Damaged Mounting Hole Repair

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Figure 1: Damaged Ejector Handle Mounting Hole.

Extra Strength! When it comes to circuit board repair, there are situations when extra strength becomes critical.

In this instance we're looking at the process to correct base board damage, or to correct a design error at an ejector handle location, see Figure 1. The ejector handle is often held in place by a roll pin that is pressed into a non plated hole on the circuit board.

When the ejector handle is retracted it's lever action will be used to press in, or eject the circuit board into, or out of a connector, or card cage. With this action, the ejector handle roll pin will put a sizable amount of stress inside the ejector handle mounting hole. Standard circuit board construction will easily withstand this stress, but what about a modified, or repaired mounting hole location?

Figure 2: Circuit Board Pinned and Secured on Milling Machine for Drilling.

There are a few repair processes that we have used to repair, or modify non plated  hole location on circuit boards. Using a standard epoxy fill process (3.3.1 Hole Repair, Epoxy Method) is often employed for mounting holes, or alignment holes in which the "extra strength" is not a requirement.

A second procedure (3.3.2 Hole Repair, Transplant Method) that is employed uses a replacement board material, instead of epoxy alone to provide added strength for those applications in which "extra strength" is a critical requirement.

The following is a refinement of the the transplant procedure to optimize the results, and get the highest strength, consistently from hole to hole. 

Figure 3: Oversized Hole Drilled to Remove Damage, and Allow Installation of Doewl Plug

1. Drill out holes using a milling machine. Ensure holes are drilled on center by utilizing data point leveling, pinning and squaring arrangement on machine table, see Figure 2, and 3.

2. Inspect the holes carefully. Holes must be smooth cut and perfectly clear of debris.

3. Seal the bottom surface of the board with Kapton tape. This will prevent the epoxy leakage, and avoid air entrapment when the epoxy coated dowel is placed in the hole from the top side of the board.

Figure 4: Dowel Plug with Slots for Hole Transplant.

4. Apply epoxy to the dowel plug and the wall of the hole. See Figure 4.

5. Insert the dowel into the hole. Ensure the dowel plug is inserted evenly, and the epoxy is present around the entire hole circumference.

6. Bake the board as required to fully cure the epoxy.

Figure 5: Dowel Plug Installed and Epoxied In Place.

7. Remove the Kapton tape and inspect for voids in the epoxy. If present, grind out the defects, re-epoxy and re-bake boards. 

8. Ensure top and bottom surfaces of dowel are level with board surface, machine if needed. See Figures 5.

9. Drill the finished hole at the specified location to meet the supplied drawing tolerance. See Figure 6.

Figure 6: Completed Repair of Ejector Handle Hole Location.

The result of this repair process is an extra strength repair capable of wistanding the typical forces applied to a normal circuit board.

Several members of the Circuit Technology Center team contributed to this feature story.