Low Dk/Df Polyimide Adhesives for Low Transmission Loss Substrates

Tasaki et al.: Low Dk/Df Polyimide Adhesives for Low Transmission Loss Substrates (1/7) [Technical Paper] Low Dk/Df Polyimide Adhesives for Low Transmission Loss Substrates Takashi Tasaki, Atsushi Shiotani, Takashi Yamaguchi, and Keisuke Sugimoto Corporate Development Dept., Research and Development H.Q., ARAKAWA CHEMICAL INDUSTRIES, LTD., 1-1-9, Tsurumi, Tsurumi-Ku, Osaka, Osaka 538-0053, Japan (Received August 1, 2017; accepted February 13, 2018, published March 30, 2018) Abstract We developed solvent-soluble polyimides with good heat resistance and low dielectric constant (Dk)/dissipation factor (Df) characteristics by optimizing the composition ratio of the aliphatic, cycloaliphatic, and aromatic groups present in the polyimide back bone. We found that the adhesives prepared using our polyimides showed good adhesion to polyimide films and low profile copper foils and low Dk/Df. Furthermore, we developed a three-layer flexible copper clad laminate (3LFCCL) with our polyimde (PI) adhesives, low-pofile copper foils, and normal PI films. This 3LFCCL showed a transmission loss similar to that of liquid crystal polymer (LCP) FCCLs at frequencies less than 20 GHz. This result indicates that by using our PI adhesives, lower cost FCCLs can be prepared, which can then be used as high-frequency substrates. Keywords: Solvent Soluble Polyimides, Low Dk, Low Df, Transmission Loss 1. Introduction The field of wireless communication and broadband technology has progressed dramatically with a growth in the market for information technology gadgets such as mobile phones and tablets. To meet the ever-increasing requirements of transmission data, the transmission frequency of circuits is increased. However, the integrity of high-frequency signals can be damaged by transmission loss. In high-frequency circuits, two factors are responsible for the transmission loss: conductor (circuits) loss and dielectric (insulating materials) loss. The former is related to the skin effect of circuits. The skin effect is the tendency for an alternating current to flow mainly near the surface of conductors. The general method for reducing the conductor loss is to make the surface of circuits smooth. However, smoothing the surface of copper circuits tends to weaken the adhesion between them and the insulating materials. Hence, it is difficult to fabricate copper circuits with a smooth surface and good adhesion to insulating materials. The dielectric loss of dielectric materials depends on their current frequency, dielectric constant (Dk), and dissipation factor (Df). Consequently, the dielectric loss increases with an increase in the current frequency. The general method for reducing this loss is to use low Dk and Df materials. Hence, low Dk and Df materials having good adhesion to the smooth copper surfaces are required for the fabrication of high-frequency printed circuit boards (PCBs). In PCBs, thermosetting insulating materials are used. For example, the prepregs for copper clad laminates (CCLs) are used as raw materials for fabricating mainboards, interlayer insulating materials for package substrates, coverlays, and bonding sheets for flexible printed circuit boards (FPCBs). These materials are mainly composed of cross-linking agents such as epoxy resins, fillers such as silica or organic phosphate to control the coefficient of thermal expansion (CTE) or to provide non-flammability, and polymers to provide flexibility and good adhesion. The cross-linking agents/fillers/polymer composition ratio varies depending upon the application. For example, in the case of the prepregs for CCLs, which are made by immersing a glass cloth in a cross-linking agents/ fillers/polymer solution, the concentrations of the crosslinking agents and fillers are higher than that of the polymer. This is done to achieve a high Tg and low CTE after curing. The same is the case with interlayer insulating materials used for fabricating package substrates. On the other hand, in the case of coverlays and bonding sheets for Copyright The Japan Institute of Electronics Packaging E17-006-1

