Presented at IMAPS 2002 Denver, Colorado September 5, 2002 (Best of Session Award) A Novel Flex Circuit Area-Array Interconnect System for a Catheter-Based Ultrasound Transducer Jeff Strole*, Scott Corbett*, Warren Lee**, Edward Light**, Stephen Smith** *MicroConnex, Inc., 34935 SE Douglas Street, Snoqualmie, WA 98065 Phone: 425-396-5707, Fax: 425-396-5861, http://www.microconnex.com **Department of Biomedical Engineering, Duke University, Durham, NC 27708 Contact author: corbetts@microconnex.com Abstract Flexible circuitry is uniquely suited as an interconnect media for medical sensor -array interconnection due to fine trace patterning capability, microvia technology and multilayer construction techniques. This paper describes a novel high density -array flex interconnect system for a catheter-based 3-D ultrasound imaging transducer. The catheter device consists of a two dimensional array of piezoelectric transducers operating at 5 MHz arrayed in a 12 x 16 matrix at 150 micron pitch. A six-layer flex substrate featuring 25 micron thick polyimide layers patterned with 25 micron wide trace features was fabricated as the first level interconnect connecting the sensor array to a cable system. The entire sensor/cable/flex system is placed within a seven French (2.33 mm o.d.) catheter. The array is used to image the internal surface of the heart from within the heart chamber itself. The flex interconnect features UV laser drilled via-in-pad technology with 50 micron vias and advanced registration technology. The top layer of the flex interconnect serves as a sacrificial layer which is post-patterned to provide acoustical and electrical isolation of the array elements after electrically interfacing the flex to the piezoelectric array. The flex circuit fabrication process and interconnect will be discussed. Keywords: Flex Circuit, Piezoelectric transducer, Array, MEMS, Ultrasound, Interconnection, BGA, PGA, Microvia, UV Laser Introduction The last twenty years have brought tremendous advances in medical imaging technologies. Ultrasound imaging in particular allows real-time images of the heart and other vital tissues to be generated, improving diagnostic outcomes and allowing therapeutic interventions to be monitored. Advanced electronic packaging technologies have played an unseen major role in this advancement, allowing solid-state phased-array technology to replace moving mechanical scanners, allowed miniaturization of scan heads for improved body access, and improved signal-to-noise ratios of transducer and electronic systems. A major recent innovation in ultrasound imaging technology is the advent of so-called 3D or volumetric imaging systems. Sensor devices for these systems contain a fully or partially populated 2D array sensor, allowing volumetric images to be captured by phased-array beam steering in the generalized space in front of the transducer. The packaging of the sensor portion of these devices is very similar to that required for semiconductor based systems and involve a hierarchy of interconnect levels including the Level 0 or 1 interconnect connecting the chip (or sensor) to the electronic imaging system. In this case the electronic interconnect must support a high-fidelity connection to each element within the arrayed sensor to allow all of the elements to operate simultaneously (in parallel) to properly beam form. The role of miniature electronic packaging is pushed to extreme limits with the advent of interventional imaging technologies. In these systems, the entire sensor device and interconnect must be packaged to fit within a small catheter which is inserted into the body to create images from within. These imaging catheters can also be combined with therapeutic functionality to allow diagnosis, treatment and monitoring in one package, ultimately reducing cost and improving health care.
