effects of bonding parameters on the reliability performance of anisotropic conductive adhesive

Effects of bonding parameters on the reliability performance ofanisotropic conductive adhesive interconnects for

flip-chip-on-flex packages assemblyI. Different bonding temperature

Y.C. Chan *, D.Y. Luk

Department of Electronic Engineering, City University of Hong Kong, 83 Tat Chee Avenue, Kowloon Tong, Hong Kong

Received 11 February 2002; received in revised form 27 March 2002

Abstract

The effects of different bonding temperatures during flip-chip-on-flex (FCOF) assembly in relation to the perfor-

mance of anisotropic conductive adhesive (ACF) interconnect were investigated. Two types of flip chips were used in

this study. It was found that Ni bumps formed better interconnections than bumpless FCOF packages. Aluminium

oxide was observed and was thought to be the main cause of the increased in contact resistance after the moisture-soak

tests. The conductive particles were not fully compressed by the bumps and pads and gaps were observed between the

conductive particles and Cu pads in bumpless packages. Conductive particles in the Ni bump FCOF packages were

tightly trapped between the bumps and pads and hence gave better connections. The performance of the ACF inter-

connects were affected by the degree of curing of the ACF, which was determined by the bonding temperature.

� 2002 Published by Elsevier Science Ltd.

1. Introduction

Before the invention of anisotropic conductive ad-

hesives (ACFs), solder alloys were used as interconnec-

tion materials in flip chip packages. These packages were

bulky, hard to work with and the lengthy assembly

processes were complicated and required high tempera-

tures. In addition, conventional lead–tin soldering used

in flip chip interconnections is incompatible with ex-

tremely fine pitch interconnection and is undesirable due

to the toxic effects of lead. ACFs possess many distinct

advantages that solder alloys can not offer, namely being

flexible, capable of fine pitch interconnections, envi-

ronmental friendly, and cheaper to manufacture as the

assembly processes are simpler, shorter and with lower

temperatures. Hence, many chip on flex electronics

packages involve interconnect applications using ACFs,

for example, mobile phones, personal digital assistants

[1] and smart cards [2].

Despite the advantages mentioned above, there are

two major drawbacks. The contact resistance of ACF

joints becomes increasingly unstable through time, par-

ticularly under high temperature (85 �C) and high hu-

midity (85% RH) conditions (so called 85/85 conditions)

[1]. This is such an important issue because these con-

ditions are well known as the qualification standards

throughout the electronic industry. Mechanisms that

thought to affect the stability of contact resistance in-

clude water absorption, electrochemical corrosion and

metal oxidation [3]. These degradation mechanisms in-

terfere with the contact resistance of ACF joints and

hence limiting the performance of electronic packages.

In order to solve this problem, one must have a clear

understanding of how exactly does the failure mecha-

nisms occur. In addition, the ACF is unable to self-align

[4] and hence high precision bonding processes are

required.

Microelectronics Reliability 42 (2002) 1185–1194

www.elsevier.com/locate/microrel

* Corresponding author. Tel.: +852-2788-7130; fax: +852-

2788-7579.

E-mail address: [email protected] (Y.C. Chan).

0026-2714/02/$ - see front matter � 2002 Published by Elsevier Science Ltd.

PII: S0026-2714 (02 )00079-3

mail to: [email protected]

ACFs consist of mixtures of conducting fillers in an

insulating matrix. This arrangement allows the material

to conduct in the z-direction while remaining insulators

in the x–y plane [3]. The aim of these adhesives is to trap

at least one conductive particle between the conductive

bumps on the flip chip and the corresponding pads on

the substrate. This has to be achieved without the oc-

currence of bridging between the pads. The particles are

randomly distributed in the matrix in most anisotropic

materials, which can cause problems especially in ultra-

fine pitch applications. This is because the concentration

of particles within the material varies at different loca-

tions, and hence may result in open or short circuit.

Epoxy resin based ACFs are thermosetting [2]. They

are temperature sensitive therefore their structure is

highly dependent upon the bonding temperature chosen.

