Impact of Young Modulus of Epoxy Glue to Copper Wire Bonding

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1 Impact of Young Modulus of Epoxy Glue to Copper Wire Bonding Tan KG 1, Chung EL 1, Wai CM 1, Ge Dandong 2 1 Infineon Technologies (Malaysia) Sdn Bhd, Malaysia 2 Infineon Technologies Asia Pacific Pte Ltd, Singapore Abstract Epoxy glue still remains as one of the main medium being used for die attach process in current semiconductor technologies. Different types of epoxy glue have different young modulus due to their own properties from different epoxy glue manufacturer. The current chip size is getting smaller and smaller due to cost saving projects from FE wafer fab. If the chip size is getting smaller but still using the same epoxy glue, then the low young modulus of the epoxy glue will become a big challenge especially for copper wire bonding process due to high Z die displacement during wire bonding process. This is due to the ultrasonic energy will not be transmitted effectively to the copper wire and bond pad surface without rigid support to the chip. In this paper, different die attached chip sizes are being studied with different young modulus epoxy glue using simulation tool to measure the die z displacement and also actual copper wire bonding process to assess the wire bonding performance. 1. Introduction In today semiconductor industries, epoxy glue still remains as one of main medium being used for die attach process due to its cost advantage and process stability in volume production. On top of this, copper wire bonding is in the increasing trend as compared to gold wire due to the cost advantage, better thermal conductivity and lower electrical resistivity performance. Due to the nature of the material properties of copper which is more harder than gold, the good bond-ability using copper wire is still remain a big challenge especially on the wire bonding process stability when bonded on the chip using lower young modulus of epoxy glue. The current chip size is also getting smaller and smaller due to cost saving projects from FE wafer fab. If the chip size is getting smaller but still using the same epoxy glue, then the low young modulus of the epoxy glue will become a big challenge especially for copper wire bonding process due to high Z die displacement during wire bonding process which will cause high NSOP and lifted ball bond issues. This is due to the ultrasonic energy will not be transmitted effectively to the copper wire and bond pad surface without rigid support to the chip. In this paper, different die attached chip sizes are being studied with different young modulus epoxy glue type using simulation tool to measure the die Z displacement and also actual copper wire bonding process to assess the wire bonding performance. 2. Approach First we selected 3 different types of epoxy glue with different young modulus and 4 different chip sizes varies from <0.5mm2 to >6.00mm2. We use simulation tool to measure the die Z displacement with different combination of chip sizes and different epoxy glue types. After simulation runs, actual wire bonding validation was being carried out with different epoxy type to access the wire bond-ability. 3. Methodology Dynamic Mechanical Analysis Model: TA-Q800 Sample dimension: 15mm length x 5mm width x 0.2mm thickness Samples cured using different cure profiles. Finite Element Analysis Only die paddle, die and glue are modelled. Die paddle is assumed to be sitting flat on heater block with clamp during wire bond process. CTE mismatch between die and die paddle is assumed to be negligible. Die is modelled as a block of bulk Si without metallization layers. All material properties are linear elastic. Vertical force is applied on die surface according to bond pad locations and die z-displacement is measured. Fig 1: 3D simulation model

2 Die Z displacement with the lowest modulus epoxy glue is the highest as compared to the highest modulus epoxy glue especially for those pads located at the edge of the chip. 5. Simulation run 2 2 nd simulation run was carried out to measure the die Z displacement vs glue curing profile on glue A with lowest young modulus. Fig 2: Die Z displacement when vertical force is applied simulating wire bond process 4. Simulation run 1 1 st simulation run was carried to measure the die Z displacement vs bond pad location on the chip bonded with three different young modulus of epoxy types. Fig 5: Die Z displacement comparison on Glue A with different dlue curing profile With higher glue curing profile of epoxy glue A with lowest young modulus, the young modulus of epoxy glue A will increase and thus improve the die Z displacement. Therefore by increasing the glue curing profile, the young modulus will become higher. Fig 3: Die Z displacement comparison between Glue B and C 6. Simulation run 3 3 rd simulation run was carried out to measure the die Z displacement comparing different bond line thickness on the lowest young modulus epoxy glue A vs highest young modulus epoxy glue C with highest bond line thickness. Die Z displacement on the pads located at the both chip edge is higher as compared to the center pads for the chip bonded with lower young modulus epoxy glue. However there is no die Z displacement on all the pads bonded with higher young modulus epoxy glue. Fig 6: Die Z displacement comparison between Glue A and C with different bond line thickness The lowest young modulus epoxy glue with highest bond line thickness gave the highest die Z displacement as compared to the lowest bond line thickness. Fig 4: Die Z displacement comparison between Glue A and C However with the highest young modulus epoxy glue, the die Z displacement still the lowest as compared to the low young modulus epoxy glue with lowest bond line thickness.

