EFFECTS OF THERMAL CYCLING PROFILES ON

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60 daisy-chained bump groups of 300 bumps that run ... and polished in order to obtain the cross sections of the bump-. Chip. ACF. Substrate. I. V. Cu trace. 856 ...
EFFECTS OF THERMAL CYCLING PROFILES ON THE PERFORMANCE OF CHIP-ONFLEX ASSEMBLY USING ANISOTROPIC CONDUCTIVE FILMS M. J. Rizvi a, C. Bailey a,*, Y. C. Chan b, H. Lu a a

School of Computing and Mathematical Sciences, University of Greenwich 30 Park Row, London SE10, 9LS, UK b

Department of Electronic Engineering City University of Hong Kong 83 Tat Chee Avenue, Kowloon, Hong Kong *Phone: +44 (0)20 8331 8660 *Fax: +44 (0)20 8331 8665 *Email: [email protected]

ABSTRACT Anisotropic Conductive Films (ACFs) are widely used in the electronic packaging industries because of their fine pitch potential and the assembly process is simpler compared to the soldering process. However, there are still unsolved issues in the volume productions using ACFs. The main reason is that the effects of many factors on the interconnects are not well understood. This work focuses on the performance of ACFbonded chip-on-flex assemblies subjected to a range of thermal cycling test conditions. Both experimental and threedimensional finite element computer modelling methods are used. It has been revealed that greater temperature ranges and longer dwell-times give rise to higher stresses in the ACF interconnects. Higher stresses are concentrated along the edges of the chip-ACF interfaces. In the experiments, the results show that higher temperature ranges and prolonged dwell times increase contact resistance values. Close examination of the microstructures along the bond-line through the Scanning Electron Microscope (SEM) indicates that cyclic thermal loads disjoint the conductive particles from the bump of the chip and/or pad of the substrate and this is thought to be related to the increase of the contact resistance value and the failure of the ACF joints.

compared to soldering not least because of the lowered bonding temperatures. In fact, ACFs are considered green elctronic materials as they are lead-free and non-toxic [5]. However, despite these advantages, the use of ACFs for volume productions are not as widely adopted as they should because ACF materials are very sensitive to environmental factors such as the temperature in every stages of the manufacturing and assembly processes [6]. ACFs consist of thermoset epoxy as the adhesive matrix and randomly dispersed conductive particles that are either of solid metal or of metal coated plastic particles.

Au -bump Entrapped Conductive Particle

Pad KEY WORDS: chip-on-flex (COF), anisotropic conductive film (ACF), finite element modelling, stress, conductive particle, contact resistance, life-time INTRODUCTION In recent years, flip chip assembly using anisotropic conductive films (ACFs) has been used to make electronic products thinner, smaller and lighter [1]. This technique not only gives good electrical performance but also shortens production time due to the simpler assembly process [2-4]. This technology is also more environmentally benign 0-7803-9524-7/06/$20.00/©2006 IEEE

Figure 1: Typical anisotropic conductive film joint. Unlike solder joints where metallurgical bonds are formed between solder and the under bump metalizations, ACF joints only mechanical contacts between the conductive particles and the bump of the chip or the pad of the substrate exist. During the bonding process, the contact spots at the bump-particle and

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particle-pad interfaces spread out as bonding pressure is applied and this creates an effective contact area through which electric conduction occurs. Both the bump and the pad exert clamping forces on the particles and the deformed conductive particle are trapped between them as shown in the Figure 1.

at both the high and the low temperatures. The test conditions are summarized in Table II. Table I: Bonding Parameters Parameters Pre-bonding Temperature (ºC) 90 Pressure (N) 10 Time (sec) 7

Since there is no metallurgical connection between the bump/pad and the particles, any relaxation of the clamping forces may induce a small gap and may dislodge the deformed particle. As a result, electric discontinuity of the bumpparticle-pad conducting path may occur. Therefore, it’s important to investigate the factors that degrade the ACF joint properties.

