Modeling, Design and Demonstration of Low-temperature, Low-pressure and

High-throughput Thermocompression Bonding of Copper Interconnections without Solders

Ninad Shahane*, Scott McCann, Gustavo Ramos+, Arnd Killian+, Robin Taylor+, Venky Sundaram,

Pulugurtha Markondeya Raj, Vanessa Smet, and Rao Tummala

3D Systems Packaging Research Center

Georgia Institute of Technology Atlanta, GA, USA

+Atotech GmbH, Berlin, Germany

Email: ninad.shahane@gatech.edu

Abstract

High-throughput assembly technologies to form Copper

(Cu) interconnections without solders at below 200oC, and

pitch below 40µm has been a major challenge in the

semiconductor industry. A unique solution has been

demonstrated by Georgia Institute of Technology to overcome

this grand challenge. This technology utilizes

thermocompression bonding to form copper interconnections

with process tolerances to accommodate non-coplanarities of

bumps and warpage of the substrate, without solders. The

bonding pressure applied for thermocompression was 365MPa,

to enable Cu bump collapse by 3µm. As thermocompression

bonders are generally force-limited to 400N, such high bonding

pressures may hinder scalability of this technology to fine

pitches with higher I/O densities. This paper addresses this

manufacturability challenge with the novel Electroless

Palladium Autocatalytic Gold (EPAG) surface finish instead of

the standard Electroless Nickel Immersion Gold (ENIG) or

Electroless Nickel Electroless Palladium Immersion Gold

(ENEPIG) finish, previously used to prevent Cu oxidation for

bonding load reduction down to 120MPa.

Finite element modeling was carried out to understand the

bonding mechanism and deformation behavior of Cu bumps

and pads. Compensation of non-coplanarities and warpage by

collapse of the Cu bumps was found to be the prevalent limiting

factor for pressure reduction. New interconnection material and

structure innovations were studied for their deformation

behavior as a function of the applied pressure in

thermocompression. The EPAG surface finish enables a 3X

reduction in bonding pressure, by the elimination of Ni, and

redistribute plastic deformation more equally between bumps

and pads.

The proposed innovations thus address both

manufacturability and scalability of copper interconnections to

20µm pitch, while maintaining compatibility with current

production-level thermocompression tools and processes.

1. Introduction

The need for higher speed and bandwidth at lower power

consumption for high-performance applications is expected to

drive off-chip interconnection pitch to 20µm and below in the

coming decade. The recent trend to 2.5D integration, where a

single large die may have to be partitioned into two or more

smaller devices and reconstructed on the substrate, with high-

density wiring, further reinforces the requirements for ultra-

fine pitch interconnections in advanced smart mobile and high-

performance systems.

Ever since IBM’s invention of the C4 solder bump flip-

chip technology [1], numerous advances in interconnection

technologies have been made to keep up with increasing I/O

densities on one hand, and IC size reduction on the other, thus

necessitating pitch scaling. The Cu pillar with solder cap

technology developed by APS [2] and more recently by Texas

instruments in tandem with Amkor in 2011, was a major

breakthrough in fine-pitch interconnections, dominating

mobile and high-performance applications at 40µm pitch in

production [3] and 30µm in research and development. Further

pitch scaling leads to a reduction of the bump standoff height

and diameter, which subsequently leads to a reduction in solder

volume. This raises severe reliability concerns due to increased

stresses at the solder-intermetallic interfaces and poor current-

handling capability of solders. Solid-liquid Interdiffusion

(SLID) bonding has been proposed as an all-intermetallic

interconnection solution to extend the scalability of solder-

based technologies to finer pitches [4]. SLID bonding works on

the principle of forming a high melting and stable intermetallic

phase after assembly at the expense of low-melting reactants.

However, this technology faces its own set of challenges,

including low throughput due to extended transition times

required to attain fully stable intermetallics [5, 6] and

questionable reliability due to Kirkendall voiding and inherent

intermetallic brittleness [7].

As a result, interconnections formed by solid-state bonding

are gaining importance to address the fine-pitch challenge.

Such technologies are extensively used in wafer-level

packaging and gaining importance for off-chip

interconnections. Solid phases have low interdiffusion rates

because of higher activation energies required compared to

liquid phases. Consequently, metallurgical bonding in solid

state necessitates clean and oxide-free surfaces and the use of

an external driving force such as ultrasonic energy or pressure.

