Page 32 - Chip Scale Review_March-April_2024-digital

a semi-additive process (SAP). The organic substrate. Underfill material tool and was subjected to a heating
photolithography stepper used in was dispensed and cured to bridge the profile mimicking actual solder
this work has a maximum exposure solder bump gap between the RDL reflow conditions. The Shadow
field size of 44 × 26.7mm, and two- interposer and the organic substrate. Moi ré optical tech nique was
field mask stitching was utilized to Te st i ng. T h e f a b r ic a t e d R DL employed to accurately measure
create the large package size of 52 × interposers were subjected to electrical the warpage of the RDL interposer
44mm. Figure 4 displays the wafer and warpage measurement as follows: under controlled conditions.
map during photomask exposure. The
wafer map during photomask exposure 1. Meander combs with 2µm L/ Results and discussion
was illustrated in Figure 4. Chiplets S positioned at the stitched area The following sections discuss RDL
with micro-bump interconnects were between the 2 mask f ields to interposer fabrication results: 1) Cu
assembled onto the top under bump verify the continuity of the of the RDL process results, and 2) the chiplet
metallization (UBM) pads on the RDL two RDL traces. A daisy chain assembly wafer-level molding and
stacks using a mass reflow process. connecting the chiplets to the RDL RDL interposer package assembly.
Wa fe r-level e p ox y c om p r e s sio n layers was utilized to confirm the Cu RDL process results. Figure
molding was applied to encapsulate continuity of the solder bumps and 5a shows the RDL and UBM layers
the chiplets and RDL stacks with an RDL interposer. fabricated on a temporary transparent
expoxy molding compound, forming 2. Warpage measurement is crucial carrier, prior to chiplets assembly.
a reconstructed wafer. Following this, as it can impact the assembly Figure 5b shows magnified optical
the temporary carrier was removed process of the RDL interposer images of a single RDL interposer
from the reconstructed wafer. Solder to the organ ic subst rate and package. Figure 6 shows high-density
balls were affixed to the bottom UBM may result in poor solder joint 2µm L/S Cu RDL traces at different
pads of the reconstructed wafer to formation, particularly for an magnif ications to meet the high-
create the solder ball grid array. The interposer package of such large
reconstructed wafer was diced into dimensions. The RDL interposer
individual RDL interposer packages, package warpage was assessed
which were then mou nted to the in a Shadow Moiré measurement

E-Tec Interconnect AG, Mr. Pablo Rodriguez, Lengnau Switzerland
Phone : +41 32 654 15 50, E-mail: p.rodriguez@e-tec.com
Figure 5: RDL processing on a temporary carrier: a)
RDL layer fabricated on a transparent carrier; and b) a
magnified image of a single interposer package.

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