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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
Kits and Systems
201-3110
Base Board Repair Kit
Kit to repair base board and laminate damage on circuit boards.
115-1322
Circuit Bond Kit
Includes clear, low viscosity, superior strength epoxy for circuit board damage repair.
110-6380
Flex-Rack PCB Holder
Sturdy holder supports PCBs for rework, drilling, pad repair, and positioning.
110-4106
Micro Drill System
Versatile power tool for milling, drilling, grinding, cutting and sanding circuit boards.
201-7100
Precision Tool Set
Nine precision-crafted tools for detailed circuit board work.
Tools and Materials
cutting
Ball Mills, Abrasives, Cutting Tools
Ball mills, abrasives and cutting tools for working on circuit boards.
Coatings
Color Agents
Use to tint the color of epoxy used for solder mask repair or circuit base board repair.
115-3302
Circuit Bond Epoxy
Clear, superior strength epoxy in two-compartment plastic packages.
235-3050
High Temp Tape Discs
High temperature polyimide tape discs, .50" diameter.
Additional Items and Supplies
Cleaner
Cleaner
General purpose cleaner for removing contamination.
Meter
Continuity Meter
Meter and probes to test for electrical continuity.
End Mills
End Mills
Designed for end cutting and hole boring.
Microscope
Microscope
Precision microscope with stand and lighting for work and inspection.
Oven
Oven
General purpose oven for drying, baking and curing epoxies.
Drill
Precision Drill
Precision drill press for accuracy and controlled depth drilling.
Wipes
Wipes
Nonabrasive, low-linting wipes for cleanup.
Circuit Bond Epoxy
Circuit Bond is a clear, low viscosity, superior strength epoxy precisely measured out into two-compartment plastic packages, so it's easy to use, and there's no measuring. For over a decade, this high-strength epoxy has been qualified and used by thousands of high-rel electronics manufacturers across the globe.
1. Circuit Bond has a working pot life of 30 minutes. It should not be mixed until ready to use.
2. To use Circuit Bond, remove the plastic clip separating the resin and hardener. Squeeze back and forth from one half of the package to the other to mix the contents.
3. Cut a corner off the package and squeeze all the contents into a Plastic Cup. Stir the contents to ensure it is thoroughly mixed.
4. Circuit Bond may contain bubbles from the mixing process. If needed, use a vacuum system to remove bubbles.
5. Color Agent can be mixed in with Circuit Bond to match surface colors if desired.
6. Apply using a Foam Swab, Micro Probe, or Mixing Stick as required.
7. Cure Circuit Bond for 24 hours at room temperature or 4 hours at 65°C (150°F).
Specifications
Packaging2 gram pre-measured packages
Mix ratio4 parts resin to 1 part hardener
Mix Ratio by Weight (R/H)100/25
ColorClear, transparent
Pot life30 minutes
Cure cycle24 hours at room temp (25 °C) or 4 hours @ 65°C
Thixotropic Index1
Specific Gravity1.20
Percent Solids100%
Viscosity (after mixing)2000 cps
Operating temperature range-55°C to 135°C
Hardness88 Shore D
Lap Shear, Alum to Alum1100 psi
Glass Transition Temperature, Ultimate92°C
Coefficient of Expansion, cm/cm/°C6 E-05
Dielectric strength400 volts/mil
Dielectric Constant, 1KHz@25°C4
Shelf Life6 months minimum
Color Agents
Color Agent is a one-part, air-drying, semi-paste ink commonly used to print hard surfaces, including circuit board base materials. Color Agent can be used to tint the color of epoxy used for solder mask repair or circuit baseboard repair. To use, mix a small amount of color agent with 2 part epoxy as the epoxy is initially mixed. Add more color agents to increase the color intensity if needed.
Part No. Description
115-9102 115-9102 Color Agent, Yellow
115-9185 115-9185 Color Agent, Red
115-9293 115-9293 Color Agent, Blue
115-9348 115-9348 Color Agent, Green
115-9358 115-9358 Color Agent, Light Green
115-9376 115-9376 Color Agent, Medium Green
115-9424 115-9424 Color Agent, Dark Gray
115-9457 115-9457 Color Agent, Light Brown
115-9560 115-9560 Color Agent, Dark Green
115-9561 115-9561 Color Agent, Green
115-9995 115-9995 Color Agent, Black
115-9996 115-9996 Color Agent - White
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 ensure the circuit has been cut.
  8. Mix epoxy. If desired, add a 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 the 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.