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Figure 6: A 30mm die-to-die bonded assembly.
Figure 5: Fluxless TCB process.
Figure 7: A fluxless TCB sample.
and substrate in a position-limited Bonding of large-area die
force-controlled fashion. The FA vapor To demonstrate the advantages of
delivery is discontinued after the our fluxless bonding process for large
completion of the TCB step. Figure 5 area dies, we have used a Si-based test
shows the process flows. die with an area of 900mm . The die
2
was ter m i nated
w i t h s o ld e r
(S n A g) c a p p e d
copper pillars of
36µ m d ia me t e r
a n d co n t ai n e d Figure 8: A flux-based TCB sample.
t wo v a r i at i o n s
of pitches: 55µm Cu-Cu direct bonding
and 80µm. A die- The following sections discuss some
to - d i e b o n d e d of the challenges with respect to Cu-Cu
as s e m b l y w as direct bonding.
formed using FA Main challenges for Cu-Cu bonding.
vapor-based oxide Maintaining the copper surfaces to be
reduction. Figure oxide free is extremely challenging in
6 shows a 30mm a C2W or C2S machine because the
d i e b o n d e d t o surrounding environment is mostly air.
another 30mm die An inert gas such as N 2 can be used to
using the fluxless keep the environment oxide free, but
TCB process. very large flow rates (e.g., ≥1000L/min)
T h e c r o s s are required. Moreover, using an inert
s e c t i o n o f t h e gas does not reduce the existing natural
bonded assembly oxide layers. Another issue for Cu-Cu
revealed excellent bonding is to keep the surface atomically
joints as shown flat and planarized across the entire chip
i n F i g u r e 7 — area. Today, it is done through a CMP
s up e r ior t o t he process, which can only be utilized for
joints made using the cases where both chip and substrate
the conventional are made up of silicon. For laminates and
f lux-based TCB PCBs, this still remains an issue. The
process as shown CMP process adds more complexities
in Figure 8. and it is not a common practice of
outsourced semiconductor assembly
and test suppliers (OSATS), though
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40 Chip Scale Review September • October • 2020 [ChipScaleReview.com]
