This invention relates generally to circulators that can be implemented in surface mount packages and particularly to a circulator that can be fabricated in a package whose shape can be selected to conform to the requirements of a microwave circuit whose arrangement is affected by other constraints. In the production of microwave circuits the use of components that are mounted on tape reels greatly increases the speed and efficiency of production, for example in a so-called pick and place automated production line.

A circulator is a device having several ports for electrical communication with other devices in which energy entering the device through one of the ports is transmitted to a port that is adjacent to the first port. Circulators have been used for many years in coupling microwave energy transmitted in wave-guides. There are several circulators known in the prior art that utilize strip-line microwave transmission lines. For example, U. S. Patent No. 4,276,522 to Coerver describes a circulator in a strip-line microwave transmission line circuit. Additionally, there are known various methods of forming strip-line circuit components. Most notably, U. S.

Patent No. 4,821,007 describes a method of manufacturing a strip line circuit component that has particular advantages in packaging components to be soldered directly to conventional circuit boards. The assignee of the present application is also the assignee of U. S. Patent No. 4,821,007 which is incorporated herein by reference.

To date, there has not been a suitable and cost-effective circulator for use in automated manufacturing, particularly for use in reflow operations. Indeed, there is a need for a circulator that can be packaged in component form and soldered directly to conventional circuit boards. Preferably, the component form is one that allows for supplying the components to the end-user in a tape and reel thereby allowing for quick and efficient production of the circuit boards. It is also desirable to provide a circulator package that can be soldered to circuit board in such fashion to allow for visual inspection of the solder joints.

Summary of the Invention It is an aspect of the present invention to provide a circulator that is suitable for use in automated production of microwave circuits, specifically reflow operations.

It is another aspect of the present invention to provide a method of manufacturing a circulator that is suitable for automated production of microwave circuits.

It is still another aspect of the present invention to provide a surface mount circulator that includes a flat mounting surface.

It is yet another aspect of the present invention to provide a surface mount circulator that can be soldered in place on a circuit board whereby the circulator solder joints can be visually inspected.

These and other aspects are obtained by a method of manufacturing a circulator comprising the steps of providing a central substrate layer with at least one cut-out section and circuit lines. Next, providing a magnet placed within said cut-out section and a ferrite placed on each side of said substrate layer. A steel plate is placed on each side of the substrate layer. Spacer layers are provided on each side of the substrate layer. An outer shim layer is placed on each side of the central substrate layer. Laminate sheets are placed between the substrate layer, the spacer layers, and the outer shim layers. Holes are drilled through each of the substrate, the spacer layers, the outer shims, and the laminate sheets. Heat and pressure are provided to cause the laminate sheets to flow in order to join the substrate, magnets, ferrites, steel plates, spacer layers and outer shim layers into a unitary structure. Finally, the structure is plated.

Brief Description of the Drawings Figure 1 (a) is an elevated plan view of a surface mountable circulator.

Figure l (b) is an elevated three-dimensional ghost view of a surface mountable circulator.

Figure 2 is a horizontal cross-sectional view of a surface mountable circulator.

Figure 3 is a vertical cross-sectional view of a surface mountable circulator.

Figure 4 is an exploded view of a surface mount circulator embodying the present invention.

Detailed Description of the Preferred Embodiment Referring now to Figure 1A, there is shown an elevated view of the surface mount circulator 100 embodying the present invention. The overall shape of the circulator 100 is rectangular, which applicants believe is the currently preferred shape for the production of mobile cellular communications equipment. Of course, end- users of surface mount components may prefer alternative shapes, such as triangular, round or hexagonal, and one skilled in the art would recognize that the present invention could take any shape that is compatible with end-user requirements.

Referring now to Figures 4A and 4B, there are depicted the processing steps in the manufacture of the surface mount circulator that embodies the present invention.

The process herein described illustrates that a plurality of circulators can be manufactured on one panel. The manufacture of a plurality of components is preferable to a single component manufacturing process because the panel production method is obviously more efficient. In the preferred method, the multiple circulators are populated on the panel and are end laminated together.

Prior to the first illustrated step of Figure 4B, as shown in Figure 1B, the central substrate 11 has had a circuit pattern 12 laid out in ways known to those skilled in the art. For example, circuit paths 12 are depicted on circuit artwork and are used in a photolithographic process to etch patterns into the substrate 11. In addition, routers can be used to form various shapes and pathways on the substrate 11 that are selected in ways that will become more apparent in the following discussion. The substrate material is preferably polytetrafluoroethylene (PTFE), but other materials are known to those skilled in the art, such as ceramic or other plastics.

In the first step of the process of Figure 4A, the substrate has magnets 10 inserted in the previously routed cutouts. The preferred embodiment uses three magnets laid out along the circumference of the cutout as shown. Any material that has the ability to store magnetic energy can be used. As such, the magnets 10 can be 0 in a pre-formed configuration or can be in the form of a paste or powder that can be spread or formed into the desired shape. The magnets 10 or magnetic material must be arranged so that a DC magnetic field is applied.

