A Characterization Method for Interfacial Delamination of Copper/Epoxy Mold Compound Specimens Under Mixed Mode I/III Loading

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1 2017 IEEE 67th Electronic Components and Technology Conference A Characterization Method for Interfacial Delamination of Copper/Epoxy Mold Compound Specimens Under Mixed Mode I/III Loading V.N.N.Trilochan Rambhatla, David Samet, Scott R. McCann, and Suresh K. Sitaraman The George W. Woodruff School of Mechanical Engineering Georgia Institute of Technology,Atlanta,GA Abstract The objective of this work is to develop a combined mode I and mode III characterization method and to use this test method to study Copper (Cu) / Epoxy mold compound (EMC) interfacial delamination from near-mode I to nearmode III global loading. Using the developed test method, a series of experiments are done with varying loading mode conditions from near-mode I to near-mode III and successful delamination of the Cu/EMC interface is observed in many cases. Three-dimensional finite-element analysis is carried out to get compliance vs crack length relationship for different loading conditions and is used to determine the crack length indirectly. The experiments indicate that as the mode mixity increases from mode I towards mode III, the critical load increases for a given crack length, and thus, the interfacial fracture energy increases with the increasing mode mixity. Keywords: Mode I/III loading; copper epoxy bimaterial; interfacial delamination; mode III loading. most of these test techniques are either complex or ineffective [6]. Many of these techniques are proposed for test specimens composed of either one material or composites and but not suitable to delaminate bimaterial specimens. Simplicity of testing depends upon testing configuration and geometry of loading fixture. Uniaxial test stands, simple in working, have been traditionally used for characterization of mode I delamination. Therefore, this work aims to design a loading fixture which can be used on the uniaxial test machine to characterize mode I and III combined loading effect on Cu/EMC interfacial delamination. These fixtures work similar to the multiple hole fixtures suggested by Ayatollahi [5], but are comparatively simple in geometry and can be used on various test specimen geometries. The fixtures are designed such that mixed mode I-III loading is applied on the intended interface. I. INTRODUCTION Copper-epoxy mold compound (Cu/EMC) packages undergo many thermal and power cycles during their lifetime. Thermo-mechanical stresses caused by coefficient of thermal expansion (CTE) mismatch under thermal excursions can result in fracture between material interfaces. The interfacial crack-tip in multi-layered packages experience a combination of mode I, II, and III loading known as mixed-mode loading. The fracture energy of an interface varies as the mode mixity changes. Extensive work has been carried out in characterizing mode I delamination, however, less experimental work has focused on mode II and very little work has focused on mode III. This is because, among other reasons, the interfacial fracture energy under mode II or III is at least an order of magnitude higher than the value at mode I, and therefore, the mold compounds, rather than the interface, usually fracture when subjected to loading conditions closer to mode II or mode III. There are different tests to characterize mode III effects on test specimen. They are [1-4]: 1) Edge crack torsion (ECT) method, 2) Split cantilever beam method, 3) Modified split cantilever beam method, 4) Crack rail shear test, 5) Anticlastic plate method, 6) Slant notch bent method, and 7) Triple bending method. Some researchers have also suggested tensile machine compatible fixtures having multiple holes to apply varied range of mixed mode I/III loads on the specimens [5]. But, Figure 1. Copper-Epoxy mold specimens used for testing II. MODE III FRACTURE EXPERIMENTS: A. Test specimens used in the experimentation: The specimens used in this work are bimaterial strip samples consisting of a layer EMC directly attached to a copper strip. The assembly procedure is as follows: liquid epoxy is injected on a copper strip using a transfer mold and is cured at 175 o C. Then, the samples are ejected from the transfer mold to cool. Materials used are CDA194 copper alloy and Sumitomo Sumikon EME-G630AY molding compound. The epoxy mold is 40 mm long, 6 mm wide, /17 $ IEEE DOI /ECTC

2 mm thick over a copper strip of 60 mm length, 8 mm width, and mm thickness. B. Test fixtures and method In order to investigate the behavior of the test specimens under mode III loading, L shaped fixtures were designed to apply near mode III loading on copper epoxy specimens, as shown in Fig 2. The test configuration is similar to Split cantilever beam test [2]. A starter crack is created in the specimen by clamping the specimen at a known distance and bending the copper strip down to initiate the delamination. Loading fixtures are then attached on the specimen using epoxy. Figure 3. Experimental setup Figure 2. Schematic of Cu/Epoxy specimen with fixtures attached The specimen with attached fixtures is tested using a Delamination Testing System (DTS) machine. The experimental is shown in Fig 3. As seen, the bottom fixture is fixed and an upward displacement is given to the top fixture at the rate of 10 μm/s giving rise to a tensile loading. Under this displacement-controlled loading, the experiment was terminated as the EMC fractured at a load of 73N as shown in Fig. 4 Figure 4. Load (N) vs Position (μ) 1889