The mobility of the conductive particles is different at

different stages during ACF curing. During the bonding

process of flip-chip-on-flex (FCOF) assembly, the ACF

is being cured and becomes soft and rubbery. This

transformation allows the ACF to flow, which in turn

allows the conductive particles within to move and dis-

tribute themselves evenly throughout the ACF joints.

When the curing process is completed, the ACF becomes

hardened and the mobility of the conductive particles

is lost. A reliable interconnect should have sufficient

amount of conductive particles between the bump and

pad in close contact and that they do not flow away

during bonding [5]. The root cause of the instability of

contact resistance maybe due to the incorrect selection

of bonding temperature during the assembly of FCOF

packages.

This series of studies concentrate on the effects of

different bonding parameters during the assembly of

FCOF packages in relation to the reliability of the ACF

joints. The aim of this study was to investigate the effects

of different bonding temperatures on the contact resis-

tance of ACF joints, with special focus on the chip/

conductive particle metallization interface. The results

of this study would allow development of ACF joints

using fine pitch flip chips on flexible substrates with

better reliability and longer fatigue life.

2. Experimental procedure

The FCOF packages are made up of three different

materials, namely silicon (Si) chip, ACF and flexible

substrate.

2.1. Silicon chips

The dimensions of the silicon (Si) chips are

10:87 mm� 3:14 mm, with rectangular bumps (70 lm�50 lm). The bumps are arranged in sets of five as a

group; with two adjacent bumps for measuring insula-

Fig. 1. Schematic diagram showing a corner of a Si chip with daisy-chained bumps.

Fig. 2. Schematic diagram showing the structure of the ACF and their conductive particles.

1186 Y.C. Chan, D.Y. Luk / Microelectronics Reliability 42 (2002) 1185–1194

tion resistance and three for contact resistance. There

are a total of 12 sets of these daisy-chained bumps

within the chip. The layout of the chip is shown in Fig. 1.

Electroless nickel (Ni) bumping process involves al-

uminium (Al) cleaning, Al activation, electroless Ni de-

position and immersion gold (Au) coating [3]. The bump

height of the Ni bump and Al pad are 4 and 1 lm, re-

spectively. The last two steps in the bumping process of

bumpless chips were omitted.

2.2. Anisotropic conductive adhesives

The type of ACF used in this study was a double

layer ACF that consists of an epoxy layer and another

one filled with conductive and insulation particles. The

conductive particles are made up of polymers plated

with a thin layer of nickel followed by a thin layer of

gold. Fig. 2 shows the structure of the ACF and its

specifications are summarized in Table 1.

2.3. Flexible substrate

The flex substrates used in this study were about 40

lm thick and the electrode is gold/electroless nickel

coated copper (Au/Ni/Cu). Twelve micrometer thick of

copper (Cu) traces was electrodeposited onto a 25 lmthick polyimide (PI), followed by 4–5 lm thick of elec-

troless nickel (Ni) and finally sputtered with a 0.4 lmthick gold (Au) layer. Since the flex substrate is of ultra-

fine pitch (the smallest gap between the traces was 10

lm), Ni was plated onto the Cu traces to prevent Cu

migration. Au sputtering was necessary to prevent the

Ni layer from oxidation.

During the pre-bonding process, the ACF was lami-

nated onto the flexible substrates, by using the Karl Suss

manual flip chip bonder. The final bonding of flip chip

onto the ACF/flex was carried out using the Toray semi-

automatic flip chip bonder. The alignment accuracy is

�2 lm. Different bonding temperatures were used in this

study, as shown in Table 2 and the schematics of the

bonding process is shown in Fig. 3.

