«2011 Published by Elsevier Ltd. Selection and peer-review under responsibility of ICM11

Transcription

1 Available online at Procedia Engineering 10 (2011) ICM11 Mechanical Properties of Copper Thin Films Used in Electronic Devices Shengde Zhang a, *, Masao Sakane a, Takeshi Nagasawa b, Kaoru Kobayashi b a Department of Mechanical Engineering, Ritsumeikan University, 1-1-1, ojihigashi Kusatsu, Shiga, , Japan b Kyocera SLC Technologies Corporation, Head Office 656, Ichimiyake Yasu, Shiga, , Japan Abstract This paper presents tensile and low cycle fatigue properties of copper thin films used in electronic devices. Copper films tested were rolled copper, electrolytic copper, direct current plated copper and pulse plated copper films. Tensile tests and strain controlled 4-point bending low cycle fatigue tests were carried out and cycles to crack initiation and those for propagation were obtained. The direct current plated copper showed the largest Young s modulus among the copper films. The electronic copper showed the largest yield stress and ultimate tensile strength, and the pulse plated copper the largest elongation. The strongest resistance to crack initiation was found in the rolled copper but there was no large difference in the cracking resistance among the other copper films. Cracks of the direct current and pulse plated coppers propagated faster than those of the other copper films. «2011 Published by Elsevier Ltd. Selection and peer-review under responsibility of ICM11 Keywords: Copper; Thin film; Tensile properties; Low cycle fatigue; Crack. 1. Introduction Build-up circuit board has been widely used in various electronic devices to achieve high density mounting technology. The build-up circuit board is formed with laminated resin and copper thin films. Electronic devices, such as build-up circuit board, undergo cyclic temperature variation and the mismatch of the coefficient of thermal expansion of electronic parts causes cyclic thermal fatigue damage. In some cases, failure in the laminated circuit board was reported by the thermal crack propagation in copper films [1]. Numerical analysis, like finite element analysis, has been frequently used to estimate the stress and strain ranges in electronic devices and the ranges have been used to predict the thermal fatigue life. In this sense, accurate tensile properties and low cycle fatigue (LCF) endurance of copper films are essential for the quality assurance of laminated circuit board. Mechanical properties of conventional bulk materials are widely known but they are not necessarily same as those of thin films, because the properties of thin films may vary with fabrication process, texture and thickness. For example, Young s moduli of single-crystal gold film [2], Co-Ta-Zr film [3] and Cr film [4] increased with decreasing film thickness. Larger ultimate tensile strength of 304 stainless steel thin film was also reported than that of the bulk material [5, 6]. Therefore, mechanical properties as used in practice should be measured for reliable * Corresponding author. Tel.: ; fax: address: zhangsd@se.ritsumei.ac.jp Publi d by Elsevier Ltd. doi: /j.proeng

2 1498 S. Zhang et al. / Procedia Engineering 10 (2011) estimate of electronic devices. Mechanical properties of copper thin films have been extensively studied for Young s modulus [7, 8] and fatigue strength [9]. However, no systematic work has been made on the tensile properties of copper thin films. In addition, no work has studied crack initiation and propagation behavior under strain controlled fatigue test as far as the authors know. The objective of this paper is to study tensile and low cycle fatigue (LCF) properties of four kinds of copper films used in laminated circuit board. The four kinds of thin films were rolled copper, electrolytic copper, direct current plated copper and pulse plated copper. Tensile tests were conducted to obtain Young s modulus, yield stress, ultimate tensile strength and elongation. Also, strain controlled LCF tests were performed by using a 4-point bending machine and cycles of crack initiation and those for propagation were experimentally observed. The tensile and LCF properties of the four copper films were discussed. 2. Experimental Procedure This study used four types of copper films: the rolled copper (RC), the electrolytic copper (EC), the direct current plated copper (DC) and the pulse plated copper (PC) films. In the RC, rolled direction (RD) and its transverse direction (TD) were tested to examine the anisotropy of the RC film. Heat treatment (HT) at 473K for 1 hour was given to the RC and the EC films. Current density in plating process of the DC films was 1A/dm 2 and that of the PC was reversed 1A/dm 2 and 3A/dm 2 for 20ms and1ms. Figure 1 shows the shape and dimensions of a tensile specimen with 100mm parallel length, 8mm width and 18µm thickness. Two small white dots with 1mm in diameter were marked on the parallel part with a span of 90mm to indicate the gage length. The displacement of the two dots was measured by an image sensor with two CCD cameras of 1.5µm displacement resolution. Tensile tests were performed at a strain rate of 0.55%/s at room temperature using a tensile testing machine presented in the literature [10]. Five specimens were tested for a thin film under each testing condition. The shape and dimensions of the fatigue specimen are shown in Fig.2. The study used a glass epoxy substrate with 100mm length, 10mm width and 1mm thickness. Copper films with 18µm thickness were glued on each side of the glass epoxy. A circular hole with 40µm in diameter and 20µm in depth was processed on a side of the copper film. A strain foil gage was attached on a side of copper film to measure the strain range on the specimen surface. The experimental apparatus used was an originally designed 4-point bending machine for thin films. The details of the machine were published elsewhere [11] t = 18 µm Fig. 1. Shape and dimensions of the tensile specimen tested (mm) 100 φ 40µm 10 Copper thin film Glass epoxy plate Fig. 2. Shape and dimensions of the fatigue specimen tested (mm)