Transactions of The Japan Institute of Electronics Packaging Vol. 11, 2018 FPCBs, the polymer concentration is generally high to achieve good flexibility and adhesion to polyimide films, which is important for this application. In any case, polymers are required for these materials. PCBs materials contain polymers such as acrylic polymers, butadiene-acrylonitrile copolymers, and phenoxy resins. In general, the polymers used in PCBs should possess heat resistance (mainly for solder reflow process), laminating workability, good adhesion properties, and flexibility. As already discussed, low Dk and Df materials are required for PCBs. Hence, the polymers used in PCBs should also have low Dk and Df. However, it is difficult for polymers to achieve a good balance between low dielectric properties and the other properties mentioned above. In addition, good adhesion to smooth copper surfaces is necessary to minimize the conductor loss. To meet these requirements novel polymers should be developed. Therefore, we focused on solvent-soluble polyimides as polymers required for high frequency PCBs. Polyimides possess excellent thermal, mechanical, and chemical properties and have been applied to various microelectronic applications such as IC passivation overcoats, FPCBs etc. Conventional polyimides are insoluble in organic solvents because of their strong cohesive and orientation forces, which result from their linear/stiff chains and imide groups having high polarity. Hence, for the fabrication of conventional polyimides, the preparation of a poly (amic acid) solution (precursor) is necessary. Poly (amic acid) is prepared by imidization of dianhydrides and diamines. The treatment of poly(amic acid) solution results in a decrease in its molecular weight and deteriorates its absorbent property. Hence, the treatment should be carefully monitored. Imidization is a difficult reaction as it requires high temperatures (300 C or more) and causes dehydration. Hence, the applications of polyimides are limited as they are difficult to process. In order to make the processing of polyimides easy, many researchers have developed solvent-soluble polyimides. The primary objective of improving the solvent solubility is to decrease the interaction among the polymer chains and stiffness of the polymer structure by introducing non-coplanar, flexible, and kinked units.[1] Solvent soluble polyimides don t require imidization process which needs high temperatures (300 C or more) for processing from varnish to coating layers by coating and drying. So, they have good processability compared with normal polyimides. Much research has been done on decreasing the n : Imide group : Aliphatic moiety (soft) : Aromatic moiety (hard) Fig. 1 Schematic of the polyimide structure. dielectric constant of polyimides. According to the Clausius-Mossotti equation, which gives the relation between the dielectric constant and molecular polarization, the dielectric constant of a material is proportional to the polarization rate and is inversely proportional to the molar free volume content.[2] Hence, the introduction of fluorine and cycloaliphatic groups,[3] bulky groups,[4] poly(silsesquioxane),[5] hyper branched structures, and silica[6] into polyimide structures has been carried out for improving their dielectric properties. Moreover, the thermosetting insulating materials used in PCBs should be easy of heat treatment (drying, laminating, curing) at temperatures below 200 C. Hence, the softening point of the polymers used in PCBs should be controlled. The softening point of materials reduces with a decrease in the number of functional groups and by the introduction of the flexible units used for improving the solvent solubility. In polymers, a reduction in the softening point makes their processing easy. However, it deteriorates their handling properties because of an increase in the stickiness and a decrease in the heat resistance and adhesion to films and metal foils. Based on the above discussion, we tried to design optimal polyimides for adhesives; in other words, we tried to optimize the composition ratio of aliphatic, cycloaliphatic, and aromatic groups in the polyimide backbone. We developed novel solvent-soluble polyimides with low Dk and Df, high heat resistance, good adhesion, and stability against processing conditions. Figure 1 shows the schematic of the polyimide structure developed by us. 2. Properties of Polyimide Figure 2 shows the photographs of the polyimide resin (in solution and solid states) developed by us. These polyimides show good solubility in solvents with relatively low boiling points (below 160 C) such as toluene, cyclohexanone, butyl acetate, etc. They could also be re-dissolved in these solvents after drying. In the solution state, the polyimides show good stability. Their viscosity remains unchanged even after a year at room temperature. These E17-006-2