Hence, oxide-resistant, soft materials such as gold are generally

preferred. Gold (Au) is chemically noble and can be deposited

using either electroplating, electroless plating or stud bumping

at pitches as low as 50µm [8, 9]. Au-Au interconnections (GGI)

can be formed by thermosonic [10] or, more recently,

thermocompression bonding. Thermocompression bonding

conditions are a trade-off between temperature and pressure;

higher temperatures resulting in lower yield strength and higher

ductility, while higher pressures improve interdiffusion by

reducing diffusion lengths. Formation of Au-Au

interconnections has been demonstrated by thermocompression

bonding at 150oC-300oC with typical pressures in the 0.02-

8MPa range [11-14]. Despite their excellent processability and

electrical, thermal and reliability performances, the use of Au-

978-1-4799-8609-5/15/$31.00 ©2015 IEEE 1859 2015 Electronic Components & Technology Conference

based interconnections in high-volume manufacturing is

inherently limited by the prohibitive cost of Au.

Ultimately, all-Cu interconnection technologies, without

solders are highly sought after by the semiconductor industry

as the “holy grail” for ultra-fine pitch. Cu has outstanding

electrical and thermal conductivities, enabling high power-

handling capability and high-speed signal transmission, and is

compatible with standard back end of line (BEOL)

infrastructure and processes. Existing direct Cu-Cu

technologies, typically used in wafer-level 3D IC stacking,

often require extensive surface preparation or activation, such

as chemical-mechanical polishing (CMP), bonding in vacuum

or controlled atmospheres, or at temperatures far exceeding that

of traditional solder-based reflows with long annealing times at

high temperature for interdiffusion and recrystallization [15-

22], thus limiting their applicability to high-volume

manufacturing. To date, no cost-effective, manufacturable all-

Cu interconnection technology, combining low bonding

temperatures and pressures, and short assembly cycle times has

been demonstrated.

Georgia Tech PRC recently patented a novel technology to

form Cu interconnections without solders at temperatures

below 200oC, in air, without any complex surface preparation,

and with cycle times comparable to production times [23]. This

is achieved with the following attributes: a) prevention of Cu

oxidation by application of standard surface finish on Cu

micro-bumps and pads; b) reduction in bumping cost by

implementing all-Cu bonding without solder plating; c)

accommodation of bump and pad non-coplanarities by

controlled collapse of the bumps during thermocompression; d)

ultra-high current handling capability without solders and with

stable interfaces; and e) high-throughput, under 5 seconds.

Excellent thermomechanical reliability and electromigration

resistance of these interconnections at 106 A/cm2 has already

been demonstrated at pitches down to 30µm, with low-CTE

organic and glass substrates [24-26]. Recently, low-

temperature, ultra-fast die-to-panel Cu interconnections were

demonstrated by Georgia Tech PRC by thermocompression

bonding using pre-applied underfill materials at a 365MPa

nominal pressure and a bump temperature of 160oC [27, 28].

The cycle time was limited by the underfill reaction speed

which is, itself, temperature-dependent. A fast curing underfill

will decrease the cycle time and will bring about a low-

temperature high-throughput assembly.

Reduction of the bonding load with adequate tolerance to

non-coplanarities and warpage is, therefore, the last bottleneck

in advancing this interconnection technology to the

manufacturability level required for ultra-fine pitch assembly.

Since thermocompresion bonders are force-limited to 400N,

this constrains I/O densities at a given process pressure, thus

requiring innovations in both interconnection design and

surface finish materials to meet this manufacturability

challenge.

This paper reports the modeling, design and demonstration

of all-Cu interconnections and assembly technology with low

bonding pressures, down to 120MPa. Finite element modeling

was carried out to understand the bonding mechanism and

deformation behavior of the interconnection structure, leading

to the controlled collapse of the Cu bumps. Different surface

finish compositions were evaluated – ENIG, used as reference

and a novel EPAG finish – with respect to the bonding pressure.

Daisy-chain test wafers and organic substrates with a minimum

pitch of 100µm were fabricated to verify the models prediction.

The surface finish was plated by Atotech GmbH on wafers and

substrates with controlled variations in thickness of Pd and Au

layers for this parametric study. Confocal 3D microscopy was

used to characterize the bump collapse for various pressure

levels in compression tests.

