5.6 Rework & Repair (IPC-7711/7721)

ThisRework standardis distinguishesnot betweena "Rework"Undo" (processbutton; correction)it is controlled trauma. Every time you apply a soldering iron to a PCBA, you introduce thermal shock, consume the sacrificial plating, and "Repair"grow the brittle Intermetallic Compound (restoration).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 (No Permission Needed)IPC-7711)

• Definition: ReprocessingRestoring a non-conforming articlecomponent to makefull itconformity conformwith the original design drawings.
• Authority: Typically granted to the originalmanufacturer drawings.without external approval.
• Examples:Example: Re-flowingReflowing a cold solder joint,joint removing a solder bridge,or replacing a tombstoned capacitor.
• Standard: The end result must be indistinguishable from a normal process output.

Repair (Customer Approval Mandatory)IPC-7721)

• Definition: ProcessingRestoring functionality to a non-conformingboard articlethat sodoes itnot functions,conform even if it doesn't matchto the original drawing.
• Examples:
  • Trace Repair:The Bridgingphysical aform brokenfits trackor withfunction ais wire.altered.
  • Pad Repair:Authority: GluingForbidden without a lifted pad back to the FR4 using epoxy.
  • Jumper Wires: Adding a wire to correct a design bug.
• Control: Repairs must be documented on a "Deviation" or "Concession" form and signed bydeviation/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

ActionControl Point	DefinitionCritical Requirement	CustomerRisk Sign-off?Avoided
Touch-upPre-Heat	AddingSoak flux/heatboard to flow100°C joint– 120°C before rework.	NoThermal Shock / Ceramic Crack
Replace PartCycles	DesolderMax and2 replaceheating componentcycles per pad.	Brittle Joints / Pad Lifting
Repair Authority	Jumper wires/Trace cuts require Customer Sign-off.	Contract Violation
Cooling	No forced cooling (fans) immediately after reflow.	Grain Structure Fracture
Cleanliness	Flux residue removed locally before recoating.	Dendritic Growth / Leakage
Trace CutRepair	CuttingWire aoverlap copper≥ track3x trace width (if repairing track).	YES
JumperCurrent Wire	AddingDensity wire mod	YES
Pad Glue	Re-attaching lifted pad	YESFailure