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
Recommended for technicians with soldering and component rework skills and exposure to most repair/rework procedures, but lacking extensive experience.
Conformance Level - High
This procedure most closely duplicates the physical characteristics of the original, and most probably complies with all the functional, environmental and serviceability factors.
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.
Images and Figures
Deleting Inner Layer Connection at a Plated Hole
Figure 1: Precision Drill System with base plate to pin circuit board in position while drilling out plated hole.
Figure 2: Completely mill through the hole to isolate the internal connection(s).
Figure 3: Fill the hole with epoxy up to and flush with the surface.
Figure 4: Completed repair.
Procedure
Identify the hole that requires rework and clean the area.
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)
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.
Completely mill through the hole to isolate the internal connections. A microscope should be used for accuracy. (See Figure 2)
Blow away material with air and clean the area.
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.
Mask the opposite side with High-Temperature Tape or a flexible mask to prevent the epoxy from flowing out the opposite side.
Mix the epoxy.
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.
Cure the epoxy per Procedure 2.7 Epoxy Mixing and Handling.
Caution: Some components may be sensitive to high temperatures.
Clean the area.
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.
Clean the area. Inspect the new hole using a microscope.