2. Bonding Mechanism

The interconnection structure in this study is shown in

Fig. 1. The Cu micro-bumps and Cu pads on wafers and

substrates are protected from oxidation by a surface finish

layer. The bonding mechanism established in [28] was

identified as partial metallurgical bonding with pressure-

induced plastic deformation as the driving force. This was

assisted by chemical bonding through a pre-applied underfill

material.

Fig. 1. Schematic of interconnection structure with Cu bumps

and pads plated with surface finish (SF), and illustration of

bonding mechanism with controlled collapse of the bump

under compression.

Fig. 2. Deformation contours showing a maximum vertical

collapse of 3.20µm at 350MPa (left) with a maximum lateral

displacement of 1.06µm (right).

The 3D quarter symmetric finite element model of Fig. 2

was built using ANSYS software to better understand the

bonding mechanism and deformation behavior of the Cu bump

and pads during thermocompression bonding. The modeled

geometry consisted of a Cu bump, 10µm in height and 10µm

1860

in diameter, plated on a 5µm-thick Cu pad on the Si die side;

and a Cu landing pad on the substrate, 10µm in thickness. The

bump diameter was scaled using the half-pitch design rule, for

20µm pitch. ENIG surface finish was considered on Cu bump

and pads with a 3µm-thick Ni(P) and Au layer, respectively.

The Ni(P) layer acts as a diffusion barrier to prevent Au

diffusion into Cu under thermal loading to form AuCu, AuCu3

or Au3Cu intermetallics [29]; while the thin Au layer acts as

soft, plastically deformable surface to create intimate contact

between the mated interfaces. The model also includes a pre-

applied underfill material surrounding the bumps. Isotropic

elastic-plastic mechanical behavior was assumed for all

materials. Plasticity was represented with a bilinear kinematic

hardening law, with the parameters reported in Table 1. The

load was applied from the die side while the substrate pad side

was fixed. The model was uniformly heated and cooled. The

model was solved at five different time steps using birth and

death, to emulate as accurately as possible the assembly

process: 1) heat to bonding temperature of 180°C; 2) force

ramped up to the bonding pressure, in the 50-350MPa range; 3)

release of the pressure; 4) cool to underfill activation

temperature of 160°C, which corresponds to its glass transition

temperature Tg (birth of underfill); 5) cool to room temperature

at 25°C.

Table 1. Isotropic elastic-plastic material properties for Cu,

Ni(P) [30], Pd [31] and Au [32].

Materials Cu Ni(P) Au Pd

Elastic (GPa) 117 70 79 120

Poisson’s ratio 0.33 0.31 0.44 0.39

Initial Yield Stress

(MPa) 172.38 140 100 200

Tangent Modulus

(MPa) 1034.2 86666 200 13043.5

CTE (ppm/K) 17 12 14 11.8

Applied compressive forces cause plastic strain in the Cu

bump, leading to their collapse and bringing the mated

interfaces into intimate contact. The deformation contours of

Cu bumps and pads are shown in Fig. 2 The deformation

gradient in the pads and the bump implies that plastic strain is

distributed between bumps and pads. Due to the stiff Ni layer

hindering deformation of the Cu bump and substrate pad,

significant plastic strain was found in the Cu pad on the die

side, raising serious concerns over potential failures of low-K

dielectric layers stacked underneath.

Additionally, excessive plastic strain is observed in the

thin Au layers of the ENIG finish, acting as a driving force for

interdiffusion or self-diffusion of Au and subsequent localized

metallurgical bonding where atomic contact was achieved

under thermocompression. Local Au-Au metallurgical bonding

has been demonstrated at 200oC and 365MPa applied pressure

in [28], with outstanding reliability performance [26]. Due to

the high plasticity and ductility of Au, it has been shown that

Au-Au bonding can be achieved at average pressures of 0.02-

8MPa, indicating that the pursuit of metallurgical bonding is

not the limiting factor for bonding load reduction in the

considered interconnection structure.

A systematic study of the bump and pad collapse as a

function of the applied pressure was conducted, and is

summarized in the graph of Fig. 3. A bonding pressure of

350MPa yields a bump collapse of 3.20µm, which is sufficient

to compensate for non-coplanarities and achieve full assembly

yield. The lateral displacement of the bump resulting from

volume conservation was found to be only 1.06µm, suggesting

a minimal risk of bridging in fine-pitch applications.

