Research on Crucial Manufacturing Process of Rigid-Flex PCB

Transcription

1 Mar Journal of Electronic Science and Technology of China Vol.4 No.1 Research on Crucial Manufacturing Process of Rigid-Flex PCB WANG Yang 1, HE Wei 1, HE Bo 2, LONG Hai-rong 2, LIU Mei-cai 2, WU Su 3 1. School of Microelectronics and Information Engineering, University of Electronic Science and Technology of China Chengdu China 2. Zhuhai Topsun Electric Technology Co., LTD. Zhuhai Guangdong China 3. College of chemistry and Environment Protection Engineering, Southwest University for Nationalities Chengdu China Abstract The main characteristics, applications, the emphases of manufacturing process are introduced, and the research of new product of rigid-flex Printed Circuit Board (PCB) is also described. In particular, the plasma desmear process, which is the crucial problems of manufacturing process, is discussed in detail. Samsung 4-layer rigid-flex PCB has been developed successfully, and the qualification rate reaches to 89.4%. Key words Printed Circuit Borad (PCB); rigid-flex PCB; manufacturing process; plasma desmear process The rigid-flex Printed Circuit Borad (PCB) has been available for nearly 25 years. And this technology almost immediately delivered on its promise of simplifying interconnection and assembly of complex interconnection structures. Because of the superiorly mechanical and electrical reliability, the rigid-flex PCBs have been used in the fields of military, aerospace, automotive, computer peripherals and portable electronics. In recent years, this technology has been steadily developed. But in China, the research about rigid-flex is in the preparatory stage, and there are no applications produced by civil manufacturers. Currently, all rigid-flex PCBs are imported from Japan and America etc [1]. This paper introduce main characteristics, applications and some new products of rigid-flex printed circuit board. Also Orthogonal Experimental Design study been carried out to optimize parameter of plasma desmear process. 1 Characteristics and Applications of Rigid-Flex PCB There have always been four drivers that have created the need for rigid-flex: 1) weight reduction; 2) reliability improvement; 3) the need for a compact package; 4) easier automatically assemble without special fixtures [2-4]. Rigid-flex fits weight reduction needs by using a very thin substrate. Typical rigid boards will be run on 0.1 mm core or thicker. Typical flexible circuits are run on mm core material. Obviously, the thinner the core and the the circuit are, the less it weighs. Because eliminating jumper cables and connectors or flat cable would be used to connect different rigid boards together, it gets a reliability improvement. Finally, the rigid-flex allows manufacturers to assemble the product and fold it into a nice compact package [5-8]. From an historical perspective, rigid-flex applications were developed primarily as an interconnect method for military systems. In recent years, this technology has been steadily migrating into commercial applications. Application utilizing rigid-flex as interconnect have been subjected to harsh environmental conditions such as thermal shock, thermal cycling, humidity, and salt spray and it is found to maintain electrical and mechanical integrity. From early 1980 s, rigid-flex has been used on every major military aircraft platform including the newly designed F-22 fighter. Also because its high reliability and the ability to reduce weight and volume, many commercial avionics packaging applications has been designed by utilizing this technology, including the 777 aircraft from Boeing. After 1990, rigid-flex has been used in the notebook computer, computer disk drive, automotive engine control, automotive navigation system and implantable medical device [9-10]. 2 Research and Development of New Product The manufacturing processes of rigid-flex PCB are very complex. Depending on different designs, there is distinctly dissimilar process. Received

