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

Outline

This method is used on multilayer circuit boards or assemblies to disconnect an internal connection at a plated hole. A Precision Drill System uses 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.

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.

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

Wipes Nonabrasive, low-linting wipes for cleanup.

Procedure

Procedure

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)
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 connections. A microscope should be used for accuracy. (See Figure 2)
5. Blow away material with air and clean the area.
6. Check continuity to ensure 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.
   
   If desired, complete the following steps.
7. Mask the opposite side with High-Temperature Tape or a 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 temperatures.
11. Clean the area.
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.

Images

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

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

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

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

Completed repair.