3 Figure 5. Fractured Epoxy mold under pure mode III loading. It is observed that the EMC got fractured before the delamination under this loading which is near mode III. Hence, it can be inferred that the critical energy release rate required to propagate crack at interface of Cu/EMC specimen under mode III loading is very high such that the cohesive fracture of EMC occurs before the onset of interfacial delamination. The interfacial strength increases with mode mixity. At higher mode mixities, the interfaces are stronger and specimens are more vulnerable to cohesive fracture than delamination. So testing at mode III loading with the goal to capture the effects mode III loading on delamination is difficult because bulk EMC fracture occurs before interfacial delamination. To reduce such EMC cohesive fractures, the specimens are subjected to mixed mode I/III loading which reduces G values at the interface, while capturing both mode I and mode III effects. Since mode I effects are predominantly captured through double cantilever beam (DCB) tests [7-9], results from the new test can be de-convoluted to obtain mode III values. Figure 6. Schematic of the Mode I-III fixtures Fig. 6 and Fig. 7 shows schematic of fixtures attached to the specimen. The fixtures made of aluminum 6061 are positioned at an angle and are attached using epoxy. It should be pointed out that when angle α (Fig. 6) is 0, the test becomes a standard DCB test. III. MODE I-III EXPERIMENTATION: Figure 7. Schematic of specimen with fixtures A. New test fixtures. Fig. 6 shows the geometry and schematic of test fixtures. The loading fixture is pair of identical parts. A half cylinder with an angular flange forms each part of the fixture. The angle of the flange with respect to the base of the half cylinder influences the interfacial loading angle during testing. When the specimens with attached fixtures are loaded using a tensile machine, the angular flanges direct a part of loading to shear the interface (see Fig. 8). Hence it gives the capability to apply mixed mode I/III loading on the specimen. The change in angle of the flange in the geometry gives the ability to change the mode mixity of loading on the test specimens. For this work, experiments are carried with the loading orientations of = 20 o, 40 o and 70 o. 1890

4 Figure 10. Test configuration with 40 o fixtures Figure 8. Schematic of new fixtures inducing out of plane component of loading on the specimen interface. B. Delamination tests using new fixtures: The goal of the experiments is to evaluate the efficiency of the new fixtures in delaminating the specimens and to characterize the interfacial behavior under mixed mode loading. A pre-crack of approximately 5 to 6 mm is created in specimen, and then, as mentioned earlier, loading fixtures are attached on the specimen using epoxy. These specimens with the fixture are then loaded on a DTS machine. The specimens are connected to the machine using loading pins passing through the fixture holes. The holes and pins are sufficiently lubricated to provide smooth moment-free loading during testing. Displacement-controlled tensile loading is then applied on the test samples. Fig. 9 and Fig. 10 show the experimental setup with 20 o and 70 o fixtures. Two samples are tested for each loading orientation of 20 o, 40 o, and 70 o with the normal. Unlike pure mode III test, these fixtures are successful in delaminating the EMC from the copper strip without fracturing the bulk EMC. Fig. 11 shows the specimen while crack propagation during the experimentation. In these tests, a displacement-controlled loading is applied at the rate of 3 μm/s. The load increases linearly until a critical point, and after which the load drops indicating the delamination propagation. Then the sample is unloaded at the same rate of 3 μm/s. This cycle of loading, delamination propagation, and unloading is repeated for 4 to 6 times. This technique is useful to get multiple compliances of the specimen at different crack lengths. It can be seen from load vs displacement data (Fig. 12) that slope of load unload curves are increasing with increasing crack length. This indicates that the compliance of the specimen increases as the crack propagates, as would be expected. Figure 9. Test configuration with 20 o fixtures. Figure 11. Successful crack propagation under mixed I-III loading. 1891

