Flip Chip Rework
From ACI Technologies
Flip Chip Rework
Flip chip components have been gaining popularity in the electronics
industry since their introduction in the 1960s. Recent advances in
attach methods and adhesives, as well as the drive for smaller and
faster electronic devices have made the technology take off. The basic
premise of the flip chip is that the chip (semiconductor device) is
mounted flipped from the traditional position. The traditional method of
mounting a die is to mount it on a lead frame with the circuit and bond
pads face up. The bond pads then receive a bond wire which then
connects to the proper lead on the lead frame. Flip chips are mounted
face down onto a substrate using small bumps on the bond pads to make
direct electrical connection to their respective pads on the substrate.
Stay tuned for more information on attachment techniques next month.
This article will focus on how to rework flip chips.
All types of underfill and adhesives have varying setting temperatures
and rework temperatures. The techniques described below are applicable
for the generic types of flip chip bonds. Follow your specific
manufacturer instructions for optimizing rework results.
Reflow Solder Flip Chip Removal
The simplest flip chips to rework are those that have been attached
using solder bumps. These chips are installed in a similar fashion to
standard surface mount technology (SMT) components. If testing proves
that a flip chip needs to be reworked prior to underfill, the removal
process is the same as that of an SMT component. Simply apply flux and
heated gas to both the chip and substrate to bring the assembly to 10-20
degrees C above the melting point of the solder and remove the flip
chip with a vacuum removal tool. Once the flip chip has been removed,
carefully remove all excess solder from the mounting pads. Clean the
area thoroughly and inspect the substrate to ensure that it has not been
damaged. Reattach a new flip chip.
If the chip already has been underfilled, rework is dependent on the
type of underfill. The bond of most epoxy underfills can be broken
utilizing heat. Some epoxies have solvents that reduce the heat needed
to break the adhesion, but the tradeoff is the time that it takes for
the solvent to break down the epoxy. Heat the area with hot gas
according to the manufacturer's rework profile and remove the chip with a
sliding motion. Remove any epoxy residue with the appropriate solvent,
clean the substrate and attach a new chip. Some examples of flip chip
rework stations are shown in Figure 1.
Figure 1: Examples of flip chip rework stations. Features for flip
chip rework include placement accuracy of better than 10 microns and
precision thermal management.
Adhesive Flip Chip Removal
Another method of flip chip attach uses adhesives, Anisotropic
Conductive Film (ACF), Isotropic Conductive Adhesive (ICA), or Non
Conductive Adhesive (NCA). Many of these adhesives can be softened by
heating. The manufacturer’s data sheet will typically show a “Tacking
Condition” where the adhesive begins to get tacky and a "Bonding
Condition", which shows the temperature, pressure, and time needed for
the adhesive to actually form a reliable bond. Some adhesives can be
reworked at the Tacking Condition temperature, while others need to be
heated to the Bonding Condition temperature. Again, consult the
manufacturer's data sheet for rework activation temperatures and
recommended solvents for cleaning the bond area prior to setting a new
flip chip.
If the assembly cannot withstand the heat prescribed in the
manufacturer’s specifications for rework, a solvent may be used to break
down the adhesive at a lower temperature. Before using a solvent the
engineer must verify that all components can withstand exposure to the
solvent without any degradation. This solvent may be either applied to
the general area of the flip chip or the entire substrate may be
submerged in a solvent bath. As with reflow underfill, the tradeoff for
lowering the temperature of rework is the time that the solvent will
take to dissolve the adhesive bond. This time is typically 8-24 hours.
Thermosonic and Thermocompression Flip Chip Removal
Both of these methods of flip chip attach create extremely strong
inter-metallic bonds at the chip to substrate interface. There are no
rework processes that can be recommended that will not damage the
substrate pad when attempting to remove a flip chip that has been
attached with either thermosonic or thermocompression assembly
techniques.
In summary, most of the more common production methods of attaching flip
chips can be reworked. The keys to successful removal are to know which
method of attaching the chip and substrate has been used, know which
adhesives or epoxies were used, and follow the manufacturer’s
instructions for removal. In the process of flip chip rework, a little
research can go a long way.
For further information regarding flip chip rework, please contact ACI
Technologies at 610.362.1320 or via email at helpline@aci.org.
ACI Technologies, Inc.
