4.3.4 Deleting Inner Layer Connection at a Plated Hole, Spoke Cut Method

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.
   Deleting Inner Layer Connection at a Plated Hole, Spoke Cut Method
Deleting Inner layer
Connection at a Plated Hole,
Spoke Cut Method
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 4.3.4 Deleting Inner Layer Connection at a Plated Hole, Spoke Cut 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 4.3.4 Deleting Inner Layer Connection at a Plated Hole, Spoke Cut Method

Precision drill system.

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 place the circuit board on the base plate of the precision drill press. (See Figure 1).
Plated through hole with inner layer spoke connections.

Figure 2: Internal view of plated
through hole with inner layer spoke connections.
3.      
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, two or four flute high grade solid carbide.
Mill adjacent to the plated hole to sever internal

Figure 3: Mill adjacent to the plated
hole to sever internal spoke connections.
4.      
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).
   
5. Blow away material with air and clean the area
   
6.
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.
   
7. Mix the epoxy.
 
8. Fill the holes with epoxy flush with the surface. Remove excess.
   
  Note
A slight overfill of epoxy may be desired to allow for shrinkage.
 
9. Cure the epoxy per Procedure 2.7 Epoxy Mixing and Handling.
   
Evaluation
   
1. Visual and electrical examination as required.


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4.3.4 Deleting Inner Layer Connection at a Plated Hole, Spoke Cut Method