Operation Challenge

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Circuit Brain Surgery

Figure 1: X-ray of subject location with out damage.

Metal suspended in space. That phrase may evoke an image of high tension wires humming with megawatts of electricity from massive generating stations, but there are many other reasons to suspend metal in space. I'm sure you can think of a few, for example satellites in synchronous orbit, or perhaps that shower scene from "Psycho".  In our case the repair discussed in this article is a neat demonstration of the concept applied to circuit repair.

The problem that required repair? A group of boards were improperly and dramatically drilled in a spot they weren't supposed to be drilled in. That can be a tough error to recover from when applied to today's complex boards.   

Figure 2: X-ray of same area post drill damage.

Figure 1 is an x-ray of one of these boards before Mr. or Ms. Destructo went to work on it.  Figure 2 is an x-ray of what this location looked like post-drill. As a result the customer found himself with a scrap heap of worthless product where once there were very valuable and difficult to replace printed circuit boards.

What to do? Here's the quick and dirty. As you can see by the x-rays, the area drilled out is pretty busy.  Including the top and bottom surfaces, there are four layers of circuitry running through the board.  Still it can be repaired and the approach is like the answer to the old saw "How do you eat an elephant?"  

Figure 3: Hole re-drilled.

"One bite at a time."  That is we have to rebuild the location, one connection and one layer at a time.

The first step - if you can believe it - is to drill again (Figure 3). We drill to make the hole a common size so we can manufacture a step that will be just the right fit for all the holes on all the boards.  Additionally we drill to make the hole walls a consistent surface for positioning and adhering the repair material.  We drill to live.

Now that the hole is a consistent cylinder we can begin to re-construct the internal stack up.   A specially machined dowel piece is inserted and adhered into the hole to replace the core (or center plane) of the board.  Once that piece is cured in place the internal circuitry that occupied the space on either side of the core is replaced .  Every circuit in the board must be individually exposed, tinned and connected using pure copper circuit trace material.  You can see the material by following this link http://www.circuitmedic.com/products/201-3130.shtml.   The circuits also must be insulated from the power and ground planes that may run through the area. After the internal traces are connected and tested, each open side of the cylinder is filled to the top and bottom surface with high strength thermo setting epoxy.

Figure 4: Internal and suspended bottom surface circuits and inserted dowel core.

Figure 5: Internal and suspended top surface circuits and inserted dowel core

Figures 4 and 5 show the specially manufactured dowel plug adhered in-place and the replacement circuit tracks either laying on the dowel surface (internal traces) or suspended in place (top and bottom traces). In this case the circuit tracks for all layers were connected prior to the epoxy fill.

Once the cylinder is structurally sound the top and bottom surfaces are blended cosmetically to complete the repair.  Figures 6 and 7 show the bottom and top sides of repaired boards.  Sorry, these pictures are of other repaired units, but I hope you'll get the point.

Figure 6: Completed board.

Figure 7: Completed repair.

That's it!  Believe me that's one tough repair and post repair testing has proven that the repaired units work just fine.  I am mighty proud of the people who had the patience, skill and discipline to conduct such a challenging piece of work with repeatable precision.

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