Tasaki et al.: Low Dk/Df Polyimide Adhesives for Low Transmission Loss Substrates (3/7) Drying Fig. 2 Photographs of the polyimide resin. Table 1 Properties of our polyimides. Test item Unit Values Test Method Maximum Stress MPa 6.91 ASTM D882-97 Fracture Elongation %GL 100 < IPC-TM-650 Elastic modulus GPa 0.136 m2.4.19 Water Absorption Rate % 0.60 25 C 24 h Dielectric Constant at 10 GHz 2.50 Dissipation Factor at 10 GHz 0.0017 ASTM D2520 Weight Loss Temperature 1% 5% 440 properties make these polyimides suitable for PCBs. In addition, in the solid state, the polyimides show good flexibility and were not sticky. Hence, these polyimides possess good handling properties required for PCB fabrication processes such as lamination. Table 1 lists the mechanical and electrical properties of the polyimides developed by us. The fracture elongation (100% or more) and elastic modulus (136 MPa) values indicate that the polyimides have good ductility and flexibility. The water absorption rate of the polyimides is less than 1%. This value is lower than the water absorption rate of conventional polyimides (2 3%). This low water-absorption improves the insulation properties of the polyimides. Note that the dielectric properties of these polyimides at 10 GHz (Dk = 2.5, Df = 0.0017) are lower than those of normal polyimides (Dk = 3.2~3.3, Df = 0.007~0.01) and as low as those of liquid crystal polymers (LCPs) (Dk = 2.9~3.2, Df: 0.002). The thermal decomposition temperature (1%) of the polyimides is greater than 300 C. Thus, these polyimides are suitable for the solder reflow process carried out at a temperature of 288 C. The dielectric breakdown strength of the polyimides is not so high; however, we believe that this value is enough for PCB polymers. Hence, we think that these polyimides possess properties which 330 10% 455 Softening point C 80 Dielectric Breakdown Voltage TG/DTA Test results of Rheometer KV/mm 47 JIS C2110:1994 Fig. 3 Rheological properties of our polyimides. Fig. 4 The extent to which the properties (Dk, Df, and softening point) of our polyimides could be controlled. made them a good choice for PCB polymers particularly for high-frequency applications. Figure 3 shows the rheological properties of the polyimides. A drop in the modulus of rigidity (G ) indicates the softening of the sample being tested. From Fig. 3, it is clear that G continued to decline after ~80 C. This means that the polyimides begin to soften at 80 C, and the softening continued at temperatures greater than 80 C. For FPC adhesives used as coverlays or bonding sheets used for laminating circuits, the raw materials should have good fluidity and adhesion during the FPC fabrication processes such as hot pressing (generally carried out at temperatures 180 C). Hence, we believe that the polyimides developed by us are suitable for FPC adhesives because of their rheological properties. Figure 4 shows the extent to which the properties of our polyimides could be controlled. The left vertical axis shows Df, while the right vertical axis shows Dk. The horizontal axis shows the Tg of the polyimides. Red square points show Dk. Blue rhombic points show Df. We could control the Df (less than 0.0035), Dk (less than 2.8) and Tg (from 80 to 180 C) of these polyimides. We suppose that two grades less than 100 C is for FPC, and other grades of higher than 100 C is for package substrates and main boards. E17-006-3

