFNs, and fine-pitch ICs. Utilizing manual soldering irons for complex area-array component extraction is prohibited.

• Thermal Profiling: The rework nozzle’s airflow and temperature profile must replicate a validated SMEMA oven reflow profile.
  • Ramp Rate: The equipment must maintain a controlled ramp rate of < 3 °C/sec.
  • Peak Temperature: The mandatory target is 235 °C to 245 °C for SAC305 alloys.
• Component Shielding: Heat-sensitive components (plastic body connectors, electrolytic capacitors, optical elements) situated within 10 mm of the active rework zone must be shielded utilizing polyimide tape or metal baffles.
• Zero-Force Extraction: A vacuum pickup tool must be utilized to vertically extract the component only after visual or mechanical confirmation of full liquidus (float). Prying, pulling, or twisting a component prior to complete reflow guarantees pad cratering.

Pad and trace repair procedures

Damaged copper pads or severed traces must be restored utilizing approved epoxy-backed circuit frames, adhering strictly to IPC-7721 (e.g., Method 4.0).

• Adhesive: An approved, thermally-rated two-part repair epoxy is mandatory. Cyanoacrylate (super glue) is strictly prohibited; it outgasses and degrades rapidly at reflow temperatures.
• Curing: The repair epoxy must be cured under mechanical pressure at an elevated temperature (typically 60 °C for 60 minutes) per the manufacturer’s technical specifications.
• Trace Splicing:
  • Overlap: The replacement conductor must overlap the existing trace by a minimum of 3 mm.
  • Joint Formation: The lap joint must be reflowed to form a continuous fillet. Direct end-to-end “butt joints” provide insufficient mechanical strength and are prohibited.

Jumper wire installation (modifications)

Jumper wires (modification wires) are deployed to execute engineering change orders (ECOs) or bypass irreparable trace damage.

• Material: Only approved insulated solid copper wire (typically 30 AWG Kynar-insulated, silver-plated) may be used.
• Routing Guidelines:
  • Wires must be routed parallel to the board’s X or Y axes (orthogonal routing). Unstructured diagonal point-to-point routing is prohibited.
  • Wires must be staked using an approved adhesive (epoxy or UV-cure) every 25 mm along the route, and at every bend, to prevent vibration-induced fatigue.
  • Strain Relief: The wire must incorporate a defined strain-relief loop adjacent to the termination point. Routing wires under structural tension is prohibited.
• Termination: Wires must terminate at existing copper pads or designated vias. Soldering jumper wires directly to component leads or bodies (“leg-to-leg” wiring) is prohibited unless explicitly defined within an MRB-authorized procedure.

Final Checkout: Rework & Repair Techniques

Parameter	Required Standard	Objective
Bottom Preheat	100 °C to 120 °C (Mandatory for Multilayer PCBs >4 layers).	Minimizes Z-axis expansion, prevents warpage, and protects internal vias.
Thermal Cycle Limit	Maximum of 3 total reflow excursions per physical coordinate.	Preserves the structural integrity and dielectric properties of the FR-4 matrix.
Desoldering Temp	Nominal 350 °C (Maximum 380 °C requires engineering authorization).	Ensures rapid liquidus without thermally degrading the pad’s resin adhesive layer.
Vacuum Dwell	Maximum of ~2 seconds per actuation.	Prevents excessive cooling and barrel stress during extraction.
Hot Air Ramp Rate	< 3 °C/sec.	Prevents thermal shock and micro-cracking in ceramic bodies.
Proximity Shielding	Mandatory for sensitive components within a 10 mm radius.	Protects adjacent components from secondary reflow and collateral heat damage.
Pad Repair Adhesive	Thermal Two-Part Epoxy only (Cyanoacrylate prohibited).	Guarantees the repair survives subsequent operational thermal cycling.
Jumper Routing	Orthogonal X/Y routing; staked every 25 mm.	Ensures mechanical reliability under vibration and maintains signal integrity.
Primary Standard	IPC-7711 / 7721	Baseline criteria governing all rework, modification, and repair operations.