Operation Tight Squeeze

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Signals Return

Figure1: Light area between pads is where connection is missing.

In every industry there is a gap between conception and execution. It doesn't take much time in the business to learn that ideas are very important, but they are usually the easiest part of the process. Putting a worthwhile idea into action can be exciting, challenging and humbling. Our business often involves helping our customers through the challenging and humbling part of the process. 

To their dismay, one of our customers found, as they were on the cusp of shipping a new product, that there was a small but critical problem in an assembly's printed circuit fabrication.

Figure 2: Wire repair

The ground plane that should have served as a return path for signals was missing between plated through holes in a section of the board. Thousands of boards were in the assembly pipeline, and thousands in the field, when a subtle disruption in the electronic operation of the final product became clear to the customer.  As a result they discovered the assembly would not operate properly! See Figure 1.

This problem existed between pads on a long row of closely spaced, plated through holes. Fortunately, after testing a number of different fixes, the customer was able to repair the fault by restoring eight of the missing connections to re-establish proper return signals. 

Figure 3: Spot face and trough between subject pads.

That was good. Good to know there was a fix. However the initial testing was done using wires, and upon viewing the fix the customer was not comfortable with the board's appearance. See Figure 2. 

Was there a way to make this fix without it being so obvious and vulnerable to being brushed and damaged? We reviewed the possibilities and came up with a process that was both reliable and repeatable. Here it is.

Figure 4: Close-up of spot face.

The first step was to mill down to the layer requiring connection.  This was accomplished by a skilled operator using a precision, depth controlled, milling machine. The layer's copper was exposed just enough to make a connection but not break through the layer; a delicate operation. In addition, a trough was cut to permit the circuit routing to remain below the plane of the board surface. See Figures 3 and 4.

The next step was an exercise in micro-dexterity on the level of a vascular surgeon, as a technician, armed with a microscope, solder, a fine soldering iron tip and .008 inch circuitry, routed the circuit across the gap and soldered each end in place. See Figures 5 and 6.

Figure 5: Circuit routed in channel / trough and soldered at both ends.

At this point the board went to inspection to ensure the solder joints and routing were per specification.

After inspection the board was returned to technicians who applied high-temperature, thermo-setting epoxy over the connection. The epoxy was blended into the board's surface by applying color agent to the mix. The results are shown below in Figures 7 and 8. 

Figure 6. Close-up of routing.

Figure 7: Connected and masked repair.

 

Figure 8: Close-up of a repaired location.

Not bad. The repair was performed eight times per board.  It was performed reliably, repeatably and economically over many boards. The gap between conception and execution was bridged. On to the next project!

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