Case A B

In this study, a computer model of ACF-based chip-on-flex (COF) assembly is analyzed to observe the stress variations along the bond-line under different thermal cycling profiles. Based on the modelled stress distribution, the most critical sites of the COF assembly are identified and investigated further using the Scanning Electron Microscope (SEM). The effect of thermal cycling profiles on the electrical performance of ACF joints are experimentally investigated. The role of the thermal expansion coefficient (CTE) of the ACF have also been investigated with the help of computer modeling.

Table II: Thermal cycle parameters Max. T Dwell Time Min. T (minutes) (ºC) (ºC) -5 +105 6, 10, 16 -20 +125 6, 10, 16

Substrate

ACF

EXPERIMENTAL PROCEDURES In the present study, a 35 µm thick ACF consisted of randomely dispersed Au-Ni coated plastic particles (3.5 µm in diameter) is used to mount the flip chip on a 41 µm thick polyimide flexible substrate to form the COF assembly. Chips with two types of bumps are used in the experiments: Au bumps and Au-Ni bumps. The dimensions of the flip chips with both bumps are 10 mm x 3 mm x 1 mm with a total of 368 square (50 µm x 50 µm) bumps on each chip. There are 60 daisy-chained bump groups of 300 bumps that run parallelly along the length of the chip. The other 68 bumps are supporting bumps and they are located along the width of the chip. The heights of the flip chip bumps and substrate pads are 18 µm and 12 µm respectively. Prior to the final bonding, ACF is removed from the refrigerator and allowed to warm up to room temperature. Then the thin transparent protective polymer layer was peeled off and ACF is placed on the flex substrate. The pre-bonding is carried out using the Karl Suss manual bonder. The process parameters are listed in Table I. After that, an organic solvent (acetones) is used to clean the chip and the recognition marks on the substrate to eliminate any foreign particles and to ensure good alignment. Finally, the test chips are mounted on the pre-bonded substrates using Toray (FC2000) Flip Chip Bonder. The final bonding conditions are also shown in Table I. The ACF bonded Au-Ni bump and Au-bump flip chip assemblies shown in Figure 2 are then subjected to the thermal cycling tests. The total cycle duration is 60 minutes and the dwell times (the total holding duration of the samples at high and low temperatures of each cycle) are 6, 10 and 16 minutes

Final-bonding 180 80 10

Chip

Figure 2: Typical flip chip assembly with ACF.

Cu trace

I V Figure 3: Circuitry of contact resistance measurement using four-point probe method. The contact resistances for all samples are measured before and after thermal cycling tests using the four point probe method as depicted in Figure 3. During the measurement, a 1 mA current (I) is applied to the circuit and the voltage (V) is measured using a Hewlett Packard 3478A Multimeter for each bump group, and the contact resistance, R, is calculated using the Ohm’s law. After that, the samples are mounted with resin and cured at room temperature. It is then mechanically ground and polished in order to obtain the cross sections of the bump-

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particle-pad interfaces. Finally, the cross-sectioned samples are gold-coated and examined through a Philips XL 40 FEG scanning electron microscope (SEM) equipped with an energy dispersive X-ray spectrometer (EDX). COMPUTER MODELLING A computer model of the COF assembly similar to Figure 2 is built as shown in Figure 4. The model consists of a 10 mm x 3 mm x 1 mm flip chip mounted on a 48 mm x 40 mm x 41 µm flex substrate using a 35 µm ACF. Due to the symmetry of the geometry, only one quarter of the whole assembly is modelled. Since the flex substrate is very thin compared to the chip, the size of the flex substrate is reduced in the model because it will not affect the results significantly. The multiphysics modeling software PHYSICA [7] is used to analyze the model. The details of the simulations that have been carried out are listed in Table II. The thermal cycle profiles with different dwell times for the modelling as well as the experiments are shown in the Figure 5. The material properties used in the simulation are listed in Table III.

Chip

ACF

Flex-substrate

Figure 4: Computer Grid of the flip chip assembly.

