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4.2.6 Conductor Repair, Inner Layer Method
Outline
This method is used to replace damaged or missing circuits on internal layers of multilayer circuit boards.

Caution
The circuit widths, spacing and current carrying capacity must not be reduced below allowable tolerances.

Caution
The overlap joint used in this method may cause problems with high frequency circuitry.

Caution
This procedure is complicated and should be attempted only by properly skilled repair personnel using the best tools and equipment.

Minimum Skill Level - Expert
Recommended for technicians with advanced soldering and component rework skills and extensive experience in most repair/rework procedures.

Conformance Level - High
This procedure most closely duplicates the physical characteristics of the original, and most probably complies with all the functional, environmental and serviceability factors.

Acceptability References
IPC-A-600 2.0 Externally Observable Characteristics
IPC-A-600 3.0 Internally Observable Characteristics
IPC-A-610 10.0 Laminate Conditions
Procedure References
1.0 Foreword
2.1 Handling Electronic Assemblies
2.2 Cleaning
2.5 Baking And Preheating
2.7 Epoxy Mixing and Handling
IPC7721 4.2.6 Conductor Repair/Modification, Conductive Ink Method
Tools and Materials
CircuitMedic Part
Abrader
Mild abrasive for removing oxides and contaminates from conductive surfaces.
CircuitMedic Part
Ball Mills, Abrasives, Cutting Tools
Ball mills, abrasives and cutting tools for working on circuit boards.
CircuitMedic Part
Circuit Track Kit
Kit to repair circuit board damage including lands and conductors.
Cleaner
Cleaner
General purpose cleaner for removing contamination.
CircuitMedic Part
Cleaning Brushes
Disposable brushes for use with solvents for cleaning and application of coatings.
Color Agent
Color Agents
Use to tint the color of epoxy used for solder mask repair or circuit base board repair.
CircuitMedic Part
Circuit Bond Packs
Clear, superior strength epoxy in two-compartment plastic packages
CircuitMedic Part
High Temp Tape Discs
High temperature polyimide tape discs, .50" diameter.
CircuitMedic Part
Knife with #16 Blades
A must-have tool for precise cutting, scraping and trimming.
Microscope
Microscope
It is a challenge to undertake precision repair without a good microscope.
CircuitMedic Part
Micro Drill System
Versatile power tool for milling, drilling, grinding, cutting and sanding circuit boards.
CircuitMedic Part
Micro Probes
Use to dispense a minute amount of adhesive and for micro-positioning of small objects.
CircuitMedic Part
Mixing Picks
Unique mixing sticks have a paddle shape on one and sharp pick on the opposite end.
Oven
Oven
General purpose oven for drying, baking and curing epoxies.
CircuitMedic Part
Scraper
Hardened stainless steel tip for scraping solder mask and removing defects.
Soldering Iron
Soldering Iron, Solder and Flux
A well maintained soldering iron is a must at every tech bench.
Wipes
Wipes
Nonabrasive, low-linting wipes for cleanup.
Images and Figures
4.2.6 Conductor Repair, Inner Layer Method
Damaged Inner Layer
4.2.6 Conductor Repair, Inner Layer Method
Figure 1: Milling into multilayer circuit board to expose the damaged internal circuit.
4.2.6 Conductor Repair, Inner Layer Method
4.2.6 Conductor Repair, Inner Layer Method
Figure 3: cut through the base material, one layer at a time, until the desired inner layer has been reached.
4.2.6 Conductor Repair, Inner Layer Method
Figure 4: Circuit Track in place ready to be soldered.
4.2.6 Conductor Repair, Inner Layer Method
Figure 5: If spacing is critical or the circuit board uses high frequency circuits, bevel the joint as shown
4.2.6 Conductor Repair, Inner Layer Method
Figure 6: Coat the top and sides of the new circuit with epoxy. Add epoxy until flush with top surface.
4.2.6 Conductor Repair, Inner Layer Method
Figure 7: Completed repair

Procedure
  1. Locate and determine the coordinates where the repair is to be made. Use films or master drawings of the board as needed.

    Note
    Obtain as much information as possible on the conductive and non-conductive layers prior to starting the procedure.  

  2. Remove components from the immediate area if necessary and clean the area.

  3. Use the microscope and Micro-Drill System and cut through the base material, one layer at a time, until the desired inner layer has been reached. (See Figure 1 and 2)

    Caution
    Great care should be taken to prevent further damage to internal circuits. 

  4. Each internal circuit should have a flat section exposed to allow the new circuit to be soldered in place. (See Figure 3)

  5. Note
    To reduce damage to the internal circuit, complete the final exposure of the internal circuit using a knife. (See Figure 3)  

  6. Remove all loose material. Clean the area.

  7. Apply a small amount of liquid flux to the ends of the internal circuit. Tin the exposed end of each circuit using solder and a soldering iron.

  8. Clean the area.

  9. Select a Circuit Track to match the width and thickness of the circuit to be replaced. Cut a length approximately as needed. The Circuit Track should overlap the existing circuit a minimum of 2 times the circuit width.

  10. Gently abrade the top and bottom of the Circuit Track with the buffer to remove any protective coating and clean.

    Note
    If needed, the ends of the Circuit Track may be tinned with solder prior to lap soldering in place.  

  11. Place the Circuit Track in position. The Circuit Track should overlap the existing circuit a minimum of 2 times the circuit width. (See Figure 4)

    Note
    If spacing is critical or the circuit Board uses high frequency circuits, bevel the joint. (See Figure 5)

    Caution

    This bevel joint method may cause problems with circuit Boards exposed to extreme temperature fluctuations.  

  12. Apply a small amount of liquid flux to the overlap joint.

  13. Lap solder the Circuit Track to the exposed internal circuit using solder and a soldering iron. Make sure the new circuit is properly aligned.

  14. Clean the area.

    Note
    The circuit Board may be preheated prior to filling the area with epoxy. A preheated PC Board will allow the epoxy to easily flow and level out. Epoxy applied to an unheated circuit board may settle below the circuit board surface as the epoxy cures. 
  15. Mix epoxy. If desired, add color agent to the mixed epoxy to match the circuit board color.

  16. Coat the top and sides of the replaced circuit with epoxy. The epoxy bonds the new circuit to the base board material and insulates the circuit. Continue adding epoxy up to the top surface of the circuit board or to the height of the next internal circuit. (See Figure 6)

    Note
    A slight overfill of epoxy may be desired to allow for shrinkage when the epoxy cures.  

  17. Cure the epoxy per Procedure 2.7 Epoxy Mixing and Handling.

    Caution
    Some components may be sensitive to high temperature. 

  18. Add additional Circuit Tracks if needed and coat with additional epoxy.

  19. Continue completing all layers until the top surface of the circuit board is reached. (See Figure 7)

  20. Clean the board as required.

  21. Apply surface coating to match prior coating as required.

Evaluation

  1. Visual examination for alignment and overlap of new circuit.

  2. Visual examination of epoxy coating for texture and color match.

  3. Electrical tests as applicable.

Procedure for reference only.