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4.3.3 Deleting Inner Layer Connection at a Plated Hole, Drill Through Method
Outline
This method is used on multilayer circuit boards or assemblies to disconnect an internal connection at a plated hole. A Precision Drill System is used with a carbide drill, end mill or ball mill to drill out the hole. The hole may then be filled with epoxy and re-drilled to the diameter needed.

Caution
Extreme care must be taken to prevent damage to adjacent circuits. A microscope must be used during milling when extreme accuracy is required.

Minimum Skill Level - Advanced
Recommended for technicians with soldering and component rework skills and exposure to most repair/rework procedures, but lacking extensive experience.

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
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 4.3.3 Deleting Inner Layer Connection at a Plated Hole, Drill Through Method
Tools and Materials
Cleaner
Cleaner
General purpose cleaner for removing contamination.
Color Agent
Color Agents
Use to tint the color of epoxy used for solder mask repair or circuit base board repair.
Continuity Meter
Continuity Meter
Meter and probes to test for electrical continuity.
End Mills
End Mills
Designed for end cutting and hole boring.
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.
Microscope
Microscope
It is a challenge to undertake precision repair without a good microscope.
Precision Drill
Precision Drill
Precision drill press for accuracy and controlled depth drilling.
Wipes
Wipes
Nonabrasive, low-linting wipes for cleanup.
Images and Figures
4.3.3 Deleting Inner Layer Connection at a Plated Hole, Drill Through Method
Deleting Inner Layer Connection at a Plated Hole
4.3.3 Deleting Inner Layer Connection at a Plated Hole, Drill Through Method
Figure 1: Precision Drill System with base plate to pin circuit board in position while drilling out plated hole.
4.3.3 Deleting Inner Layer Connection at a Plated Hole, Drill Through Method
Figure 2: Completely mill through the hole to isolate the internal connection(s).
4.3.3 Deleting Inner Layer Connection at a Plated Hole, Drill Through Method
Figure 3: Fill the hole with epoxy up to and flush with the surface.
4.3.3 Deleting Inner Layer Connection at a Plated Hole, Drill Through Method
Figure 4: Completed repair.

Procedure 
  1. Identify the hole that requires rework and clean the area.

  2. Mark the coordinates on the board surface and pin the circuit board in place on the base plate of the Precision Drill System. (See Figure 1)

  3. Select the appropriate size end mill, drill or ball mill and insert it into the chuck of the Precision Drill System. The cutting tool should be approximately 0.50 mm (.020") greater than the plated through hole inside diameter. Set speed to high.

    Caution
    Abrasion operations can generate electrostatic charges.

    Note

    End mills are normally single end, two or four flute high grade solid carbide.

  4. Completely mill through the hole to isolate the internal connections. A microscope should be used for accuracy. (See Figure 2)

  5. Blow away material with air and clean the area.

  6. Check continuity to be sure that the internal connection has been deleted. Also check the continuity and inspect the neighboring circuits to make sure that none of them have been severed or damaged.

    If desired complete the following steps
     
  7. Mask the opposite side with High Temperature Tape or flexible mask to prevent the epoxy from flowing out the opposite side.

  8. Mix the epoxy.

  9. Fill the hole with epoxy up to and flush with the surface. Remove excess epoxy. (See Figure 3)

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

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

    Caution
    Some components may be sensitive to high temperature.

  11. Clean the area.

  12. Select an end mill or drill as needed. Insert the cutting tool into the Precision Drill System. Mill directly through the center of the cured epoxy. The surface pad remaining may be used as a target location for accuracy. A microscope should be used during milling for accuracy. (See Figure 4)

    Caution
    Be careful not to re-expose the internal layers of the hole when drilling out the epoxy.

  13. Clean the area. Inspect the new hole using a microscope.

Evaluation

  1. Visual and electrical examination as required

Procedure for reference only.