LCP - NSFC

Applied Surface Science 258 (2012) 2643–2647

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The effect of pretreatment on adhesive strength of Cu-plated liquid crystal polymer (LCP) Meisheng Zhou, Wenlong Zhang ∗ , Dongyan Ding, Ming Li State Key Laboratory of Metal Matrix Composites, Shanghai Jiao Tong University, Shanghai 200240, China

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Article history: Received 15 June 2011 Received in revised form 27 September 2011 Accepted 20 October 2011 Available online 31 October 2011 Keywords: LCP film Etching pretreatment Adhesive strength

a b s t r a c t Copper metallization on LCP was carried out by means of electroless plating followed by electroplating and the effect of pretreatment on the adhesive strength of the Cu-plated LCP was investigated in detail. Compared with the other etching agents used here, potassium permanganate was found to be the most effective and the optimum etching time is 20 min. With potassium permanganate as the etching agent, the adhesive strength could reach 12.08 MPa, which is much higher than the reported maximum adhesive strength (lower than 8.0 MPa). XPS spectra of LCP film indicated that hydrophilic groups were introduced into the LCP surface by etching, creating a nanometer-scale surface roughness and improving the wettability between copper and LCP. SEM and AFM observations revealed that the distinctly increased adhesive strength could be attributed to the improved wetting and the mechanical interlocking effect. The failure mode of Cu-plated LCP film was found to be dependent on the etching time. When the etching time was short, the failure mode of Cu-plated LCP film was mainly adhesive. As the etching time increased, cohesive failure gradually occurred, causing an adhesive/cohesive mixed failure mode. © 2011 Elsevier B.V. All rights reserved.

1. Introduction Recently, the rapid development of electronic devices has imposed higher requirements on electronic design and advanced packaging applications especially in high frequency field. Although the conventional low temperature co-fired ceramic (LTCC) technology has allowed the fabrication and integration of 3D circuits and printed circuit boards (PCBs) for high frequency applications, the integration capability is still limited. In addition, LTCC technology has higher cost as compared to an organic material-based technology [1]. Thus, as an alternative to LTCC technology, organic materials, which possess low dielectric loss, low cost, easy fabrication and high reliability, have been demanded recently [2]. Liquid crystal polymer (LCP), one of the next generation thin film organic dielectrics, is regarded as future substrate and packaging material for flexible PCB and 3D packaging due to its attractive properties such as low dielectric constant, low moisture absorption, low coefficient of thermal expansion and chemical resistance [3–5]. LCP is an ideal candidate of conventional PCB and 3D packaging materials and is very suitable to be used for high frequency (above 5 GHz) and high speed data transmission in wireless network and microprocessors. Generally, Cu-coated LCP films are mainly produced by lamination method and coating method including electroless plating,

electroplating, sputtering and chemical vapor deposition [6–9]. It is difficult to obtain a thick metal layer and the cost of rolled copper foil is high for the lamination method [6,7]. The electroplating method, wet chemical treatment and dry treatment (plasma treatment and reactive ion etching) are usually used to pretreat LCP surfaces. For the wet chemical treatment, the commercial chemicals like Circuposit Etch 3336 (Candor) are usually used. In this case, the adhesive strength is poorer compared with reactive ion etching (RIE) and plasma pretreatments [9]. The etching pretreatment with common acids like sulfuric acid and chromic acid has been also proved to be less useful for adhesion enhancement [10]. Therefore, the poor adhesion problem is always a big concern in the application of plated LCP films. For improving the adhesion strength, in this study, three etching agents were used for LCP film and then the etched LCP film was electroless plated, followed by electroplating. The effects of the different etching agents and etching time on the adhesion strength of the Cu-plated LCP film were investigated. The optimum etching agent and etching time for good adhesive strength were determined and the maximum adhesive strength of up to 12.08 MPa was obtained. The reasons for the improved adhesion were discussed. 2. Experimental 2.1. Materials and methods

∗ Corresponding author. E-mail address: [email protected] (W. Zhang). 0169-4332/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2011.10.111

The LCP film (Vecstar CTZ-50 ␮m) was supplied by Kuraray Co. Ltd headquartered in Japan. The film was cut into strips with 2 cm

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M. Zhou et al. / Applied Surface Science 258 (2012) 2643–2647

Fig. 1. Roughness of LCP surface and adhesive strength between copper and LCP by different pretreatment chemicals: M1 etched by KMnO4 , M2 by NaOH and carboxylic acid derivatives followed by CrO3 and H2 SO4 , M3 by CrO3 and H2 SO4 , M4-original LCP.