3 7. Simulation run 4 4 th simulation run was to compare the die Z displacement with different chip size bonded with lowest and highest young modulus of epoxy glue. Fig 9: Die Z displacement comparison on glue B with different bond force The die Z displacement bonded with higher bond force on glue B with slightly higher young modulus than glue A shown lower as compared to glue A. Fig 7: Die Z displacement comparison between Glue A and C with different chip size The smallest chip size bonded with lowest young modulus epoxy glue has highest die Z displacement as compared to the highest young modulus epoxy glue. However there is not much difference of die Z displacement between the Glue A with lowest young modulus and Glue C with highest young modulus for the bigger chip size. Fig 10: Die Z displacement comparison on Glue C with different bond force No die Z displacement observed on glue C with highest young modulus even bonded with highest bond force. 8. Observations 8. Simulation run 5 5th simulation run was to compare the die Z displacement bonded with different bond force using three different young modulus epoxy glue type. Fig 11: Observation after simulation runs with different combination. Fig 8: Die Z displacement comparison on glue A with different bond force The higher the bond force, the higher the die Z displacement on glue A with lowest young modulus. The observations after simulation runs with different combinations are as follows:- The lowest young modulus epoxy glue will give highest die Z displacement especially at the both edge of the chip. With higher curing profile will increase the young modulus of glue and thus improve the die Z displacement. Lower bondline thickness will also improve the die Z displacement even bonded with lower young modulus of epoxy glue. Chip size also play an important role to improve the die Z displacement. The larger chip size the better. Bond pad located at the edge of the smaller chip size has higher die Z displacement as compared to those bond pads located at the center of the chip.

4 9. Wire bond validation runs Wire bond validation runs was carried out to study the impact of different young modulus of epoxy glue to the copper wire bond process quality responses such as ball shear and ball pull test. Two test vehicles was being selected for this study bonded with 30µm, 50µm and 65µm Cu wire respectively. Fig 14: Ball shear strength bonded with 65µm Cu wire with Ball shear strength increased with the increasing of young modulus of epoxy glue when bonded with 65µm Cu wire. Fig 12: Ball shear strength bonded with 50µm Cu wire with Ball shear strength increased with the increasing of young modulus of epoxy glue when bonded with 50µm Cu wire. Fig 15: Ball pull strength bonded with 65µm Cu wire with Fig 13: Ball pull strength bonded with 50µm Cu wire with Fig 16: Ball shear strength bonded with 30µm Cu wire with

5 Fig 17: Ball pull strength bonded with 30µm Cu wire with 10. Conclusions Lower young modulus of epoxy glue has very high impact to Cu wire bonding especially with thicker wire of 30µm and above. If the chip size is smaller and the bondline thickness is thicker bonded with lower young modulus of epoxy glue, the die Z displacement is the highest and this will give very big impact towards wire bonding with Cu wire due to ultrasonic energy will not be transmitted effectively to the copper wire and bond pad surface without rigid support to the chip. If we increase the glue curing profile with higher curing temperature and longer curing time, it will helps to increase the young modulus of epoxy glue. However we cannot keep on increase the curing temperature because the glue molecules will decompose and caused lifted die after reliability stress. Therefore if we want to wire bond with thicker Cu wire on the smaller chip size and higher bond line thickness, it is highly recommended to use the higher young modulus epoxy glue. References 1. B. Senthil Kumar, Acuesta Albert, Lim Lay Hong Geraldine, Effect of Die Attach Material on Heavy Cu Wire Bonding with Au Coated Pd Bond Pad in Automotive Applications, J. Clerk Maxwell, A Treatise on Electricity and Magnetism, 3rd ed., vol. 2. Oxford: Clarendon, 1892, pp Megan Chang, Anderson Li, Low Stress Die Attach Material Challenges for Critical Si Node with Cu Wire, K. Elissa, Title of paper if known, unpublished. 3. Fui Yee Lim, C. L. W. Yew, Die Attach Materials Impacts to Copper Wire Bonding: New Challenges, Y. Yorozu, M. Hirano, K. Oka, and Y. Tagawa, Electron spectroscopy studies on magneto-optical media and plastic substrate interface, IEEE Transl. J. Magn. Japan, vol. 2, pp , August 1987 [Digests 9th Annual Conf. Magnetics Japan, p. 301, 1982]. 4. Yu Hin Chan, Hang-Kyo Kim, Deming Liu, Peter C.K. Liu, Yiu Ming Cheung, Ming Wai Ng, Comparison of Wire Bondability on Rigid and Flexible Substrates, 5. Raymund Y. Augstin, Janet M. Jucar, Jefferson S. Talledo, The Effect of Glue Bond Line Thickness (BLT) and Fillet Height on Interface Delamination, 6. H.T. Wang, Y. C. Poh, An Analysis on the Properties of Epoxy Based Die Attach Material and the Effect to Delamination and Wire Bondability, Acknowledgments The author would like to express his gratitude to Mr. Garbin Giovanni Ragasa and Mr. Chong Hoe Jian from Infineon Technologies UPD die bond and wire bond group for samples preparation to carry out wire bond validation runs with different epoxy glue type and also Ms.Ge Dandong for her technical input and advise on the epoxy glue.