Curing and Bonding Behaviors of Anisotropic Conductive Films (ACFs) by Ultrasonic Vibration for Flip Chip Interconnection

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Curing and Bonding Behaviors of Anisotropic Conductive Films (ACFs) by Ultrasonic Vibration for Flip Chip Interconnection

Ki Won Lee, Hyoung Joon Kim, Myung Jin Yim, and Kyung Wook Paik Dept. of Materials Science and Engineering, KAIST

373-1, Guseong-dong, Yuseong-gu, Daejeon, 305-701, Korea phone: +82-42-869-3386, fax:+82-42-869-3310, email: kiwonlee@kaist.ac.kr

Abstract

In this study, the curing and bonding behaviors of ACFs by ultrasonic vibration for flip chip interconnection were investigated using a 40 kHz ultrasonic bonder with longitudinal vibration.

In-situ temperature of the ACF layer during thermosonic (TS) bonding was measured to investigate the effects of substrate materials and substrate temperature. Curing of the ACFs by ultrasonic vibration was investigated by dynamic scanning calorimetry (DSC) analysis in comparison with isothermal cure. Die adhesion strength of TS bonded specimens was compared with that of thermo-compression (TC) bonded specimens.

The temperature of the ACF layer during TS bonding was significantly affected not by substrate temperature but by substrate materials. The temperature of the ACF layer increased up to 300℃ within 2 seconds on FR-4 substrates but 250℃ within 4 seconds on glass substrates at room temperature. ACFs were fully cured within 3 by ultrasonic vibration, because ACF temperature reached at 300℃ within 3 seconds. Die adhesion strengths of TS bonded specimens were as high as those of TC bonded specimens both on FR-4 and glass substrates.

As a summary, TS bonding of ACF will significantly reduce the ACF bonding times at several seconds, and also make the bonding possible at room temperature compared with tens of seconds bonding times and more than 180℃

bonding temperature of TC bonding.

1. Introduction

Anisotropic Conductive Films (ACFs) are well known adhesive interconnect materials consisting of conducting particles and adhesive polymer resins in a film format. And they have been widely used as interconnect materials in flat panel display applications such as Out Lead Bonding (OLB), flex to PCB bonding, chip-on-glass (COG), and chip-on-film (COF) for last decades [1], and also in flip chip semiconductor packaging applications [2]. ACF interconnects are simple and lead-free processing as well as cost effective packaging method.

For ACF interconnection, thermo-compression (TC) bonding is the most common method, however it is necessary to reduce the bonding temperature, time and pressure, because TS bonding is often limited by slow thermal cure, uneven cure degree of adhesive, large thermal deformation of the assembly and high bonding pressure for high I/O interconnection. Therefore, new ACF bonding method such as TS bonding is needed.

Thermosonic (TS) bonding for metal-to-metal interconnection is widely used in flip chip, TAB, or surface mount technologies due to its low cost, simplicity, and fast assembly time at low temperature with reduced bonding pressure [3]. TS bonding can be an attractive alternative of the ACF bonding process because it is generally known that polymer materials generate a large amount of heat under cyclic deformation due to its large loss modulus which leads to heat dissipation of energy. However, few studies have been performed on TS bonding using ACFs.

In this study, experimental investigations of the curing and bonding behaviors of ACFs by ultrasonic vibration are described. Longitudinal ultrasonic vibration was used to rapidly cure ACFs, and its curing characteristics were investigated. And the die adhesion strength of ACF joints after TS bonding was examined in comparison with that of TC bonding.

2. Experiments

2.1 Materials preparation

A Test Si chip had a dimension of 3 mm(w) X 3 mm(l) X 680um(t). FR-4 and glass with 1mm thickness were used as substrate materials. ACFs were epoxy based adhesive film with 45um thickness, and they contained Ni/Au coated polymer balls with 5um diameter as a conductive particle.

Table 1 summarizes the specifications of test specimens.

