Abstract:
A fabrication process for ferrite toroids which utilizes ferrite ceramic tape having an improved elongation characteristic. The process utilizes a set of rigid mandrels which are employed in the final lamination to support the rectangular cross section of the internal cavity of a respective ferrite tube, thereby reducing stress concentration and permitting the highest lamination pressure to be used in the final step. The mandrels are removed prior to panel densification. The tape and mandrels operate together to minimize cracks and pores in the toroids and provide an added advantage of maintaining high tolerances in the internal cavity dimensions as well as the cavity-to-cavity alignment.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates generally to a method of fabrication of ferrite phase shifters and more particularly to a method of fabricating ferromagnetic toroids using mandrels for controlling the toroidal shape during a lamination process of ferrite ceramic tape used to form the toroids. 
     2. Description of Related Art 
     Ferrite phase shifters are well known and comprise devices in which the phase of an electromagnetic wave at a given frequency propagating through a transmission line can be altered. Such devices have been extensively used in radar applications for electronic beam steering and phased array applications. 
     The most costly item in the fabrication of toroid ferrite phase shifters is the fabrication of ferromagnetic toroids where thin walled ferrite ceramic tubes having a rectangular cross section are conventionally made by pressing a ferrite powder into a mold followed by sintering a solid body and/or diffusion-bonding solid ferrite plates together. In order to maintain the dimensional tolerances required, particularly at high frequencies used for radar applications, for example, the ferrite tube still required some type of machining. For example, a Ka band toroid typically has walls of only 0.014 in. thick, with tolerances of ±0.001 in. Machining of these individual ceramic tubes, particularly the relatively weak ferrite type ceramics, is inherently expensive because of the touch labor involved which results in relatively poor yields due to breakage. Furthermore, the use of commercially available ferrite materials has led to magnetic and dielectric properties that are neither well controlled nor optimized. 
     One known fabrication process of which the present invention is an improvement, is shown and disclosed in U.S. Pat. No. 5,876,539 entitled “Fabrication of Ferrite Toroids”, issued to Alex E. Bailey, et al. on Mar. 2, 1999. This patent is assigned to the assignee of the present invention and is specifically incorporated herein by reference. 
     The process shown and described in U.S. Pat. No. 5,876,539 uses a ceramic tape having a predetermined dielectric and magnetic properties which is formed in the three contiguous slabs of ferrite. The ferrite slabs are then laminated at relatively high temperatures and pressure. The center slab is routed to form longitudinal slots which later comprise square openings of a toroid. The base slab and the slotted slab are laminated at moderate pressures and finally the top slab is added. The final lamination step used to attach the top slab is done at a lower pressure than that used for any of the pre-lamination steps. It uses a lower pressure for the final lamination in order to avoid collapse of the internal, unsupported cavity formed between the slots of the center slab. Use of such lower pressures results in poor bonding of the cavity walls to the upper and lower slabs, thereby forming a transition layer that is lower in density and in some instances contains rows of pores along the laminated interface after densification/sintering is complete. Cracks formed at this interface and poor bonding of the top slab have resulted in poor reliability and yield. Moreover, inclusion of pores the toroid structure is disastrous in terms of magnetic properties of the toroid and may not perform the required phase shift. The result has been extremely high cost accompanied by moderate performance. Because of this, the assignee of the present invention has developed its own process for making toroidal phase shifters. 
     SUMMARY 
     Accordingly, it is an object of the present invention to provide an improvement in the method of fabricating ferrite toroids. 
     It is another object of the invention to provide improved method of fabricating ferrite toroids which improves the dimensional tolerances and therefore enhances performance of ferrite phase shifters utilizing thin walled ferrite ceramic tubes. 
     It is a further object of the invention to provide an improved method of fabricating relatively delicate square ferrite toroids which improves yields and therefore lowers the cost of manufacturing. 
     The foregoing and other objects of the invention are achieved by a fabrication process which utilizes ferrite ceramic tape having an elongation characteristic of 15-25%, and the utilization of one or more rigid mandrels which are employed in the final lamination to support the rectangular cross section of the internal cavity of a respective ferrite tube, thereby permitting the highest lamination pressure to be used in the final step, and wherein the mandrel(s) are removed prior to panel densification. These modifications work together to minimize cracks and pores in the toroids and provide an added advantage of maintaining high tolerances in the internal cavity dimensions as well as the cavity-to-cavity alignment. Such process improvements lead to increased yields and lower costs. After the final tape panel is densified, the top and bottom faces are ground and polished to provide an exact dimension on the top and bottom walls. A dual ferrite toroid can be made thereafter by aligning ferrite tube cavities and joining a pair of such toroids one on top of the other. 