width and 5 cm length. Firstly, LCP samples were cleaned with a degreasing fluid consisted of trisodium phosphate, sodium carbonate, sodium hydroxide and emulsifier OP to get rid of smear and grease. After being rinsed with distilled water, the samples were pretreated by three wet chemical methods. In the first method, KMnO4 was used as the etching agent, and three steps were followed: swelling, etching and reduction. The LCP surface was etched with KMnO4 and NaOH after being immersed into swelling solution consisted of NaOH, ethylene glycol monomethylether and hexylene glycol for swelling, and then dipped into ammonium oxalate

Fig. 3. Adhesive strength of copper to the LCP as a function of etching time.

solution for reduction. In the second method, the samples were etched first with NaOH and dimethylformamide, and then with CrO3 and H2 SO4 . In the last method, the samples were just chemically etched with CrO3 and H2 SO4 . Each LCP film was treated for 5–30 min at different temperatures. Afterward, the treated samples were immersed in a colloidal palladium–tin catalyst solution at 40 ◦ C for 5 min. In this process, Sn2+ reacts with Pd2+ , causing Pd2+ reduction: Pd2+ + Sn2+ → Pd + Sn4+ The catalyst thus generated was colloidal and adhered to the surface of LCP. After the activation step, the samples were then immersed into a dilute fluoroboric acid solution to remove the excess sn4+ from the surface, leaving Pd particles on the LCP surface. Then the samples were rinsed with distilled water. The last step was to put the samples into an electroless plating solution at 45 ◦ C for 2 min, resulting in a 1 ␮m copper layer on both sides of the LCP film. The composition of the electroless plating solution is shown in Table 1. After that, electroplating was performed in order to produce a thicker copper layer. Electroplating bath consisted of 40 g/L of copper sulfate, 10 g/L of sulfuric acid and 50 mg/L of sodium chloride. The electroplating process was carried out at 30 ◦ C with current density of 3.0 A/dm2 for 15 min to obtain the thickness of 10 ␮m copper layer. 2.2. Characterization The Rhesca PTR-1011 bonding tester was used to measure the shear-off strength between LCP and copper layer. The Cu-plated samples were firstly bonded on the hard copper substrate. An aluminum stamp with a contact area of 9 mm2 was then glued onto the copper layer. The Loctite adhesive produced by Henkel was used here and had been proven to have enough strength to bind the samples and the stamp/substrate together. The distance between the blade of machine and the top of the copper layer was fixed Table 1 Electroless plating solution.

Fig. 2. XPS C1s spectra for LCP surface: (a) original LCP and (b) etching treatment.

Substance

Content

Copper sulfate pentahydrate Potassium sodium tartrate Sodium hydroxide Potassium ferrocyanide Formaldehyde (37%) pH Temperature Agitation

10 g/L 35 g/L 8.5 g/L 10 mg/L 10 ml/L 12.5 45 ◦ C Recommended


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Fig. 5. SEM picture of the copper layer on LCP (cross sections): (a) 2 min electroless copper plating followed by 15 min electroplating and (b) 2 min electroless copper plating followed by 15 min electroplating (the side with higher roughness).

groups caused by the chemical etching. A monochromatic aluminum K alpha line was used as an excitation source. Survey spectra were acquired from 0 to 1400 eV, with a pass energy of 50 eV and a step size of 0.5 eV. The core level spectra were obtained with a pass energy of 20 eV and a step size of 0.05 eV. 3. Results and discussion 3.1. Comparison of different etching methods

Fig. 4. AFM pictures in different etching time: (a) 10 min; (b) 20 min; and (c) 30 min.

to 200 ␮m with the shear speed of 0.5 ␮m/s. The data of adhesive strength can be read out from the machine. A FEI-Sirion 200 scanning electron microscope was used to observe the topographies of cross sections of copper-coated LCP film and fracture surfaces of both LCP and copper sides. The surface roughness of etched and un-etched LCP films was examined with a Multimode Nanascope IIIa atomic force microscope (AFM). The roughness hence obtained was the arithmetic mean of the roughness within a scanning length of 15 ␮m. X-ray photoemission spectroscopy (XPS) was used to monitor the change of function

In the electroless plating process, a copper layer was chemically deposited on the LCP surface with the help of a Pd/Sn catalyst as a result of redox reaction. Fig. 1 shows the adhesive strength under shear and the roughness of several samples. The figure that the adhesive strength between the copper layer and LCP film is the highest for the pretreatment with KMnO4 (M1), the second is for the pretreatment with NaOH (M2), the third is for the pretreatment with CrO3 and H2 SO4 (M3) and the lowest one is without pretreatment. The highest roughness of 136.42 nm corresponds to the etching with KMnO4 (M1) and in this case the highest adhesive strength of up to 12.06 MPa can be obtained, which is much higher than the previously reported highest adhesion strength of 8.0 MPa [10,11]. It can be concluded that KMnO4 is the most effective etching agent. The change in roughness with the different pretreatment methods has the same trend as the adhesive strength. The same change trend between adhesive strength and roughness indicates that the increased adhesive strength could be attributed to the increased roughness. It can be also found from Fig. 1 that the roughness of 98.2 nm for the case of M2 is higher than that of 84.8 nm for the case of M3, indicating that the additional NaOH and carboxylic acid derivatives can further increase the roughness.