Fig. 3. Collapse of Cu bump with ENIG surface finish as a

function of bonding pressure.

Formation of Cu interconnections, 10µm in diameter,

plated with ENIG surface finish, has been previously

demonstrated at 30µm pitch by thermocompression at 200°C

and 365MPa bonding pressure. Compression trials carried out

at this nominal pressure led to a collapse of the bump by 3.5µm,

as indicated by the SEM images of Fig. 4, before and after

compression. At this pressure level, the model predicts a total

collapse of the bump by 3.44µm. The empirical data correlate

well with the predictions, with a 1.7% error margin. Deviations

between model and experiments can be attributed to the

discrepancies between the materials properties used in the

model with respect to the actual ones. Characterization of the

mechanical properties of the Cu bumps, including elastic and

tangent moduli and yield strength, is ongoing for more accurate

modeling. The cross-section of an assembly in nominal

bonding conditions shown in Fig. 4 confirms the deformation

behavior of the interconnection structure, showing deflection

of the Cu landing pad on the substrate side alongside collapse

of the bump. Very little deformation of the pad on the die side

was observed as compared to the substrate side, contrary to the

model predictions, as the substrate pad was directly resting on

a soft, highly-deformable polymer layer, shifting the strain

distribution.

Since Au-Au solid-state bonding can be achieved at much

lower pressures than considered, the limiting factor for bonding

pressure reduction is the offset from non-coplanarities. A key

innovation in surface finish has been proposed to address this

challenge, as detailed in the following sections.

0

0.5

1

1.5

2

2.5

3

3.5

100 150 200 250 300 350C

oll

ap

se (μ

m)

Bonding Pressure (MPa)

ENIG

1861

(c)

Fig. 4. SEM images of a Cu micro-bump array a) before and

b) after thermocompression bonding at 200C – 365MPa for 60

sec; and (c) of the cross-section of a copper interconnection

formed in the same conditions [28].

3. Novel EPAG Surface Finish

3.(a)Process Modeling

Based on previous results, elimination of the Ni interface

would be desirable to improve the deformation behavior of the

interconnection structure. The Ni layer degrades electrical

performance, especially at high frequencies due to skin effect

and can also cause shorting due to Ni spread, thus limiting its

use for fine line technologies [33]. It is, however, beneficial as

a thick barrier layer to prevent interdiffusion of Cu and Au,

which could be detrimental to the thermo-mechanical stability

of the joint [34].

A new surface finish, EPAG recently developed by

Atotech GmbH, is considered for this study. The EPAG process

allows deposition of 50-200nm Palladium (Pd) layers directly

on Cu, and subsequently 40-400nm thick Au layers on the Pd.

The EPAG process has been shown to be particularly

suitable for fine-line applications. No extraneous Pd or Au

deposition is observed even with only 15µm spaces between

Cu traces, while the autocatalytic nature of the process prevents

the attack of the sub-15µm fine-line Cu structures. With solder,

the thin Pd and Au layers are entirely dissolved yielding similar

IMC formation and reliability than with Immersion Sn and OSP

finishes [33]. Also, compatibility of EPAG with Au, Cu or Ag

wire bonding was demonstrated. An additional advantage of

EPAG over ENIG is its better high-frequency performance for

signal transmission. Losses along traces were measured to be

substantially lower. In case of thermocompression bonding, the

electroless Pd layer is expected to act as a diffusion barrier

preventing intermixing of Cu and Au, while autocatalytic Au

still provides a soft, low-modulus contact surface to enhance

contact plasticity and metallurgical bonding.

The finite element model from the previous section was

altered to evaluate the effect of surface finish composition on

the Cu interconnect deformation behavior under

thermocompression. The ENIG surface finish, used as

reference, was replaced with thin layers of Pd and Au. Further,

the Pd and Au thicknesses of the EPAG surface finish were

varied in the 50-100nm and 0-250nm range, respectively, in the

configurations reported in Table 2, to optimize the surface

composition with regards to metallurgical bonding and cost.

The mechanical behavior of Pd was considered isotropic

elastic-plastic, and represented with a bilinear kinematic

hardening law with the parameters shown in Table 1.

