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

Conformance Level - High

Tools and Materials

Ball Mills, Abrasives, Cutting Tools Ball mills, abrasives and cutting tools for working on circuit boards.

Circuit Bond Epoxy Clear, superior strength epoxy in two-compartment plastic packages.

Cleaner General purpose cleaner for removing contamination.

Color Agents One part, semi-paste ink used to tint the color of epoxy and for direct printing on circuit board surfaces.

Continuity Meter Meter and probes to test for electrical continuity.

End Mills Designed for end cutting and hole boring.

Flex-Rack PCB Holder Sturdy rack for PCBs used for rework and positioning.

High Temp Tape Discs High temperature polyimide tape discs, .50" diameter.

Micro Drill System Versatile power tool for milling, drilling, grinding, cutting and sanding.

Microscope Precision microscope with stand and lighting for work and inspection.

Oven General purpose oven for drying, baking and curing epoxies.

Precision Drill Precision drill press for accuracy and controlled depth drilling.

Precision Tool Set Nine precision-crafted tools for detailed circuit board work.

Wipes Nonabrasive, low-linting wipes for cleanup.

Procedure

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.

Images

Circuit Cut, Inner Layer Circuits

Precision Drill System with base plate to pin circuit board in position while cutting circuits.

Mill into circuit board at proper coordinates to cut inner layer circuits as required.

Fill the milled hole with epoxy up to and flush with the surface.

Completed repair.