Effect of Direction of Ultrasonic Vibration on Flip-Chip Bonding

Transcription

1 Transactions of The Japan Institute of Electronics Packaging Vol. 6, No. 1, 13 [Technical Paper] Effect of Direction of Ultrasonic Vibration on Flip-Chip Bonding Mutsumi Masumoto*, Yoshiyuki Arai*, **, and Hajime Tomokage* *Department of Electronics Engineering and Computer Science, Fukuoka University, Nanakuma, Jonan-ku, Fukuoka , Japan **Research and Development Div., Toray Engineering Co., Ltd., Oe, Otsu, Shiga , Japan (Received May 14, 13; accepted October 29, 13) Abstract Flip-chip bonding has several advantages, such as high precision, high density, short interconnections, and small parasitic elements. However, creating reliable interconnections between chips and substrates is the key issue in flip-chip bonding. In this work, bonding states are investigated using test element group (TEG) dies with gold stud bumps. Ultrasonic bonding is applied to a substrate with longitudinal and lateral leads, and the die-pull mode is investigated systematically. For conventional flip-chip bonding equipment, the die-pull test shows different bonding states for longitudinal and lateral leads. However, we have developed a flip-chip bonder with a rotational vibration head, where the direction of the angle of the vibration with respect to the die configuration can be changed. For a head rotated to 45 degrees, uniform bonding is established on both the longitudinal and the lateral leads. A wide process margin for flip-chip bonding is obtained, with a high yield. Keywords: Flip-chip bonding, Die-pull test, Bump shear strength, Ultrasonic vibration 1. Introduction The development of packaging technologies such as system-in-a-package (SiP) has greatly contributed to making electronic devices smaller and more multifunctional. Smaller, high-density packages require micro-interconnections between the die and substrate. Wire bonding and flip-chip bonding are the main techniques used to make these interconnections. Flip-chip bonding is a process in which bumps are fabricated on a die and interconnected to the package substrate, with the active die face down. Flipchip bonding has many advantages, such as high precision, high density, short interconnections, and small parasitic elements. However, the reliability of the interconnections is crucial for flip-chip bonding.[1] Low-k materials are used for insulating layers for large-scale integration, in order to reduce the signal delay; however, because low-k materials are mechanically fragile, the bonding force must be reduced. Ultrasonic vibration is commonly applied to the bonding head to decrease the force. It has been reported that the structure and power of the ultrasonic vibration horn affects the bonding state.[2] However, failure analysis of flip-chip bonding is not easy, because it is difficult to observe the connected area under the face-down condition. Arai et al.[3] have reported on a method to evaluate the bonding status using a die-pull tester, which can be used to evaluate whether the bonding conditions are suitable for the production environment.[4] It is known that the directions of the leads on the substrate also affect the bonding state,[5] i.e., the bonding behavior of longitudinal and lateral leads is different under the horizontal vibration condition. In this paper, the bonding states are investigated using test element group (TEG) dies with gold stud-bumps. Ultrasonic bonding is applied to substrates with longitudinal and lateral leads, and the die-pull mode is investigated systematically. The die is physically removed from the substrate, and the failure mode is analyzed. The bonding behavior with a vibrational head rotated by 45 degrees with respect to the die configuration is compared with that of a conventional head, and the process margin of flip-chip bonding for both heads is discussed along with the failure modes. 2. Experimental Procedure The conventional flip-chip bonding equipment has a fixed vibration direction with respect to the die configuration, as shown in Fig. 1. The flip-chip bonder we developed has an ultrasonic vibration head that can be rotated in advance and operated with constant weight and frequency, as shown in Fig Copyright The Japan Institute of Electronics Packaging

