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Katy Crist
Director, Marketing & Communications
Tokyo Electron
Fluxless TCB of large-area dies with localized in situ

oxide reduction

By Bob Chylak, Adeel Bajwa, Tom Colosimo, Tom Palumbo [Kulicke & Soffa Industries, Inc.]
T o meet the demands for to remove oxides and is accomplished altogether and it will further facilitate the

ever growing data, greater
standoff heights. The second approach is
computing performance, lower via dipping the solder-capped pillars into transition to finer pitches and smaller die
a cavity filled with flux, or by spraying
power consumption, high bandwidth, and it on the surface of the substrate. A key to develop methods that can reduce the
low latency in computationally challenging requirement of a reliable assembly process oxides on interface materials, such as
applications such as server, mobile, is to completely clean the flux residues solder and copper, by means other than
graphics, and artificial intelligence, etc., before the bonded die is underfilled. In using flux. Inert environment chambers
modern semiconductor chips often include general, it is very hard to completely (e.g., nitrogen gas), plasma treatments (e.g.,
large amounts of complex circuitry. remove these residues, especially when Ar, H 2 ), and noble metal finishes (e.g.,
This leads to large chip sizes, which for they are trapped underneath the die Au), are some of the prominent approaches
performance scaling reasons often come Several factors, such as flux residue that are being investigated both in
YOU with reduced interconnect sizes and chemistry, die standoff height, die area, academia and industry.
pitches that are usually assembled in a flip-
and interconnect density, etc., can affect
C chip fashion by either mass reflow, or more the cleaning process. For large-area die Previous work on fluxless bonding
2
commonly through a thermal compression (e.g., ≥1000mm ) with high interconnect A number of research papers highlight
M bonding (TCB) process. Both methods density and tight interconnect pitch, the challenges with fluxless bonding. A
SEE typically require the application of flux the entrapped flux residues are almost few are discussed in the sections below.
Y
CM and post-bonding flux residual removal impossible to clean. Even more so, the Reducing and inert environment
steps, which in general adds process trends in increasing interconnect densities approaches. Although inert gases such
MY complexity. Even more so, the shrinking and decreasing die standoff heights will as Ar and N 2 provide an excellent inert
WHAT’S
CY contact size and pitch results in short die make flux residue cleaning more and more medium for soldering, they are still
CMY standoff heights, e.g., 30-50µm, and this challenging. Furthermore, the underfill limited in terms of removing the existing
makes it extremely difficult to clean the material does not adhere to the surface, oxides on the bonding surfaces. Some
K flux residues. which is contaminated with flux residues. commercially-available tools are equipped
NEXT. size die (e.g., 32mm X 28mm) would The inability to clean residues can result with inert environment chambers, but they
To put this into perspective, a full reticle
in formation of voids during the molding
do not prevent the use of flux materials.
contain approximately 442,000 contacts
challenges because the solder will
at 45µm pitch. We are proposing a fluxless process. These voids pose serious reliability Furthermore, they often require very high
gas flows (i.e., ≥1000L/min) and the gas
TCB bonding process solution, which potentially leak into these voids during the consumptions are not economically viable.
eliminates the need for flux application reflow, and the solder will often short to the The flux application on a substrate
and therefore, post-bonding flux residual adjacent interconnect resulting in a failure. is generally always required before it
cleaning steps. These flux residuals pose If the failure is not caught in testing, then makes its way to the bonding chamber.
severe package reliability concerns. it can potentially occur when the chip is The reducing gas (e.g., formic acid [FA])
Our proposed fluxless method relies on soldered to the printed circuit board (PCB), vapor-based mass reflow technology has
WE Attract, hire and retain your 21st century localized in situ reduction of oxides, using again, resulting in a field failure. been around for quite some time, which
In response to these issues, f lux
formic acid vapors, from the contact
eliminates the need for pre-reflow fluxing
workforce with SEMI.
manufacturers have developed “no-clean”
surfaces just prior to and during the
bonding process. This method prevents fluxes. While these fluxes do leave less and post-reflow flux residue cleanup steps.
Noble metal finishes. Intel introduced
residues, the residues that are left are
flux process-related overheads and at the
ATTRACT No one does MORE to fill the talent pipeline for the same time, achieves the same result. even more difficult to clean. The no-clean intended to be used for fluxless bonding
the concept of solder preforms that are
fluxes, when used improperly, can lead to
applications. These preforms consist of a
semiconductor industry. SEMI connects industry, talent and
education for members worldwide. To access the next Challenges: large area dies/high- electrochemical migration and dendritic low melting point metal (e.g., tin, indium,
growth [1]. Currently, the semiconductor
etc.) and are further capped with a more
density interconnects
WHO’S generation of leading minds, visit semi.org/semiismore. flip-chip based TCB process, regardless industry is looking at two potential noble metal finish (e.g., gold, palladium,
The fundamental requirement for any
approaches to address these issues. First,
etc.) to protect it against the oxidation.
of the die size and the interconnect pitch, there has been R&D activity to eliminate The preforms are sandwiched between the
is to get rid of oxides from the mating solder-based materials by developing direct mating surfaces and the entire assembly is
NEXT. SEMIISMORE interfaces so that they can wet and bond eliminate the requirement for fluxing brought to the melting point of the preform,
Cu-to-Cu bonding technology. This would
which upon melting, dissolves the noble
reliably. Fluxing is the most common way
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