Page 36 - Chip Scale Review_March-April_2024-digital

the bond tool and pre-heated to 100°C
to reduce the interposer warpage.
Subsequently, the RDL interposer
was aligned and gently pressed onto
the organic substrate with a low bond
force. The bond tool was then heated
to a peak bond temperature of 300°C
for a short duration and subsequently
cooled down. During the cooling
phase, the bond force was maintained
to minimize the warpage of the RDL
interposer. Figure 11c shows the RDL
interposer successfully assembled
to the organic substrate with the
TCB process and X-ray inspection
(Figure 16) shows good solder joint
formation between the RDL interposer
and organic substrate, showcasing
the effectiveness of this method
in addressing the RDL interposer
Figure 14: Designation of the chiplets and daisy chain locations. warpage during package assembly.
T he Shadow Moiré assessment
of the assembled RDL inter poser
on the organic substrate displays a
380µm convex profile, surpassing the
warpage outcomes of the unassembled
RDL interposer package of 277µm
(Figure 17). This elevated warpage
is attributed to the CTE mismatch
between the RDL interposer and the
organic substrate materials. It should
be noted that a metal stiffener frame
is absent on the organic substrate
used in the st udy. Incor porating
a stiffener frame into the organic
substrate is anticipated to alleviate the
warpage results of the assembled RDL
interposer package.

Summary
A large RDL interposer measuring
Figure 15: Package profile of an RDL interposer measured by Shadow Moiré at a key reflow temperature. 52 x 44mm was successfully fabricated
using the RDL-first FOWLP process,
accommodating 12 embedded chiplets.
The fabrication process involved a
2-mask stitching process to produce
the sizable interposer, with 6 layers
of high-density RDL featuring 2µm
L/S on a temporary carrier. Chiplets
with micro-bumps were accurately
placed on UBM bonding sites and
soldered through mass reflow. X-ray
inspection verified the alignment
and successful solder joint formation
bet ween ch iplet s a nd R DL. T he
chiplets were then encapsulated with
epoxy mold compound to complete the
RDL interposer package. Electrical
Figure 16: X-ray inspection of solder joints between the RDL interposer and the organic substrate: a) Plan testing demonstrated good continuity
view of the entire RDL interposer package; b) A magnified and tilted view of the solder joints.

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34 Chip Scale Review March • April • 2024 [ChipScaleReview.com]