Medusa's Head Rework

Operation BGA Reroute

Medusa's Head Rework

Figure 1: BGA site requiring Medusa's Head rework.

A customer was in a tight spot, literally and figuratively. He needed 23 connections to be pulled out from under a BGA location. The BGA component involved was a 676 pin, 1 millimeter pitch gem (see placement location in Figure 1) that required rewiring to entice the placed component to perform it's newly designed function. That's a goodly amount of re-routing under any circumstances. It would have been easier if the pads that required re-routing were on the perimeter of the BGA layout, but such was not the case. The newly routed pads had to extend from the center of the component to outside of the package footprint so that wires could be added to the newly placed pattern of surface mount pads.

All of this is easy enough to consider and doable, but delivering the goods requires attention to detail. First the design engineers have to determine (electronically) what goes to where, then layout engineers have to convert that to usable physical information. In this case they came up with something that

Figure 2: Proposed routing.

looks like Figure 2.

That's a good start, but it's only a start. What looks good in a point-to-point drawing may still need some tweaking when it comes to actually laying circuit connections on a board surface. Already existing surface etches and oversized or shifted pads may cause technicians to re-route the circuit paths. As you can see in Figure 3, there are a couple of etches (circuits / traces) on the board that are immediate cause for concern. To route the new circuits in the fashion laid out in the supplied drawing would cause the new circuits to be perilously close to the ones on the board and would also build up the new circuit to a height that may interfere with future solder paste stenciling operations. Re-routing is called for.

Figure 3: Circuits that interfere with routing proposal.

Once the new route assignments are drawn up the technician will have to scribe the board to indent the surface. The newly laid circuit must run in such a way as not to interfere with the solder paste stenciling and BGA placement. Figure 4 is a close up of the the detailed scribe and trough operation. A technician will use an Xacto knife and a high-speed, precision hand drill to excavate the routes.

The pad that is originally on the surface of the board must be separated from its via (if it's attached to a via) and the original pad removed from the board. The original pad may be used on occasion, but more often than not the circuit that runs off the pad is either not long enough to provide a proper overlap for the the newly routed circuit. Either that or the circuit that runs off the original pad is running in the wrong direction. In either of those cases (which is most of the time) the old BGA pad must come off and be replaced with a new pad that has a circuit that runs in the proper direction and provides an adequate overlap length.

Figure 4. Scribed routes.

After the pad is placed and bonded on the board the new circuit must be run. Using procedure 4.2.1 Conductor Repair, Foil Jumper, Epoxy Method, the circuit is routed out to the perimeter of the BGA footprint.

Routing the copper trace material takes patience. Not only does it have to be conformed to the many twists and turns of the new path, it must be kept down in the trough. That can be tricky as the more the copper is worked the more it wants to rise off the surface of the board.

Figure 5: Close-up of completed circuit routing.

The last procedure is to place a new surface mount pad in the perimeter footprint previously scribed on the board, using procedure 4.7.2 Surface Mount Pad Repair, Film Adhesive Method. As part of this procedure, a film of high-strength, thermo-setting epoxy is deposited all along the length of the newly laid circuit securing the circuit in place and allowing it to withstand the rework envirionment.

Figures 5 and 6 show the final results. Close-up one can see the delicate touch required to properly perform this detailed set of instructions. The backed off shot shows the scope of this task.

That's all it takes!