Manufacturing of the Folded Flexible Circuit Board

Transcription

1 Manufacturing of the Folded Flexible Circuit Board Sanjiv Samant, Ph.D. Medical Physicist, Physics Division Department of Radiation Oncology St. Jude Children s Research Hospital 332 N. Lauderdale St. Memphis, TN Tel: Fax: sanjiv.samant@stiude.org Jinesh Jain, MS University of Memphis Research Assistant, Physics Division Department of Radiation Oncology St. Jude Children s Research Hospital 332 N. Lauderdale St. Memphis, TN Tel: Fax: jjain@memphis.edu Abstract Multilayering technique using a flexible folding circuit board is useful in any application requiring the routing of high-density conductors through a narrow available space. It offers the following advantages over the contemporary multilayering technique using rigid circuit boards: higher compactness, increased conductor density, simpler fabrication and assembly, and lower cost. Flexible circuit has the unique property of being a three-dimensional circuit that can be shaped in multi-planar configurations and reinforced in specific areas for specific applications. The key is structured successive folding in complimentary directions of the various parts causing a number of intermediate portions to be stacked, thereby reducing the final area. The final folded multilayer circuit is fabricated from a single flexible circuit comprising of a series of foldable strips, rigid as well as non-rigid, which are interconnected, The International Journal of Microcircuits and Electronic Packaging, Volume 25, Number 1, First Quarter, 2002 (ISSN ) 88

2 folded, and bonded into a composite structure. Such a folding results in a dramatic saving in the area needed to draw out the conductors associated with a planar circuit board, by slightly increasing the thickness. The added thickness is not an issue for space requirements since in any device there is ample space available in the thickness dimension for heat dissipation from the circuit board. With the uniform application of adhesives and sealant, the multi layer flexible board can also be made non-permeable. This paper presents the CAD design for the flexible circuit board, its folding process, and the manufacturing steps utilized in the fabrication of the prototype board. The process exploits the advantages offered by a flexible circuit in an optimum way to achieve high compactness and performance. In our application, we are able to manufacture a board, non-permeable in the direction of the folding up to 1500 psi, with a saving of 67% in area. Additional compactness is possible depending on the application. Keywords Flex, Circuit, Imaging, Multilayer, Portal, Compact 1. Introduction The kinestatic charge detector (KCD) [1] is a scanning, multi-electrode, and high-pressure ionization chamber with a distinctive feature of analog integration of the collected signal, resulting in a high contrast resolution. Ionizing radiation creates ions in the chamber, which drift towards the collecting electrodes, in the process inducing current in them. This current is measured at regular intervals, and the obtained signal is processed to yield the digital image. The megavoltage portal imaging system utilizing the KCD requires a large number of channels to achieve high resolution over a large field of view. However, the space available to route these channels out of the chamber to a data acquisition system, where they can be read, is highly limited. Modem devices, universally, will face this problem because technology aims at miniaturization causing interconnectivity between devices to get increasingly complex and demanding. The interconnections must not only accommodate miniaturized devices but they themselves should be miniaturized. The contemporary solution to routing a large number of conductors through a narrow space is multi layering. The idea behind multi layering is to distribute the conductors on multiple layers, thereby slightly increasing the thickness, but limiting the width. Currently multi layering is achieved by bonding multiple rigid circuit boards, each fabricated separately, into one composite multilayer circuit. However, this technique is expensive due to the high precision required for multiple board alignment, and the need for multiple electroplating processes. The conductor density is limited due to the presence of vias on the board for connectivity between multiple layers. The vias need a pad around them, much larger than the dimension of the hole, because of drilling imperfections. These pads also reduce the spacing between the conductors, increasing the capacitance, which is unacceptable in high-speed pulsed applications. Due to the above limitations, this technique is not always optimum. 89

