IEEE Catalog Number: CFP1159B-USBISBN: 978-1-4577-1388-0

Feasibility Study for Copper Bump with Tin Plating Bonding by Using NCP Adhesive and Ultrasonic Bonding Technique

Jardar Yang and Hill Huang

ChipMOS TECHNOLOGIES INC.

No. 1 R&D Rd. 1, Hsinchu Science Park, Hsinchu, Taiwan, R.O.C.

E-mail: jardar_yang@chipmos.com

Abstract

The aim of this project is to develop an alternative material for bumps of LCD driver ICs and to establish a feasible packaging process for the alternative material. With the continuously escalating price of gold, an alternative material to replace the widely used gold bumps in LCD driver IC packaging is eagerly demanded. Therefore, copper bumps are selected for evaluation in this study. To comply with the oxidation-prone and higher hardness characteristics of Cu bumps, nonconductive paste (NCP) resin and ultrasonic bonding are introduced as well in the packaging process. Compared with the commonly used thermo compression bonding technology for Au-bumped LCD driver IC packaging, the ultrasonic bonding method with NCP resin employs relatively lower temperature and bonding force.

In the feasibility evaluation of the Cu bumps, the objective is to achieve a bonding reliability and adhesion quality similar to or even better than that of Au bumps under thermo compression bonding. In the experiment, fine tuning of some operative parameters was executed in order to get the positive results. These operative factors include the following: (a) NCP resin viscosity, (b) NCP resin dispense pattern, (c) dispense temperatures, (d) bonding sequence, (e) bonding temperature, (f) ultrasonic power, (g) bonding force, and (h) bonding time, which play key roles in the study.

Key words: copper bumps, ultrasonic bonding, Non-Conductive

Paste (NCP)

Introduction Currently, chip-on-film (COF) is the most commonly used

method of packaging LCD driver ICs. In typical COF packaging technology, a gold bumped flip chip is mounted to a flexible substrate consisting of polyimide (PI) base film and tin-plated/gold plated copper leads by using thermo compression bonding. The relatively high temperature (around 380℃ ~ 550℃) and pressure connect the gold bumps with the tin-plated/nickel-gold-plated copper leads by forming Sn-Au/Au-Au eutectics. The thermo compression bonding between Au bumps and Sn/Ni-Au plated Cu leads has been proven to have a highly reliable performance and is extensively used for mass production of COF packages. However, with the gold price continuously soaring and packaging cost competitiveness, copper has been taken into consideration as an alternative material for bumps due to its advantages in electrical characteristics and cost efficiency. Nevertheless, oxidization and higher hardness of copper raise challenges to electrical interconnection. Moreover, while the electronic devices focus on meeting the requirements of fine pitch, smaller form factor, high density along with better performance, the attenuated electrical terminals become more sensitive to heat and humidity. Therefore, high temperature working

environment is getting unfavorable to the modern devices. In this study, Au bumps are replaced with Cu bumps for LCD driver ICs and Non-Conductive Paste (NCP) resin and ultrasonic bonding method are introduced to provide better bonding strength and lower temperature bonding environment.

For the conventional COF packaging, the typical process flow mainly includes Inner Lead Bonding (ILB), Potting (POT), Curing, Marking (MK) and Final Test (FT). The most critical two processes “ILB” and “Potting” are briefly described below for better understanding. In ILB process high temperature of around 380℃ ~ 550℃ and high bonding force applied to the chips and COF tape make the gold bumps firmly bonded to the tin plated/nickel-gold plated Cu leads by forming Sn-Au/Au-Au eutectics. This is the most decisive process of COF packaging. Fig. 1 shows the interconnection between Au bumps and Cu leads.

Fig. 1. Interconnection between Au bumps and Cu leads

Fig. 2. Encapsulated device after potting and curing processes

Next, in the potting process after ILB the underfill resin is filled into the gap between the chip and the tape by capillary action to protect the electrical interconnection. After the underfill filling, the devices are placed into an oven at 100℃~150℃ for fully curing the underfill. Fig. 2 shows the encapsulated device after potting and curing processes.