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Fig. 6. SEM micrographs of LCP and copper surfaces after shearing test: (a) LCP side, (b) copper side, (a1) and (a2) etching time = 0 min, (b1) and (b2) etching time =10 min, and (c1) and (c2) etching time = 20 min.

The chemical bonding state of LCP surface with and without etching treatment was analyzed by XPS and the result of C1s spectra are shown in Fig. 2. The C1s spectra can be deconvoluted into several peaks which characterize the possible bonding states. In this case, the peaks centered at 285.96, 286.36, 289.08 and 291.72 eV are assigned to hydrocarbon bond (C–C/C–H), carbon–oxygen bond (C–O), carboxyl group (O–C O) and carbon dioxide group (O C O), respectively. Carbon dioxide group is considered to come from air contamination. The C1s spectrum of the treated LCP surface shows a notable increase in the intensity of C–C/C–H and O–C O and a obvious decrease in the intensity of C–O. These are expected to result from the destruction of ester bonds which produces oxygencontaining groups including phenolic hydroxyl groups and carboxyl groups. The interfacial chemical bonds like oxygen-containing groups can interact with metals producing metal–oxygen–carbon type bonding [12]. The oxygen-containing groups produced (carboxyl groups and phenolic hydroxyl groups) are hydrophilic and beneficial to wettability between electroless plating solution and LCP surface and hence can contribute to the adhesive strength [11,13,14]. The effect of etching time on adhesive strength has been investigated for the case with pretreatment of KMnO4 and NaOH. Fig. 3 shows the adhesive strength of copper to the LCP as a function

of etching time. As a comparison, the change in roughness with etching time is also shown in Fig. 3. It can be seen that the adhesive strength between LCP film and copper layer clearly increases with etching time first and then keeps stable after 10 min, while the roughness constantly increases with etching time. The big difference between the two change trends is that the roughness increases greatly after 20 min while the adhesive strength almost remains unchanged after 10 min. The role of etching in this case seems to increase the roughness, thus increasing the adhesive strength. Surface roughening is important in adhesion improvement because the higher the surface roughness of LCP is, the more the contact area between LCP and copper is. A larger contact area can enhance the anchoring effect and hence the adhesive strength between LCP and copper. As for the unchanged adhesive strength after 10 min, this may be attributed to interface defects and stress concentration caused by sharp angles. Fig. 4 shows surface morphologies of LCP films after different etching time. It can be seen that the longer the etching time, the more and sharper the polymer protuberances on the surface. The sharp angles of the protuberances may cause interface defects and stress concentration on the LCP/Cu interface, resulting in a decreased adhesive strength. A combined effect of the increased surface roughness and the protuberances with sharper angles may be the reason for the unchanged adhesive strength.


3.2. SEM analysis Cross sections of Cu-plated LCP film with KMnO4 as the etching agent were examined by SEM, as shown in Fig. 5. The thickness of 10 ␮m was achieved after 2 min electroless plating and 15 min electroplating processes. It was found that the copper layer thickness and surface roughness of LCP film are different on the two surfaces of LCP film although the treatment condition is the same for the two surfaces of the LCP film. The LCP surface with lower roughness has thinner thickness than the surface with higher roughness. It also shows that the copper layer and the LCP surface are in mutual engagement, forming the mechanical interlocking. Shearing fracture surfaces of both the copper side and the LCP film side exposed after the shearing test are shown in Fig. 6. From Fig. 6, a typical adhesive/cohesive mixed failure mode can be clearly seen. The typical adhesive failure was generated on LCP and copper sides in the case without etching treatment (Fig. 6(a1) and (a2)). As the etching time increases, the fracture surfaces of LCP side showed a fibrillar-like morphology and became rougher and rougher, while more and more polymer fibrils remain attached to the copper side, indicating that cohesive failure gradually occurred. It can be easily understood that the adhesive strength becomes higher when the cohesive failure mode plays a more and more important role in the shearing failure. 4. Conclusions In this study, copper metallization on LCP was carried out by means of electroless plating followed by electroplating and the effect of pretreatment on the adhesive strength of the Cu-metalized LCP was investigated. The conclusions can be drawn as follows: (1) A homogenous electroless copper layer of 0.5–1.0 ␮m was achieved on LCP surface while the thickness increased to 10 ␮m after electroplating process. (2) Good adhesion was achieved by etching with potassium permanganate. The optimum etching time for potassium permanganate etching is 20 min and the maximum adhesive strength is up to 12.08 MPa, much higher than the reported maximum adhesive strength of 8.0 MPa. (3) Oxygen-containing groups including phenolic hydroxyl and carboxyl groups are introduced into LCP surface by etching, creating a nanometer-scale surface roughness and improving the wetting between copper and LCP. The distinctly increased adhesive strength can be attributed to the improved wettability and mechanical interlocking. The shearing failure mode of Cu-plated LCP film was found to depend on the etching time. When the etching time was short, the shearing failure mode