Table 1

Specifications of the ACF

Description Specification

Film thickness (lm) 30

Conductive particle Au/Ni coated polymer

Insulation coated No

Particle size (lm) 3

Pre-bonding temperature

(�C)110

Pre-bonding time (s) 5

Pre-bonding force (MPa)

per unit area of bump

10

Bonding temperature (�C) 180

Bonding time (s) 10

Bonding force (MPa) per

unit area of bump

100

Tg (�C) 145

Table 2

Bonding temperatures used

Bonding temperature (�C)

Standard 180

Tests 160, 200, 220, 240

Fig. 3. Schematic diagram showing the formation of flip chip interconnections with (a) bumped chip and (b) bumpless chip using

ACFs.

Y.C. Chan, D.Y. Luk / Microelectronics Reliability 42 (2002) 1185–1194 1187

The contact resistance of the ACF joints of the

FCOF packages was measured by using the four-point

probe method as shown in Fig. 4.

In the four-point probe test, 1 mA was applied to the

circuit constantly and the voltage was measured for each

set of bumps using the Hewlett Packard 3478 A Multi-

meter. The contact resistance was calculated by using

R ¼ V =I .One set of samples was stored under ‘‘dry’’ conditions

(20 �C/30% RH) and another set of samples was mois-

ture-soaked under 60 �C/95% RH conditions for 336 h.

The samples were then mounted in epoxy resin and

cross-sectioned. The Philips XL40 FEG scanning elec-

tron microscope (SEM) equipped with energy dispersive

X-ray (EDX) was used to inspect and analyse the

microstructure and microjoints of the FCOF packages,

especially the chip/conductive particle metallization in-

terface.

To simulate the curing reaction of ACF during

FCOF assembly, the ACF was laminated onto the flex

substrate followed by curing for 10 s at the temperatures

selected as shown in Table 2. The degree of curing of

ACF was measured by using the Perkin–Elmer spectrum

one Fourier transform infra-red (FT-IR) Spectrometer

as demonstrated by Chiu et al. [6].

3. Results and discussion

From Fig. 5, one can see that the contact resistance

for both Ni bump and bumpless (flip chips with Al pads

rather than Ni bumps) FCOF packages show the same

trend. The contact resistance of the packages bonded at

160–200 �C decreased steadily but then increased when

the bonding temperature used was above 200 �C. FCOF

packages assembled at 200 �C gave the best contact

when compared to those assembled at a different tem-

perature. For the Ni bump and bumpless FCOF pack-

ages assembled at 200 �C, the initial contact resistance

was 2.77 and 4.18 mX, respectively. After 336 h of ‘‘dry’’

storage, the contact resistance dropped only slightly to

2.56 and 4.06 mX, respectively. After 336 h of moisture

absorption, the contact resistance increased slightly to

2.88 and 4.93 mX, respectively.

Fig. 5 shows that bumpless FCOF packages gave

higher contact resistance values than Ni bump packages,

especially with bonding temperatures above 200 �C, thecontact resistance doubled. This may be due to the heat

being absorbed by the FCOF packages and acting as a

catalyst for oxidation of the Al pads. Humid environ-

ments favour oxidation and the increase in contact

Fig. 4. Contact resistance measurement of ACF joints using the

four-point probe method (I ¼ 1 mA).

Fig. 5. Average contact resistance of Ni bump and bumpless (b/less) FCOF packages assembled at various temperatures with different

storage conditions.


Fig. 6. Schematics showing the path of an electron flowing through the (a) non-oxidised and (b) oxidised Al pad.

Fig. 7. SEM micrograph showing the ACF interconnect chose for EDX analysis at the (a) middle of the Ni bump and (b) Ni bump/

ACF interface.

Fig. 8. SEM micrograph showing the ACF interconnect chose for EDX analysis at the (c) middle of the Al pad and (d) Al pad/ACF

interface.


resistance with FCOF packages after 60/95 treatment

was evident, as shown in Fig. 5. Materials like alumin-

ium oxidises very easily and the aluminium oxide layer

formed may act as a barrier between the Al pad-con-

ductive particle and conductive particle-Cu pad, making

it difficult for the electrons to flow through this Al pad-

conductive particle-Cu pad path. This phenomenon is

shown in Fig. 6.