Materials Thickness

Si chip 3mm X 3mm Si test chip 680um Substrate 1 3cm X 3cm bare FR-4

substrate 1mm

Substrate 2 2.5cm X 2.5cm bare glass

substrate 1mm

Base film Epoxy based adhesive 45um ACF Conductive

particle NI/Au coated polymer balls 5um diameter Table 1. The specifications of test specimens.

2.2 Equipment

Figure 1 shows the ultrasonic bonder which consists of an ultrasonic transducer, a horn, a hot-plate and a load cell.

Longitudinal vibration frequency of ultrasonic bonder was 40 kHz. The output power of the ultrasonic bonder was 100W and its peak-to-peak amplitude was 10um. To prevent the damage of Si chips, such as cracking and fracture, a teflon cap of 500um thickness was applied at the end of the ultrasonic horn.

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Figure 1. An ultrasonic bonder with longitudinal vibration.

2.3 In-situ temperature of the ACF layer during TS bonding In-situ temperature of the ACF layer during TS bonding was measured to investigate the heating characteristic of the ACF during TS bonding. To investigate the effects of substrate temperature on the temperature of the ACF layer, FR-4 substrates were maintained at room temperature, 50℃, and 80℃. And glass substrates at room temperature were also used to investigate the effects of substrate materials on the temperature of the ACF layer. Figure 2 shows the in-situ ACF temperature measurement set-up.

Substrate Si chip

ACF Thermo

couples Thermometer

Ultrasonic vibration

ACF Thermo

couples Thermometer

Figure 2. A schematic diagram of in-situ ACF temperature measurement

K-type thermocouples and a thermometer with 125ms sampling period were used to measure the initial heating rate, the maximum temperature and the cooling rate of the ACF layer.

ACF Jig

Figure 3. A shematic diagram of the flip chip assembly structure using a jig for TS bonding.

The surfaces of FR-4 and glass substrates were engraved to place the 40um thick k-type thermocouples right beneath the ACF layer. ACFs were pre-bonded on the substrates at 80

℃, and test Si chips were ACF flip chip assembled using a jig to prevent misalignment of chips during TS bonding as shown in figure 3. Then, in-situ temperature of the ACF layer was monitored during TS bonding for every 125ms.

2.4 TS and isothermal cures of ACFs

Curing times of ACFs cured by TS bonding were measured by DSC analysis in comparison with those of ACFs under isothermal TS bonding conditions.

For isothermal TC bonding, ACFs were pre-bonded on 18um thick Al foils, and the pre-bonded ACFs were isothermally cured at 150℃, 230℃, and 300℃ on a hot-plate and rapidly cooled on an Al block at room temperature. The cured specimens were analyzed by DSC to calculate their degree of cure.

For TS bonding, release films were used between ACFs and substrates to easily detach the ACF layer from FR-4 substrates after TS bonding. These detached ACFs were analyzed by DSC. In both cases, DSC analysis was performed with a heating rate of 5℃/min.

2.5 Die adhesion strengths of ACFs after TS and TC bondings Die adhesion strength of ACFs after TS bonding was measured in comparison with that of TC bonding.

ACFs were pre-bonded on FR-4 and glass substrates at 80

℃, and test Si chips were aligned on substrates using a jig.

Then, pre-aligned chips on substrates shown in figure 3 were TS bonded for 0.3s, 0.6s, 1s, 2s, 3s, 5s and 7s, and their die shear strengths were measured.

Die shear test was performed with a shear rate of 3cm/min, and the maximum die shear strengths were measured as shown in figure 4.

2 1 .8

Kgf Load cell

Display

Substrate holder Shear direction Shear tip

TS bonded Si chip on substrates

2 1 .8

Kgf Load cell

Display

Substrate holder Shear direction Shear tip

TS bonded Si chip on substrates

Figure 4. A shematic diagram of die shear test

2.6 ACF thickness changes during TS bonding at room temperature

For the application of TS flip chip bonding at room temperature, ACF thickness should be effectively reduced to make stable contacts of conducting particles between chip bumps and substrate pads before full curing of ACF.

