4.2.6 Conductor Repair, Inner Layer Method

Procedure

1. Locate and determine the coordinates for the repair. Use films or master drawings of the board as needed. 
   Note: Obtain as much information as possible on the conductive and non-conductive layers before 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 Figures 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. 
   Note: To reduce damage to the internal circuit, complete the final exposure of the internal circuit using a knife. (See Figure 2)
5. Remove all loose material. Clean the area.
6. 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.
7. Clean the area.
8. 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.
9. 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 before lap soldering in place.
10. 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 the spacing is critical or the circuit Board uses high-frequency circuits, bevel the joint.
    Caution: This bevel joint method may cause problems with circuit Boards exposed to extreme temperature fluctuations.
11. Apply a small amount of liquid flux to the overlap joint.
12. Lap solder the Circuit Track to the exposed internal circuit using solder and a soldering iron. Make sure the new circuit is properly aligned.
13. Clean the area.
    Note: The circuit Board may be preheated before 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.
14. Mix epoxy. If desired, add a color agent to the mixed epoxy to match the circuit board color.
15. 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 the height of the next internal circuit. (See Figure 5) 
    Note: A slight overfill of epoxy may be desired to allow for shrinkage when the epoxy cures.
16. Cure the epoxy per Procedure 2.7 Epoxy Mixing and Handling.
    Caution: Some components may be sensitive to high temperatures.
17. Add additional Circuit Tracks if needed and coat with additional epoxy.
18. Continue completing all layers until the top surface of the circuit board is reached. 
19. Clean the board as required.
20. Apply surface coating to match prior coating as required.

Evaluation

1. Visual examination for alignment and overlap of the new circuit.
2. Visual examination of epoxy coating for texture and color match.
3. Electrical tests as applicable.