Operation Super Strong

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Extra Strength Repair, Not Just Hype

Figure 1: Damaged Ejector Handle Mounting Hole.

Extra Strength! How often do you hear it in your everyday life?  Extra strength medication, extra strength glue, extra strength (fill in the blanks)! It's a favorite term of many an advertiser, and in some cases it is more hype than substance. 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 utilized 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.

Recently we have worked side-by-side with a long time customer in refining the transplant procedure to optimize the results, and get the highest strength, consistently from hole to hole. Below is an overview of the procedure.

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

1. Drill out holes to .190 inches using turret 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 hole carefully for any debris using back light.  Hole must be smooth cut and perfectly clear of debris.

Figure 4: Dowel Plug with Slots for Hole Transplant.

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

4. Apply epoxy to the dowel plug and the wall of the hole. The dowel plug will be cut to match the board thickness and notched, see Figure 4.

5. Insert the .1875 inch 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 for two hours at 165F to fully cure the reworked locations.

Figure 5: Dowel Plug Installed and Epoxied In Place.

7. Remove the Kapton tape and inspect for any voids in the epoxy. If present, grind out defects on top and bottom of board.  Re-epoxy and re-bake board for two hours at 165F to cure the reworked locations. 

8. Ensure top and bottom surfaces of dowel are adequately epoxied and are level with board surface, see Figures 5.

9. Drill the finished hole at the specified location to meet the supplied drawing tolerance. The circuit board will be pinned, and secured on the fixture plate to ensure the board is held firmly in the correct position for drilling, see Figure 6.

Figure 6: Completed Repair of Ejector Handle Hole Location.

The result of this repair process allowed a significant number of assembled boards to be reliably repaired and shipped to customer sites. The "extra strength" was needed, not just the hype!

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