FAB Stress Analysis Comparison Between Au and Al on Paladium Bond Pad

Transcription

1 Australian Journal of Basic and Applied Sciences, 6(9): , 2012 ISSN FAB Stress Analysis Comparison Between Au and Al on Paladium Bond Pad Vithyacharan Retnasamy, Zaliman Sauli, Steven Taniselass, Wan Mokhdzani Wan Nor Haimi and Norazeani A. Rahman School of Microelectronic Engineering, Universiti Malaysia Perlis (UniMAP), Kampus Alam, Pauh Putra Perlis, Malaysia. Abstract: In this paper, the comparion between aluminum free air ball (FAB) impact and gold FAB impact on palladium bond pad during wire bonding process is done. The intial impact stage between the wire FAB and bond pad were simulated. The simulation was carried out using Ansys version 11. Impact forces of 1N to 5N were utilized in this study. The simulated results showed that with increase impact force, the stress response on wire FAB, palladium bond pad and copper lead frame increased. Stress on the wire FAB is higher compared to the stress on bond pad and lead frame. The Aluminum FAB induced higher stress response. Maximum stress response for both wire materials were obtained at impact force of 5N Key words: Palladium bond pad, impact stage, gold FAB, aluminum FAB INTRODUCTION Electronic packaging is significant manufacturing process for microelectronic devices as signal and current distribution are executed during the packaging process. The present advancement in integrated circuit fabrication process requires enhance electrical interconnection which has significant effect on the reliability of the microelectronic devices. In many of these devices, wire bonding is extensively utilized as interconnection method. Micro dimension wires are used to perform the bonding process due to the miniature size devices. Currently, the packaging industry is driving the wire bonding process towards increased yield, dwindle in pitch size and lower cost of production (Harman, 2010). However, the reliability aspect is not compromised to achieve these targets as tremendous efforts are being placed in improving and maintaining the reliability of the wire bonded packages. One of the techniques used by various researches to characterize and enhance the reliability of wire bonded packages is simulation. Results obtained from the simulation are can used as guidance to improve the wire bonding process in reality. The impact of wire bonding force on borophosphosilicate glass (BPSG) layer in a power package during wedge bonding was examined by (Yong, Irving, Luk, Rioux, & Qiuxiao, 2008) through simulation. The effects of mechanical force on BPSG layer during bonding process were scrutinized. The results of their study showed that the deformed aluminium wire induces maximum stress on the bond pad and spreads the bond force on to the BPSG layer and metal layer which has a significant effect on the reliability of power package. Thermal stress analysis on chip on board package was done by (Pang, Tan, Leonard, & Chen, 1997) using simulation. The effects of varying the encapsulant coefficient of thermal expansion (CTE) for two types materials and the occurance of the delamination near the wire bond were investigated. The results of their study exhibited a complex bending mode which generates a cyclic fatigue effect in the wire bond interconnections during thermal cyclic test.the effects of Free air ball(fab) size, constant velocity, bonding force and ultrasonic energy level on the first bond formation were scrutinized by (Jun & Ping, 2007) using simulation. The impact stage and ultrasonic were simulated in their study. The results of their study showed that the bond force and ultrasonic had a significant effect on the formation of first bond and it was identified that ultrasonic energy had the most significant effect.the influence of bonding parameters on wire bond strength were investigated by (Jun, Yongjin, & Zhongqin, 2007) by the means of simulation. The interaction among bonding interfacial and bond strength were examined. It was demonstrated that augmentation of impact velocity increases the segment extension and bond strength of the wire. It was also identified that bond force has an significant on the bond strength of the wire.the effects of bond pad design and residual stress on wire bonding failures such as cratering were investigated by (Yong,Irving,& Luk, 2004) using simulation. Influence of ultrasonic energy amplitude and frequency on the first bond formation and stress distribution on the bond pad device and silicon layer were evaluated. Results of their study showed that the ultrasonic amplitude had a more significant on bond pad cratering when compared with the ultrasonic frequency.the stress and deformation of Ball stitch on Bump (BSOB) wire bond on a laminate substrate was evaluate by (Yong, Allen, Timwah, & Irving, 2006) through simulation. Influence of bonding parameters such as bond force, ultrasonic frequency and laminate material on the bonding process were examined for optimized bonding condition. The results their study showed that high bond forces and ultrasonic frequency inflate stress at the bonding interface. Corresponding Author: Vithyacharan Retnasamy, School of Microelectronic Engineering, Universiti Malaysia Perlis (UniMAP), Kampus Alam, Pauh Putra Perlis, Malaysia. vc.sundres@gmail.com 229