     Further scope of applicability of the present invention will become apparent from the detailed description provided hereinafter. It should be understood, however, that the detailed description, while indicating the preferred method of the invention, is provided by way of illustration only, since various changes, alterations and modifications coming within the spirit and scope of the invention will become readily apparent to one skilled in the art. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention will become more fully understood when the detailed description of the invention provided hereinafter is considered in conjunction with the accompanying drawings, which are provided by way of illustration only and thus are not meant to be considered in a limiting sense, and wherein: 
     FIGS. 1-11 are perspective views illustrative of the fabrication steps utilized in accordance with the preferred method of forming ferrite toroids in accordance with the subject invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The present invention starts with a plurality of 8.5 mil. layers of ferrite tape having a B r  between 3360-3650 gauss and having an elongation characteristic of 15-25%. 
     The first step in the fabrication process is shown in FIG.  1  and involves laying up three sections of ferrite tape, including a top section  10 , a center section  12  and a base section  14  on a copper laminate layer  16 . The top section  10  includes four ferrite tape layers  18   1 , . . .  18   3 , a center section  12  which includes three ferrite tape layers  20   1 ,  20   2 , and  20   3  and the base section includes four layers of ferrite tape  22   1 , . . .  22   4 . Further as shown in FIG. 1, a top layer or sheet of polyester film  26  such as “Mylar” (a trademark of Dupont) is positioned atop the top section  10 , an intermediate layer  28  of Mylar™ is positioned between the top and center sections  10  and  12 , and a third layer  30  of Mylar™ is placed between the center and base sections  12  and  14 . A bottom layer  32  of Mylar™ is located between the base section  14  and the copper laminate layer  16 . 
     With the various component layers laid up in a stack as shown in FIG. 1, they are then isostatically laminated in a laminating fixture, such as an evacuated bladder  34  shown in FIG. 2, at a pressure of 1500 psi and temperature of 72° F. for fifteen (15) minutes. 
     Following this, a router device is used to simultaneously form tooling holes  36  in all three sections  10 ,  12 , and  14 , of ferrite tape  18 ,  20  and  22  as well as the Mylar™ layers  26 ,  28 ,  30 , and  32 . The Mylar™ layers  26 ,  28 ,  30 , and  32  with tooling holes  36  are thus usable for later use as will be shown hereinafter. 
     Referring now to FIG. 4, following the first lamination step, the top and base sections  10  and  14  are removed leaving the center section  12  with the layer  28  of Mylar™ on top. This is followed by forming a plurality n of parallel elongated grooves or slots  38   1 , . . .  38   n  in the center section  12  using the same router used to form the tooling holes  36 . In a preferred embodiment n=27. Following the formation of the slots  38   1 , . . .  38   n , the Mylar™ layer  28  including the slots formed therein is removed from the center section  12 . 
     Following the removal of the slotted Mylar™ sheet  28 , the slotted center section  12  is visually inspected and any debris in the slots is removed so that slot edges are clean and square. This is followed by a second lay-up procedure as shown in FIG. 5, where the slotted center section  12  and base section  14  are laid up on the copper laminate layer  16 , with the Mylar™ sheet  32  with tooling holes  36  being in place between the base section  14  and the copper laminate  16 . A new strip of Mylar™  40  is next placed between the center and base sections  12  and  14  so that it extends inwardly past the tooling holes  36  to the proximate ends of the slots  38   1  . . .  38   n  as shown by the dotted line  42 . Also, the strip of Mylar™  40  as shown in FIG. 5 also extends out from the edges of the center and base sections  12  and  14 . 
     Referring now to FIG. 6, the previously slotted piece of Mylar™  28  is now placed over the slotted center section  12  along with a piece of latex  42  which is placed over the entire lay up. This is followed by a second lamination step in the evacuation bladder  34  at 1500 psi. and 72 20   F. for 15 minutes. 
