Circuit Board Repair Guide > Conductor Repair Procedures > 4.3.2 How to Cut Inner Layer Circuits or Conductors

How to Cut Inner Layer Circuits or Conductors

Repair Circuit Boards | Repair SMT Pads | Repair BGA Pads | Repair Lands | Repair Edge Contacts
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This repair procedure 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. A Precision Drill System is used with a carbide end mill. After milling, the area is sealed with epoxy.

This procedure is recommended for surface or inner layer circuit cuts only.
Inner layer circuit needs to be cut
Circuit Cut, Inner Layer
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.

IPC 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

Related Procedure References
CTC 1.0 Foreword - Circuit Board Repair Guide
CTC 2.1 Handling Electronic Assemblies
CTC 2.2.1 How to Clean a Circuit Board
CTC 2.2.2 Cleaning Circuit Boards, Aqueous Batch Process
CTC 2.7 Epoxy Mixing and Handling
IPC7721 4.3.2 Conductor Cut, Inner Layer Conductors

Tools and Materials
Color Agents
Continuity Meter      
End Mills
Pin Clamps
Precision Drill System      
Tape, Kapton

4.3.2 Circuit Cut, Inner Layer Circuits

Printed Board Type: R/F  |  Skill Level: Advanced  |  Conformance Level: High  |  Rev.: E  |  Rev. Date: Jul 7, 2000

Precision Drill System for cutting circuits

Figure 1: Precision Drill System
with base plate to pin circuit board
in position while cutting circuits.

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)
Cut inner layer circuits

Figure 2: Mill into circuit board at
proper coordinates to cut inner
layer circuits as required.

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.
End mills are normally single end, two or four flute high grade solid carbide
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.
Fill milled hole with epoxy

Figure 3: Fill the milled hole with
epoxy up to and flush with the surface.
8.  Mix epoxy. If desired, add color agent to the mixed epoxy to match the circuit board color.
Fill the milled hole with liquid circuit board 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).
Examine milled hole to be sure all material is removed from the hole prior to filling the hole with epoxy.
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.
Circuit repair completed

Figure 4: Completed repair.
1.  Visual examination of cuts for spacing, and unintended damage to surrounding circuits.
2.  Electrical tests as applicable.

Links to Related Products

Circuit board epoxy for ciruit repair  
Circuit Bond Kit 115-1322: Liquid circuit board epoxy
Circuit Bond Kit contains 10 packages of clear, low viscosity, superior strength epoxy ideal for circuit board use. The epoxy is precisely measured out into two-compartment plastic packages. This adhesive is easy to use and there's no measuring.

Color Agents for Circuit Boards
Matching epoxy to the color of the circuit board being repaired is easy when you use a color agent. Color agents may also be applied directly to the circuit board surface for color matching.

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