2 The stress distribution in wire bonding joint under extreme conditions were simulated by (Ramminger, Türkes, & Wachutka,1998). The enlargements of cracks in the wire bond joints were evaluated. The results of their study showed that cracks occurred in the joint of wire bond weaken the wire bond. The crack growth path was also observed in their study. The occurrence of metal lift off during wire bonding process was investigated by (Qiuxiao,Yong, Timwah, & Irving, 2008) using simulation. The effects of bonding parameters which includes ball diameter, material properties of bond wire, bonding temperature and capillary profile on the stress distribution between bond pad FAB interfaces were evaluated. The results of their study showed that the stress on the bond pad and device increased with increment of ball diameter. This is due to higher bonding force utilized for larger wire diameter. Their study also indicated that harder FAB induces more of bond wire metal lift off.the existence of chip out under the bump(codb) failure more during copper wire bonding process were evaluated by (Spaan et al.,2010). Simulation was done to understand and improve the bonding process. Results of their study indicated cracks initiate CODB occurrence due to high bonding force and high shear force range. High stress regions were observed at the edge of the bond pad and this contributes to the CODB failure.therefore it can be seen that simulation plays a significant part in characterizing and improving the wire bonding process. At present, the augmentation of microelectronic device technology requires the inception of new bond pad material to enhance the reliability of the devices. One of the suggested materials are palladium bond pads (Harman, 2010). However, the reaction of the palladium bond pad towards wire bonding process has to be interpreted so that optimum bonding parameters can be obtained.hence, in this study, the comparison of two types of wire bond material impact on palladium bond pad during the initial impact stage of wire bonding is investigated. The stress response of Gold FAB and Aluminum FAB, palladium bond pad and copper lead frame during impact stage are evaluated through simulation. In this study, only the impact stage between the free air ball(fab) and bond pad is considered and simulated. The impact force for both wire material were ranged from 1N to 5N respectively. Simulation was performed using Ansys version Methodology: The impact two types of wire FAB on the palladium bond pad during wire bonding process is compared through simulation in this study. Ansys version 11 was utlized for the simulation. A 3D model was developed to perform the simulation. The 3D model consists of wire FAB, palladium bond pad and copper lead frame. 10-node Quadratic Tetrahedron solid element (SOLID 187) was used to design the model. Surface-to-surface target element (TRAGE170) and contact element (CONTA174) pair with pure penalty formulation was used to define the contact region of FAB and the bond pad. The 3D model is illustrated in Fig. 1. Fig. 1: The 3D Fab Impact model The dimension of the 3D model is stated as follow. The diameter of wire and FAB is 25µm and 70 µm respectively. The thickness of the palladium bond and copper lead frame pad is set 1.5µm and 3µm respectively. The 3D model consisted of elements and nodes. Table 1 presents the material properties of the 3D model. Subsequently, the impact force is applied on the wire FAB in the negative Y axis and constrains is applied to the lower part of the lead frame and sides of the bond pad to resist movemnet during impact stage. The impact force of the wire FAB was varied from 1N to 5N correspondingly. Several assumption were applied to simplify simulation. They include assuming the temperature of the FAB is equal to the bond pad. Second, the 230

