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

Conformance Level - High

Tools and Materials

Abrader Mild abrasive for removing oxides and contaminants.

Ball Mills, Abrasives, Cutting Tools Ball mills, abrasives and cutting tools for working on circuit boards.

Circuit Bond Epoxy Clear, superior strength epoxy in two-compartment plastic packages.

Circuit Tracks Copper conductors to repair circuit board damage including traces and conductors.

Cleaner General purpose cleaner for removing contamination.

Cleaning Brushes Disposable brushes for solvent cleaning and application of coatings.

Color Agents One part, semi-paste ink used to tint the color of epoxy and for direct printing on circuit board surfaces.

Flex-Rack PCB Holder Sturdy rack for PCBs used for rework and positioning.

High Temp Tape Discs High temperature polyimide tape discs, .50" diameter.

Knife with #16 Blades A must-have tool for precise cutting, scraping and trimming.

Micro Drill System Versatile power tool for milling, drilling, grinding, cutting and sanding.

Micro Probes Sharp probes for dispensing adhesive and positioning small objects.

Microscope Precision microscope with stand and lighting for work and inspection.

Mixing Picks Disposable mixing sticks for mixing and applying adhesives.

Oven General purpose oven for drying, baking and curing epoxies.

Precision Tool Set Nine precision-crafted tools for detailed circuit board work.

Scraper, Curved Tip Hardened stainless steel tip for scraping solder mask and removing defects.

Soldering Iron Properly maintained soldering iron and properly sized soldering iron tips.

Wipes Nonabrasive, low-linting wipes for cleanup.

Procedure

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.

Images

Conductor Repair, Inner Layer Method

Milling into multilayer circuit board to expose the damaged internal circuit.

cut through the base material, one layer at a time, until the desired inner layer has been reached.

Circuit Track in place ready to be soldered.

If spacing is critical or the circuit board uses high frequency circuits, bevel the joint as shown.

Coat the top and sides of the new circuit with epoxy. Add epoxy until flush with top surface.

Completed repair