Intracardiac imaging is among the newest of these modalities. These transducer/catheter systems are threaded through the venous system directly into the heart chambers, where they can create continuous images of the inner heart wall. These images allow real-time monitoring of interventional procedures, in particular catheter ablation, an increasingly common procedure to treat conduction related heart dysfunction. While existing catheter transducer systems capable of creating conventional 2D images of the heart wall are commercially available, this paper discusses recent advances allowing a 2D transducer to be packaged within the catheter footprint allowing true three-dimensional imaging of the interior heart wall. miniature coaxial cable is used to provide the highest fidelity signal transmission path to the sensor elements in ultrasound arrays, in this case the tight size constraint of the 7 French catheter (2.33 mm O.D.) required the use of a new non-coax ribbon cable technology (microflat ribbon cable, W.L. Gore). By using every other cable channel as a ground, sufficient isolation of individual signal channels was achieved. The micro cable is shown in Figure 3. Sensor Packaging System The generalized sensor probe is shown schematically in Figure 1. Each element within the 2D sensor system must be connected individually to a beam forming channel. In addition the entire sensor system must be packaged to fit within a flexible catheter. Figure 1 Schematic of 2D array sensor showing 3D volumetric scan planes. Figure 2 Sensor packaged withing catheter showing side scan (a) and angle scan (b) options. Figure 3 Top and end detail of ribbon cable. Flex Circuit Interconnect To facilitate interconnection of the cable system to the sensor device a specially designed flex circuit was developed. The flex performs several functions in the electronic packaging. Primarily it serves as the interconnection transition medium from the cable to the sensor itself. Rows of cable termination pads on the flex facilitate easy termination of multiple individual ribbon cables to the flex by mass termination means. The flex itself acts as a pitch converter to transition the signal to the tight array pitch of the sensor elements. The flex also transitions the geometry of the interconnect footprint to a 2D pad grid array matched to the exact sensor footprint allowing efficient interconnection to the piezoelectric sensor array. In addition to the traditional electronic packaging functions of the flexible circuit, the design of the circuit allows the top layer to serve as a sacrificial layer which is diced into after bonding of the piezoelectric sensor to increase acoustic isolation of the channels (See Figure 21). Finally the flex is designed to be acoustically transparent to allow sound to pass through the interconnect to an absorptive backing to reduce acoustic ringdown, which can adversely affect the image. The catheter itself is approximately 3 ft long requiring the use of a high performance flexible cable interconnect as the primary signal transmission medium to the sensor device. While traditionally,
Flex Circuit Architecture A multilayer flex circuit architecture was required in order to meet the geometry requirements of the 2D sensor system and the requirement of the top sacrificial layer. The fabrication process of a typical multilayer flex circuit is outline below: 1. Inner Layer Circuit Fabrication Inner layer circuits are fabricated using subtractive photolithography on pre-sputtered and electroplated polyimide media. 2. Lamination Inner layer circuits and unpatterned outer layers are laminated together using an acrylic bondply. Registration of the inner layer circuits is a critical element of the design and is based on a proprietary alignment method [2]. 3. Via Drilling and Metalization Vias are drilled through the laminated multilayer flex circuit and metalized using an electroless copper process followed by electoplated copper. Via alignment to inner layer circuits is critical. 4. Outer Layer Circuit Fabrication Outer layer circuits are fabricated using subtractive photolithogaphy. Any required covercoats are applied over outer layers. Figure 4 shows the cross section of the flex circuit designed for the 7 French catheter array. complexity of trace routing in the lead escape region in the sensor array. The routing density in the array (0.15 mm fully populated 12 x 16 array) is significantly greater than current chip scale packaging densities (typically 0.4 mm or greater array pitch). The result was a unique, tightly registered 6 layer design utilizing microvia technology and fine-line traces which combined the density of a chip scale circuit with a conventional multilayer flex. Figure 5 shows the overall footprint and dimensions of the flex circuit. Traces are routed out both ends of the array of the flex which is then folded back on itself to fit into the 7 French catheter package. Array detailed below Figure 5 Dimensions of flex circuit Figure 4 Cross section of the six layer flex circuit Flex Circuit Design While multi-layer flex circuit fabrication is common thoughout industry today, specific design constraints of the 7 French catheter made this circuit unique. First, the tight cross sectional of the catheter restricted the width of the flex circuit, reducing the amount of space available to route traces. A controlled impedance parallel lead configuration (every other trace grounded) was adopted to minimize electical crosstalk which in effect doubled the number of conductors required in the circuit. Acoustic impedance constraints limited the number of layers in the circuit. In addition, the overall length of the flex circuit (76.6 mm) was relatively long compared to the density and Figure 6 shows the top sacrifical layer of the flex in the array. No signal traces can be routed on this layer because of subsequent dicing of the sensor and top interconnect layer in both the x and y dimensions (see Figure 21). Laser drilled via-in-pad microvias connect the top pads to buried trace layers within the circuit. Figure 6 Top layer in the array.
