Page 26 - ChipScale_Jul-Aug_2020-Digital

Cu /Sn /Cu system has 60 minutes held at 200°C.
been reported, however The resultant bond material
weak interfaces formed thermal conductivity would
across intermetallics, ultimately be in the range
together with reduced of 70-150W/mK, depending
elast icit y because of on the particle chemistry
the intermetallic´s high and particle size distribution
mo d u l u s , c a n l i m i t (PSD) loading, together with
application of TLPS in the epoxy formulation.
very high-power density Semi-sintering media
applications. have found good use in mid-
to high-power applications
Hybrid pastes/semi- w h e r e t h e s i g n i f i c a n t
sintering pastes processing time is countered
Compared to soldering b y t h e e l i m i n a t i o n of
and TLPS, the sintering intermetallics, good thermal
process does not require c o n d u c t iv it y, t u n a b l e
a phase change of the mo du lu s r a ng i ng f rom
metal components and no approximately 2–20GPa,
melting occurs. Sintering and the relative simplicity of
can be achieved either curing without the need of a
without using any pressure sinter press.
when using hybrid semi-
sintering pastes, or by Pressure sintering
using pressure for a fully Time, temperature and
metallic bond. pressure are key enablers for
Hybrid semi-sintering neck formation and fusion of
pastes are broadly similar particles in pressure-assisted
to ICAs as the sinter sintering. The externally
material stock consists of Figure 5: Cross-sectional SEM micrographs of Cu/Sn/Cu TLP bonded interfaces subjected applied force is required to
to bonding at 300°C for various durations adapted from [6].
three parts: silver/copper push the particles closer to
solid particles in micro- each other, thereby squeezing
flake form, liquid solvents, out voids, promoting cohesion,
and an epoxy component and ultimately achieving a
[8]. The key differentiator fully-metallic sinter layer.
is the metallic loading and For nano-sinter particles,
final structure achieved. In necking can initiate at room
ICAs, conductive particles temperature and atmospheric
allow electrons to move pressure because of the very
across the bulk either large surface area to volume
through direct contact of the particles and resultant
between the particles, high surface activity [4].
o r t h r ou g h q u a n t u m In order to achieve room-
tunneling of proximal t e mp e r a t u r e st a bi l it y,
m e t a l li c i n t er f ac e s . nanoparticles are engineered
Hybrid sintering, however, to incorporate proprietary
goes one step fur ther surface stabilizers, which are
by having the metallic designed to protect against
fillers diffuse into each Figure 6: Cross section of semi-sintered micron-sized flake particles in a cured epoxy self-sintering. The typical
other (Figure 6), thereby resin. COURTESY: MacDermid Alpha process profile for sinter
significantly improving property degradation as T J approaches the paste products includes
the thermal and mechanical properties epoxy T G . a drying profile of printed paste at
of the attachment layer. Very careful The typical process prof ile for 130°C-150°C, followed by die placement
formulation is necessary in order to allow pressure-less sintering includes a 60 and actual sintering. For a sinter cycle
metallic diffusion to occur simultaneously minute pre-cure at 130°C where the of 60s, nanoparticle sinter products can
as the polymer resin cures. Ultimately, solvent evaporates, and the solid and non- be processed at a temperature as low as
in-service, the resin matrix strength is volatile organics parts remain constant, 230°C and a pressure of 10MPa to achieve
augmented by the fully linked metallic thereby pushing the particles closer a die shear value of well over 50MPa and
network, which makes the bulk material together. This is followed by a resin a bulk thermal conductivity in the range
less susceptible to sudden material cure/sinter profile for approximately of 250-300W/mK. Micron-based sinter

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