Transactions of The Japan Institute of Electronics Packaging Vol. 11, 2018 3. Properties of Adhesives Used in Coverlays A coverlay is an adhesive-backed film (normally a polyimide film is used). It is used in FPCs for covering the circuits. First, we tested our polyimides to check whether they could be used as a component of coverlay adhesives. Conventional FPC adhesives have mainly three components: cross-linking agents (epoxy and hardener), polymers (elastomers), and flame retardants. As discussed earlier, these polyimides had a polymer (elastomer)-like structure. We used these polyimides in the form of a 30 wt% varnish containing cyclohexanone and methylcyclohexane as solvents. We found that an improvement in the compatibility between our polyimides and the cross-linking agents led to an improvement in the adhesion and heat resistance of the FPC adhesives. Hence, in order to improve the compatibility of our polyimides with the crosslinking agents, we mixed two types of polyimides (high Mw-type and low Mw-type) developed by us. We used multifunctional liquid epoxy resins as cross-linking agents. As flame retardants, we used phosphorus and silica fillers. Figure 5 shows the composition of the adhesive solution used for the coverlay test. The concentration of the adhesive solution was made 30% by mixing solvents such as toluene and cyclohexanone. We prepared the coverlay test sample (Fig. 6) according to the production flow diagram shown in Fig. 7. First, we coated a 12 μm thick polyimide film made in DU PONT- TORAY CO., LTD., Kapton EN with the adhesive solution. The coated sample was then baked at 150 C for 3 min to obtain a B-stage adhesive-backed polyimide film with a 10 μm thick adhesive layer. Finally, we prepared the adhesive test sample by bonding the B-stage adhesive-backed polyimide film to the shiny side of an electrolytic copper foil made in Furukawa Electric Co., Ltd., F2-WS at 120 C and curing the adhesive at 170 C for 30 min. We investigated the properties of the resulting sample. Table 2 lists various properties of the adhesive. The peeling strength value (1.0 N/mm) indicates that the adhesive had a good adhesion to the polyimide film and the shiny side of the electrolytic copper foil. Moreover, the adhesive had low Dk (2.7) and Df (0.0035) values. However, these values are greater than the Dk and Df values for the polyimide. Considering the incombustibility and solder heat resistance values of this adhesive, it can be concluded that it is suitable for the fabrication of coverlays for FPCs operating at high frequencies. Our polyimides The mix of 2 varieties 66.9% Cross linking agents Multifunctional epoxy Phenol novolac resin 3.1% Fillers Phosphorus type Silica type 30.0% Solvents: Cyclohexanone, Toluene etc Fig. 5 Composition of the adhesive solution used for the coverlay test. Electrolytic copper foil(18 μm) Adhesive layer(10 μm) Polyimide film(12 μm) Fig. 6 Structure of the coverlay test sample. Step Temp.( C) Time 1 150 3 min 2 120-3 170 30 min Coating 1. Drying of the adhesive solution 2. bonding to the 3. ring of the adhesive Foil Foil AD (C-stage) Conduct of tests Fig. 7 The production flow diagram for the coverlay sample. Table 2 Test results of the coverlay adhesive. Items Targets Results Peeling strength (90, 50 mm/min) 0.8 N/mm < 1.0 N/mm Dk at 10 GHz* < 2.8 2.7 Df at 10 GHz* < 0.005 0.0035 Incombustibility V-0 V-0 (Corresponding) Solder heat resistance (288 C 3 min) * We tested the hardened adhesive alone. 4. Properties of the Adhesive for A Three-layer Flexible Copper Clad Laminate (FCCL) FCCLs are used as substrates for FPCs. Polyimides are conventionally used as insulating layers in FCCLs and FPCs operating at high frequencies. There have been reports of using LCPs in substrates and adhesives for FCCLs because of their low Dk and Df values. However, special apparatus is required along with high temperature and pressure conditions to realize the lamination between LCPs. It is known that the adhesion of LCPs to copper circuits is not so good. Because of these problems, the applications of LCPs are limited. Therefore, we compared our polyimides with LCPs as raw materials for FCCLs to check E17-006-4