The SEM-EDX results showed that the oxygen con-

tent in bumpless packages was much higher than those

with Ni bumps. Referring to Fig. 7, there was no oxygen

detectable at point (a), the middle of the Ni bump, butFig. 9. Curing percentage of ACF at various temperatures

(curing time ¼ 10 s).

Fig. 10. Optical micrographs showing the morphology of ACF (a) before curing and after curing for 10 s at (b) 160 �C (c) 180 �C(d) 200 �C (e) 220 �C and (f) 240 �C (magnification ¼ �200).


11.5 wt.% oxygen was present at point (b), the Ni bump/

ACF interface. The oxygen is suspected to come from

the ACF rather than the thin layer of Au on the Ni

bump being oxidised, as gold is inert to oxidation.

Comparing these values to the results obtained from the

locations shown in Fig. 8, point (c), the middle of the Al

pad, contained 22.4 wt.% of oxygen and 49.3 wt.% at

point (d), the Al pad/ACF interface. In addition, from

the SEM micrograph shown in Fig. 8, it is clear that the

Al pad consists of two layers; one being the Al pad itself

and the other is suspected to be the aluminium oxide

layer.

The degree of curing of ACF plays an important role

in determining the reliability of the ACF joints. ACFs

are thermosetting polymers, which deform and degrade

easily at high temperatures. The degree of curing of

ACF is very much dependent upon the bonding tem-

perature. Previous study shows that 80% curing of ACF

would be achieved for ACF prepared at 180 �C for 10 s

[1,6]. From the curing percentage of ACF results shown

in Fig. 9, we found that the ACF was about 74% and

83% cured at 180 and 200 �C for 10 s, respectively. When

the bonding temperature was below 160 �C, the ACF

was only 26% cured. The cross-linkage within the

Fig. 11. SEM micrographs showing the morphology of ACF (a) before curing and after curing for 10 s at (b) 160 �C (c) 180 �C(d) 200 �C (e) 220 �C and (f) 240 �C.


polymer may be incomplete. In contrast, when the

bonding temperature was above 240 �C, the ACF was

about 95% cured, which would not be desired to use as a

bonding temperature. This is because the epoxy may set

too quickly without sufficient flowing at this tempera-

ture and hence the conductive particles would not have

enough time to distribute themselves in between the

bumps and pads in close contact.

The glass transition temperature, Tg, of the ACF used

is 145 �C. When this temperature is reached, the ACF

softens and begins to flow. It becomes hardened when

the curing reaction is completed [5]. This is because as

the curing of ACF proceeds, the linear polymer chain in

the epoxy resin grows and branches to form cross-links

[5]. The polymer chain is no stronger than its weakest

link, and the temperature of initial degradation is usu-

ally the temperature at which the least thermally stable

bonds fail. The bulk of the polymer may be stable, but

the failure of the weakest bonds often produces results

such as discolouration [7]. When the polymer has

reached its failure point, it will decompose and its

physical integrity would be lost. These effects were ob-

served in the ACF being cured at different temperatures

for 10 s, as shown in Fig. 10––as the temperature in-

Fig. 12. SEM micrograph showing the conductive particles trapped within the ACF joint of Ni bump FCOF assembled at 200 �C.

Fig. 13. SEM micrograph showing the conductive particles unable to distribute within the ACF joint of Ni bump FCOF assembled at

240 �C.


creased, the epoxy started to break and eventually the

epoxy layer was degraded into lumps.

The appearance of the ACF being cured at 160 and

180 �C, Fig. 10(b) and (c) respectively, was similar to the

uncured ACF, Fig. 10(a)––no significant break down of

the ACF was observed. When the curing temperature of

ACF was at 200 �C, the linear polymer chains within

started to grow and its physical appearance began to

change as shown in Fig. 10(d). At 220 �C, the growing

polymer chain branched out to form cross-links, Fig.

10(e), until the chemical bonds within the cross-links

extended to their maximum and broke and lumps were

observed as shown in Fig. 10(f). This change in mor-

phology was examined by using the SEM and the mi-

crographs are shown in Fig. 11.

