5.6 Rework & Repair (IPC-7711/7721)

Rework is not a "Undo" button; it is controlled trauma. Every time you apply a soldering iron to a PCBA, you introduce thermal shock, consume the sacrificial plating, and grow the brittle Intermetallic Compound (IMC) layer. The goal of IPC-7711 (Rework) and IPC-7721 (Repair) is to restore functionality without sacrificing long-term reliability. If the rework process causes invisible damage (measling, pad lifting, barrel cracking), the board will pass the functional test today and fail in the field next month.

Rework vs. Repair: The Legal Distinction

You must understand the contractual difference before touching the board.

Rework (IPC-7711)

• Definition: Restoring a component to full conformity with the original design drawings.
• Authority: Typically granted to the manufacturer without external approval.
• Example: Reflowing a cold solder joint or replacing a tombstoned capacitor.

Repair (IPC-7721)

• Definition: Restoring functionality to a board that does not conform to the original drawing. The physical form fits or function is altered.
• Authority: Forbidden without a signed deviation/concession from the customer.
• Example: Installing a jumper wire to bypass a broken trace or using epoxy to secure a lifted pad.

Thermal Management & Pre-Heating

The number one cause of rework failure is "Thermal Shock." Touching a room-temperature (25°C) ceramic capacitor with a 350°C iron creates a localized expansion fracture.

Thermal Logic:

• If reworking ground planes or heavy copper (> 1 oz) → Then bottom-side preheating is mandatory.
• If board thickness > 1.6 mm → Then preheat to 100°C – 120°C before applying top-side heat.
• If using Hot Air → Then ramp rate must not exceed 4°C/sec. Explosive outgassing (popcorning) occurs if moisture trapped in the component expands too fast.

Pro-Tip: Calibrate the "dwell time." Do not rely on the operator's feeling. Use a stopwatch. If a joint does not reflow within 5 seconds of iron contact, the thermal capacity of the iron is too low, or the tip is oxidized. Do not just push harder; stop and change tools.

The "Three Strike" Rule (IMC Control)

Solder is not glue; it is a metallurgical bond formed by the Intermetallic Compound (Cu6Sn5). This layer is brittle.

1. First Reflow: Primary assembly. IMC layer forms (Good).
2. Second Reflow: Rework/Replacement. IMC layer grows thicker (Acceptable).
3. Third Reflow: Failed rework attempt. IMC layer becomes excessive, prone to vibration fracture (High Risk).

Control Limit:

Define a maximum of 2 rework attempts per location. If the component fails a third time, scrap the board. Do not allow operators to "cook" a joint indefinitely.

Conformal Coating Removal

You cannot rework through coating. It must be removed without damaging the Soldermask or the component markings.

Removal Methods:

• Thermal: Best for thick coatings (Epoxy/Urethane). Use a controlled hot knife. Risk: Scorched board surface.
• Chemical: Best for Acrylics/Silicones. Use solvent spot-cleaning. Risk: Solvent entrapment under components.
• Micro-blasting: abrasive blasting for hard coatings (Parylene). Risk: Static generation (ESD) and stray abrasion.

Pro-Tip: When replacing coating after rework, feather the edges of the new coating into the old coating. Do not leave a hard ridge where moisture can pool.

Final Checklist