Therefore, the changes of ACF thickness during TS bonding

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at room temperature were measured by cross-sectioned scanning electron microscopy (SEM).

3. Results and discussion

3.1 In-situ temperature of the ACF layer during TS bonding Figure 5 shows the changes of the ACF temperature on FR-4 substrates during TS bonding at various substrate temperatures.

0 2 4 6 8 10 12 14

0 50 100 150 200 250 300 350 400

In-situ temperature of ACF ()

Bonding time (seconds) 20 50 80

Ultrasonic vibration Cooling

0 2 4 6 8 10 12 14

0 50 100 150 200 250 300 350 400

Bonding time (seconds) 20 50 80

Figure 5. In-situ temperatures of ACF layer on various FR-4 substrate temperatures during TS bonding.

During TS bonding at room temperature, the temperature of the ACF layer on FR-4 substrate reached to 300℃ within 2 seconds with an extremely high initial heating rate of 340℃/s.

This result shows that the ACF layer can be rapidly heated by ultrasonic vibration. In general, the temperatures of the ACF layer reached steady-states above 300℃ within 3 seconds, and were not significantly affected by substrate temperatures, because ACFs were heated not by heat conduction from substrates but by self heat generation by ultrasonic vibration.

0 2 4 6 8 10 12 14

0 50 100 150 200 250 300 350

Bonding time (seconds)

FR-4 Glass

0 2 4 6 8 10 12 14

0 50 100 150 200 250 300 350

Bonding time (seconds)

FR-4 Glass

Figure 6. In-situ temperatures of the ACF layer on FR-4 and glass substrates during TS bonding at room temperature.

Figure 6 shows in-situ temperatures of the ACF layer on FR-4 and glass substrates during TS bonding at room temperature.

The temperature of the ACF layer on glass substrates showed slower heating rate and lower maximum temperature than that on FR-4 substrates, and it reached a steady-state above 240℃ which is lower than 300℃ of FR-4. Generally, glass has a thermal conductivity of 1 W/mK while FR-4 has a value below 0.3 W/mK. Therefore, the heat energy of the ACF layer can easily be dissipated to surroundings through glass substrate due to the higher thermal conductivity of glass than that of FR-4. However, further study on the heat generation of various substrate materials during TS bonding is needed, because it can also affect the temperature of the ACF layer.

3.2 TS and isothermal cures of ACFs

Figure 7 shows the degrees of cure of isothermally cured ACFs at various temperatures.

0 5 10 15 20

0.0 0.2 0.4 0.6 0.8 1.0

Degree of Cure

Heating Time (sec) 300 230 150

Figure 7. Isothermal cures of ACFs at various curing temperatures.

As the temperature increased, curing time of ACFs significantly decreased. At 300℃, ACFs were fully cured within 3 seconds. In the previous result, the temperature of the ACF layer on FR-4 during TS bonding at room temperature reached to 300℃ within 2 seconds and showed a stead-state above 300℃. Therefore, rapid TS cure of ACFs by TS bonding can be explained by the temperature effect. To examine the degree of cure of ACFs by TS bonding, degree of cure of ACFs cured by TS bonding for 0, 1, and 3 seconds at room temperature were analyzed as shown in Figure 8.

The area of curing peak which represents the non-cured part of ACFs were significantly reduced, as the TS bonding times increased. And the curing peak was completely disappeared in the ACF cured by TS bonding for 3 seconds.

This result shows that the ACFs were fully cured within 3 seconds during TS bonding at room temperature. The curing behavior of the ACFs by TS bonding at room temperature was the same as that of ACF isothermally heated at 300℃.

