Fatigue Crack Growth Behavior of Nanocrystalline Copper for Chip-to-Package Interconnects Cody Jackson* Dr. Ashok Saxena** (advisor), Rahul Rajgarhia** (graduate student) *Arkansas Tech University, **University of Arkansas, Fayetteville Mechanical Engineering REU project presentation, July 23 2007

Outline Background and Problem Statement Proposed Solution Experimental Procedure Research Results Discussion Conclusions

Interconnects in Flip-Chip Chip-to-package interconnect Silicon Under fill height Substrate pitch Due to the mismatch in the CTE of substrate and chip, interconnects experience cyclic stress (fatigue) due to fluctuations in current. In mobile electronics (laptops etc.) there is a greater demand for reliable interconnects due to possible damage when devices are dropped. From the above discussion, the electrical and mechanical properties of interconnects determine their feasibility for an application. Source: Garner et al., Intel Technology Journal, v9, n4 (2005).

Current Technology Lead solder is being gradually phased out due to environmental and legal requirements. Aluminum cannot meet the electrical requirements for fine pitch interconnects. Lead free solder also cannot meet the challenges of mechanical reliability and electrical conductivity for pitch sizes ~ 20µm (Aggarwal et al., 2002) There is a need to address the future challenges of reduced size of electronic packaging, specifically: Mechanical reliability due the reduced size and increased stress Electrical conductivity (current density, efficiency) Environmental concerns and Reduced cost Source: A. O. Aggarwal, P. Markondeya Raj, R. J. Pratap, A. Saxena, and R. R. Tummala, "Design and fabrication of high aspect ratio fine pitch interconnects for wafer level packaging," 2002 IEEE Conference at Singapore, pp. 229-34 (2003)

High-cycle fatigue resistance Proposed solution Nanocrystalline copper with grain size < 100nm Higher mechanical strength than lead free solder and high cycle fatigue resistance. Very good electrical conductivity Coarse grained Cu Nanocrystalline Cu Nf (cycles to failure) Ultrafine grained Cu 100 200 300 400 500 0.005 0.01 0.015 0.02 Strain (mm/mm) Stress (MPa) nanocrystalline copper microcrystalline copper Tensile test High-cycle fatigue resistance Source: S. Bansal, "Characterization of Nanostructured Metals and Metal Nanowires for Chip-To-Package Interconnections," in Materials Science and Engineering. Ph.D. dissertation, Atlanta: Georgia Institute of Technology, (2006). Zama, S., D. F. Baldwin, et al. "Flip chip interconnect systems using copper wire stud bump and lead free solder." Electronics Packaging Manufacturing, IEEE Transactions on 24(4): 261-268 (2001).

The Problem For nanocrystalline Cu to be a potential material for interconnects, it is critical to evaluate its performance under cyclic stress. Fatigue crack growth (FCG) behavior of nanocrystalline Cu is not yet known. Also, compare the results to FCG rate of microcrystalline Cu

Mechanical Properties Research Laboratory Fatigue testing was conducted on Cu101 (99.99%) and nanocrystalline (NC) copper in a 2.2 kiP servo-hydraulic machine made by Test Resources®. Cu101 was annealed at 500oC for 50 min to normalize it. Nanocrystalline Cu was produced using Equal Channel Extrusion Process Tests were conducted as per E647-05 “Standard Test Method for Measurement of Fatigue Crack Growth Rates”, ASTM International (2005). v 03.01.

Test specimen The specimen type is Compact Tension (CT). Specimens were designed as to ASTM E647. a W = 0.8”, B = 0.4”, a/W = 0.25 1.25W B

Research cont. Resolution = 0.001mm Range = + 2 mm Load, kN Time, ms The waveform used was sinusoidal. All samples were pre-cracked to produce a sharper initial crack. Crack mouth opening (v) is measured using an extensometer. Compliance is calculated from the unloading portion of the v/P plot using about 35 data points. Crack size (a) is calculated using the measured compliance as per ASTM E647. An average of 75 such crack size measurements is taken for each data point recorded. Crack increments (Δa) of 0.25 mm are recorded. Resolution = 0.001mm Range = + 2 mm Load, kN Time, ms

Research cont. vs ΔK plot is known as Paris’ Law. When plotted on a log scale, there is a linear segment known as the Paris regime which allows the stress intensity factor (ΔK) to be related to the sub-critical crack growth independent of the specimen geometry. Using this plot, you can estimate the life remaining in components due to fatigue cracking. Source: http://www.kuleuven.be/bwk/materials/Teaching/master/wg12/l1300.htm

Research cont. Samples were pre-cracked at constant load. Fatigue crack growth tests were conducted using decreasing ΔK method. ΔK = Kmax – Kmin (applied stress intensity parameter) ΔKo= intial ΔK (constant for a test) Δa = ao – a (crack increment) α = -0.08 mm-1 (normalized K gradient)

Data Analysis a The a vs. N plot was used to calculate da/dN. da/dN was then plotted against the calculated ΔK values. N

Results

Results

Comparison

Crack Size Correction To account for crack tunneling, the crack was visually measured. The visual measurement was compared with the compliance measurement.

Conclusion Due to increased demand for smaller, more portable electronic devices, current interconnect technology is not sufficient. To meet mechanical and electrical property requirements new materials are needed for interconnects. Nanocrystalline copper has a higher strength than microcrystalline copper and appears to be a material which can meet the new demands. Threshold values are very important because a common form of current technology (e.g. laptops and cell phones) will normally undergo a number of fatigue cycles that are in the threshold regime. For example, a laptop may be turned on/off twice a day, five days a week, for five years. This is a total of ~2600 cycles, on interconnects 25 x 10-3 mm. This gives an approximate da/dN value of 9.6 x 10-6mm. Manufacturers can take estimates similar to this and compare to current interconnect sizes to determine the estimated life of certain components in their products. Based on scenarios as before mentioned, it is important to know that through fatigue crack growth testing, nanocrystalline copper has shown increased resistance to fatigue crack growth, but with a slightly lower threshold value.

Acknowledgments Dr. Saxena (advisor) Rahul Rajgarhia (graduate student) Jeff Evans Sau Wee Koh Jeff Knox Dr. Zou Henry Wang