2 No.1 WANG Yang, et al: Research on Crucial Manufacturing Process of Rigid-Flex PCB 25 Topsun Electronic Technology Co., LTD had developed several research on Rigid-Flex PCBs. First product is shown in Fig.1. It s is a double-sides rigid-flex PCB. The construction consists of a single-side flex and a layer of single-side rigid together with a modified epoxy prepreg. Rigid Area Flex Area Rigid Area Flexible base material Fig.1 Double-sides rigid-flex construction The second product is Samsung 4 layer flex-rigid PCB. This construction, which is shown in Fig.2., is comprised of a double-side Flexible Printed Circuit (FPC) with a single-side rigid laminated on each side. The adhesive is also modified epoxy prepreg. Flex area Fig.2 4 layer flex-rigid construction PI Coverlayer Flexible base material PI Coverlayer During the research, there are several processes should be noticed. 1) At the stage of the photochemical patterning of circuit, the quality of the image depend primarily the thickness of resist and the light source. A highly collimated, intense lights source is critical to generating a straight, clean image. The thinner the resist is, the easier it is to generate a fine image. In our country, there has been an increase in the availability of very thin dry-film resists. 2) The formation of microvia. In this stage, rigid manufacturers have made tremendous strides in the size and quality of drilled holes that they can generate with today s equipment. Flex, however, offers a variety of methods (mechanical drilling, punching, plasma etching, laser drilling) for via generation. This operation is very critical, since the rigid-flex is built-up with materials at very different hardness. Whereas the rigid material is relatively hard, the polyimide and in particular the adhesive are very soft. 3) It uses acrylic adhesive materials that have high Thermal Coefficient of Expansion (TCE). Because of the difference of TCE, the flexible portion can form groove. A new family of low- and no-flow prepreg epoxies can decrease the tensility, which is induced by adhesive. 4) After drilling, the polyimide and in particular the adhesive generate some smear which can t be removed by conventional chemical disposal and can induce low-reliability of rigid-flex PCBs. Ordinary desmearing with potassium permanganate, as used in conventional rigid board production, is not feasible, since the adhesive is not compatible with strong alkaline chemicals. The plasma surface treatment systems are now being used to deal with the problem. Not only removes desmear, but also improves plating adhesion and reduces the likelihood of circuit failure. However, Plasma etching system presents some challenges to the engineer to decide which attributes of via should be measured and controlled. We can set up designed experiments to discuss which parameters are crucial to characterizing the performance of plasma etching system and how to optimize these parameters, which are our emphases. 3 Emphases of Research To characterize the factors of plasma etching system is by conducting a Orthogonal Experimental Design (OED) study. The first stage is to decide the crucial parameters. There are five variables affecting the performance of plasma etching system. The relation between factor and level is shown in Tab.1. Level υ O2 /cc min 1 (A) Tab.1 -level list υ CF4 / c min 1 (B) RF/kW (C) T/( ) (D) t/min (E) 1 A1 (400) B1 (100) C1 (1500) D1 (110) E1 (8) 2 A2 (600) B2 (200) C2 (1200) D2 (130) E2 (10) 3 A3 (800) B3 (300) C3 (1600) D3 (150) E3 (6)

3 26 Journal of Electronic Science and Technology of China Vol.4 Under the same condition, two kinds of materials (epoxy, polyimide) are tested: the first material consists of a layer of adhesive material with single-side rigid laminated on each side; the other is comprised by two layers flex lamination. Sample size: 5 cm 5 cm. Measurement: before plasma etching weight its weigh (w 1 ) using electronic balance and after plasma etching weight (w 2 ) again. The equation for the etching rate is υ=(w 1 w 2 )/t, where t is time. The experiment scheme and result of experiment are shown in Tab.2. Tab.2 The experiment scheme and the related results Number υ O2 /cc min 1 (A) υ CF4 /cc min 1 (B) RF/kW (C) T/( F) (D) t/min (E) EP(epoxy) υ /mg min 1 PI (polyimide) υ/ mg min 1 1 A1 B1 C1 D1 E A1 B1 C1 D1 E A1 B1 C1 D1 E A1 B2 C2 D2 E A1 B2 C2 D2 E A1 B2 C2 D2 E A1 B3 C3 D3 E A1 B3 C3 D3 E A1 B3 C3 D3 E A2 B1 C2 D3 E A2 B1 C2 D3 E A2 B1 C2 D3 E A2 B2 C3 D1 E A2 B2 C3 D1 E A2 B2 C3 D1 E A2 B3 C1 D2 E A2 B3 C1 D2 E A2 B3 C1 D2 E A3 B1 C3 D2 E A3 B1 C3 D2 E A3 B1 C3 D2 E A3 B2 C1 D3 E A3 B2 C1 D3 E A3 B2 C1 D3 E A3 B3 C2 D1 E A3 B3 C2 D1 E A3 B3 C2 D1 E k k2 EP k R k k2 PI k R

