Chip Scale Review September • October • 2019[ChipScaleReview.com]
First, it can be noted that more than
87.5% of daisy chains were functional
after bonding, which is a very good result
for a new development. Moreover, the
percentages of valid central daisy chains are
excellent—100% for the three wafers. The
most remarkable result from this study is that
no failures occurred after thinning. It can be
observed that yields are slightly reduced after
coating, and few daisy chains failed. However,
more data are needed to draw conclusions.
Two flexible labels were diced using a
laser and removed from the wafer carrier. A
printed circuit board (PCB) was designed
and manufactured to facilitate electrical
characterization. A ZIF connector was
used to interconnect the label on the
). Six test patterns were
measured. The first two patterns were just
electrical tracks on the polymer without
contact with the silicon die. The goal was to
ensure that metal lines were not damaged
by removing the label from the carrier.
Peripheral and central daisy chain patterns
of large and small dies were measured.
Electrical results are summarized in
and compared with calculated values.
It has to be pointed out that all central
daisy chains in the study were functional.
Moreover, measurements closely agree with
calculated values. More tests are ongoing on
new labels to confirm these results.
With ChipInFlex, a new paradigm was
introduced for integrating ultra-thin silicon
bare dies within a flexible label made on the
wafer carrier. ChipInFlex is a generic wafer-
level process for manufacturing flexible
labels and integrates silicon components.
This process is the first to offer f lip-
chip silicon dies interconnected within a
flexible film. The electrical interconnection
is achieved with gold stud bumps made
The final average resistance of a single
contact was found to be from 12 to
14mOhms for wafers with an 80µm-thick
bottom polymer, 9mOhms for the wafer with
a 30µm-thick bottom polymer and 3mOhm
for the wafer with no bottom polymer. The
presence of a bottom polymer layer helped
absorb the force on the stud bump during the
thermocompression process and probably
reduced the resistance value of the contact.
No differences were observed between the
center and the periphery of dies.
shows the mapping of a central four-point
Kelvin pattern measured on a small die
on the periphery after final coating. The
continuity of all the daisy chains was tested
and the functionality rates are presented in
after each step.
Global average resistance of Kelvin patterns measured after main steps of the process.
Resistance value mapping of a central four-point Kelvin pattern measured on small die in the periph-
ery after final coating.
Percentage of functional daisy chain after the main steps of the process.
Resistance (in Ohm) measured of test patterns and compared with calculated values.
Printed circuit board to interconnect the
label using a ZIF connector.