Rapid curing of the ACFs by TS bonding can be explained

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because the temperature of the ACF layer reaches above 300

℃ after 2 seconds by ultrasonic vibration.

80 100 120 140

-0.5 0.0 0.5

Heat flow (arb. unit)

Temperature ( )

3s 1s 0s

Figure 8. DSC curing peaks of ACF cured by TS bonding for 0, 1, and 3 seconds.

The significant meaning of this result is we can do ACF flip chip bonding at room temperature rather than elevated temperature more than typical 180℃ bonding temperature.

And at the same time, the bonding times can be also significantly reduced to several seconds by TS bonding compared with tens of seconds bonding times of TC bonding.

3.3 Die adhesion strengths of ACFs after TS and TC bonding Figure 9 shows die adhesion strengths of TS bonded specimens using FR-4 and glass substrates at room temperature.

0 1 2 3 4 5 6 7

0 100 200 300 400 500 600

Die adhesion strength (Kgf/cm

2 )

Bonding time (seconds) Glass FR-4

T/C bonding, glass substrate

T/C bonding, FR-4 substrate

0 1 2 3 4 5 6 7

0 100 200 300 400 500 600

2 )

Bonding time (seconds) Glass FR-4

T/C bonding, glass substrate

Figure 9. Die adhesion strengths of TS bonded specimens on FR-4 and glass substrates.

Die adhesion strengths of TS bonded specimens using glass substrates were as high as those of TC bonded specimens. And those of TS bonded specimens using FR-4 substrates were about 80% of TC bonded specimens. In both cases, die adhesion strengths rapidly increased by curing of ACFs, and reached the maximum values within 5 seconds.

After the maximum values, die adhesion strengths slightly decreased despite the ACFs were fully cured. Considering that the temperature of the ACF layer on FR-4 substrates reached above 300℃ during TS bonding, decrease in die shear strength might be due to degradation of both FR-4 and ACFs. To measure the degradation temperatures of FR-4 and ACFs, DSC analysis was performed.

150 200 250 300 350 400

-60 -40 -20 0 20 40 60 80

Temperature ( )

ACF

Degradation

Curing

150 200 250 300 350 400

-60 -40 -20 0 20 40 60 80

Temperature ( )

ACF

Degradation

Curing

250 260 270 280 290 300 310 320 330 340

10 12 14 16 18 20 22 24 26 28 30

Temperature ( )

FR-4

Degradation

250 260 270 280 290 300 310 320 330 340

10 12 14 16 18 20 22 24 26 28 30

Temperature ( )

FR-4

Degradation

Figure 10. DSC degradation peaks of FR-4 substrate and ACF.

As shown in figure 10, FR-4 degraded above 290℃ which is earlier then ACFs, and ACFs were decomposed above 340

℃. And a few FR-4 substrates showed glass fibers exposed on the surface due to degradation of epoxy resin of PCB substrates after TS bonding for several seconds as shown in figure 11.

Therefore, decrease in die adhesion strengths especially at PCB substrates might be due to the degradation of epoxy resin of the substrates during TS bonding. And the decrease in die adhesion strengths at glass substrates might be due to the degradation of ACFs themselves.

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Figure 11. Surface degradation of FR-4 substrate after TS bonding.

To solve the decrease in die adhesion strength caused by FR-4 degradation, TS bonding using FR-4 substrates was performed by a pulse vibration mode. The pulse vibration mode was controlled by bonding times of 1 second and delay times of 0.5 second to lower the substrate temperature during TS bonding. For example, 3 second of pulse vibration consisted of 1 second bonding, 0.5 second delay, 1 second bonding, 0.5 second delay, and 1 second bonding to apply totally 3 second of bonding time. Figure 12 shows the result of pulse vibration.