3 An alternate technique to achieve multi layering is presented here. It utilizes flexible circuit boards in conjunction with a folding geometry. Flexible circuits have the unique property of being a three-dimensional circuit that can be shaped in multi-planar configurations. An organized and structured folding of such boards can achieve multi layering with a high degree of compactness, without the disadvantages of typical rigid multi layer boards. A dramatic savings is achieved in the area needed to draw out the conductors, associated with a single rigid circuit board, by slightly increasing the thickness. The added thickness is not an issue for space requirements since in any device, there is ample space available in the thickness dimension for heat dissipation from the circuit board. Increased conductor density can be achieved due to elimination of vias, and the folding of the board becomes a relatively trivial task with a flexible circuit. This technique can also be significantly cheaper than rigid multi layer circuit board multilayering depending on the application. However, to ensure that the folded flexible multilayer board achieves high compactness, non-permeability, and tight tolerances in the physical dimensions and electrical properties, it is necessary to have high precision in the folding, the right choice of sealant, and uniform application of the sealant between the successive layers of folds. 2. Design The design consists of a double-sided circuit laid out on a flexible substrate, Kapton Ô, with a uniform thickness of 0.007" (inches). The double-sided circuit constitutes of the circuit layout on one side and a ground plane on the other. The ground plane is a uniform thin layer of copper to minimize capacitance between layers when folded and stacked. The circuit layout of the front surface is shown in Figure 1. The size of the circuit board is 12" x 28", of which a portion of size 12" x 3.5" is needed inside the chamber, and the remaining portion is for routing the conductors out of the chamber from the width (3.5- inch side) of the board. A series of rigid and non-rigid parts are utilized to facilitate appropriate folding and stacking. A description of the various parts is as follows: Part A is the actual Collector Board, having 576 (N) conductors, and stays inside the chamber. It is made rigid by laminating a piece of FR4 material of same size on its backside, before the folding is performed to avoid any creases. Part B consists of four separate strips of flexible circuit that get folded along the yellow line, and under the collector board, during the folding process. The four strips are accomplished by cutting a thin slot in the flex circuit along the blue lines. Each strip contains 144 (N / 4) conductors, connected to corresponding conductors in the Parts A & C. Part C again consists of four separate strips that are reinforced with FR4. Each strip connects to the corresponding flexible strips of Parts B & D. The extra tab on C4 only carries extra conductors for voltage supplies in the chamber and is not involved in the folding. After the folding is complete, the strips are stacked on top of each other and they pass through the pressure seal in the end cap of the chamber. Part D is similar to Part B having four flexible strips and similar conductor details. This part is folded, along the yellow line, and under the connector board, during the folding process. 90

4 Part E contains multiple connectors through which the signal is read out and hence called the Connector Board. The conductors are routed in an organized manner and arranged in groups to facilitate a connector. The donuts represent the termination of conductors in vias for the connector. 3. Folding Process The first step is to fold the Part B strips along the 45-deg (yellow) lines, all in the same direction. Corresponding to the folds in Part B, a counter fold is performed in the opposite direction in Part D for each strip. Due to this, the strips in Part C get stacked on top of each other. These steps are graphically demonstrated in Figure 2. The folding must be performed very carefully using guides to achieve maximum alignment of the Part C strips for applications involving pressure seals or if the flex board is extremely long. Otherwise, the folding can be done manually with reduced precision avoiding unnecessary costs. Once these steps are carried out, the Collector Board, Part A, is folded back on to the folded Part B. Likewise, the Connector Board, Part E, is folded flat on to the folded strips of Part D. After folding, as clearly seen, conductors occupying a 12-inch length are routed through 4-inch width, with a slight increase in the thickness dimension, due to the stacking of layers. The following steps were followed while manufacturing the prototype board: Step 1: The flex circuit is laid on a flat surface and both its surfaces are cleaned thoroughly with 70% isopropyl rubbing alcohol. Step 2: The front surface of Part A and the tab on Strip C4 is covered completely with a high temperature KaptonTM tape (3M #5413). This is done to prevent any moisture from oxidizing the exposed tin-plating on the Part A conductors. While taping, care must be taken to not overlap any two adjacent layers of tape. This is very important since any overlapping of tape will create a depression on the collector surface, when the folded circuit is held under compression in a mechanical press. Step 3: Pieces of PSA (3M-486MP Pressure Sensitive Adhesive), cut to the size 3" x 3", are pasted to the Part B regions of the circuit on the ground side (bottom surface). The exact placement of the PSA is very important (measured 4.125" from the open end of Part A) to attain the folding at the right place. Step 4: Similar pieces of PSA are stuck to the Part D regions on the conductor surface (front surface). Again, the placement of the PSA should be precisely in the correct area (measured 5" from the open end of Part E). Step 5: The FF4 stiffener for Part A must be prepared by bonding PSA on its bottom surface. This stiffener is then carefully bonded to the bottom surface of Part A. High precision is required and exact superimposition of the stiffener on Part A is imperative since any misalignment will be multiplied along the length of the flex circuit and the folds will not overlap. Hence, the stiffener is kept on a flat surface and its PSA cover is peeled off. U sing a flat edge to align the flex, Part A of the flex is carefully positioned on the stiffener. Once the correct placement is established, the bond is secured by pressing the board with a roller. Step 6: The entire circuit is folded flat on itself, along the edge of Part A stiffener. Again, folds are secured using a roller. Care must be taken to not break any of the conductors. 91