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of Cu-plated LCP film was mainly adhesive. As the etching time increased, cohesive failure gradually occurred, causing an adhesive/cohesive mixed failure mode. Acknowledgements This work was supported by National Science and Technology major projects: Very Large Scale Integrated Circuit Manufacturing Equipment and Complete Process (No. 2011ZX02702) and by National Natural Science Foundation of China (No. 51171108). We would like to thank Kuraray Co. Ltd for supplying LCP films (Vecstar CTZ-50 ␮m). References [1] X. Zhang, Q. Zhang, G. Zou, L. Chen, Z.N. Chen, J.H. Liu, Development of SOP module technology based on LCP substrate for high frequency electronics applications, in: ESTC 2006: 1st Electron. Systemin. Technol. Conf., 2006, pp. 118–125. [2] D.C. Thompson, O. Tantot, H. Jallageas, G.E. Ponchak, M.M. Tentzeris, J. Papapolymerou, Characterization of liquid crystal polymer (LCP) material and transmission lines on LCP substrates from 30 to 110 GHz, IEEE Trans. Microw. Theory Tech. 52 (2004) 1343–1352. [3] X.F. Wang, L.H. Lu, C. Liu, Micromachining techniques for liquid crystal polymer, in: 14th IEEE Int. MEMS Conf., 2001, pp. 21–25. [4] N. Kingsley, G.E. Ponchak, J. Papapolymerou, Reconfigurable RF MEMS phased array antenna integrated within a liquid crystal polymer (LCP) system-onpackage, IEEE Trans. Antennas Propag. 56 (2008) 108–118. [5] N. Kingsley, S.K. Bhattacharya, J. Papapolymerou, Moisture lifetime testing of RF MEMS switches packaged in liquid crystal polymer, IEEE Trans. Compon. Packag. Technol. 31 (2008) 345–350. [6] T. Suga, A. Takahashi, M. Howlader, K. Saijo, S. Oosawa, A lamination technique of LCP/Cu for electronic packaging, in: 2nd Int. IEEE Conf. Polym. Adhes. Microelectron. Photon., 2002, pp. 177–182. [7] M. Howlader, T. Suga, A. Takahashi, K. Saijo, S. Ozawa, K. Nanbu, Surface activated bonding of LCP/Cu for electronic packaging, J. Mater. Sci. 40 (2005) 3177–3184. [8] R.N. Dean, J. Weller, M. Bozack, C.L. Rodekohr, B. Farrell, L. Jauniskis, J. Ting, D.J. Edell, J.F. Hetke, Realization of ultra fine pitch traces on LCP substrates, IEEE Trans. Compon. Packag. Technol. 31 (2008) 315–321. [9] J. Ge, M.P.K. Turunen, J.K. Kivilahti, Surface modification of a liquid-crystalline polymer forcopper metallization, Polym. Sci. B: Polym. Phys. 41 (2003) 623–636. [10] L. Chen, M. Crnic, Z.H. Lai, J.H. Liu, Process development and adhesion behavior of electroless copper on liquid crystal polymer (LCP) for electronic packaging application, IEEE Trans. Electron. Packag. Manuf. 25 (2002) 273–278. [11] T. Sugiyama, Y. Iimori, K. Baba, M. Watanabe, H. Honma, Surface metallization on high temperature liquid-crystal-polymer film by UV-irradiation process, J. Electrochem. Soc. 156 (D360) (2009). [12] E.M. Liston, L. Martinu, M.R. Wertheimer, Plasma surface modification of polymers for improved adhesion: a critical review, J. Adhes. Sci. Technol. 7 (10) (1993) 1091–1127. [13] B. Wang, W. Eberhardt, H. Kuck, Adhesion of PVD layers on liquid crystal polymer pretreated by oxygen-containing plasma, Vacuum 79 (2005) 129–133. [14] Y. Kurihara, H. Ohata, M. Kawaguchi, S. Yamazaki, K. Kimura, Improvement of adhesion and long-term adhesive reliability of liquid crystalline polyester film by plasma treatment, J. Appl. Polym. Sci. 108 (2008) 85–92.