Page 35 - Chip Scale Review_March-April_2024-digital

illustrated in Figure 13b, indicating
a leakage current in the range of
pico-amperes, which aligns with the
target specifications.
Chiplets to R DL daisy chain.
Figure 14 shows the daisy chain
locations between RDL and chiplets.
Electrical testing was performed on the
respective UBM pads connected to the
RDL and chiplets; and all daisy chains
showed good electrical continuity with
no open connections, indicating good
assembly yield of the chiplets using the
mass reflow process.

Warpage measurement
The warpage of fabricated RDL
interposer packages was measured
Figure 12: a) Meander comb test structure at the stitch line; and b) a magnified image of meander traces at using the Shadow Moiré technique
the stitch line.
d u r i ng a r e f l o w p r o c e s s . T h i s
assessment was conducted to evaluate
the RDL interposer package’s warpage
whe n mou nt e d ont o t he orga n ic
subst r at e. F ig ure 15 illu st r ates
the package war page th roughout
the ref low process. The fabricated
package was measured with the bump
side facing down (live-bug), where
positive values indicate a convex
profile and negative values indicate
a concave prof ile. I n it ially, t he
RDL interposer exhibited a convex
warpage of 213µm. As the temperature
i n c r e a s e d , t h e p a c k a g e p r of i l e
transitioned from convex to concave,
reaching a maximum war page of
-736µm at a peak temperature of
275°C, before decreasi ng as t he
tempe r at u re wa s lowe red. Upon
cooling to room temperature, the final
package warpage measured 277µm.
This significant change in warpage is
attributed to the coefficient of thermal
expansion (CTE) mismatch between
the RDL layers, chiplets, and mold
compound. The observed warpage
variation poses a significant challenge
du r ing the assembly of the R DL
interposer to the organic substrate
using a reflow approach, potentially
leading to issues such as inadequate
solder joint formations.
To address the challenge of large
package warpage, a TCB process was
implemented for assembling a large
RDL interposer package. The process
involved several steps to mitigate the
warpage issue effectively. Initially,
Figure 13: Electrical test results of a stitched meander comb structure: a) an I-V curve of meander Cu trace; the RDL interposer was picked up on
and b) intra-layer leakage current of meander comb.

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