3 wire FAB material is assumed bilinear rate dependent elastic plastic material. As for the bond pad and copper lead frame, elastic plastic material property is asumed. Lastly, the ultrasonic energy effect is not included in this study and only the initial impact stage between the FAB and bond pad were simulated. The results of this study are based on Von Mises stress criterion. Table 1: Material Properties Material Density, ρ (g/cm 3 ) Young's modulus,e (Pa) Poisson ratio, v Gold x Aluminum x Palladium x Copper x RESULTS AND DISCUSSION In this paper, the simualtion of gold FAB and aluminum FAB impact on palladium bond pad are compared and presented. Impact forces ranging from 1N to 5N were applied and stress response at each impact force was attained. All the simualtion results are based on von Mises stress. Figure. 2 illustrates comparison of stress on gold FAB and aluminum FAB at varied force of impact stage. From Figure. 2, it is observed that as the stress response on both FAB on the increases with increment of impact force. However, stress on aluminum FAB is higher at all impact force when compared with the gold FAB Von Mises Stress (MPa) Gold wire Aluminum wire Force (N) Fig. 2: Stress on Gold FAB and Aluminum FAB versus applied force Fig. 3 illustares the stress response comparison on palladium bond pad for gold and aluminum wire. The stress response on palladium bond pad increases with increased impact force for both wire material. Aluminum wire induces higher stress on the palladium bond pad surface compared to gold wire. A sharp rise in stress response from 2N to 4N on the palladium bond pad surface is observed in Fig. 3 for both wire materials. Von Mises Stress(MPa) Gold wire Aluminum wire Force (N) Fig. 3: Stress on the bond pad versus applied force 231

4 Fig 4 illustrates the stress response on copper lead frame for both wire materials. It is observed that stress response of the copper lead frame increases with increasing impact force. Aluminum wire exbihits higher stress on the copper lead frame when compared with gold wire. Von Mises Stress (MPa) Gold wire Aluminum wire Force (N) Fig. 4: Stress on the leadframe versus applied force The overall simulated results are exhibited in Table 2. Based on the results in Table 2, it is noticed that the stress on the wire FAB is higher compared to the stress on the palladium bond pad and copper lead frame for both wire materials.this is because, the wire FAB impacts the bond pad, deformation occurs at the point of impact between the wire FAB and palladium bond pad. As the impact force increased, the deformation rate of wire FAB and the bond pad increases. Table 2: Simulation Results Force (N) Ball Stress (MPa) Bond Pad Stress(MPa) Lead Frame Stress(MPa) Au Al Au Al Au Al The stress contour of the palladium bond pad for impact force of 1N,2N,3N,4N and 5N for both wire materials is shown in Fig. 6(a-j). When impact force of 1N is applied, the palladium bond pad experinces intial deformation which results in cratering on the bond pad for both wire materials. When the impact force of 2N is applied, the deformation and cratering region on the palladium bond pad surface increases radially for both wire materials. As impact force of 3N, 4N and 5N, the stress distribution and cratering region on the palladium bond pad surface reached a steady state and showed less increment. However, the stress response of the palladium bond kept increasing for 3N,4N and 5N as observed in Fig. 3. consequently, Fig.6(a-j) reveals that the cratering and deformation region on the palladium bond pad surface increases untill impact force of 2N and exhibited minimal increment for impact force of 3N, 4N and 5N for both wire materials. On the other hand, the stress response of the palladium bond pad increased with increasing impact force and maximum impact force was obtained at 5N for both wire materials. 232

5 (a) (f) (b) (g) (c) (h) (d) (i) (e) (j) Fig. 6: (a-j) Stress contour of the bond pad: (a-e) for Gold Fab, impact force of 1N to 5N, (f-j) for Aluminum FAB for impact force 1N to 5N Fig 7(a-j) shows the stress contour of copper lead frame for impact force of 1N,2N,3N,4N and 5N for both wire materials. For impact force of 1N, a willowy deformation and craterion region is induced in the copper lead frame surface by both wire materials. When impact force is increase to 2 N, the deformation and cratering region on the surface of copper lead frame lengthened in both horizontal and vertical direction for both wire materials. A auxiliary minor growth of deformation and cratering region was observed for impact force of 3N, 4N and 5N on the surface of copper lead frame for both wire materials. On whole, the copper lead frame exhibited lower stress response when compared with the palladium bond pad for both wire materials. This is due to the acting of force from the bond pad which is in the opposite direction of the fixed force that holds the leadframe. Hence, it can be conlcuded that the bond pad has absorbed the some of the impact force during the FAB impact stage and partial force was transfered to the copper lead frame. Thus, Fig. 7(a-j) acknowledges that with the increase of impact force, deformation and cratering region on the surface of copper lead frame increases gradually. 233