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IEEE Catalog Number: CFP1159B-USBISBN: 978-1-4577-1388-0

In this study, the challenge Cu bumps may raise is that Cu-Sn or Cu-Au eutectics are not easily formed between Cu bumps and Sn/Au plated leads under current bonding conditions since they require a higher temperature and voids and Cu oxides that are likely to form obstruct the formation of eutectics. Hence, NCP is introduced to enhance the adhesion strength. The NCP resin shrinks under a thermal treatment and generates negative pressure between the chip and the tape so that the metal-to-metal bonding is made between the Cu bumps and the leads. The NCP connections do not need severe conditions for both temperature and pressure; hence, ultrasonic bonding with relatively lower temperature and pressure is employed. The ultrasonic power not only can remove some of the NCP resin on the surface of the copper leads during bonding, but also facilitate the metallurgical bonding between the bumps and the leads. With the combination of Cu bumps, NCP and ultrasonic bonding, the packaging process flow is rearranged to “NCP dispensing→ ILB → MK → FT”.

The NCP resin is first dispensed on the chip bonding area of the COF tape. A thermo-ultrasonic bonding is then applied to connect the chip with the COF tape, and in the meanwhile the NCP is also cured. With this process flow, a separated curing step can be omitted resulting in reduction of manufacturing time. A schematic diagram of NCP dispensing and bonding apparatus is shown in Fig. 3a, and Fig.3b shows a schematic diagram of NCP dispensing and ILB processes.

Fig. 3a. A schematic diagram of NCP dispensing and bonding

apparatus

Fig. 3b. A schematic diagram of NCP dispensing and ILB

processes

Experiment

a. Material preparation A typical COF tape which is extensively used in thermo

compression bonding is selected. The polyimide base film of the COF tape has a thickness of 38 um and the inner leads are

of around 8 um thick copper with 0.16 um thick tin plating. The total pin count of the leads amounts to 1,298 of test vehicle chip, and the thermal compensation of the inner leads is designed in accordance with the thermo compression bonding conditions of relatively high temperature. The test vehicle chip is 17.28 mm * 1.68 mm. The Cu bumps disposed on the test vehicle chip are with a height of 14 um and deployed at a min. pitch of 25 um in linear pattern.

An A-type NCP adhesion resin is selected to be used in this experiment. The workable temperature range of this type of NCP adhesion resin is within 180℃ ~ 260℃. The profile of viscosity vs. temperature is shown as Fig. 4.

Fig. 4. Viscosity vs. temperature profile of A-type NCP resin

b. Equipment Fig. 5 shows the ultrasonic bonding equipment used in

this experiment. The bonding machine is a flip chip bonder, in which the chips are flipped down while bonding so that the bonding tool contacts with the chip backsides and the COF tape is positioned on the bonding stage. The ultrasonic vibration amplitude was set within the range of 0.5 um ~ 2.0 um, and the frequency of ultrasonic vibration was held constant at the range of 40~60 KHz.

Fig. 5. Ultrasonic bonder apparatus

c. The parameter setting of NCP and ultrasonic bonding

The amount of NCP resin to be applied on the chip bonding area of the tape is determined according to the chip size, the dispense needle nozzle size and the dispense pattern. In this experiment, the G type needle nozzle and the line-type dispense pattern were selected. The package unit of the tape was rolled to the position of the dispense stage and the NCP

Dispense Unit UltraSonic

Bonding Unit

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IEEE Catalog Number: CFP1159B-USBISBN: 978-1-4577-1388-0

resin was applied in the center of the chip bonding area of the package unit in a straight line pattern along the long side of the chip bonding area. The package unit was then moved to lie on the preheated leveling stage so as to have the resin evenly spread before bonding. The amount of NCP resin applied was about 1~3 mg.

