Several multilayer backplanes were mistakenly manufactured with missing plating for 24 holes on each board. The top and bottom side pads were in place, but the holes had no plating thus the inner layers we not connected. See Figure 1.
Each of the holes required an internal connection to either layer 2 or layer 3. Making connections to various layers complicated the rework solution and of course, these backplanes were critical so a reliable rework method was required ASAP.
Figure 2. Copper ribbon soldered to exposed inner layer.
Due to ground shielding on the bottom side of the backplane, all rework needed to be completed from the top side. Figure 1 shows the top side of the board showing some of the locations requiring rework to establish missing internal connections.
After consultation with our engineers, our rework operators, and the customer, it was decided that the best rework approach would be as follows.
Use a Precision Drill system, fitted with a stereo microscope and a 0.080" carbide end mill, and carefully machine down to the respective internal layer and expose the copper signal trace. See figure 1.
Figure 3. Connections completed and areas filled with epoxy.
Select a length of 2 oz. pure copper ribbon that matched the width of the signal trace.
Tin the copper ribbon and the exposed inner layer with solder.
Solder the copper ribbon from the exposed internal layer to the top side surface pad and electrically test to confirm the connection. See Figure 2.
Complete the rework by filling each milled hole using color-matched high-strength 2 part epoxy and cure as required. See figure 3.
Several members of the Circuit Technology Center team contributed to this feature story.