Circuit Technology Center

Inner Layer Conductor Modification

Inner Layer Conductor  Modification
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 long in this business to learn that ideas are essential, but they're usually the easiest part of the process. Putting a worthwhile idea into action can be exciting, challenging, and humbling. One of our customers found a small but critical error to their dismay.

The ground plane that should have served as a return path for signals was missing between plated through holes in a section of their circuit 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.

Inner Layer Conductor  Modification
Figure 2: Initial rework using a jumper wire.
This problem existed between pads on a long row of closely spaced, plated through holes. Fortunately, after testing several different rework options, the customer was able to correct the fault using a short jumper wire. However, upon viewing the rework with a wire, the end customer would not accept it. See Figure 2.

Was there a way to rework these boards without the obvious surface wires? We reviewed the possibilities and came up with a reliable and repeatable process. Here it is. The first step was to mill down to the layer requiring the connection. This was accomplished by a skilled operator using a precision, depth-controlled milling machine. The inner layer of copper was exposed; a delicate operation.

Inner Layer Conductor  Modification
Figure 3. Close-up of new conductor at one location.
In addition, a shallow groove was machined in the circuit board surface to permit the new conductor routing to remain below the circuit board surface. The next step was an exercise in micro-dexterity. Our technician soldered new conductors across the gap.

The tools and materials used included a microscope, solder, a fine-tipped soldering iron, and copper foil conductor .008" wide. See Figure 3. At this point, the circuit boards were sent to inspection to ensure the solder joints and routing were per specification.

Inner Layer Conductor  Modification
Figure 4: Completed rework with area filled with color-matched, high strength epoxy.
After inspection, the boards were returned to the technicians who filled the machined groove with high-temperature, thermo-setting epoxy. The epoxy was blended into the board's surface using a coloring agent. The results are shown below in Figure 4.

The repair was performed at 8 locations on each circuit board. It was performed reliably and economically. 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.