Changing a Conductor Path at a BGA Site
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.
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
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.
|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.|
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).
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).
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 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.|
- 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).
- 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).
- 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).
- Clean the area with and overcoat the new circuit and solder joints with a thin layer of high-strength, high-temperature epoxy.
- Test the new circuit and replace the BGA component.
the remaining end of the wire to complete the new circuit. Complete the
final inspection using x-ray inspection equipment.
|Figure 3: Solder a wire to the extending circuit and overcoat with epoxy.|
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
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
Several members of the Circuit Technology Center team contributed to this feature story.