4 No.1 WANG Yang, et al: Research on Crucial Manufacturing Process of Rigid-Flex PCB V O2 RF T t V CF4 υ/mg min A1 A2 A3 B1 B2 B3 C1 C2 C3 D1 D2 D3 E1 E2 E3 Fig.3 Main Effects Plot-PI Fig.5 SEM before plasma V O2 RF T t V CF4 11 υ/mg min A1 A2 A3 B1 B2 B3 C1 C2 C3 D1 D2 D3 E1 E2 E3 Fig.4 Main Effects Plot-EP Fig.6 SEM after plasma The relation between experiment index and factor of OED study is shown in Fig.3 and Fig.4. The plasma etching optimal parameter A3B3C3D1E1 is obtained by experiment analysis of OED study. The parameter of plasma etching parameter (A3B3C3D1E1) is optimization parameter or not should set up another experiment to verify. The acceptance of smear removal by plasma process is no smear found under SEM. The contrast between before plasma and after plasma is shown in Fig.5 and Fig.6. The final objective of desmear plasma is to enhance the hole wall adhesion which just be verified by plating adhesion check. Fig.7 is cross section metallography micrograph after check. Fig.7 shows that after plating and thermal stress (260, 10&20&30sec), cross section of the hole wall have no void & hole wall pull away. Fig.7 Cross section metallography micrograph after thermal stress Base upon all of above checks and standard of IPC, the desirable parameters of desmear plasma is indeed A3B3C3D1E1(υ O2 800 cc/min υ CF4 300 cc/min RF kw T 110 F T 8 min) which has been done in experiment No.25 in OED study. 4 Conclusions After the development of 2-layer rigid-flex PCB,

5 28 Journal of Electronic Science and Technology of China Vol.4 the experience of production had been accumulated. The discovery of crucial process and optimization of plasma etching system give engineers forethought and insight. About the Samsung 4-layer rigid-flex PCB, 1200 products had been made, the quantity of certified products is High qualification rate (89.4%) indicates the success of this project. References [1] King J. Rigid-flex boards/looking at a firm future[j]. FPC Fabrication, 1996, 19(9): 26. [2] McKenney D J. Cost-eEffective flex/high-volume multilayer flex and rigid-flex technology[j]. PC Fabrication, 1995, 18(6): [3] Maynard B, Irvine, Sheldahl, et al. Reliable plated though holes for rigid flex boards[j]. Electronic Packaging&Production, 1988, 28(9): [4] Joseph F. Improving rigid-fex plated through hole reliability[j]. Circuit Tree, 1997, 10(3): 78. [5] Joel Y. Fine-line flex circuitry/a glance into the future of flexible circuitry[j]. PC Fabrication Asia, 1995, 3(3): [6] Markstein H W. A manufacturing technology/improved fabrication technologies have made rigid-flex circuitry more reliable and affordable[j]. Electronic Packaging&Production, 1996, 36(2): 61-62, 64, 66. [7] Keating J, Larmouth R. High Volume Applications of Commercial Rigid-Flex[Z]. In IPC Printed Circuits Expo '96/Proceedings, San Jose, California, [8] Fjelstad J. Improving rigid-flex plated through hole reliability[j]. Circuit Tree, 1997, 10(3): 78. [9] Forman C. Advances in flex/rigid flex circuit prototyping technology[j]. IPC Printed Circuits, 1995, 7(3): [10] Mckenney D J. A Cost Effective High Volume Multi-Layer Flex and Rigid-Flex Technology[Z]. IPC Printed Circuits Expo'95/Proceedings, San Diega. CA, Brief Introduction to Author(s) WANG Yang ( 汪 洋 ) was born in Anhui Province, China, in He is a master student at University of Electronic Science and Technology of China (UESTC), majoring in applied chemistry. From Sept to Jun. 2005, He was a coagent of hole metallization of rigid-flex PCB project with Topsun Electric Technology Co., LTD. His current research fields include flex PCB and rigid-flex PCB manufacturing process. HE Wei ( 何 为 ) was born in Sichuan Province, China, in He received the M.S. degree in applied chemistry from Chongqing University in He is now a professor and master advisor at UESTC. His research interests include applied chemistry and electric chemistry HE Bo ( 何 波 ) is technology manager of Topsun Electric Technology Co., LTD. He has a lot of experience and professional knowledge in flexible printed boards. Now he is in charge of the design, manufacturing process, research and development of FPC. LONG Hai-rong ( 龙 海 荣 ) received the B.S. degree in Applied chemistry from UESTC in Now he is a superintend of Topsun Electric Technology Co., LTD. His reserch interests include electroless plating, plasma desmear and chemical analysis. LIU Mei-cai ( 刘 美 才 ) received the B.S. degree in Applied chemistry from UESTC in Now he is an engineer of Topsun Electric Technology Co., LTD. His research interests include manufacturing process of FPC, lamination, etching process. WU Su ( 吴 苏 ) was born in Anhui Province, China, in 1983, She is a bachelor student at SouthWest University for Nationalities. Her interests include environmental design and analysis.