Tasaki et al.: Low Dk/Df Polyimide Adhesives for Low Transmission Loss Substrates (5/7) Our polyimides The mix of 2 varieties 95.0% Cross linking agents Multifunctional epoxy Cyanate ester 5.0% Solvent: Cyclohexanone, Toluene etc Fig. 8 Composition of the adhesive solution for a three-layer FCCL. Rolled copper foil (12 μm) Adhesive Adhesive layer (6-7 μm) PI film Polyimide film (12 μm) Adhesive Adhesive layer (6-7 μm) Rolled copper foil (12 μm) Fig. 9 Structure of the adhesive test sample for a three-layer FCCL. Step Temp. ( C) Time 1 150 3min 2 170-3 180 4hrs Coating, 1. Drying of the adhesive solution 2. bonding to the polyimide film 3. ring of the adhesives PI Film AD (C-stage) PI Film AD (C-stage) Conduct of tests Fig. 10 The production flow diagram for the adhesive test sample for a three-layer FCCL. whether our polyimides can overcome the problems of LCPs. We prepared an adhesive solution for a three-layer FCCL, as shown in Fig. 8. As in the case of the coverlay test, we mixed two types of polyimides (high Mw-type and low Mw-type) developed by us for improving the adhesion, heat resistance, and compatibility with the cross-linking agents. We used multifunctional epoxy and cyanate ester as cross-linking agents. The concentration of the adhesive solution was made 30% by using toluene and cyclohexanone. We prepared the three-layer FCCL samples (Fig. 9) according to the production flow diagram shown in Fig. 10. First, we coated low-profile rolled copper foils made in JX Nippon Mining & Metals Corporation, BHFX and BHFXV2 with an Rz (ten point mean roughness) of 0.9 μm and a thickness of 12 μm with the above-mentioned adhesive solution. Next, the coated samples were baked at 150 C for 3 min to obtain B-stage adhesive-backed copper foils Table 3 Test results for the adhesive test sample for threelayer FCCLs. Items Targets Results Peeling strength (90, 50 mm/min) 0.8 N/mm < 1.0 N/mm Dk at 10 GHz* < 2.8 2.5 Df at 10 GHz* < 0.005 0.0022 Solder heat resistance (288 C 3 min) * We tested the hardened adhesive alone. Transmission path Thickness x Width x Length : 12µm x 100µm x 10cm Insulation Impedance: materials 25µm 50Ω GND 12µm Fig. 11 Specifications of the micro-strip line used for the transmission loss test. with a 6 7 μm thick adhesive layer. The B-stage adhesivebacked copper foils were bonded to the top and bottom surfaces of a 12 μm thick polyimide film made in DU PONT-TORAY CO., LTD., Kapton EN at 170 C and curing the adhesive at 180 C for 4 h to obtain the adhesive test samples. We investigated the properties of these samples. Table 3 lists the test results of both samples using BHFX and BHFXV2. The peeling strength (1.0 N/mm) indicates that this adhesive showed good adhesion to the polyimide film and low-profile rolled copper foils (BHFX and BHFXV2). Moreover, the Dk (2.5) and Df (0.0022) values for this adhesive layer were low. On the basis of the test results listed in Table 3, we can conclude that this adhesive test sample is suitable for the three-layer FCCLs used in FPCs operating at high frequencies. We also investigated the transmission loss of a microstrip line fabricated by using the PI-type three-layer FCCL developed by us (represented as Our PI adhesive samples ). We used BHFXV2 in addition to BHFX as a copper foil in Our PI adhesive samples. By changing surface treatments, BHFXV2 is improved in conductive loss compared to BHFX in spite of having the same Rz (0.9 μm) as BHFX. At the same time, we also tested a normal PI-type FCCL and an LCP-type FCCL as a reference. In these reference FCCLs, BHFX is used as a copper foil. The specifications of the micro-strip line used in this test are shown in Fig. 11, and the structure of the samples used is shown in Fig. E17-006-5

Transactions of The Japan Institute of Electronics Packaging Vol. 11, 2018 Total thickness of insulating layer: 50µm, Circuit width: 100µm, The length of transmission path: 10cm, Impedance: 50Ω Fig. 12 Structure of the samples used in the transmission loss test. S21 (db) Our PI adhesive samples Adhesive(6-7µm) PI film(12.5µm) Adhesive(6-7µm) Table 4 Dk/Df of each sample used in the transmission loss test at 10 GHz. Sample type : BHFX or BHFXV2 Reference samples * Thermoplastic polyimide resins TPI* PI TPI* LCP : BHFX Normal PI LCP Dk/Df@10 GHz 12. Table 4 lists the dielectirc properties of each sample used in this test. Figure 13 shows the results of the transmission loss test (insertion loss, S21) carried out using a network analyzer. The S21 of our polyimide adhesive samples (represented as Our PI (BHFX) or Our PI (BHFXV2) in the figure) were lower than that of the normal PI-type FCCL (represented as Normal PI (BHFX) in