During the bonding process of FCOF assembly, the

ACF is being cured and becomes soft and rubbery. This

transformation allows the ACF to flow, which in turn

allows the conductive particles within to move and dis-

tribute themselves evenly throughout the ACF joints.

When the curing process is completed, the ACF becomes

hardened and the mobility of the conductive particles is

lost. When the curing temperature was at 160 �C, theconductive particles carried within the epoxy layer were

able to move but only in a slow rate. At a higher tem-

perature, 180 �C, the particles gained more energy and

Fig. 14. SEM micrograph showing the conductive particles being compressed by the Ni bump and Cu pad leaving no gaps.

Fig. 15. SEM micrograph showing the conductive particles not being fully compressed by the Al pad and Cu pad leaving small gaps.


hence became more mobile and were able to move

slightly faster. At 200 �C, the ACF softened and was

flowing at the correct rate. At this temperature, the

conductive particles within the epoxy layer were able to

distribute themselves throughout the ACF joints, and

hence creating the best contact between the chip bumps

and substrate Cu pads, as shown in Fig. 12. A reliable

interconnect should have enough conductive particles

within the ACF joint and that they do not flow away

during bonding [5]. However, when the bonding tem-

perature was above 200 �C, the ACF was cured and

set too fast preventing the conductive particles from

spreading evenly between the bumps and pads, therefore

not creating good interconnections as shown in Fig. 13.

When compared to the Ni bump FCOF packages, the

bumpless ones gave a higher contact resistance owing to

the way the conductive particles within are trapped. As

shown in Fig. 14, the conductive particles are being

compressed slightly and trapped within the bumps and

pads. There was hardly any space in between the con-

ductive particles and bump or pad. This combination

gave a better contact and hence easier for the electrons

to flow through. However, in the bumpless FCOF

packages, the conductive particles are not being fully

compressed by the bumps and pads, and hence leaving a

gap in between, as shown in Fig. 15. This combination

may impede the flow of electrons through the intercon-

nects therefore bumpless FCOF gave a higher contact

resistance.

4. Conclusions

Most materials increase in resistance with tempera-

ture, since the higher the temperature the more vigor-

ously the atoms vibrate, so the more they hinder the

passage of drifting electrons. It was found that there was

only a gradual increase in contact resistance after the

FCOF packages were being moisture soaked. Since the

order of magnitude in contact resistance did not change,

one would consider the packages were reliable.

In this study, the optimum temperature for bonding

FCOF with ACF was concluded to be at 200 �C. Thebonding temperature determines the degree of curing of

the ACF. At temperatures below 200 �C, the degree

of curing was <80% and the flow of ACF provided

the conductive particles sufficient mobility to distribute

themselves evenly between the bumps and pads. In

contrast, if the bonding temperature was >200 �C, 95%curing of the ACF is achieved and the ACF would set

too quickly before the conductive particles have a

chance to locate themselves throughout the intercon-

nects. Hence, the ACF joints of the FCOF packages

assembled at 200 �C performed better with lower contact

resistance values when compared to those assembled at

different temperatures, especially at temperatures above

200 �C. It is therefore concluded that the performance

of the ACF interconnects is greatly influenced by the

bonding temperature during the assembly of FCOF

packages.

In addition, it was found that the conductive particles

were trapped tightly between the Ni bumps and Cu pads

in the Ni bump FCOF packages. However, the con-

ductive particles within the bumpless FCOF packages

were not fully compressed between the Al and Cu pads

and hence leaving small gaps. This finding was thought

to be another factor that caused the ACF interconnects

in bumpless FCOF packages to be less effective and may

be influenced by the bonding pressure. Therefore, to

fully understand the performance and reliability of the

ACF interconnects, other bonding parameters should

also be considered.

Acknowledgement

The authors would like to acknowledge the Strate-

gic Research Grants (project no. 7001080) of the City

University of Hong Kong.

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