Abstract:
A vacuum-tight cable feedthrough device includes a metallic first flange that is penetrated by a slot. Passing through the slot is a flat stripline cable that includes a plurality of conductive signal channels encompassed by a dielectric material on whose upper and lower surfaces is disposed a conductive material includes a ground. The stripline cable is sealed within the slot to provide a substantially vacuum-tight seal between the cable and the first flange. In a preferred embodiment, the cable feedthrough device includes a plurality, at least 16, of stripline cables. In a further preferred embodiment, the device includes a second flange and a bellows sealably connecting the first and second flanges, thereby providing a substantially vacuum-tight, flexible housing for the plurality of cables.

Description:
This application is a divisional of U.S. patent application Ser. No. 08/958,834, filed Oct. 28, 1997 for VACUUM-TIGHT CONTINUOUS CABLE FEEDTHROUGH DEVICE, now issued as U.S. Pat. No. 6,093,886. 
    
    
     FIELD OF THE INVENTION 
     The invention relates to a cable feedthrough device, and more particularly to a substantially vacuum-tight device having stripline cables that pass through and are sealably connected to a metallic flange that can be subsequently mounted on a bulkhead or a vessel wall. 
     BACKGROUND OF THE INVENTION 
     In the liquid argon calorimetry system of the ATLAS experiment at the CERN Large Hadron Collider in Geneva, Switzerland, nearly a quarter million signal and calibration lines are required to pass through the walls of the ATLAS calorimeter cryostats. Signal feedthroughs for such applications can be constructed using individual pins sealed in glass or ceramic, but the conductor density required for the ATLAS calorimeter greatly exceeds the densities typically achieved using pin-based feedthrough technology. The two feedthrough planes, one extremely cold, the other relatively warm, would have to be very large to accommodate the required number of lines, resulting in a bulky device. The bulky design would complicate the assembly of the device and the installation of the requisite plumbing services in its vicinity. In addition, the large number of connectors required along the readout path would add to the construction expense and also result in degradation in signal quality. 
     An alternative to sealed pin technology for the fabrication of cable feedthrough devices entails the use of epoxy materials for the formation of vacuum-tight seals, as described in W. D. Wood and W. L. Wood, “Hermetic Sealing with Epoxy” in Mechanical Engineering, March 1990, Pave Technology Co. This technology, however, is suitable only for devices exposed to temperatures down to about −65° C. 
     Thus, there continues to be a need for a cable feedthrough device of compact design that is readily and inexpensively fabricated, and is also capable of maintaining vacuum-tightness even at very low temperatures. The device and process of the present invention meet this need. 
     SUMMARY OF THE INVENTION 
     In accordance with the present invention, a vacuum-tight cable feedthrough device comprises a metallic first flange that is penetrated by a slot. Passing through the slot is a flat stripline cable that comprises a plurality of conductive signal channels encompassed by a dielectric material on whose upper and lower surface is disposed a conductive material comprising a ground. Solder seals the stripline cable within the slot to provide a substantially vacuum-tight seal between the cable and the first flange. 
     In a preferred embodiment of the invention, the cable feedthrough device comprises a plurality, at least 16, of stripline cables. In a further preferred embodiment, the device includes a second flange and a bellows sealably connecting the first and second flanges, thereby providing a substantially vacuum-tight, flexible housing for the plurality of cables. 
     Further in accordance with the invention is a process for making a cable feedthrough device that, in a preferred embodiment, provides for applying a first solder by heating the first flange to a temperature about 20° to 50° C. below the first solder fusing temperature over about 25 minutes to 35 minutes, followed by heating the flange to about 20° to 50° C. above the first solder fusing temperature over about 2 minutes to 6 minutes, and then cooling the flange below the fusing temperature of the first solder. 
     The continuous cable feedthrough device of the present invention has several substantial advantages over the use of individually sealed pins: it provides desirable compactness; the uninterrupted passage of the cables results in a constant, controlled characteristic impedance along the entire signal path; the absence of pins and their mating connectors significantly lowers cost and simplifies installation; the continuous conductor through the soldered cable-flange interface provides improved electrical reliability. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic cross-sectional view of a stripline cable inserted in a slot. 