0 1 2 3 4 5 6 7

0 50 100 150 200 250 300

2 )

Bonding time (seconds) Continuous TS bonding Pulse TS bonding

0 1 2 3 4 5 6 7

0 50 100 150 200 250 300

2 )

Bonding time (seconds) Continuous TS bonding Pulse TS bonding

Figure 12. Improved die adhesion strength by pulse vibration (FR-4) The result shows die adhesion strengths increased by pulse vibration. Therefore, it was clearly shown that pulse vibration could solve the PCB substrate degradation caused by the high temperature during TS bonding, and die adhesion strengths as high as those of TC bonding were successfully achieved by TS bonding.

3.4 ACF thickness measurement

Figure 13 shows the thickness changes of the ACF layer on FR-4 substrates during TS bonding at room temperature.

0 1 2 3 4 5

0 10 20 30 40 50 60

ACF thickness (µm)

Bonding time (seconds) ACF thickness

Figure 13. Thickness of the ACF layer on FR-4 substrate during TS bonding at room temperature.

Thickness of the ACF layer rapidly decreased to 8um within 1 second during TS bonding. Considering the heights of substrate pads and chip bumps, flip chip interconnections by TS bonding using ACFs is possible at room temperature, because the thickness of the ACF layer effectively decreased before the ACF is fully cured. However, further study of the effects of the pads and the bumps on ultrasonic vibration is needed to demonstrate the real bump contact by TS bonding.

4. Conclusion

The curing and bonding behaviors of ACFs by ultrasonic vibration were experimentally investigated using a 40 kHz ultrasonic bonder with a longitudinal vibration.

The temperature of the ACF layer during TS bonding was significantly affected not by substrate heating temperatures but by types of substrate materials. The temperatures of the ACF layer reached up to 300℃ within 2 seconds on FR-4 substrates, and 250℃ within 4 seconds on glass substrates maintained at room temperature. Lower temperature of the ACF layer on glass substrates was due to the higher thermal conductivity of glass than that of FR-4.

ACFs were fully cured within 3 seconds at 300 ℃ isothermal TC bonding and also within 3 seconds by TS bonding meaning that TS bonding quickly reached to 300℃

within 3 seconds. However, at TC bonding, 300℃ heating is not allowed because of damages of substrates and chips and also severe dimensional changes at high temperature. Rapid curing of the ACFs by TS bonding is presumably due to rapidly reaching to high temperature of the ACF layer above 300℃ after 2 seconds.

Die adhesion strengths rapidly increased by curing of the ACFs and showed the maximum values within 3 seconds on FR-4 substrates and within 5 seconds on glass substrates. The maximum die adhesion strength of TS bonded specimens using glass substrates was as high as that of TC bonded specimens. However, that of using FR-4 substrates was about 80% of TC bonded specimens despite the ACFs were fully cured because of degradation of FR-4 substrates. Pulse

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vibration could solve the FR-4 substrates degradation caused by the high temperature during TS bonding, and die adhesion strengths as high as that of TC bonding were achieved by pulse TS bonding.

As a summary, TS bonding of ACF will significantly reduce the ACF bonding times at several seconds, and also make the bonding possible at room temperature compared with tens of seconds bonding times and more than 180℃

bonding temperature of TC bonding.

5. References

1. I. Watanabe et al., “Packaging Technologies using Anisotropic Conductive Adhesive Films in FPDs”, Proc.

Asia Display/IDW, pp. 553~556, 2001

2. J. Liu, A. Tolvgard, J. Malmodin, and Z. Lai, “ A Reliable and Environmentally Friendly Packaging Technology-Flip Chip Joining Using Anisotropically Conductive Adhesive”, IEEE Trans. Comp. Packag., Manufact.

Technol., Vol. 22, No. 2, pp.186~190, 1999

3. P. Lawyer, D., Choudhury, M. Wetzel and D. Rensch,

“Thermosonic Bonding of High-power Semiconductor Devices for Integration with Planar Microstrip Circuitry”, Proc. Inter. Electron. Manuf. Technol. Sym., pp.390~393, 1998