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4.3.2 Circuit Cut, Inner Layer Circuits
Outline
This method is used to sever a circuit or short. A small section of the circuit is removed forming a break. The width of the break should be at least as wide as the minimum conductor spacing. The Precision Drill System is used with a carbide end mill. This method is recommended for surface or inner layer circuit cuts. After milling, the area is sealed with epoxy.

Note
This method is recommended for surface or inner layer circuit cuts.

Caution
Extreme care must be taken to prevent damage to adjacent or underlying inner layer 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
IPC-A-610 10.0 Laminate Conditions
Procedure References
1.0 Foreword
2.1 Handling Electronic Assemblies
2.2 Cleaning
2.7 Epoxy Mixing and Handling
IPC7721 4.3.2 Conductor Cut, Inner Layer Conductors
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.
Oven
Oven
General purpose oven for drying, baking and curing epoxies.
Precision Drill
Precision Drill
Precision drill press for accuracy and controlled depth drilling.
Wipes
Wipes
Nonabrasive, low-linting wipes for cleanup.
Images and Figures
4.3.2 Circuit Cut, Inner Layer Circuits
Circuit Cut, Inner Layer
4.3.2 Circuit Cut, Inner Layer Circuits
Figure 1: Precision Drill System with base plate to pin circuit board in position while cutting circuits
4.3.2 Circuit Cut, Inner Layer Circuits
Figure 2: Mill into circuit board at proper coordinates to cut inner layer circuits as required
4.3.2 Circuit Cut, Inner Layer Circuits
Figure 3: Fill the milled hole with epoxy up to and flush with the surface.
4.3.2 Circuit Cut, Inner Layer Circuits
Figure 4: Completed repair

Procedure 
  1. Identify the circuit or short to be cut. Determine from the artwork or drawings where the best location is to make the break. The width of the break should at least match the minimum required electrical spacing.

  2. Clean the area.

  3. If the cut is on an inner layer circuit, mark the coordinates on the circuit board surface or set up a fixture to precisely locate the board in the Precision Drill System. (See Figure 1)

  4. Select the appropriate size end mill or ball mill and insert it into the chuck of the Precision Drill System. The milling cutter should be slightly larger in diameter than the circuit to be cut. Set speed to high.

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

  5. Mill down into the board at the proper coordinates to cut the inner layer circuits or to break the inner layer short. Do not mill deeper than needed. A microscope should be used for accuracy. (See Figure 2)

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

  7. Check continuity to be sure that the circuit has been cut.

  8. Mix epoxy. If desired, add color agent to the mixed epoxy to match the circuit board color.

  9. Fill the milled hole with epoxy up to and flush with the surface. An epoxy dispenser may be used to accurately control the application of epoxy. Remove any excess epoxy. (See Figure 3)

    Caution
    Examine milled hole to be sure all material is removed from the hole prior to filling the hole with epoxy.

    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.

Evaluation

  1. Visual examination of cuts for spacing, and unintended damage to surrounding circuits.

  2. Electrical tests as applicable.

Procedure for reference only.