Next, as depicted in Figure 4B, the substrate 11 is covered with a pre-routed laminate sheet 42 with a desired dielectric characteristic in order to stack-up the structure of the circulator. Again, one skilled in the art would recognize that the laminate 42 can be constructed of various materials, such as glass reinforced

hydrocarbon/ceramics. Next, ferrite 24 is placed on top of the laminate and in between the magnets 10. Any material with gyro-magnetic characteristics can be utilized. The ferrites 24 can be pre-formed, in powder or paste form which can be spread or placed into the desired position and shape.

The next step, still referring to Figure 4B, is the addition of spacer layers 35 and additional laminate 42. The spacer layers 35 and laminate 42 are provided in order to match the height of the ferrite 24, magnets 10 and the steel plates 28 that are subsequently placed on top of the previously arranged pieces. The plates 28 are comprised of any material that is able to conduct magnetic energy, preferably steel.

Of course, powders or pastes that can be spread or placed in the correct position can be used. Depending on the desired performance characteristics there can be a varying number of plates utilized.

An additional laminate sheet 42, as shown in Figure 3, is placed over the existing structure. This laminate sheet 42 has been pre-drilled to provide vias in order to allow electrical contact from the steel plates to an electrical ground located outside of the circulator device. An outer shim layer 40 is provided over the laminate sheet 42. The shim material is preferably PTFE. The outer shim 40 also includes vias 20 that align with the vias in the laminate sheet in order to allow for contact to an electrical ground located outside the circulator device. The outer shim material encloses the magnets 10, ferrite 24, and steel plates 28.

The process described to this point provides for the stack-up of constituents on one side of the central substrate. At this point, the part is flipped over and the steps are repeated. Specifically, a first laminate sheet is placed over the central substrate followed by the addition of the ferrite material, the spacer layers, the steel plates, the second laminate sheet and the outer shim.

After both sides of the circulator have been stacked up, the panel is placed into a lamination press for temperature and pressure treatment in order to finally form the circulator, as is well known in the art. The temperature applied to the device must be high enough to cause the laminates to flow and the pressure must be sufficient to form the circulator while taking into account the need to avoid cracking any of the constituents.

After the lamination process is complete, several holes are drilled through the panel in order to form vias 20 or through-holes. After the lamination process, the device is plated with a copper plating solution and then with a tin-alloy. Finally, the

panel is singulated to provide individual components in the desired shape, preferably rectangular. The singulation is known to those skilled in the art in order to arrive at a particular size and shape product. In addition, the singulation process bisects some of the holes thereby forming semicircular indentations along the periphery of each device. Three of the indentations form input/output ports and the remaining indentations are used as soldering sites for the end-user products. This surface mounting technique is of particular advantage because the end user can perform quality assurance checks of the port connections and solder sites using a direct line of sight inspection technique due to the unique configuration of the circulator.

Referring now to Figure 2, there is shown a horizontal cross-sectional view of the surface mount circulator 100 embodying the present invention. The cross section is taken through the mid-line of the circulator device 100. The circulator 100 is comprised of a substrate 11 that is manufactured from PTFE, or other similar materials as described above. The substrate 11 has a cutout 14 that is formed through the use of routers as is known in the art. Magnets 10 are placed around the periphery of the circular cutout 14. As mentioned in the discussion above, the magnetic field that is required to be created in order for the circulator to operate correctly can be created by conventional magnets or by other materials such as pastes or powders, so long as the materials have the necessary magnetic properties. In addition, although the preferred embodiment includes a magnet, a specialized application does exist for 'below resonance'circulators wherein the circulator can be formed without a magnet.

The magnets 10 create a magnetic field biasing the ferrite material around the conductor lines 12 that have been previously etched or photolithographed onto the substrate 11 as described above. Of course, the conductor lines 12 may assume various shapes and geometric configurations without departing from the scope and spirit of the present invention. The conductor lines 12 are in electrical communication with input/output ports 16 that are formed along the periphery of the circulator 100 by the drilled holes 20 and the singulation process described above wherein the holes 20 are bisected. There are also drilled holes 20 that are placed at various locations about the circulator 100 that provide for an electrical connection from the steel plates 28 to ground (not shown).

Referring now to Figures 3,4A and 4B, the substrate 11 that has been pre-cut as described above accommodates the magnets 10. The layers 35 of PTFE that are used to stack-up the circulator 100 as described above are also cut out to

accommodate the magnets 10, the ferrite 24 and the steel plates 28. The outer shim 40 contains holes or vias 20 to the steel plates 28 to provide an electrical contact to ground (not shown). There are laminar sheets 42 located between each of the layers 35 and the substrate 11 as described above. In some instances, temperature compensation plates (not shown) can be added to the circulator 100. The compensation plates are preferably a nickel-steel alloy. As noted above, after the stack-up and the lamination processes have been completed the circulator 100 is plated, preferably with a tin alloy solution.

It is apparent to those skilled in the art that the invention can be modified in arrangement and detail without departing from the scope and spirit of the following claims and equivalents thereof. For instance, one skilled in the art would recognize that the circulator could easily be modified to act as an isolator by merely supplying a termination at an output port.