2 Masumoto et al.: Effect of Direction of Ultrasonic Vibration on Flip-Chip Bonding (2/5) The test element group (TEG) dies used in this measurement were mm with 500 gold stud-bumps per die. The pad size and pad pitch were 45 μm and 50 μm, respectively. The substrates to be connected were FR-4 type with copper leads plated with nickel and gold. The thickness of the nickel and gold were 0.08 ± 0.04 μm and 0.50 ± 0.25 μm, respectively. The lateral and longitudinal lead patterns were placed equally on the substrates. The conditions for flip-chip bonding were as follows: trigger force 12 N (or 2.5 g per bump), bonding force 34.5 N, bonding time 0.7 s, and amplitude of ultrasonic vibration 1.3 μm. The reflow test was performed at 2 C, three times. The direct-current resistance of a daisy-chain connection between a die and a substrate was measured, and any sample which increased in resistance by more than % after a reflow test was classified as not good (NG). The die-pull test was performed using the mode counting system.[3] The die was physically removed from the substrate in the vertical direction. The pull speed was 1 mm/s, and the maximum force was 300 N. Then the pulloff images were observed, and classified into four modes. The different failure modes are shown schematically in Fig. 2, along with the pull-off marks observed on the pad and substrate sides for each mode. Mode A indicates weaker bonding between the chip pad and stud-bump compared with that between the stud-bump and lead. Mode B is fracture of the bump, and implies strong bonding, while mode C indicates incomplete bonding between bump and lead. Finally, mode D is fracture between the lead and substrate material. This also implies strong bonding, but sometimes mode D also occurs because of copper delamination problems when making the substrates. The shear test was performed with a conventional shear tester. To measure the shear strength of the bonds between bumps and leads, the aluminum pads on a TEG Ultrasonic vibration Fig. 1 Ultrasonic bonding head and die configuration: conventional flip-chip equipment and equipment with head rotated with respect to the die. TEG Substrate Pad-side Substrateside (c) (d) Fig. 2 Die-pull mode and typical photographs at pad side and substrate side: mode A, mode B, (c) mode C and (d) mode D. 39

3 Transactions of The Japan Institute of Electronics Packaging Vol. 6, No. 1, 13 TEG chip Bump Lead Substrate (c) Fig. 3 Process for carrying out a bump shear test: after bonding, aluminum pad etched off chemically and (c) shear test. die were chemically etched off, as shown in Fig. 3. The etching was performed by immersing the die into a KOH 23% solution at 50 C for min. We ascertained that this chemical treatment did not affect the shear strength of the stud-bump. 3. Results and Discussion The typical failures observed after the reflow tests are shown in Fig. 4. From the resistance measurements, an individual connection was determined as NG, and then cross-sectioning was performed. The cracks usually occurred between the die pad and bump, and between the bump and lead. In order to obtain the distribution of bondfailure modes on a wafer, the die-pull test was performed with conventional flip-chip equipment. Figure 5 shows the distribution of the bond-failure modes obtained for five dies. The results for the longitudinal lead and the lateral lead are shown in Fig. 5 and, respectively. Mode B is dominant for the longitudinal lead (the proportions of modes A, B, C and D were 2%, 96%, 0% and 2%, respectively). On the other hand, the lateral lead connection usually failed in either mode A or B (the proportions of modes A, B, C and D were 46 %, 53%, 0% and 1%, respectively), as shown in Fig. 5. Mode B indicates strong bonding between a bump and a lead. However, for the lateral lead under this condition, there is a significant proportion of mode A failures, meaning that excess vibrational energy was applied to the pad during the lateral-direction bonding.[3] Mode A is supposed to correspond to the presence of a crack between the pad and bump after the reflow test. Figure 6 shows the distribution of bump shear strengths obtained from the shear tests on the dies bonded using conventional flip-chip bonding equipment. The mean value and standard deviation are tabulated in Table 1. The mean shear strength and variation in shear strength are smaller for the longitudinal leads than for the lateral leads. The small variation in shear strength for the longitudinal lead is consistent with the dominance of die-pull Mode B in Fig. µ m Fig. 4 Typical defects observed after reflow test: crack between a pad and a bump, crack between a bump and a lead. Table 1 Shear test for conventional and 45-degree-rotated bonders. Sample number Mean value (g) Standard deviation (g) Conventional 45 degree rotated Lateral Lateral For the lateral leads, on the other hand, almost half of failures correspond to Mode A, which accounts for the larger value of the mean bump strength, and also the larger variation in bump strength. Although Mode A corresponds to a crack between a pad and a bump, the result of Fig. 6 was obtained with the bumps etched off from the substrate. That might be the reason why the shear strength for the lateral leads was larger than for the longitudinal ones. We consider that the ultrasonic vibration first induces a low bonding force; the bonding strength then reaches its maximum value, followed by fracture, as the vibration continues. Figure 7 shows the flip-chip bonding process as a function of bonding time. When the bonding time is short, lead open failures occur, because of the low bonding force.