     FIG. 2 is a plane view of a flange provided with two slots for the insertion of stripline cables. 
     FIG. 3 is a schematic cross-section of a stripline cable. 
     FIG. 4 is a graph of the temperature of a flange as a function of heating time. 
     FIG. 5 depicts a flange containing 32 slots. 
     FIG. 6 includes a cross-section and end views of a cable feedthrough device comprising two flanges, a bellows, and 30 stripline cables. 
     FIG. 7 is a view which is not to scale of a flange with a slot and a stripline cable which contains sixty-four channels located in the slot in accordance with one embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     In accordance with the invention, a stripline cable passes through a slot in a metallic flange, to which the cable is sealably connected by solder. The slot is cut through the flange using Electrical Discharge Machining (EDM). In one embodiment as shown in FIG. 7, the flange is 0.375 inch (9.53 mm) thick, the slot  81  dimensions are 1.4 inches×0.030 inch (35.6 mm×0.76 mm), and the stripline cable  82  contains 64 signal channels. The inner edge surfaces of the slot are preferably pre-plated first with a nickel layer and then with an overlying solder layer, and the assembly is heated above the fusing temperature of a relatively low-melting solder while applying the low-melting solder to sealably bond the cable to the plate. Construction of a vacuum-tight feedthrough device requires a leak-tight cable and a solderable cable and slot, along with a solder of sufficiently low fusing temperature to ensure that the dielectric component of the cable, which is preferably a Kapton™ polyimide resin, is not damaged during the soldering process. 
     In a preferred embodiment, copper is electrodeposited on a portion of the edge surfaces of the stripline cable that are to be situated within a flange slot and sealably soldered therein. The outer surfaces of the cable where the low melting solder is to be applied are then preferably electroplated with 63/37 tin-lead solder, which provides good corrosion resistance and ease of solderability. As previously noted, the slots in the flange are preferably also electroplated with tin-lead solder, applied on top of a very thin (0.001 inch, 25 μm) underlying layer of nickel. 
     Among relatively low-melting solders, an indium-tin solder is preferred. A 50/50 indium-tin solder having a fusing temperature of about 250° F. (121° C.) is especially preferred for its excellent properties under cryogenic conditions. Indium-tin solder, furthermore, has good malleability and wettability as well as good resistance to thermal fatigue. To improve the solderability of the surfaces to be soldered, a low temperature activating (180° F. 82° C.), water soluble flux is preferably used to remove the metal oxides immediately before the solder flows, thereby ensuring complete solder wetting. 
     FIG. 1 is a schematic, not to scale, cross-sectional view of a stripline cable  1  inserted in a slot  10  cut through a metal flange (not shown). Cable  1  includes copper signal lines  2  having a width of about 150 μm (6 mils) and a thickness of about 35 μm (1.4 mils), copper edge traces  3  having a width of about 375 μm (15 mils), and a dielectric layer  4 , preferably Kapton™ polyimide having a total thickness of about 275 μm (11 mils). (The specific dimensions just recited and those that follow are illustrative.) 
     Encompassing dielectric layer  4  is a deposited copper ground layer  5  having a thickness of about 50 μm (2 mils). A thin (about 13 μm, 0.5 mil) layer  6  of solder , preferably a tin-lead solder, more preferably about 63/37 tin-lead, is deposited on ground layer  5 . 
     On the inner edge surface  11  of slot  10  is deposited a very thin (about 2.5 μm, 0.1 mil) layer  12  of nickel; a thin (about 13 μm, 0.5 mil) layer  13  of solder, preferably 63/37 tin-lead solder, is then deposited on nickel layer  12 . 
     The space within slot  10  between solder layer  13  and solder layer  6  on cable  1  is filled with solder bond  14 , which has a relatively low fusing temperature and preferably comprises 50/50 indium-tin solder. Solder bond  14 , which has a thickness of about 125 μm (5 mils), sealably connects stripline cable  1  within slot  10 . 