5 Figure 12. Force (N) vs. Position or Displacement (μ) in 20 o loading case. Decreasing slope of load/unload curves with increasing crack length. IV. FINITE ELEMENT ANALYSIS: In order to compute crack length from the compliance, a 3D finite-element model (See Fig. 13) of the test configuration is created in ANSYS. The model is meshed using SOLID 185 and 186 elements. The nodes facing the base of the fixtures are merged with EMC and copper volumes. The non-frictional contact is provided between the pin and hole to facilitate smooth rotation during simulation. The merging of the nodes of EMC and copper facing each other along the length of the specimen from crack position to backend creates a crack front between EMC and copper. A mesh convergence analysis is first performed to get results which are almost uninfluenced with the mesh size. The critical regions like crack front region and pin regions are meshed with finer mesh and other parts are meshed with comparatively coarser mesh. The compliance of the system for the crack length ranging from 7 mm to 21 mm under corresponding to 20 o loading case is obtained by simulating load vs. displacement, and such compliance vs. crack length is shown in Fig. 14. The compliance is the displacement per unit force of top pin in the direction parallel to flange angle. Figure 14. Simulated compliance versus crack length for 20 o loading case. The compliance can be fitted as a cubic polynomial of crack length, as given below: C = a a a V. RESULTS AND DISCUSSION Fig. 14, Fig. 15, and Fig. 16 show experimental results (load vs. displacement) for 20, 40, and 70 fixture angles, respectively. Figure 15. Force (N) vs. Position or Displacement (μ) in 20 o fixture case Figure 13. Finite element model of the test configuration Figure 16. Force (N) vs. Position or Displacement (μ) in 40 o fixture case 1892

6 to thank NXP for providing test specimens. Also, the authors would like to thank Dr. Sam Graham at Georgia Tech for providing access to the DTS machine for conducting experiments. Figure 17. Force (N) vs. Position or Displacement (μ) in 70 o fixture case The following can be deduced from the above results; 1. It can be observed that the slopes of load-unload curves in each case, decreases with increase of crack length. This means that the compliance of the system increases with crack length in all cases. 2. As the global mode mixity increases from 20 o towards 70 o, the interface requires larger forces to delaminate at a given crack length. For example, the average critical force required to delaminate starting pre-crack of 7 mm is 7.55 N for 20 o, 11.7 N for 40 o, and 16 N for 70 o. This indirectly shows that the interfacial fracture energy increases with mode mixity. The interfacial fracture energy and the exact mode mixity for various cases will be presented in a future publication. VI. CONCLUSIONS It is found that delamination of the interface of bimaterial test specimens like Cu/EMC specimens at pure mode III loading is difficult. Such a loading leads to cohesive fracture of EMC before the onset of delamination. Simple loading fixtures are designed to apply mixed mode I- III loading on bimaterial test specimens. Experiments are conducted on copper-epoxy specimens using the designed fixtures. The fixtures are successful in delaminating the interface at higher mode mixities without fracturing the EMC. It is found that critical force required to delaminate the EMC/copper interface increases with increasing mode mixity. The developed test technique can be used to characterize the influence of mixed mode I-III loading on the interfaces of bimaterial specimens. ACKNOWLEDGEMENTS The authors acknowledge Semiconductor Research Corporation for funding this work. The authors would like REFERENCES [1] Lee, S. M. "An Edge Crack Torsion Method for Mode III Delamination Fracture Testing." Journal of Composites Technology & Research, 15(3), pp , [2] Martin, R.H., Evaluation of the split cantilever beam for mode III delamination testing, Composite Materials: Fatigue and Fracture. Vol. 3, ASTM STP 1110, September, pp [3] Farshad, M., Flueler, P., Investigation of mode III fracture toughness using an anti-clastic plate bending method, Engineering Fracture Mechanics, 1998, 60, pp.5 6. [4] Becht, G., Gillespie Jr JW., Design and analysis of the crack rail shear specimen for mode III interlaminar fracture, Composites Science and Technology, 1988, 31, pp [5] Ayatollahi, M.R., Saboori, B., A new fixture for fracture tests under mixed mode I/III loading, Eur J Mech- A/Solids, pp , [6] Crien, G., Perrete, M., A new fixture to analyze mode III toughness of bonded assemblies, Engineering Fracture Mechanics, pp.3-6, [7] Krieger, W. E. R., Raghavan, S., and Sitaraman, S. K., Experiments for Obtaining Cohesive-Zone Parameters for Copper-Mold Compound Interfacial Delamination, IEEE Transactions on Components, Packaging, and Manufacturing Technology, Vol. 6, No. 9, Sep. 2016, pp [8] Samet, D., Kwatra, A., and Sitaraman, S. K., Compliance-Based Approach to Study Fatigue Crack Propagation for Copper-Epoxy Interface, ASME - International Technical Conference and Exhibition on Packaging and Integration of Electronic and Photonic Microsystems (InterPACK2015) and the13 th International Conference on Nanochannels, Microchannels and Minichannels (ICNMM), San Francisco, CA, July 2015, InterPackICNMM [9] Samet, D., Kwatra, A., and Sitaraman, S. K., Cohesive Zone Parameters for a Cyclically Loaded Copper-Epoxy Interface, 66 th Electronic Components and Technology Conference, IEEE- CPMT and EIA, Las Vegas, NV, May