160

160

i

140

Max.T

120

120

100

100

80

80

0

Temperature ( C)

0

Temperature ( C)

140

60 40 20 0

40 20 0 -20

-40

-40

1 Cycle 0

20

Min. T

-60

40

60

80

0

20

Time (minutes) 160 140

RESULTS AND DISCUSSIONS Figure 6 represents the modelled stress Syy distribution pattern in the COF assembly for case A at the end of the first temperature rise while the whole assembly is exposed to the highest temperature. Stress concentrations are observed along the edge sides of the chip-ACF interfaces. Since the CTE for ACF is much higher than both the chip and the flex substrate, ACF will try to expand more than that of chip and flex substrate. Moreover, higher modulus (E) of the chip will resist the expansion between the chip-ACF interfaces. The substrate is flexible, but due to its lower CTE it will still resist the expansion of the ACF. Therefore, there is a strong CTE mismatch among the chip, ACF and substrates. As it has been stated, ACFs are mainly made of thermosetting epoxy-based polymeric materials, and the properties of these materials are very sensitive to the temperature. As the heat is applied, ACFs melt and become viscous fluid during the bonding process and three-dimensional cross-linkage happens and the ACFs harden. After this curing process, if the assembly is exposed to even higher temperatures, the ACFs may swell and as a result the clamping force provided by the bump of the chip and pad of the substrate may no longer exist. This loss of the clamping force damages the contact between the entrapped conductive particles and the bump/pad. On the other hand, when the thermal cycle temperature changes from the upper to the lower bound, the whole assembly is subjected to a rapid cooling. During the dwell time at the lower temperature the CTE mismatch also causes stress in the assembly. It is the cyclic nature of the stress which will lead to the weakening of the bond-line.

60

-20

-60

ii

Table III: Material properties used in the simulation Materials Modulus, Poisson’s CTE E (MPa) ratio ν α (ppm/K) Chip 131,700 0.3 2.7 Flex substrate 4000 0.3 20 ACF 1450 0.4 133

40

60

80

Time (minutes)

iii

120

80

0

Temperature ( C)

100

60 40 20 0

Higher stresses

-20 -40

Dwell Time

-60 0

20

40

60

80

Time (minutes)

Figure 5: Thermal cycle profiles for dwell time (i) 6 min (ii) 10 min and (iii) 16 min.

Figure 6: Stress ( Syy) distribution pattern in the flip chip assembly for case A after 20 minutes of thermal cycle (dwell time =10 min). Figure 7 shows the vertical displacement of ACF for case A and B at various time steps during a load cycle. It is evident 857

that under the Case B (-20, +125) conditions the displacement is larger than Case A (-5, +105). It is also clear that in response to the temperature change, the COF assembly deforms periodically. It is the cyclic deformation and the cyclic stresses that dislodge the conductive particles from the bump and the pad. This failure mode could explain the appearance of the gap between the pad and the conducting particle as shown in the SEM image in Figure 8. The existence of the conduction path of an ACF joint is dependent on the metallic contacts between conductive particles and bumps/pads. The greater the contact area the less the contact resistance [8, 9]. In one of our previous studies [10] it was found that the expansion of ACF materials can dislodge conductive particles, reduce the contact areas and increase the contact resistance. The computer modelling results are consistent with those findings.

CTE mismatch among the materials of the conductive particle is expected. This is consistent with experimental observations as shown in the Figure 10. This figure shows that the outermost metallic layer has broken as the core material expands and pushes the metal coatings. This type of disruption increases the contact resistance values and lowers the electrical performance.

Ni-layer

Au-layer

Plastic Particle (Resin)

Vertical Displacement of ACF (m)

0.0000040 0.0000035

Case A (-5, +105) Case B (-20, +125)

0.0000030 0.0000025

Figure 9: A conductive particle of anisotropic conductive film.

0.0000020 0.0000015 0.0000010

Bump

0.0000005 0.0000000 500

1000

1500

2000

2500

3000

3500

4000

4500

Time (sec)

Figure 7: Vertical displacement of ACF for case A and B (dwell time = 10 min).

Bump

Pad

Pad

Conductive Particle

Uneven expansion of particle materials due to local mismatch

Figure 10: Disruption of metallic layer due to uneven expansion during the thermal cycling tests.

