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Changing a Conductor Path at a BGA Site
Here's a situation that you may experience someday, if you haven't already. A start-up electronics company discovered that they had a problem. Their contract assembler had just delivered the first 100 boards. Despite a thorough design review and initial prototype, they found that each board needed rework at several ball grid array (BGA) sites.

The rework involved changing the circuit paths at the BGA sites, which required not only adding new circuits, but also severing the old circuits.

The procedure that was developed to handle this problem follows: After removing the BGA components and cutting the connections from the subject BGA pads to the connection vias, the BGA pads were removed. New BGA pads were then thermally bonded to the board surface, and copper circuits tracks were connected to these new pads. The circuit tracks were routed to the perimeter of the BGA sites. Wires were then attached to these new tracks.

This type of rework can be difficult because it requires demonstrated capabilities in multiple disciplines, and in this case it involves three different skill sets. First is the expertise needed for BGA removal and replacement at replacement. The second involves proficiency in milling and cutting operations. The third skill set involves the addition of the new BGA pads, copper tracks, and jumper wires.

The Detailed Procedure Follows
  1. Changing a Conductor Path at a BGA Site
    Figure 1: Cut the connection to the via using a precision drill system. Remove the BGA pad. Then mill a shallow channel in the board surface from the BGA pad area to the perimeter of the BGA site.
    Remove the subject BGA components using standard BGA rework equipment. Typically, this would be a hot-air rework system. If the density of the assembly mandates the use of bottom-side heating or pre-heating, an area source convective heater, hotplate, or bottom-side hot air nozzle can be used depending upon the configuration and make of the equipment.

  2. Cut the short conductor (dog bone) connecting the BGA pad to the connecting via using a high-speed precision drill. Use a carbide end mill approximately 0.015" diameter. Exercise care not to cut into the circuit board surface more than is necessary to sever the conductor. (See Figure 1).

  3. Remove the subject BGA pad. The pad can be gently peeled off, with the aid of a knife. Apply heat from a soldering iron if needed to lesson adhesion to the board. (See Figure 1).

  4. Changing a Conductor Path at a BGA Site
    Figure 2: Bond a new replacement BGA pad in place. Then lap solder a copper circuit track to the tail extending from the BGA pad. Run this new track along the milled path out beyond the perimeter of the BGA site.
    Use a milling machine to make a shallow channel in the board surface from the BGA pad area to the perimeter of the BGA site. Tight spacing may restrict the width of the channel to 0.002" or less. Use a carbide end mill approximately 0.002 in wider that the new connecting circuit. (See Figure 1).

  5. Bond a replacement BGA pad in place using a bonding system. The new BGA pad must have a tail to line up with the circuit track to be added next in this procedure. (See Figure 2).

  6. Lap solder a copper circuit track to the tail extending from the BGA pad. Run this new track along the milled path out beyond the perimeter of the BGA site. (See Figure 2).

  7. Solder one end of a fine gauge wire to the extending circuit. The opposite end of the wire will be soldered later. (See Figure 3).

  8. Clean the area with and overcoat the new circuit and solder joints with a thin layer of high-strength, high-temperature epoxy.

  9. Test the new circuit and replace the BGA component.

  10. Changing a Conductor Path at a BGA Site
    Figure 3: Solder a wire to the extending circuit and overcoat with epoxy.
    Solder the remaining end of the wire to complete the new circuit. Complete the final inspection using x-ray inspection equipment.
Conclusion
Adding engineering change orders (ECO's) to BGA sites need not be a nightmare, although it is certainly a challenging rework procedure. It can be done reliably and safely by following the procedure outlined above.

The result will be a robust, reliable connection that can be depended upon until the appropriate design changes in the board are implemented to correct the situation. Performed correctly, it is a reliable fix that will save time, money and countless otherwise perfect assemblies from the recycle bin.

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