Typical bump and pad collapse displacement contours

obtained with the EPAG surface finish are presented in Fig. 5

for bonding pressures varying in the 100-150MPa range. This

contour suggests that plastic strain was distributed more

uniformly between bump and pads compared to the ENIG

model of Fig. 3. This indicates a lower risk of failure in the sub-

wafer dielectric layers. The collapse of the Cu bump and pad as

a function of the applied bonding pressure is shown in Fig. 6,

for all given surface finishes. The model predicted an almost

3X decrease in the bonding pressure required to bring about a

3µm total collapse at 20µm pitch, from the reference 350MPa

with ENIG to 120MPa in any of the EPAG configurations.

Fig. 5. Bump and pad collapse displacement contour of the Cu

interconnection structure with EPAG4 surface finish, after

thermocompression at 100MPa (left), 120MPa (middle) and

150MPa (right), with a corresponding bump collapses of

2.39µm, 3.64µm and 5.58µm, respectively.

Fig. 6. Collapse of the Cu bumps and pads with respect to

ENIG and EPAG surface finishes as a function of the applied

bonding pressure, obtained by modeling of the

thermocompression process at 180°C.

0

1

2

3

4

5

6

7

8

9

0 50 100 150 200 250 300 350

Co

lla

pse

(μ

m)

Bonding Pressure (MPa)

EPAG legs

ENIG

1862

The EPAG slope is higher than that from the ENIG

structure because of its higher sensitivity to loading. Since the

Pd-Au layers are relatively thin and more ductile than stiff Ni,

higher plasticity is achieved the EPAG model at a given

pressure, hence a higher bump collapse. The lateral

displacement of the Cu-EPAG interconnection at 120MPa was

found minimal, a mere 0.88µm, thus eliminating the chance of

bridging for fine-pitch applications. The lateral displacement

was slightly lower as compared to that of the reference ENIG

model, due to the low bonding pressure applied.

At 120MPa, the total bump collapse varied only by 3.5%

with different bump stack designs, in the 3.51-3.64µm range.

A highest bump and pad collapse of 3.64µm was observed at

120MPa with 100nm Pd and 250nm Au (EPAG4). It was also

observed that an increase in thickness of the Au layer on both

bump and pad interfaces resulted in a higher total collapse at a

given bonding pressure. This can be attributed to the low yield

strength of Au, leading to higher plasticity levels at the

interface with increased thickness. Lastly, an EPAG surface

finish with only Pd showed a similar behavior as other EPAG

compositions. A pure Pd surface finish as such could be a cost-

effective solution to bring this technology to finer pitches,

provided that the thin electroless Pd layer can effectively

prevent Cu oxidation with a reasonable shelf life, and that

direct Pd-Pd metallurgical bonding can be achieved at

temperatures below 200°C.

Finite element modeling proves that the novel EPAG

surface finish would bring significant bonding load reduction,

while maintaining high plasticity levels at the mated interfaces

to enable metallurgical bonding. A thorough experimental plan

was carried out to confirm these simulation results.

3.(b) Test Vehicle Fabrication

For this parametric study aimed at the optimization of the

EPAG surface finish composition, a simple daisy-chain test

vehicle was fabricated using standard industry processes, with

a minimum pitch of 100µm, to ensure excellent assembly yield.

The test vehicle design comprises of a 5mm x 5mm Si die,

600µm in thickness. The test die features 760 Cu micro-bumps,

30µm in diameter and 10µm in height, arranged in three

peripheral rows at 100µm pitch and a central area array at

250µm pitch. The daisy-chain pattern is divided into individual

chains, with four two-point probe structures for corner daisy-

chains and eight for half-edge daisy-chains, while the area array

is split into four individual daisy-chains as shown in Fig. 7.

Fig. 7. Test vehicle design with daisy-chain structures for

interconnection yield and reliability evaluation: 5mm x 5mm

die (left) and 30mm x 20mm substrate with probing pads

(right).

A 600µm-thick 6” Si wafer was utilized to fabricate the Si

dies using semi-additive plating processes. A 2µm-thick SiO2

layer was first deposited using Plasma-Therm PECVD. Then,

a 30nm Ti – 500nm Cu seed layer was sputtered over the oxide

layer. The dogbone routing layer was patterned by

photolithography followed by electrolytic plating of 2.5µm. An

additional lithography step was required to pattern the micro-

bumps, which were then electroplated to a 10µm height. After

photoresist stripping and seed layer etching, surface finish is

applied to prevent Cu oxidation. Test substrates were fabricated

from 6” x 6” Cu-clad FR-4 organic laminates for ease of

subtractive processing. The Cu cladding on the 1mm-thick FR-

4 core was etched down from 50µm to 10µm, followed by a

standard photolithography and back-etch processes to build the

dogbone Cu pattern without a seed layer. After photoresist

stripping, surface finish was applied.

