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Rework for Internal Conductor Mismatch
Rework for Internal Conductor Mismatch
Figure 1: Initial milling step.
The team at Circuit Technology Center was put to the test when a large OEM discovered a designed problem with fifty prototype boards. Two pairs of .005" wide traces had their polarity inverted, needing to be crossed, and time was of the essence. 

The rework of these fully populated, 18 layer boards was made even more difficult because these were internal traces, with one pair on layer 4 and the other on layer 18.

Rework for Internal Conductor Mismatch
Figure 2: Initially milling.
Circuit Technology Center began the rework procedure drilling to expose each pair of internal traces. An x-ray system was used to assist in locating the traces.

Next the boards were fixture and pinned on a milling machine equipped with a stereo microscope and a .050" carbide end mill. Holes were milled down to reveal the traces, but not cut through. See Figure 1 and 2.

Rework technicians then meticulously scraped away the remaining thin layer of base material to expose the copper traces. Using a special knife the small area was delicately scraped. A small cut was made in each trace and the remainder of the traces were tinned with solder. 

Rework for Internal Conductor Mismatch
Figure 3: Conductor overlap rework.
A piece of copper ribbon was also tinned, and then soldered to cross one trace to the other.  A second piece of tinned copper was soldered to the other trace to complete the crossover. See Figure 3.

Finally the technicians filled each milled area with a high strength epoxy tinted with a coloring agent.

The entire process took a knowledgeable team of technicians with steady hands to complete this rework. The final results astounded the customer.

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