3 S. Zhang et al. / Procedia Engineering 10 (2011) Strain controlled 4-point bending LCF tests were performed at room temperature. The strain wave was a fully reversed triangle at 0.4Hz and normal strain ranges on the specimen surface were set to 0.5%, 0.7% and 1.0%. Fatigue tests were interrupted at the preset number of cycles and the crack length was observed by a CCD camera placed under the specimen. Figure 3 shows the definition of the crack length used in this paper. The crack length (2c) is defined as 2c=l 1 +l 2 +d including the diameter of the initial circular hole, where l 1 and l 2 are the crack lengths shown in Fig.3. The number of cycles to crack initiation ( c ) is defined as c =( )/2; 1 is the number of cycles at which the crack is firstly found and 2 the number of previous observation cycles to 1. l2 d 2c l1 Fig. 3. Definition of crack length 3. Experimental Results and Discussion 3.1. Tensile properties Figure 4 depicts the stress-strain curves of the copper films with that of annealed bulk copper [12]. All the copper films as well as the bulk copper showed clear strain hardening and only the bulk copper gave stress drop at the final stage presumably caused by local necking of the specimen. Elongations of the copper films were smaller than that of the bulk copper. Comparison of Young s moduli of copper films is presented in Fig.5. Young s moduli of the RC(RD) and the RC(TD) are ranged from 70.7GPa to 73.8GPa and were somewhat smaller than that of the EC. There is a little difference in Young s modulus between the RC(RD) and the RC(TD) so that there exits little anisotropy of Young s modulus in the RC. The largest Young s modulus was found in the DC and Young s modulus of the PC was similar to that of the RC(TD). Comparing Young s modulus between the DC and the PC, the difference of the fabrication of the two films is only the current waveform. The DC was made with a constant current while the PC with a reversal current so that the plating current reversal has an influence to reduce Young s modulus of the copper film. Since Young s modulus of the bulk copper was about 130GPa, Young s moduli of the copper films were all smaller than that of the bulk copper, which is consistent with the results of other papers [7, 8]. EC DC PC Bulk copper [12] RC(RD) RC(TD) Fig. 4. Stress-strain curves of copper films Fig.5. Comparison of Young s moduli of copper films

4 1500 S. Zhang et al. / Procedia Engineering 10 (2011) (a) Fig. 6. Comparison of (a) yield stress and (b) ultimate tensile strength of copper films (b) 1 2 σ 0. 2 = 228.9d Fig. 7. Relationship between yield stress and grain size of copper films Fig. 8. Comparison of elongations of copper films Figures 6 (a) and (b) compare the yield stress and the ultimate tensile strength for the copper films. The yield stresses of the RC(RD) and the RC(TD) were around 90MPa and were the smallest among the copper films tested. The yield stress of the EC was about 190MPa and was the largest. Comparing the yield stress between the DC and the PC, the yield stress of the DC was slightly larger than that of the PC, which means that the plating current does not significantly have an influence on yield stress of the copper films. The general trend of the ultimate tensile strength in Fig.6 (b) was same as that of the yield stress shown in Fig.6 (a). Figure 7 shows the relationship between the yield stress and the grain size. A linear relationship holds between σ 0.2 and d -1/2 called Hall-Petch relationship [13]. The relationship is expressed by the following equation. σ 0.2 =228.9d -1/ (1) The relationship indicates that the yield stress is mainly related to the grain size. The result that the film with larger yield stress has larger ultimate tensile strength, suggests that the ultimate tensile strength of the films is also related to the grain size. The elongations of the copper films are shown in Fig.8. The elongations of the RC(RD) and the RC(TD) were 12.6% and 8.1%, indicating that there is a little anisotropy in elongation of the RC. The elongation of the EC was 8.7%, which is similar to that of the DC. The elongation of the PC was 18.7% and was the largest among the copper films. The difference of processing between the DC and the PC is only the current reversal so that it also had a significant effect on increasing the elongation of the copper film.