5 Figure 1. Front surface of the fully extended (unfolded) Collector Board. The gray portion represents the conductors, fanned in and out to route the conductors. Part A; Actual Collector Board: Part B; Foldable Strips: Part C; Rigid Strips: Part D; Foldable Strip: Part E; Connector Board. Cutting along the blue lines forms the strips and the folds are performed along the yellow lines. 92

6 Step 1: Folding the flexible parts B&D such that the rigid strips (Part C) are stacked on top of each other. Steps in Folding (Conceptual) As indicated in the paper model Step 2: Fold the collector board (Part A) on to the stacked rigid strips Step 3: Fold the connector board (Part E) on to the stacked rigid strips Figure 2. The strips in Part B are all folded in the same direction along the 45- degree line. Corresponding to the folds in each strip of Part B, a counter fold is performed in the opposite direction in Part D. Due to this, the strips in Part C are stacked on top of each other. Careful folding with the help of guides achieves maximum alignment of the Part C strips. Step 7: Pre-folds are performed on Part B in the appropriate direction. These folds are only temporary and to facilitate an easy final folding of the complete circuit. Step 8: Similar pre-folds are performed on Part D in the appropriate direction (counter to the folds in Part B). These folds must be loose too. Step 9: The final fold for Part B4 is performed in such a way that the tab on strip C4 exactly aligns with the short edge of part A. Once this is done, the fold is finalized. While performing the fold, the adjacent strips should be carefully held so that the tear does not go beyond the tear stop. Step 10: Then the folds for Parts B3, B2 and Bl are finalized such that after folding, the strips C3, C2 and Cl align with the straight edge of strip C4. The folds are made permanent by using a roller. Step 11: The folds on Part D are performed using Part E to align with the straight edges. These folds should be made permanent taking care that all the strips of Part C are aligned with the straight edge of part A. If not, they should be forced to align before making the crease permanent. 93

7 Step 12: The PSA covers on the Parts B and Parts D are carefully peeled off to expose the glue, and the folded circuit is pressed again with a hand roller, making the folds permanent. The folding process is complete. Step 13: Once the folding process is complete, the seal area and the fold area have to be filled with epoxy to give a composite circuit board. Adequate amount of low viscosity Torr SealTM (Epoxy Resin Sealant by Varian Vacuum Technologies) is made homogeneous and kept ready for application. This choice of sealant was a result of factors suitable for the imaging system, which required a leak proof pressure seal across the circuit board up to 1500 psi. However, any sealant with a high bonding strength and viscosity can be used successfully. The work-time associated with Torr Seal is only min. so the next five steps must be completed within that time. Step 14: The backside of Part A (with stiffener) is laid on a flat clean surface and then a uniform layer of epoxy, roughly 0.005" thick, is applied on it. Then the folded flex Part Bl and strip Cl is placed on it. Next, a layer of epoxy is spread uniformly on the flex and the stiffener for C 1 is placed on it. The stiffener must be carefully aligned with the flex. Step 15: The above step is repeated for the Parts B2, B3 and B4 using stiffeners C2, C3 and C4. The alignment of various flex strips and stiffeners must always be kept in check. Sufficient epoxy must be applied in the tab area. Step 16: After the folded circuit is filled with epoxy, the sides of the circuit are covered with the high temperature Kapton tape to prevent the glue from squirting out from the sides. Now the circuit is ready to be hard-pressed in the mechanical press. Step 17: Two sheets of cardboard are kept on the steel base plate of the press. Then a sheet of Mylar (0.005" thick) is placed on the cardboard sheets. Then the folded circuit board is kept on the Mylar sheet with Part A facing up. The circuit board is then covered with another sheet of Mylar and the FR4 shims are arranged around the circuit board. Mylar is used because the epoxy does not bond to it. Yet another sheet of cardboard is placed, followed by the top steel base plate. The cardboard ensures the uniform distribution of pressure by deforming readily. Step 18: The entire sandwich is then put in a press at 9000 pounds and 140 c for 30 min, curing temperature and time for Torr Seal. The final thickness of the circuit board is determined by the thickness of the shims. Step 19: After the folded circuit is taken out of the press, it is set-aside for 24 hours for the epoxy to harden. Step 20: The circuit board is brought back to the required dimensions by machining off the excess epoxy squirted out in the press. Since the application requires a permanent fold, the general design constraint of the folding radius being 10 times the material thickness [3], does not apply. The plated through holes should be placed at least 0.05" away from the fold area to prevent undue stresses on the copper, leading to failure [2]. Various parts may need reinforcements, as per the specifications e.g. Collector board, Part A, must be made rigid to achieve flatness and precise positioning in the chamber; Part E needs to be reinforced so that it has the mechanical strength to hold the connectors; and Part C must be reinforced to effectively withstand the pressure seal. The FR4 stiffeners, not indicated in Figure 1, are laminated to the flexible circuit after the folding is completed [6]. Figures 3 & 4 show the final 94