6 (a) (f) (b) (g) (c) (h) (d) (i) (e) (j) Fig. 7: (a-j) Stress contour of the copper lead frame: (a-e) for Aluminum Fab, impact force of 1N to 5N, (f-j) for Gold FAB for impact force 1N to 5N Consequently, it can summarized that during the impact stage of wire bonding process, the impact force exerts stress onto the wire FAB, the bond pad and lead frame as demonstarted the simulated results. Thus, proper impact force has to be utilized to prevent high stress on the bond pad and lead frame as high stress may damage the bond pad and the structures below. From the comparion of wire material, it was observed that aluminum wire induces highr stress when compared with the gold wire. This is due to gold wire properties which is softer and less rigid than aluminum wire. Conclusion: In this study the comparison of two types of wire bond material impact on palladium bond pad during the initial impact stage of wire bonding were investigated. Comparison were done using gold and aluminum wire. The stress response of wire FAB, palladium bond pad and copper lead were examined for both wire materials. In this study, the only the initial impact stage between the FAB and bond pad were simulated. Simulation was carried out using Ansys version 11 The impact force were varied from 1N to 5N. Von Mises Stress was used as a criterion to intrepret the results. The results of the simulation showed that when the applied impact force increases, the stress level on wire FAB, palladium bond pad and copper lead frame increased for both wire materials The stress induced by the aluminum FAB is higher compared to the gold FAB. Cratering and 234

7 deformation regions were observed on the bond pad and lead frame. ACKNOWLEDGEMENTS The author would like to thank and acknowledge School of Microelectronic Engineering, Universiti Malaysia Perlis for their support and facilities REFERENCES Harman, G., Wire bonding in microelectronics: McGraw-Hill. Jun, He, & Ping, Zhu Parameter Sensitivity Analysis for Thermosonic Bonding Process by Finite Element Method. Paper presented at the Electronic Packaging Technology, ICEPT th International Conference on., Jun, He, Yongjin, Guo, & Zhongqin, Lin., Numerical Study on the Effects of Bond Parameters on Thermosonic Bond Strength. Paper presented at the Electronic Packaging Technology, ICEPT th International Conference on Aug Pang, H.L.J., T.L. Tan, J.F. Leonard, & Y.S. Chen, Reliability assessment of a wirebond chip-on-board package subjected to accelerated thermal cycling loading. Paper presented at the Electronic Packaging Technology Conference, Proceedings of the st. Qiuxiao, Qian, Yong, liu, Timwah, Luk, & S. Irving, Wire bonding capillary profile and bonding process parameter optimization simulation. Paper presented at the Thermal, Mechanical and Multi-Physics Simulation and Experiments in Microelectronics and Micro-Systems, EuroSimE International Conference. Ramminger, S., P. Türkes & G. Wachutka, Crack mechanism in wire bonding joints. Microelectronics Reliability, 38(6 8), Spaan, E., E. Ooms, W.D. van Driel, C.A. Yuan, D.G. Yang & G.Q. Zhang, Wire bonding the future: a combined experimental and numerical approach to improve the Cu-wire bonding quality. Paper presented at the Thermal, Mechanical & Multi-Physics Simulation, and Experiments in Microelectronics and Microsystems (EuroSimE), th International Conference on April Yong, Liu, Allen, H., Timwah, Luk, & S. Irving, Simulation and experimental analysis for a ball stitch on bump wire bonding process above a laminate substrate. Paper presented at the Electronic Components and Technology Conference, Proceedings. 56th. Yong, Liu, S. Irving, & T. Luk, Thermosonic wire bonding process simulation and bond pad over active stress analysis. Paper presented at the Electronic Components and Technology Conference, Proceedings. 54th. Yong, Liu, S. Irving, T. Luk, M. Rioux, & Qiuxiao, Qian, Impact of wedge wire bonding and thermal mechanical stress on reliability of BPSG/poly layer of a silicon die. Paper presented at the Electronic Components and Technology Conference, ECTC th. 235