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10

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

PCB (

Figure 9

). 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

Table 3

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.

Summary

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.

Figure 8

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

Table 2

after each step.

Figure 7:

Global average resistance of Kelvin patterns measured after main steps of the process.

Figure 8:

Resistance value mapping of a central four-point Kelvin pattern measured on small die in the periph-

ery after final coating.

Table 2:

Percentage of functional daisy chain after the main steps of the process.

Table 3:

Resistance (in Ohm) measured of test patterns and compared with calculated values.

Figure 9:

Printed circuit board to interconnect the

label using a ZIF connector.