4.3.4 Deleting Inner Layer Connection at a Plated Hole, Spoke Cut Method
Outline
This method is used on multilayer circuit boards or assemblies to disconnect an internal connection at a plated hole. A precision drill press is used with a carbide end mill to make precise cuts at the spokes or internal circuits extending from the hole.
Caution
Extreme care must be taken to prevent damage to adjacent circuits. A microscope must be used during milling when extreme accuracy is required.
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Minimum Skill Level - Expert
Recommended for technicians with advanced soldering and component rework skills and extensive experience in most repair/rework procedures.
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.
Oven
General purpose oven for drying, baking and curing epoxies.
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, Spoke Cut Method
Figure 1: Precision Drill System with base plate to pin circuit board in position while cutting inner layers.
Figure 2: Internal view of plated through hole with inner layer spoke connections.
Figure 3: Mill adjacent to the plated hole to sever internal spoke connections.
Procedure
Identify the hole that requires rework and clean the area.
Mark the coordinates on the board surface and place the circuit board on the base plate of the precision drill press. (See Figure 1)
Select the appropriate size end mill or drill and insert it into the chuck of the Precision Drill System. The cutting tool should be approximately .010 - .025 mm (.005" - 010") greater than the width of the spoke or circuit to be cut. Set speed to high.
Caution
Abrasion operations can generate electrostatic charges.
Note
End mills are normally single end, 2 or 4 flute solid carbide.
Mill into the circuit board surface adjacent to the plated hole. The milled holes should be aligned directly above the internal spoke connections. Mill down just deep enough to sever the internal spokes connecting the plated hole to the internal plane. A microscope must be used for accuracy. Up to 4 milled holes may be required. Do not drill deeper than needed. (See Figure 3)
Blow away material with air and clean the area.
Check continuity to be sure that the internal connection has been deleted. Inspect the neighboring circuits to make sure that none of them have been severed or damaged.
Mix the epoxy.
Fill the holes with epoxy flush with the surface. Remove excess.
Note
A slight overfill of epoxy may be desired to allow for shrinkage.
Cure the epoxy per Procedure 2.7 Epoxy Mixing and Handling.
