4.3.2 Circuit Cut, Inner Layer Circuits

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 only.

Circuit Cut, Inner Layer

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.

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
CTC	1.0	Foreword
CTC	2.1	Handling Electronic Assemblies
CTC	2.2.1	Cleaning, Local
CTC	2.2.2	Cleaning, Aqueous Batch Process
CTC	2.7	Epoxy Mixing and Handling
IPC7721	4.3.2	Conductor Cut, Inner Layer Conductors see procedure below

Tools and Materials
Cleaner
Color Agent
Continuity Meter
End Mills
Epoxy
Microscope
Oven
Pin Clamps
Precision Drill System
Tape, Kapton
Wipes

Procedure 4.3.2 Conductor Cut, Inner Layer Conductors

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

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)

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

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.

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.

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.

Figure 4: Completed repair.

Evaluation

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

2.	Electrical tests as applicable.