8 folded circuit board with the required reinforcements. See Figure 5 for a cross sectional view of the final folded circuit board. 4. Results The final thickness of the flexible circuit with the copper and the polyimide coverlay is 0.011" (0.001" top coverlay " circuit copper " Kapton substrate " ground plane copper " bottom coverlay). The final circuit board, due to careful and precise folding and uniform glue thickness, achieved highly precise alignment up to 0.005" of the Part C strips. Of the 7 successive circuit boards folded during the prototype fabrication, 5 circuit boards had an acceptable degree of flatness with less than 0.005" variation. The first two boards were damaged due to initial setup problems related to improper placement of the shims in the mechanical press. Successive folding attempts yielded a 100% success. A conductivity test assured the continuity of all the conductors after performing the folding. The average channel-channel impedance was measured to be about 280 gigaohm. Figure 6 shows the impedance measured between adjacent conductors, randomly across the board. The minimum impedance of 50 gigaohm falls within the acceptable specifications on the board. The ground copper layer ensures a low capacitance between the conductors, measured to be 48 pf between adjacent channels. Figure 7 is a plot of capacitance measurements at four random locations across the board with the capacitance decreasing with increasing distance between channels, as expected. Significant savings in the area for conductor routing can be achieved with this technique and is demonstrated as follows. The circuit board measures approximately 28" x 12.2" when fully extended, amounting to an approximate total area of 342 inch2. After the third folding step the length and the width measures approx. 28" and 3.5" respectively, or a total area of only 115 sq. inches, amounting to a reduction of approximately 67% of the unfolded area. Even higher savings can be achieved, depending on the application, since the collector board (Part A) can be eliminated. The prototype folded circuit board has been successfully tested for the megavoltage KCD based imaging system, which is used for imaging radiotherapy treatment fields involving megavoltage photon energies. The circuit board encompasses a pressure seal while traversing from the inside to the outside of the detector. This pressure seal has been tested to withstand a pressure of 1500 psi. The currents flowing through the circuit board are on the order of nanoamperes, and hence there is no problem associated with heat dissipation. 5. Conclusions and Discussions The folding geometry using a series of rigid and flexible circuits achieves multilayering with significant compactness of area and a multitude of benefits e.g. lower cost, higher compactness and conductor density, and simplicity. The flexible folding multi layer board may offer a significant saving in the cost, depending on the application (a factor of four to five per board for this application, as compared to rigid multilayer boards). A major advantage is that the conductor density can be higher since it is not limited by the size of the vias, as in rigid multilayer circuit boards (as of act, 2001, for large boards, the hole and pad sizes are 0.010"/0.020" [2]). The elimination of vias also makes the fabrication economical since the vias are expensive to fabricate, electroplate and align. The flex circuits are touch and abrasion resistant due to the sandwiched construction and the top and 95