Surface finish on both wafers and substrates was plated by

Atotech Germany. The Au and Pd layer thicknesses were

designed to evaluate the effect of the interconnection stack on:

a) bonding pressure reduction while maintaining a 3µm bump

collapse to compensate for non-coplanarities, and b)

metallurgical bonding, confirmed by detailed interfacial

characterization, and compared to the ENIG control reference.

For that purpose, the various compositions of EPAG reported

in Table 2 were considered. The as-plated Si die with the

EPAG4 surface finish can be observed in Fig. 8. An XRF

measurement of the EPAG4 surface finish on the dicing mark

of the wafer indicates an average thickness of 282nm of Au and

124nm of Pd, which correlate reasonably with the assumptions

from the model.

Table 2. EPAG surface finish Pd-Au thickness legs for Si die

and FR4 substrate

Si Die

Leg Pd (nm) Au (nm)

Wleg1 100 -

Wleg2 100 250

Substrate

EPAG1 100 -

EPAG2 50 50

EPAG3 100 100

EPAG4 100 250

Fig. 8. Optical micrographs of as-plated Si die with EPAG4

surface finish.

3.(c)Experimental Model Validation

Initial compression tests were carried out on the Si dies

with EPAG surface finish to understand the evolution of the

bump height with changes in bonding pressure. It can be seen

from Fig. 9 that the as-plated bumps had rounded tips and

showed collapse of the initial dome shape to give complete

1863

planarization at 120MPa (64N) and 300MPa (161N)

respectively.

Fig. 9. Confocal images of Cu bumps with EPAG finish as-

plated showing a rounded-tip shape, and after compression at

300MPa and 120MPa, showing complete bump planarization,

and partial planarization with slightly reduced contact area,

respectively.

Further, assembly was performed by thermocompression

bonding using a semi-automatic Finetech Fineplacer Matrix

flip-chip bonder with a placement accuracy of ±3µm.

Thermocompression bonding was carried out at 190oC and

120MPa, applied for 3s, to form Cu-EPAG interconnections.

Electrical measurements of the daisy-chain resistances

confirmed perfect electrical yield. Optical images of the cross-

section of an assembly formed in such conditions are presented

in Fig. 10. Flattening of the initially rounded bump is observed,

as well as a slight deviation of the Cu pad on substrate side, as

predicted by the model.

Fig. 10. Cross-sections of Cu interconnections formed by

thermocompression bonding at 120MPa – 3s – 190oC.

Conclusions

This paper models, designs and demonstrates, for the first

time, low-pressure, low-temperature all-Cu interconnections

without solders at multi-fine pitch. With previous studies on

Cu-Cu interconnections using standard ENIG surface finish,

metallurgical bonding is achieved with high bonding pressures.

In order to reduce the bonding pressure, innovations in the

areas of surface finish are explored. The bonding mechanism

and deformation behavior at the interface during

thermocompression bonding was studied using finite element

models. The novel EPAG surface finish without a thick Ni(P)

barrier layer showed 3X reduction of bonding pressure

compared to ENIG. For the same bonding pressure, the total

bump and pad deformation increased with a marginal increase

in Au thickness from 100nm to 250nm. This is due to the low

yield strength of Au, giving higher plasticity at the interface.

The distribution of deformation between bumps and pads

however raises concerns for low-K dielectric failures. To

address this fundamental challenge, novel interconnection

concepts are required, where the pressure-induced deformation

is completely confined within the bump, while still providing a

sufficient driving force to enable metallurgical bonding of the

mated interfaces. Manufacturable solutions to this technical

challenge at bonding pressures below 50MPa are currently

under investigation by GT PRC.

In summary, a new breakthrough in all-Cu interconnection

design, material and manufacturable assembly process

technology is presented, with reduced bonding pressures for

scalability to 20µm pitch and below.

Acknowledgements

This study was supported by the Interconnections and

Assembly industry program at Georgia Tech PRC. The authors

are grateful to the industry sponsors and mentors for their

funding and technical guidance. The authors would like to

thank Atotech GmbH, especially Mrs. Maja Tomic and her

team, for surface finish plating.

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