Figures 7 12 below show the signal routing on each of the five trace layers within the array footprint region (approximately 2.5 mm x 2.5 mm). Traces and spaces of 25 micron are typical in the array. The dots show where the laser drilled microvias are placed throughout the layers. Figure 10 Layer 5 signal traces in the array Figure 7 Layer 2 signal traces in the array Figure 11 Layer 6 signal traces in the array Figure 12 show the composite of all 6 layers in the array. Registration is clearly a critical factor in this design. Figure 8 Layer 3 signal traces in the array Figure 9 Layer 4 signal traces in the array Figure 12 Composite of layers 1 6 in the array
Figure 13 below shows a typical pad configuration used for terminating the microflat ribbon cable. Guard traces routed between the signal traces on each layer and a top ground plane in the termination help provide the required electrical isolation. Figure 16 Layer 5 in the array Figure 13 Signal traces in the ribbon cable termination Figure 17 shows the top view of the circuit after lamination and outer layer processing. Note traces on multiple layers are visible and the tight registration between all layers. Flex Circuit Fabrication Figures 14 16 show the inner layer circuits after fabrication and before lamination. The circuit is fabricated by laminating a total of 3 layer pairs to yield six patterned layers. Trace layers 3 and 4 are patterned on opposite sides of the innermost layer while layers 2 and 5 are patterned on the inner side of the two outer layer pairs. Outer trace layers 1 and 6 are patterned after lamination, via drilling and plating. Figure 17 Top view of array termination region of flex circuit. Finally, Figure 18 shows the completed flex circuit in the region of the ribbon cable termination region. Figure 14 Layer 2 in the array Figure 18 Cable termination region of flex Figure 15 Layers 3 and 4 in the array Array Lamination and Post Processing After the flex circuit was fabricated, the microflat ribbon cable was terminated to the flex circuit by wire bonding or soldering (see Figure 19) and the piezoelectric sensor was laminated to the flex substrate and post prosessed. All manufacturing and
testing subsequent to the flex circuit fabrication was performed by the Duke University affiliated authors. Figure 19 Detail of cable-to-flex termination region. Figure 20 shows the array portion of the device after the piezoelectric sensor device has been terminated to the flex and diced into individual sensor elements. The sensor region contains a 10x14 array with 112 active elements operating at a frequency of 5 MHz. Figure 21 shows schematically the dicing process into the sacrificial top layer of the flex circuit. Figure 23 Final catheter assembly after potting into array Images were generated using a commercial ultrasound scanner (Volumetrics Medical Imaging, Durham, NC, USA). The scanner uses 16:1 parallel receive processing and generates 4100 B-mode lines at up to 30 volumes per second. Figure 24 below shows in vivo images in a sheep heart produced by the array. Figure 20 Piezoelectric sensor after termination to array region of flex and post dicing to separate elements. Figure 24 In vivo sheep heart images: 120 degree, 6 cm B-scans (A) and (B) and real-time 3D rendered view (C) of the left atrial chamber (LA) generated from the catheter array. Conclusions Figure 21 Schematic diagram showing trace breakout to upper array region and dicing saw cuts through top layer of flex interconnect creating acoustic and electric isolation of array elements. Figure 22 shows the entire cable/flex assembly during insertion into the 7 Fr catheter and Figure 23 shows the array assembly after potting of the sensor assembly into the catheter forming the final assembly. Figure 22 Cable/Flex/Sensor assembly before insertion into catheter In this paper we discuss a novel ultrasound catheter and interconnect system. The interconnect consists of a micro ribbon cable terminated to a custom designed multilayer flex circuit with high layer count and extremely tight registration accuracy. [1] Corbett, S., et al, Advanced Multilayer Polyimide Substrate Utilizing UV Laser Microvia Technology, Proceedings, IMAPS 2000 Symposium on Microelectronics, September 2000, pp. 212-216. [2] Strole, J. et al, Method of Creating an Electrical Interconnect Device Bearing an Array of Electrical Contact Pads, U.S. Patent 6,354,000, March 12, 2002. [3] Lee, W., et al, "Catheter 2D Arrays for Real- Time 3D Intracardiac Imaging: Increased Channel Count, Miniaturization and Tool Integration", 2002 US Navy Meeting on Acoustic Transduction Materials and Devices.