     Stripline cables are available from various commercial sources, including Flex-Link Products, Inc., San Fernando Calif., and Parlex Corporation, Methuen Mass. Twenty-inch (50.8 cm) lengths of flat cables were purchased from three independent manufacturers. These cables were individually tested before incorporating them into feedthrough devices. Cables from two of these companies satisfied all mechanical and electrical requirements; those from the third vendor were insufficiently vacuum-tight. 
     A soldering station was designed and constructed to assemble prototype feedthrough devices. These prototypes included a full size flange plate that was used to demonstrate that, despite the low heat conductivity of stainless steel, such a plate can be heated evenly to the temperature required for soldering. 
     Two single-flange feedthrough devices that contained two cables, each having 128 channels, were constructed. FIG. 2 depicts a circular flange plate  20  provided with tapped holes  21  and two EDM-cut slots  22 . 
     Both of the two-cable, single-flange devices were thermally cycled from room to liquid N 2  temperature at least 3 times ( one was cycled 20 times), then tested using pressurized helium at 4 bar. No leaking around the solder-sealed cable-flange interface was detected at 10 −9  bar cc/sec. One of these devices was also pressurized up to 20 bar, with no observed failure of the components. 
     The stripline cables included in the feedthrough device of the present invention should have the following characteristics: 
     Flexibility for smooth bending and twisting 
     Dimensional stability for a temperature range from −200° C. to +200° C. 
     Vacuum-tightness to better than 10 −9  bar cc/sec 
     Minimal heat transfer along the cable (through the metallic component) 
     Controlled characteristic impedance of about 50Ω per line 
     Minimal DC resistance (less than about 1 Ω/ft) 
     Minimal cross talk (less than 1%) between adjacent lines for very fast signals 
     Tolerances within ±10% 
     Calculation and optimization of the characteristic cable impedance and the cross talk between adjacent lines was carried out using analytical formulae given in B. C. Wadell, Transmission Design Handbooks Reading Mass., Mar. 25, 1990. The transverse cross section of one embodiment of a stripline cable  30  is depicted in FIG.  3 . Preferably, both the lines (signal channels)  31  and the ground  32  of cable  30  are formed from copper. The thickness of the copper should be large enough to minimize the line resistance but low enough to minimize etching effects. A thickness of 35 μm (1.4 mils, 1 oz. copper) was selected as the line thickness t. 
     Further specifying the signal conductor, lines  31  should be wide enough to minimize DC resistance while achieving the desired characteristic impedance, selected to be 50Ω per line. For a given impedance, the larger the width, the thicker should be the dielectric material. To keep the cable flexible and minimize cross talk, a dielectric  33  having a thickness d between lines  31  and ground  32  of 138 μm (5.5 mils), a dielectric  33  total thickness b of 310 μm (12.4 mils), and a channel width w of 125 μm (5 mils) were selected. 
     Once the thickness of the dielectric material is selected, the remaining important parameter for controlling cross talk between adjacent lines  31  is the spacing between them. To keep the cross talk in the specified range, the spacing c between lines  31  was chosen to be 250 μm (20 mils), center-to-center. 
     The stripline cables were manufactured by commercial vendors and tested in-house. A brief description of the feedthrough device fabricating procedure follows: 
     The metal flanges were machined to the appropriate dimensions in-house; the slots were cut and solder-plated by outside companies. After inspection of all components, the cables, pretested for vacuum tightness, were sealed into the flanges by soldering. The solder joints were leak-tested using a vacuum tester that was designed and constructed in-house. 
     The stripline cables containing Kapton™ polyimide dielectric material were manufactured using standard printed circuit technology. The cables were constructed of inert materials laminated so as to preclude trapped air molecules, thereby ensuring a leak-tight bond. The materials employed in one embodiment include: 
     Kapton™ polyimide resin, selected for its flexibility and manufacturability; 
     A modified polyimide adhesive having high viscosity at the bonding temperature 
     1 oz. copper for signal traces and ½ oz. copper for the ground 
     To avoid out-gassing, the lamination is typically carried out in the following steps: heating in an oven to remove absorbed water; removing reaction condensation; and laminating in a constant temperature press at a temperature of about 250° C. and a pressure of 4 MPa for approximately 30 minutes. 