the figure) and Our PI Dk Df Our PI (adhesive) 2.5 0.0022 Normal PI 3.2 0.007 LCP 3.0 0.0016 Frequency (GHz) 0 5 10 15 20 Fig. 13 Transmission loss test results (insertion loss, S21). Table 5 The S21 at 20 GHz of each sample in the transmission loss test. Sample type S21@20 GHz LCP (BHFX) -5.24 db Our PI (BHFXV2) -5.47 db Our PI (BHFX) -5.93 db Normal PI (BHFX) -7.35 db (BHFXV2) was close in value to that of the LCP-type FCCL (represented as LCP (BHFX) in the figure). Table 5 lists the S21 at 20 GHz of each sample in this test. The S21 at 20 GHz of Our PI (BHFXV2), -5.47 db is almost same as that of LCP (BHFX), -5.24 db. This result suggests that FCCLs with low transmission loss like LCP FCCL can be manufactured even in the case of using normal PI films as core substrates by using our PI adhesives and low profile copper foils. The FCCL obtained by using the PI adhesives developed by us and low profile copper foils was lower cost than the LCP-based FCCL, and thus can be used for the widespread commercialization of highfrequency FCCLs. In addition, the Df of our polyimides adhesive was found to be 0.0022. This value is higher than the Df of the LCP-based adhesive (0.0016). The transmission loss could be further minimized if we could further reduce the Df value (less than 0.002) of our PI adhesive. Hence, we are working on developing a novel adhesive composition to attain a Df value of less than 0.002. 5. Conclusions We developed solvent-soluble polyimides with good heat resistance and low Dk/Df characteristics by optimizing the composition ratio of the aliphatic, cycloaliphatic, and aromatic groups present in the polyimide backbone. We found that the adhesives prepared using our polyimides showed good adhesion to polyimide films and copper foils, good heat resistance, and low Dk/Df. Furthermore, we developed a three-layer FCCL with our PI adhesives, lowprofile copper foils, and normal PI films. This FCCL showed a transmission loss similar to that of LCP FCCLs at frequencies less than 20 GHz. This result indicates that by using our polyimide adhesives, lower cost FCCLs can be prepared, which can then be used as high-frequency substrates. Hence, we believe that the polyimides developed by us are suitable to be used as raw materials for high-frequency PCB substrates. E17-006-6

Tasaki et al.: Low Dk/Df Polyimide Adhesives for Low Transmission Loss Substrates (7/7) Acknowledgment We would like to thank H. Arai and R. Fukuchi (JX Nippon Mining & Metals Corporation) for collaboration and advice on this work. References [1] D. Liaw, Chem. Mater., Vol. 10, 734 739, 1998. [2] D. W. Van Krevelen, Properties of Polymers, 232, 590 596, Elsevier Scientific Publishing Company 1976. [3] M. Hasegawa and M. Koyama, High Performance Polymers, Vol. 15, 47 64, 2003. [4] Y. Watanabe, Y. Shibasaki, S. Ando, and M. Ueda, Journal of Polymer Science: Part A: Polymer Chemistry, Vol. 42, 144 150, 2004. [5] M. Tsai and W. Whang, Polymer, Vol. 10, 734 739, 2003. [6] S. Kim, X. Wang, S. Ando, and X. Wang, RSC Adv., Vol. 4, 27267 27276, 2014. Takashi Tasaki received B.S. and M.S. degrees in Materials Physics and Chemistry from the University of Kyushu, Japan, in 2001 and 2003, respectively. Since 2003, he has worked as an engineer at ARAKAWA CHEMICAL INDUSTRIES, LTD. He is now in charge of developing and marketing new polyimide materials. Atsushi Shiotani received the B.E. degree in 2005 from Nagasaki University, M.E. degree in 2007 and the Dr. degree in 2010 from Kyushu University. He joined ARAKAWA CHEMICAL INDUS- TRIES, LTD. and engaged in development of polymer for electronic materials. Now he works the development of adhesives for electronic materials. Takashi Yamaguchi received the B.E. degree in 2009, M.E. degree in 2011 from Okayama University. He joined ARAKAWA CHEMICAL INDUS- TRIES, LTD. and engaged in development of polymer for electronic materials. Now he works the development of adhesives for electronic materials. Keisuke Sugimoto received the B.E. degree in 2008 from Kyoto Institute of Technology, M.E. degree in 2010 from Osaka University. He joined ARAKAWA CHEMICAL INDUS- TRIES, LTD. and engaged in development of polymer for electronic materials. Now he works the development of adhesives for electronic materials. E17-006-7