Discontinuity of Conduction path

Figure 8: SEM image showing the discontinuity of conduction path due to thermal mismatch. In this study, a conductive particle in the ACF consists of a plastic sphere coated with a 0.15 µm layer of Ni and a 0.05µm layer of Au. The structure of the particle is shown in Figure 9. Therefore, the conductive particle has a structure that has materials with dissimilar properties. The CTE values of the plastic sphere, the Ni-layer and the Au-layer are 70 ppm/K, 13.14 ppm/K and 14.2 ppm/K respecively. As a result, a local

Figure 11 shows the contact resistance values of the samples before and after the thermal cycling test. Both Au-Ni bumped and Au bumped COF assemblies have shown similar trends in the increase of the contact resistance. The increase in the contact resistance is thought to be caused by the damage suffered by the conduction path as described earlier. The contact resistance values are much higher for the samples subjected to thermal cycle with a temperature range of –20 to +125 0C (case B) than that with a temperature range of –5 to +105 0C (case A). This is expected because the samples subjected to thermal cycling under case B experienced greater expansion and contraction. Figure 12 shows the modeled stresses in the ACF when the COF is heated from the low temperature to the high temperature dwell region. Here, case A shows lower stress values compared to case B. This indicates that there is a correlation between the maximum stress levels and the increases in the contact resistance observed in the experiments and COF assemblies subjected to

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Contact Resistance (mohm)

thermal cycling condition with wide temperature ranges would experience early failures.

Contact Resistance (mohm)

28

Au-Ni Bump Au Bump

26

Cycling Range -20 to +125 deg C

24 22 20

Cycling Range -5 to +105 deg C

25

20

15

10

5

0

18 16

Initial Values

After 100 Cycles

Au-Ni Bump Au-Bump

6

10

Dwell Time (minute)

16

Figure 13: Contact resistance for case A after 100 cycles (initial contact resistance for Au-Ni and Au bumps are 16.68 m and 18.35 m respectively).

After 100 Cycles

Figure 11: Comparison in contact resistance values for two cases (Dwell time = 10 min).

Compressive

Contact Resistance (mohm)

35

Tensile

Case B

Case A

-10

-8

-6

-4

-2

0

2

4

6

30

Au-Ni Bump Au-Bump

25 20 15 10 5 0

6

10

Dwell Time (minute)

16

Figure 14: Contact resistance for case B after 100 cycles (initial contact resistance for Au-Ni and Au bumps are 16.52 m and 18.29 m respectively).

8

Stress, Syy (MPa)

Based on the above discussions, it is clear that the temperature range and the dwell time affect the ACF-joint life-time. The number of cycles to failure of ACF-joint can also be estimated using the equation [11]

Figure 12: Stress, Syy in the ACF (at the beginning of higher dwell region) for samples subjected to transfer from low to high temperature (dwell time 10 min). The effects of the thermal cycle dwell time on the contact resistance have also been studied and the results are shown in Figure 13 and 14 for case A and case B respectively. Both figures show that dwell time has a great impact on the ACFjoint performance. The contact resistance increases with the increase in the dwell time regardless of the bump materials. One of the possible mechanisms of this phenomenon is that a longer dwell time allows the assembly to expand (at higher temperature zone) and to contract (at the lower temperature zone) for longer time so that the polymeric chains may suffer more damages and this weakens the adhesion strength. This weakening may leads to the increase in contact resistance.

Νf =

Α  τ exp10  τ adh

  

f 0.13

where Nƒ is the number of cycles to failure of ACF-chip joint, A is a positive constant, τ is the peak shear stress across the bond area, τadh is the adhesion strength of the as-bonded sample and ƒ is the cyclic frequency in Hz[11]. This equation links the stress, and therefore the temperature range, to the life-time of ACF-joints.