The temperature setting of the dispense stage and the leveling stage was decided according to the viscosity vs. temperature profile of the NCP resin. They both were held at the temperature around 60~80℃. The bonding force was first determined according to the total contact area between the copper bumps and the inner leads and was tuned in light of the actual bonding condition. The bonding process was accomplished in three sequential steps. In the first step, the ultrasonic vibration was applied and the bonding force increased gradually from 20 NT to 100 NT and lasted for about 1~5 seconds. Then, the bonding force was held at 100~150 NT for another 1~5 seconds with ultrasonic vibration in the second step. The ultrasonic vibration amplitude not higher than 5 um was applied in these two steps. In the last step, the bonding force remained at 100~150 NT without ultrasonic power for about 6~10 seconds for fully curing the NCP resin. Therefore, the bonding time depends mainly on the fully curing time of NCP resin. As a result, the bonding process can be completed within 20 seconds in this experiment.

Moreover, the bonding tool was heated to around 200~300℃ while the bonding stage was first held at 80~100℃ . However, in this experiment the ultrasonic bonding temperature was lower than that in the thermo compression bonding while the bonding tool dropped from about 400℃ to 300℃. The much lower temperature of the bonding tool leaded to a smaller expansion of the chips compared to thermo compression. However, with the slightly decreased temperature of the bonding stage, the tape expansion did not change too much compared to thermo compression. Hence, the degree of expansion of the COF tape was larger than that of the chip. The total inner lead offset of thermal compensation designed in accordance with the thermo compression bonding became too large. As a result, lead shift and misalignment occurred. Therefore, the temperature of the bonding stage was reduced to around to 40℃ and the over expansion of the tape and the misalignment problems were then resolved.

d. Results The OM inspection results as shown in Figs. 6~ 7, and an

acceptable level of positional accuracy and interconnection quality between Cu bumps and inner leads are shown in the Figs. Moreover, a 100% pass rate in the O/S test was achieved.

Fig. 6. Overview of Inner Lead Bonding result

Fig. 7. Cross Section of Inner Lead Bonding result

Conclusion Basically, the study for the copper bumps with Tin-plated

inner lead bonding along with NCP and ultrasonic bonding showed a positive result. The challenge of bonding Cu bumps with the Tin plated leads due to the high hardness and oxidation-prone characteristics of copper can be resolved by the use of NCP adhesive and ultrasonic bonding. The negative pressure generated when the NCP adhesive shrinks under a thermal curing process facilitates the metal-to-metal bonding between the Cu bumps and the leads. The ultrasonic power also stimulates the metallurgical bonding between the bumps and the leads.

In this experiment, the total pitch offset value of inner leads of the COF tape was designed in accordance with the thermo compression conditions, in which the temperatures of bonding head and bonding stage are about 400℃ and 100℃, respectively. However, the ultrasonic bonding introduced a lower temperature bonding environment so that the temperature of the bonding head dropped from 400℃to around 300℃. The extent of expansion of the COF tape is smaller under lower temperature bonding condition so the pitch offset value should be redesigned to comply with the ultrasonic bonding to avoid the lead shift and misalignment problems. Moreover, the solder resist opening of the chip bonding area should also be redefined since the NCP resin formed a fillet smaller than the conventional underfill resin formed, which may lead to incomplete coverage of the exposed inner leads within the chip bonding area. The NCP resin plays a key role in this application so to find a type of NCP with the characteristics more applicable to this process is also an important task.

References [1] S.C. Tan, Y.C. Chan, and Nelson S.M. Lui, “Process

Optimization to Overcome Void Formation in Nonconductive Paste Interconnections for Fin-Pitch Applications” – Journal of ELECTRONIC MATERIALS, Vol. 34, No. 8, pp. 1143-1149, August 2005

[2] Kenji Takahashi, Mitsuo Umemoto, Naotaka Tanaka, Kazumasa Tanida, Yoshihiko Nemoto, Yoshihiro Tomita, Masamoto Tago, and Manabu Bonkohara, “Ultra-high-density interconnection technology of three-dimensional packaging” – Microelectronics Reliability 43, pp. 1267-1279, 2000

[3] Hiroki Maruo, Yoshihito Seki, Yoshiharu Unami, and Tadanori Ominato, “Ultrasonic Flip Chip Bonding on FPC” – Fujikura Technical Review, pp. 36-39, 2004

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