4 Masumoto et al.: Effect of Direction of Ultrasonic Vibration on Flip-Chip Bonding (4/5) Mode A Mode B Mode C Mode D Mode A Mode B Mode C Mode D Fig. 5 Proportions of mode A, B, C and D failures for conventional flip-chip bonding: longitudinal lead, lateral lead Lateral Bump shear strength (g) Fig. 6 Distribution of bump shear strengths. lead Lateral lead Bonding time (s) Process margin Fig. 7 Diagram showing bonding states and times for longitudinal and lateral leads bonded using a conventional flip-chip bonder. Note the small process margin Fig. 8 Proportions of mode A, B, C and D failures for 45-degree-rotaed flip-chip bonding: longitudinal lead, lateral lead. On the other hand, excess bonding energy causes cracks when the bonding time is too long. For the lateral leads, the ultrasonic energy can be absorbed more easily than for the longitudinal leads. Therefore, the bonding states obtained for the longitudinal and lateral leads can be explained as shown in Fig. 7. The bonding energy, which is the product of vibration power and bonding time, is different between the longitudinal and the lateral leads, which can cause a time-lag in forming firm bonds, resulting in a reduced process margin for effective ultrasonic bonding. To change the bonding energy applied by the ultrasonic vibration, the bonding head was rotated with respect to the die configuration. Figure 8 shows the results for die-pull tests and Fig. 9 shows the bump shear strength distribution for a head rotated 45 degrees. The mean value and 41

5 Transactions of The Japan Institute of Electronics Packaging Vol. 6, No. 1, Lateral Bump shear strength (g) Fig. 9 Distribution of bump shear strengths for bonds made with the 45-degree-rotated bonder. bonding. Using the 45-degree-rotated head improved the yield for flip-chip bonding. For example, in one product, the yield on 3,000 dies increased from 85.1% to 99.4%. Low-k Black Diamond TM material was used for the insulating layer. After open and function tests, dies bonded using the 45-degree-rotated head also showed high reliability in tests such as preprocessing humidity resistance level 3, heat cycle test 55/125 C, high temperature and humidity preservation test 110 C/85%, and high temperature self-test at 150 C. We are still doing research on other factors such as flip chip bonding temperature, and gold stud bump shape and will publish the results in a next paper. Fig. 10 Mode B pull-off patterns for the longitudinal and lateral leads. lead 4. Conclusion Ultrasonic flip-chip bonding of TEG dies with gold studbumps was applied to substrates with longitudinal and lateral leads, and the die-pull mode was investigated systematically. When conventional flip-chip bonding equipment was used, the die-pull test showed different bonding states for the longitudinal and lateral leads. A flip-chip bonder with a vibration head that could be rotated with respect to the die configuration was developed. For a 45-degreerotated head, uniform bonding was established on both longitudinal and lateral leads. A wide process margin for flip-chip bonding was obtained with a high yield. Lateral lead Bonding time(s) Process margin Fig. 11 Diagram showing bonding states and times for longitudinal and lateral leads bonded using a 45-degree-rotated bonder. Note the increased process margin. standard deviation are tabulated in Table 1. The curves for the longitudinal and lateral leads have the same mean strength and variation, and the overall variation is small compared with that obtained using conventional flip-chip equipment. SEM images of the longitudinal and lateral leads after the die-pull test are shown in Fig. 10. Mode B pull-off patterns, with similar structures, were formed on both the leads. Figure 11 shows a schematic of the bonding status versus bonding time for a 45-degree head. The ultrasonic energy was delivered to both longitudinal and lateral leads, resulting in a much wider process margin for effective References [1] Y. Jin, Z. Wang, and J. Chen, Introduction to microsystem packaging technology, Science Press, pp , 11. [2] A. Yamauchi, S. Kuwauchi, S. Sato, and S. Nakai, Enhancements in ultrasonic flip chip bonding to flexible printed circuit substrate (in Japanese), MES 03 Symposium Proceedings, pp. 0 3, 03. [3] Y. Arai, W. Jimyung, S. Aoki, K. Imai, and Y. Miyamoto, Die pull tester for flip-chip bonding, ICEP11 Proceedings, pp , 11. [4] M. Masumoto, N. Nakanishi, and A. Okazaki, Japanese Patent JP A, 11. [5] Y. Arai, Y. Miyamoto, S. Aoki, and K. Shimatani, Mode counting system for die pull test, ICEP12 Proceedings, pp , 12. Mutsumi Masumoto Yoshiyuki Arai Hajime Tomokage 42