5 S. Zhang et al. / Procedia Engineering 10 (2011) Crack initiation and propagation behavior Figure 9 shows photographs of surface crack observed at the circular hole part at ε=1.0% for all the copper films. Figure 9 (a) is the photograph where the crack was firstly found and Fig.9 (b) the photograph after fatigue test. Numbers in the figure indicate the observation cycles for each copper film. Cracks initiated at the circular hole and propagated normal to longitudinal direction of the specimen. This means that the crack propagated normal to the direction of maximum principal stress. Figure 10 correlates the numbers of cycles to crack initiation with strain range for the copper films. Lines in the figure are those drawn by a least square method. The RC showed the stronger cracking resistance than the other copper films. The number of cycles to crack initiation of the RC(RD) were slightly smaller than that of the RC(TD) but there is no large difference between the two results, which indicates that the difference of the rolled direction has a little effect on cracking. The numbers of cycles to crack initiation of the EC and the PC were smaller than that of the DC especially at high strain ranges. Figures 11 shows crack propagation curves of the copper films at ε=0.5%. Slopes of crack propagation curves of the RC(RD) and the RC(TD) were smaller than those of the other copper films. The propagation curve of the RC(RD) has nearly the same slope as that of the RC(TD) independent of the rolling direction. The slope of the EC was steeper than that of the RC. The DC and the PC showed the fastest crack propagation rate among all the copper films. RC(RD) RC(TD) EC DC PC (a) =1300 =1650 =300 =500 =200 (b) Longitudinal direction of the specimen =6300 =6000 =4000 =3700 =2000 Fig. 9. Cracks observed at the circular hole part at ε=1.0% (a) the crack was firstly found, (b) the test was stopped

6 1502 S. Zhang et al. / Procedia Engineering 10 (2011) Fig. 10. Correlation of the c with the strain range Fig. 11. Crack propagation curves of copper films at ε=0.5% 4. Conclusions (1) Young s modulus was about 100GPa for the direct current plated copper, but was only 70-75GPa for the other copper films. Young s moduli of all the copper films were smaller than that of bulk copper. (2) The largest yield stress was found in the electrolytic copper and the smallest in the rolled copper. Yield stress of the direct current copper was slightly larger than that of the pulse plated copper. The film with larger yield stress showed larger ultimate tensile strength. The Hall-Petch relationship held between the yield stress and the grain size for the copper films. (3) The pulse plated copper showed the largest elongation and it was around 8-12% for other copper films. (4) The strongest cracking resistance was found in the rolled copper but there was no large difference of the resistance among the other copper films. Cracks of the direct current and the pulse plated coppers propagated faster than those of the other copper films. References [1] Tsukada Y. Introduction of build-up circuit board. Nikkan Kogyo Shimbun Ltd: 2000, p [2] Catlin A, Walker WP. Mechanical Properties of Thin Single-Crystal Gold Films. J App Ph 1960; 31(12): [3] Hashimoto K, Sakane M, Ohnami M. Young s Modulus of Thin Films Measured by High Resolution Three-Points Bending Machine. ASME Mechanics & Mater Elect Pack 1994; 187: [4] Hashimoto K, Sakane M, Ohnami M. Development of Vibrating Reed Machine for Measuring Young s Modulus of Thin Films. JSMS 1995; 44(507): [5] Arai M, Ogata T. Development of Small Fatigue Testing Machine for Film Materials. JSME 2002; 68(669): [6] Fukushi M, Miyata H, Murakami A. Development on the Tensile Fatigue Test Apparatus and Strength Evaluation of Thin Metal Film. JSME 2006; 72(718): [7] Hong, S.H., Kim, K.S., Kim, Y.M., Hahn, J.H., Lee, C.S. and Park, J.H., Characterization of Elastic Moduli of Cu Thin Films Using Nanoindentation Technique. Com Sc Tech 2005; 65: [8] Tamakawa, K. and Miura, H., Microstructure and Mechanical Properties of Electroplated Copper Thin Films, Proc Mechanical Eng Cong 2006; p [9] Murata N, Tamakawa K, Suzuki K, Miura H. Fatigue Strength of Electroplated Copper Thin Films Under Uni-Axial Stress. JSMME 2009; 3(3): [10] Zhang S, Oka M, Nagasawa T, Terada K, Kobayashi K, Sakane M. Tensile Properties of Polyamide Thin Films for Electronic Devices. JSMS 2011; 60(2): [11] Umemura Y, Sakane M, Tsukada Y, Terada K. Low Cycle Fatigue Crack Initiation and Propagation for Copper Thin Films. Proc 20th JIEP [12] Howard E.B. Atlas of Stress-Strain Curves. ASM International, Metals Park, OH: 2000, p [13] Petch N.J. The Cleavage Strength of Polycrystals. J Iron & Steel Inst 1953; 174:25-8.