4.3.3 Deleting Inner Layer Connection at a Plated Hole, Drill Through Method

This method is used on multilayer circuit boards or assemblies to disconnect an internal connection at a plated hole. A Precision Drill System is used with a carbide drill, end mill or ball mill to drill out the hole. The hole may then be filled with epoxy and re-drilled to the diameter needed.

Caution
Extreme care must be taken to prevent damage to adjacent circuits. A microscope must be used during milling when extreme accuracy is required.

Deleting Inner Layer Connections at a Plated Hole

Deleting Inner Layer
Connection at a Plated Hole

Acceptability References
IPC-A-600	2.0	Externally Observable Characteristics
IPC-A-600	3.0	Internally Observable Characteristics
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	2.5	Baking and Preheating
CTC	2.7	Epoxy Mixing and Handling
IPC7721	4.3.3	Deleting Inner Layer Connection at a Plated Hole, Drill Through Method 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 4.3.3 Deleting Inner Layer Connection at a Plated Hole, Drill Through Method

Figure 1: Precision Drill System with base plate to pin circuit board in position while drilling out plated hole.

1.	Identify the hole that requires rework and clean the area.

2.	Mark the coordinates on the board surface and pin the circuit board in place on the base plate of the Precision Drill System. (See Figure 1).

Completely mill thorugh the hole to isolate the internal connection(s)

Figure 2: Completely mill through
the hole to isolate the internal connection(s).

3.	Select the appropriate size end mill, drill or ball mill and insert it into the chuck of the Precision Drill System. The cutting tool should be approximately 0.50 mm (.020") greater than the plated through hole inside diameter. Set speed to high.

	Caution Abrasion operations can generate electrostatic charges.

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

4.	Completely mill through the hole to isolate the internal connection(s). A microscope should be used for accuracy. (See Figure 2).

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

6.	Check continuity to be sure that the internal connection has been deleted. Also check the continuity and inspect the neighboring circuits to make sure that none of them have been severed or damaged.

Figure 3: Fill the hole with epoxy up
to and flush with the surface.

If desired complete the following steps

7.	Mask the opposite side with Kapton tape or flexible mask to prevent the epoxy from flowing out the opposite side.

8.	Mix the epoxy.

9.	Fill the hole with epoxy up to and flush with the surface. Remove excess epoxy. (See Figure 3).

	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.

	Caution Some components may be sensitive to high temperature.

11.	Clean the area

Figure 4: Completed repair.

12.	Select an end mill or drill as needed. Insert the cutting tool into the Precision Drill System. Mill directly through the center of the cured epoxy. The surface pad remaining may be used as a target location for accuracy. A microscope should be used during milling for accuracy. (See Figure 4).

	Caution Be careful not to re-expose the internal layers of the hole when drilling out the epoxy.

13.	Clean the area. Inspect the new hole using a microscope.

Evaluation

1. .	Visual and electrical examination as required