4.7.1 Surface Mount Pad Repair, Epoxy Method

Printed Board Type: R/F/C
Skill Level: Advanced
Conformance Level: Medium
Revision: E
Revision Date: Jul 7, 2000

OUTLINE

Damaged Surface Mount Pad

This method is used to replace damaged surface mount pads with commercially available replacement pads. The new pads are bonded to the circuit board surface using liquid epoxy.

CAUTION
It is essential that the board surface be smooth and flat. If the base material is damaged see appropriate procedure.

NOTE
This method uses commercially available replacement surface mount pads. The new pads are fabricated from copper foil. They are available in hundreds of sizes and shapes and are generally supplied solder plated. If a special size or shape is needed it can be custom fabricated.

ACCEPTABILITY REFERENCES
IPC-A-600 2.0	Externally Observable Characteristics
IPC-A-610 10.0	Laminate Conditions
PROCEDURE REFERENCE
1.0	Foreword
2.1	Handling Electronic Assemblies
2.2	Cleaning
2.5	Baking and Preheating
2.7	Epoxy Mixing and Handling
IPC7721 4.7.1	Surface Mount Pad Repair, Epoxy Method

Figure 1: Remove the defective pad and remove soldermask from the connecting circuit.

TOOLS & MATERIALS
Circuit Frames, Pads
Cleaner
Epoxy
Flux, Liquid
Knife
Microscope
Oven
Scraper
Solder
Soldering Iron
Surface Mount Pad Repair Kit
Tape, Kapton
Tweezers
Wipes

PROCEDURE

1. Figure 2: Select a commercially available replacement pad that matches the missing pad.

   Clean the area.
   
   
2. Remove the defective pad and a short length of the connecting circuit. (See Figure 1).
   
   
3. Use knife and scrape off any epoxy residue, contamination or burned material from the board surface.

   
   CAUTION
   Abrasion operations can generate electrostatic charges.
   
   

4. Scrape off any solder mask or coating from the connecting circuit. (See Figure 1).
   
   

5. Figure 3: Cut out the new surface mount pad.

   Clean the area.
   
   
6. Apply a small amount of liquid flux to the connection area on the board surface and tin with solder. Clean the area. The length of the overlap solder connection should be a minimum of 2 times the circuit width.
   
   
7. The area for the new pad on the board surface must be smooth and flat. If internal fibers of the board are exposed or if there are deep scratches in the surface they should be repaired. Refer to appropriate procedure.
   
   
8. Select a commercially available surface mount pad that most closely matches the surface mount pad to be replaced. If a special size or shape is needed it can be custom fabricated. (See Figure 2).
   
   

9. Figure 4:Place the new surface mount pad in place using Kapton tape.

   Cut out and trim the new pad. Cut the length to provide the maximum allowable circuit overlap for soldering. Minimum 2 times the circuit width. (See Figure 3).

   
   NOTE
   The new replacement surface mount pad may be trimmed from copper sheet.
   
   

10. Mix the epoxy and apply a small amount to the surface where the new pad will be placed.
    
    
11. Place a piece of Kapton tape over the top surface of the new pad. Place the new pad into position on the circuit board surface using the Kapton tape to help in alignment. (See Figure 4).
    
    

12. Figure 5: Completed repair.

    Cure the epoxy per Procedure 2.7 Epoxy Mixing and Handling.

    
    CAUTION
    Some components may be sensitive to high temperature.
    
    

13. After the epoxy has cured remove the Kapton tape used for alignment. Carefully clean the area and inspect the new pad for proper alignment.
    
    
14. If the new pad has a connecting circuit apply a small amount of liquid flux to the lap solder joint connection area and solder the circuit from the new pad to the circuit on the circuit board surface. Use minimal flux and solder to ensure a reliable connection. Kapton tape may be placed over the top of the new pad to prevent excess solder overflow.

    
    NOTE
    If the configuration permits, the overlap solder joint connection should be a minimum of 3.00 mm (0.125") from the related termination. This gap will minimize the possibility of simultaneous reflow during soldering operations. Refer to 7.1 Soldering Basics.
    
    

15. Mix epoxy and coat the lap solder joint connections. Cure the epoxy per the manufacturer's recommended instructions.

    NOTE
    Additional epoxy can be applied around the perimeter of the new pad to provide additional bond strength.
    
    

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

EVALUATION

1. Visual examination
   
   
2. Measurement of new pad width and spacing.
   
   
3. Electrical continuity measurement.