Circuit Board Repair Guide > Conductor Repair Procedures > 4.2.6 How to Repair an Inner Layer Conductor

How to Repair an Inner Layer Conductor

Repair Circuit Boards | Repair SMT Pads | Repair BGA Pads | Repair Lands | Repair Edge Contacts
Repair a Conductor | Plated Hole Repair | Base Board Repair | Replace Solder Mask or Coatings

This advanced procedure 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.
 
Damaged inner layer conductor needs replacement
Damaged Inner Layer

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

IPC 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

Related Procedure References
CTC 1.0 Foreword - Circuit Board Repair Guide
CTC 2.1 Handling Electronic Assemblies
CTC 2.2.1 How to Clean a Circuit Board
CTC 2.2.2 Cleaning Circuit Boards, Aqueous Batch Process
CTC 2.5 Baking and Preheating of Printed Circuit Boards
CTC 2.7 Epoxy Mixing and Handling
IPC7721 4.2.6 Conductor Repair/Modification, Conductive Ink Method

Tools and Materials
Ball Mills
Buffer
Circuit Tracks      
Cleaner
Color Agents      
Epoxy
Flux, Liquid
Knife
Micro Drill System      
Microscope
Oven
Scraper
Soldering Station
Solder
Tape, Kapton
Wipes

4.2.6 Conductor Repair, Inner Layer Method

Printed Board Type: R/F  |  Skill Level: Expert  |  Conformance Level: High  |  Rev.: E  |  Rev. Date: Jul 7, 2000

Expose damaged internal circuit

Fig. 1: Milling into multilayer
circuit board to expose the
damaged internal circuit.


Micro Drill System used for milling, drilling, grinding, cutting and sanding circuit boards

Fig. 2: A high quality, Micro Drill System is recommended for this delicate operation.


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.
Cut until inner layer reached

Fig. 3: Cut through the base
material, one layer at a time,
until the desired inner layer has
been reached.


4. 
Each internal circuit should have a flat section exposed to allow the new circuit to be soldered in place. (See Figure 3).
 
5. 
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.
Circuit Track ready to be soldered

Fig. 4: Circuit Track in place
ready to be soldered.


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 (PCB Track) should overlap the existing circuit a minimum of 2 times the circuit width. (See Figure 4).
 
High frequency circuits, bevel joint as shown

Fig. 5: If spacing is critical or the circuit board uses high frequency circuits, bevel the joint as shown.


 
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 liquid circuit board 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.
   
Coat circuit with epoxy

Fig. 6: Coat the top and sides of
the new circuit with epoxy. Add
epoxy until flush with top surface.


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.
   
Conductor repair completed

Fig. 7: Conductor repair completed.


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.




Links to Related Products


Micro Drill System used for drilling, grinding, cutting circuit boards  
Micro Drill System 110-4105
The Micro Drill System is a repair technician's dream and is ideal for milling, drilling, grinding, cutting and sanding circuit boards. It removes coating, cuts circuits, cuts leads, drills holes, cuts slots, shapes FR4 and performs many other procedures using various interchangeable bits. Unlike most hand-held tools, the Micro Drill has a tiny, high speed DC motor in the hand piece, eliminating cumbersome drive cables and giving you better control. A separate power supply keeps the hand piece lightweight and reduces operator fatigue. Base unit has controls for forward/reverse and speed up to 45,000 RPM.


Circuit Track Kit used to repair damaged PCB tracks  
Circuit Track Kit 201-3130
Thanks to Circuit Tracks, it's now easier than ever to repair damaged circuits. Circuit Tracks are rectangular shaped conductors made of 99.9% pure copper. The rectangular ribbons closely conform to the original conductor dimensions. The replacement Circuit Track is bonded in place using a prepackaged epoxy which is included in this kit. The Circuit Track Kit is a great time and labor-saver and an easy solution to repairing missing or damaged circuits and conductors.



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