Chip Scale Review November • December • 2017[ChipScaleReview.com]
Cleaning fine-pitch copper pillar flip-chip packages
By Mike Bixenman
he miniaturization of modern
ele c t r on ic s c on t i nue s t o
challenge the effectiveness of
common cleaning processes and the ability
to obtain desired cleaning performance and
optimal yield. When soldering the flip-chip
die to the ball grid array (BGA) interposer,
water-soluble paste fluxes are used to ensure
a strong metallurgical bond. Ineffectiveness
in removing f lux residues can lead to
contamination, underfill voiding, poor yield,
and reliability issues in the field.
Devices utilizing copper pillar technology
have more interconnects per surface area,
which results in tighter pitch and lower
standoff gaps. As standoff gaps lower, flux
residues have less area to outgas during
reflow. This results in more active residues
under the die. A longer wash time using water
with a low concentration of cleaning agent is
typically required to properly clean die under
these lower standoff gaps.
Aqueous saponified cleaning agents diluted
in deionized (DI) water have commonly been
used to remove flux residues under the flip-
chip bottom termination. Saponified cleaning
agents are mildly alkaline to form an attractive
force to the flux residue. However, alkaline
cleaning agents can react, consume and
corrode reactive metals. Longer time in the
cleaning solution increases chemical attack,
which can cause failures due to mechanical
stresses. Potential metal incompatibilities
and insufficient rinsing represent the most
common challenges for engineering improved
cleaning agents for cleaning copper pillar flip-
Current saponified cleaning solutions
adversely react with many metals (Al, Cu,
Sn, Ag, Ni, etc.) that are present on the
copper pillar die. Especially in alkaline
solutions, Al is readily attacked causing
galvanic corrosion reactions. Also, there is the
potential for defects and discoloration on Cu
and SAC alloys during the cleaning process.
Crevice corrosion from cleaning solution
reactions with exposed metals can weaken
interconnects and reduce mean time to failure.
Cu pillar technology
Copper pillar bumping is a growing design
trend in electronics packaging. Copper
pillar technology offers many advantages
in speed and line pitch. Building integrated
circuits with copper and aluminum reduces
the potential for electromigration while
improving current carrying capacity.
Likewise, copper pillar is more cost effective
than Au stud bumps for high bump designs.
Finer pitch can be achieved, which translates
into higher performance over a smaller
surface area (
Unlike the traditional bumping process,
the copper pillar design exposes a number
of reactive metals to the cleaning process.
Many of these metals react and dissolve
when exposed to saponified cleaning agents.
Corrosion inhibitors can reduce, and in some
cases, prevent this interaction. The challenge
is designing cleaning agents that are effective
at inhibiting the different exposed metals
with which the cleaning agent comes in
contact. If longer cleaning time is needed,
the risk of metal interaction is greater.
Aluminum is highly reactive when
exposed to alkaline cleaning agents.
Common saponified aqueous cleaning
agents that are used to clean traditional
bumped die work well. These cleaning
agents are not suitable for cleaning copper
pillar die due to their propensity to attack
aluminum and copper. Aluminum is
a diffusion barrier metal for copper.
Common alkaline cleaning agents can
attack, dissolve and crack the aluminum
is an example of how a
traditional saponified cleaning agent
at tacks, cor rodes and dissolves the
aluminum pad. Copper is also reactive
when exposed to alkaline cleaning agents.
Cleaning solutions with poor copper
inhibition will tarnish and oxidize the side
of the copper pillar (
Nickel functions as the intermetallic
layer between the copper pillar and SAC305
alloy. Cleaning agents high in alkalinity
have been shown to undercut the copper/
nickel adhesion layer (
alloy can pit and darken when exposed to
highly alkaline cleaning agents. Longer
exposure time increases the interaction
). The problem is that cleaning
agents that react with the exposed metals
result in various forms of pitting, galvanic
and crevice corrosion. The corrosion effect
not only affects electrical performance
Copper pillar design.
Al pad dissolves, cracks and turns white.
Copper pillar oxidation.
Nickel adhesion layer undercut.