9 This edge of all the rigid strips (Part C) are aligned to this edge of the collector board (Part A) Rigid Strips (Part C) stacked on top of each other Collector Board Part A This edge of all the rigid strips (Part C) aligns with the end of these lips on the collector board (Part A) Figure 3. Final folded multilayer circuit board depicting the alignment of part C strips. Folding is performed using guides to achieve maximum alignment of the Part C strip edges with Part A. However, depending on the application, this alignment may not be critical and manual folding can be performed. Figure 4. A perspective view of the conceptualized folded circuit board illustrating the uniform thickness due to the FR4 stiffeners and the critical alignment of various edges. Gray cooler indicates the conductors, green represents the flexible folding strips, and yellow represents the FR4 stiffeners. 96

10 Collector Board Folded Flexible Strip Rigid Base Rigid Base for Part C1 Figure 5. Cross-sectional view of the final folded flexible circuit board with stiffeners. Red color indicates the collector board, blue represents folded Part B strips, yellow represents the stacked Part C strips, and gray represents the FR4 stiffeners. Figure 6. Plot of channel-channel impedance measured across the folded circuit board. The minimum measured impedance is 50 Gigaohm, which is higher than that required for the application. Each channel measurement represents the impedance between adjacent pairs, which have been chosen sequentially at pseudo-regular intervals across the circuit board s 576 channels. 97

11 Figure 7. Plot of capacitance measured between a randomly chosen (center) fixed channel and adjacent channels at increasing distance on either side. Each series represents a separate randomly chosen center channel. The interchannel distance is 0.020". As expected, the capacitance drops as the distance between the channels increases. bottom coverlay [4]. Flexible circuitry also offers excellent electrical performance due to the low dielectric constant of the substrate. Uniform glue thickness and precise folding ensure non-permeability under high pressure, as was demonstrated with our assembly technique and application. In case of high-current applications involving significant heat dissipation, one of several heat relief mechanisms can be used. These mechanisms include using water-capillaries around the heat producing areas, or a thermoplate extending beyond the circuit board area providing more cooling surface, or a thermocouple plane as part of the insulation layer to produce thermoelectric refrigeration or a fan. The CAD design for a flexible circuit is considerably faster and simpler than the design of a rigid multilayer circuiu8] A rigid multilayer circuit has to be designed in several layers and the layers then need to be connected with electroplated vias. The interconnection complexity shoots up rapidly as the number of layers increase, increasing the potential for design errors. Flexible circuitry offers the benefit of planar point-to-point connections eliminating a lot of design complications related to interconnectivity between layers. Multilayering, then, is reduced to a simple and organized folding of a single layer with the added benefits of reduced assembly costs and simplified manufacturing. 98

12 The folding flexible board geometry provides an inexpensive and easy method to route highdensity conductors through a small space in a structured, precise and reproducible manner. This concept can be useful in any application that involves routing of high- density conductors through inadequate space. Its application potentially spans many modem electronic devices, since the one of the current trends in electronics is miniaturization.[5,7] The extent of miniaturization is limited, not only by the individual electronic components, but also by the issue of interconnectivity between electronic components and interfaces to peripheral devices. The use of a folded flexible circuit board provides one solution to this interconnectivity problem. Acknowledgements [5] K. Casson, A Perspective On Flexible Circuitry As An Enabling Technology, Technology of Flexible Circuitry & Electronic Packaging, June, [6] J. Keating, Rigid Flex: An Answer To Dimensional Stability, Technologyof Flexible Circuitry & Electronic Packaging, January, [7] T. Costlow, Flex Gaining Favor In Chip- Package Realm, Electronic Engineering Times, (976),47, October, [8] T. J. Mack, Flexible Circuits Are Easier To Make, Electronic Engineering Times, (837),52, February, [9] F. Clyde, Printed Circuits Handbook: Fourth Edition, McGraw-Hill Publishers, New York, This work was supported by a grant (CA 76061) from the National Cancer Institute. References [1] F. A. DiBianca and M. D. Barker, Kinestatic Charge Detection, Med. Phys., 12, , [2] All Flex Inc., Design Guide Home [Online], Available: htill:// [3] J. Fjelstad, An Engineer s Guide To Flexible Circuit Technology, Bristol, England: Electrochemical Publications Ltd., [4] J. Fjelstad, Flexible Circuit Technology 2, Location: Silicon Valley Publishers Group,