     Stainless steel (SS) has a relatively low coefficient of thermal conductivity compared, for example, to copper. Therefore, solder sealing stripline cables to SS flanges requires special care. A soldering station capable of providing sufficient heat for the process was constructed, and a technique for performing the heating process efficiently was developed. 
     To achieve even heating of the SS plate, all elements are slowly preheated to about 230° F. , below the fusing temperature of 50/50 indium-tin solder. Then a faster heating rate is used to raise the temperature rapidly to about 275° F. As soon as the solder is completely melted and the cable-plate assembly attains thermal equilibrium, it is quickly cooled, using fans, to below the solder fusing temperature, and then allowed to cool further naturally. This results in a typical temperature profile as a function of heating time, as shown in FIG.  4 . 
     The described technique has been applied to sealing a multiplicity of cables in a single flange to yield a feedthrough device having many channels. FIG. 5 depicts plane and cross-sectional views of a circular flange  50  having 32 slots  51 . Sealing a stripline cable (not shown), each containing 64 signal channels, into each slot  51  of flange  50  provides a feedthrough device having 2048 channels. 
     The continuous feedthrough components were tested individually before assembly. Upon receipt from the manufacturers, the stripline cables were individually tested, using an in-house constructed cable tester, before being assembled with a flange. For this testing, the cables were temporarily sealed into test plates using bees-wax; the cable tester allowed for the testing of all possible leak paths, including end-to-middle and end-to-end. 
     A soldering station constructed in-house, which is capable of providing sufficient heat to raise the temperature of stainless steel flanges to the soldering temperature, employs six 200 W electrical heaters mounted on each side of the flange plates. These heaters provide up to 4.8 kW of heat to enable quick soldering. The vacuum tightness of the completed feedthrough devices was checked using an in-house fabricated vacuum tester. 
     FIG. 6 includes cross-sectional and plane views of a further embodiment of the present invention. Cable feedthrough device  60  comprises a first flange  61 , a second flange  62 , and a flexible bellows  63  sealably connected to flanges  61  and  62  to provide a substantially vacuum-tight enclosure  64  for  30  stripline cables  65 . Feedthrough device  60  is well-suited for use with a cryostat (not shown) connected to first flange  61 , which serves as a “cold flange.”Second flange  62 , which serves as a “warm flange,” is optionally equipped with heaters  66  for maintaining the temperature of flange  62  above the dew point and a thermocouple  67  for monitoring the temperature. Both flanges  61  and  62  are suitably formed from stainless steel plate having a thickness of ⅜ inch (9.5 mm). 
     An arrangement of  30  slots  68 , parallel to one another, is EDM cut through first flange  61 . A similar pattern of  30  slots  69 , parallel to one another but orthogonal to slots  68 , is cut through second flange  62 . Each of the stripline cables  65  is inserted in a slot  68  in first flange  61  and in a corresponding slot  69  in second flange  62 . The 30 cables  65  are subsequently sealably soldered over a single heating cycle, as illustrated in FIG. 4, to both flanges  61  and  62 . Because each slot  68  is orthogonal to its corresponding slot  69 , each cable  65  must be twisted through an angle of 90 degrees within enclosure  64 . This arrangement provides needed flexibility required during contraction and/or expansion of bellows  63  and enclosure  64  as the cryostat temperature varies over a wide range. 
     Bellows  63 , which provides flexibility to cable feedthrough device  60 , is preferably formed from thin stainless steel, a suitable thickness being 8 mils (200 μm). Bellows  63  is sealably connected, preferably by welding, to flanges  61  and  62 . Vacuum is established and maintained within enclosure  64  via port  70  in second flange  62  to a vacuum pump (not shown). 
     The invention has been described in detail with particular reference to preferred embodiments thereof, but it will be understood that variations and modifications can be effected within the spirit and scope of the invention.