     Following the lamination step shown in FIG. 6, the assembly is removed from the bladder  34 . The latex layer  42 , the Mylar™ layer  28  and the underlying strip of Mylar™  40  is then cut at the ends of the slots  38   1  . . .  38   n  along the dotted line as shown by reference numeral  42 . The strips of Mylar™  28  and  40  as well as the overlying latex sheet  42  are removed, exposing the ends of the slots  38   1  . . .  38   n , as shown in FIG.  7 . 
     Next as shown in FIG. 8, a set of slightly rounded mandrels  44   1  . . .  44   n  for reducing stress concentration in the ferrite tape by supporting the internal cavities of the slots and being equal in number to the slots  38   1  . . .  38   n  and having a length longer than that of the slots  38   1  . . .  38   n  are coated with lecithin oil using a sponge wipe and placed in the slots  38   1  . . .  38   n . Although not shown, when desired the slotted area on top of the center section  12  can be covered with a sheet of Mylar™, not shown, so that only the slots that already contain a mandrel are exposed. Each mandrel  44   i  is slid into its respective slot  38   i  beneath the Mylar™ sheet. This will avoid smears of oil on top of the slotted center slab  12 . 
     Once all of the mandrels  44   1  . . .  44   n  are placed in the slots  38   1  . . .  38   n , the top section  10  which was laminated in the first lamination step shown in FIG. 2 is trimmed so that it matches the current size of the slotted center section  12 . The top section  10 , thus trimmed, is placed over the center section  12  as shown in FIG.  9 . 
     Referring now to FIG. 10, the entire assembly shown in FIG. 9 is covered with a full sheet of Mylar™ containing a set of tooling holes  36 . The Mylar™ sheet  46  extends to the edges of the assembly so that all ferrite layers are covered. Next, the area where the mandrels  44   1  . . .  44   n  are covered with a strip  48  of rubber to guard against tears in a lamination bladder  50  into which the entire assembly is in place as shown in FIG. 10. A third isostatic lamination process is then effected at 72° C. at a pressure between 3000 psi and 6000 psi, e.g., 4500 psi for fifteen minutes. 
     Following the third lamination step, the assembly is removed from the bladder  50  and the copper laminate layer  16  is carefully separated from the assembly as shown in FIG.  11 . This is followed by cutting the base slab  14  along with the underlying Mylar™ layer  32  from beneath the mandrels  44   1  . . .  44   n . Next, the portion of the assembly remaining is clamped on a fixture, not shown, and the mandrels  44   1  . . .  44   n  are pulled out one or more at a time, typically two or three at a time. 
     Following removal of the mandrels  44   1  . . .  44   n , the laminated assembly is densified by firing and the top and bottom faces are ground and polished to provide an exact dimension on the top and bottom walls. This is followed by dicing into a desired slab size. 
     Note that the foregoing procedure is more efficient than individual machining of toroids, since numerous toroids can be ground in one operation. The size of the slab that can be used for grinding depends on the alignment of the cavities with the outside surfaces and the flatness or camber of the assembly shown, for example, in FIG.  11 . Where a panel size used for grinding includes one quadrant of a 27-slot panel, such a size provides for the grinding of 27 toroids at once. Each quadrant would then be diced in half (to bisect the length of each slot) and folded over onto itself, aligning the two sets of cavities, one above the other. By gluing the required dielectric plate between these aligned cavitied structures, a dual toroid assembly is formed. Final dicing into individual dual toroids follows. Use of state-of-the art commercial dicing equipment ensures that dimensional tolerances are maintained on the diced faces. 
     Once the dual toroids are obtained, wire windings must be added to provide the magnetic field used to shift phase. This has traditionally been an expensive and tedious process, as it involves multiple windings of a one-mil diameter gold wire through each cavity opening. In the present invention, ceramic tape technology is used and avoids the wire winding problem by using thick film metalllization to form the windings directly in the toroid walls. Vias are used for the vertical legs and screen-printing for the horizontal legs of the windings. The windings are thus cofired with the ferrite when the panel is densified, eliminating the potential for breaking fine wires as well as the air gap between the ferrite and winding that is present when wire is wound by hand. This intimate contact between windings and ferrite adds to the performance of the device by forcing the highest concentration of magnetic field into the ferrite. Elimination of the tedious hand winding saves further cost in the assembly process. 
     The foregoing detailed description merely illustrates the principles of the invention. It will thus be appreciated that those skilled in the art will be able to devise various arrangements which, although not explicitly described or shown herein, embody the principles of the invention and are thus within its spirit and scope.