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Moreover, as shown in the Table IV the CTE value of ACF is another dominating factor for the failure of ACF interconnects. The stress value increases as the ACF’s CTE value increases. Other researchers [12,13] also have noticed that more reliable ACF joints can be achieved by using ACFs with lower CTE values. Table IV: ACF CTE value and the maximum stress in the ACF joints for case A. Max. Stress, Syy in the ACF (Pa) CTE, α (ppm/K) 113 0.134E7 133 0.154E7 143 0.162E7 153 0.174E7

CONCLUSIONS Both finite element computer modeling and the experimental methods are employed to investigate the reliability of COF assemblies under a number of different cyclic temperature loading conditions. It has been found that at the edge of the chip-ACF interfaces the atresses are the highest and ACF joint failure is likely to happen in this region. The cyclic deformation and the stresses in the ACF joints are thought to be the driving forces that dislodge the conductive particles from the pad resulting in the increase in the contact resistance or even open joints. The dwell time, the temperature range and the ACF’s CTE value are important factors affecting the lifetime of COFs under thermal cycling test conditions. Experimental studies have revealed that both the global CTE mismatch in the COF assembly and the local CTE mismatch within each conductive particle may contribute to the ACF joint failue when cyclic thermal loads are applied.

[5] V. A. Chiriac and T. Y. T. Lee, “Transient Thermal Analysis of an ACF Package Assembly Process”, IEEE Trans. on C. P. T., vol. 24, no. 4, pp. 673-681, 2001. [6] Y. C. Chan and D. Y. Luk, “Effects of Bonding Parameters on the Reliability Performance of Anisotropic Conductive Adhesive Interconnects for Flip-Chip-on-Flex Packages Assembly I. Different Bonding Temperature”, Microelectronics Reliability, vol. 42, no. 8, pp.1185-1194, 2002. [7] PHYSICA, Multi-physics software limited, University of Greenwich, London, UK. http://www.gre.ac.uk/~physica [8] G. B. Dou, Y. C. Chan and J. Liu, “Electrical conductive characteristics of anisotropic conductive adhesive particles”, Journal of Elec. Pack., vol. 125, no. 4, pp. 609-616, 2003. [9] J. Maattanen, “Contact resistance of metal-coated polymer particles used in anisotropically conductive adhesives”, Solde. & Surf. Mount Tech., vol. 15, no. 1, pp.12-15, 2003. [10] M. J. Rizvi, Y. C. Chan, C. Bailey and H. Lu, “Study of anisotropic conductive adhesive joint behavior under 3-point bending”, Microelectronics Reliability, vol. 45, no.3-4, pp.589-596, 2005. [11] A. Gladkov and A. B. Cohen, “Parametric dependence of fatigue of electronic adhesives”, IEEE Trans. on C. P. T., vol. 22, no.2, pp. 200-208, 1999. [12] C. Y. Yin, M. O. Alam, Y. C. Chan, C. Bailey and H. Lu, “The effect of reflow process on the contact resistance and reliability of anisotropic conductive film interconnection for flip chip on flex applications”, Microelectronics Reliability, vol. 43, no. 4, pp.625-633, 2003. [13] M. J. Yim, Y. D. Jeon and K. W. Paik, “Reduced Thermal Strain in Flip Chip Assembly on Organic Substrate using Low CTE Anisotropic Conductive Film”, IEEE Trans. on E. P. M., vol. 23, no.3, pp.171-176, 2000.

ACKNOWLEDGEMENT The authors would like to acknowledge the technical support from the EPA centre of the City University of Hong Kong. REFERENCES [1] W. S. Kwon and K. W. Paik, “Fundamental Understanding of ACF Conduction Establishment with Emphasis on the Thermal and Mechanical Analysis”, Int. Journal of Adh. & Adh., vol. 24, no. 2, pp.135-142, 2004. [2] Y. W. Chiu, Y. C. Chan and S. M. Lui: “Study of ShortCircuiting between Adjacent Joints Under Electric Field Effects in Fine Pitch Anisotropic Conductive Adhesive Interconnects”, Microelectronics Reliability, vol. 42, no. 12, pp.1945-1951, 2002. [3] G. Sarkar, S. Mridha, T. T. Chong, W. Y. Tuck, S. C. Kwan, “Flip Chip Interconnect using Anisotropic Conductive Adhesive”, Journal of Mat. Proces. Tech, vol. 89-90, pp. 484490, 1999. [4] R. A. Islam, Y. C. Chan and B. Ralph, “Effect of drop impact energy on contact resistance of anisotropic conductive adhesive film joints”, J. Mater. Res., vol. 19, no. 6, pp.16621668, 2004.

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