High capacity SDRAM memory module with stacked printed circuit boards

[0002] This application is related to copending U.S. patent applications Ser. Nos. 09/932,525 filed Aug. 17, 2001; 09/932,654 filed Aug. 17, 2001; and 10/077,057 filed Feb. 19, 2002, all of which are hereby incorporated by reference.

[0003] The present invention relates to high input/output (I/O), high density, low cost electronic modules and, more particularly, to the high I/O, high density, low cost packaging of high performance, high density memory devices such as Double Data Rate (DDR) Synchronous Dynamic Random Access Memory (SDRAM) and having impedance-controlled buses for maintaining high electrical performance.

[0004] In data processing and network systems, it can always be certain that the demand in memory capacity will increase at a high rate. Such increase has in recent years taken on a new dimension; while memory demand increased, the space available for mounted memory devices has become increasingly restricted. The Electronic Industries Alliance (EIA) has set up a standard for the dimensions for rack-mountable equipment. Traditionally, a piece of rack-mountable equipment has a standard width of 19 inches and a height in increments of 1.75 inches. This is also known as “1U.” However, a trend has begun to reduce the height for the servers in a server rack to dimensions appreciably lower than 1U.

[0005] This equipment height restriction has also placed height restrictions on other components such as memory modules. The traditional SDRAM dual inline memory modules (DIMMs) are simply too tall to be able to be mounted vertically on the system board. Special sockets have been designed to allow DIMMs to be mounted either at an angle or even parallel to the system board. As the speed of memory devices increases to greater than 200 megahertz, for example, the electrical performance of such DIMM sockets is becoming inadequate.

[0006] One method being used today to solve the need to increase both memory capacity and density is to stack two, thin small outline package (TSOP) SDRAM devices on top of each other on a DIMM. An alternate approach is to stack two devices within a chip scale package (CSP). These stacking schemes, while increasing memory density, are not easily reworkable.

[0007] It is desirable to find a packaging solution that resolves both the capacity and the height issues. In addition, the solution must also be low in cost, readily manufacturable, upgradable with ample granularity, have improved electrical performance even at high frequencies, and have good reliability. Ample granularity allows the amount of memory on a given memory module to be increased or decreased in smaller increments (e.g., in increments of 256 megabytes, instead of one gigabyte).

[0008] It is therefore an object of the invention to provide a high capacity, high density, low profile SDRAM memory module for high performance memory devices.

[0009] It is another object of the invention to provide a high capacity, high density, low profile SDRAM memory module that is readily manufacturable and upgradable.

[0010] It is still another object of the invention to provide a high capacity, high density, low profile SDRAM memory module that provides improved electrical performance at high frequencies and good reliability.

[0011] The present invention is a family of specialized embodiments of the modules taught in the referenced copending U.S. patent applications. A memory module is desired with granularity, upgradability, and a capacity of two gigabytes using 256 MB SDRAM or DDR SDRAM memory devices in CSPs in a volume of just 4.54 inches by 2.83 inches by 0.39 inch.

[0012] Each module includes a substrate, having contact pads and memory devices on its surfaces, and impedance-controlled transmission line signal paths to support high speed operation. The substrates may be conventional printed circuit cards preferably with CSP packaged memory devices along with other components attached directly to both sides of the substrates.

[0013] The inclusion of spaced, multiple area array interconnections allows a row of memory devices to be symmetrically mounted on each side of each of the area array interconnections, thereby reducing the interconnect lengths and facilitating matching of interconnect lengths. The footprints for the interconnections between the substrates and to the system board are the same to reduce part number and reliability and qualification testing. Short area array interconnections, including ball grid array (BGA), pin grid array (PGA), and land grid array (LGA) options, or interchangeable alternative connectors, provide interconnections between modules and the rest of the system. The distance between the spaced multiple area array interconnections is preferably chosen to ensure that the solder joints in the BGA interconnection option are reliable.

[0014] Driver line terminators may be included on the substrates for maintaining high electrical performance. Thermal control structures may also be included to maintain the memory devices within a reliable range of operating temperatures.

[0015] A complete understanding of the present invention may be obtained by reference to the accompanying drawings when taken in conjunction with the detailed description thereof and in which:

[0016] FIG. 1ais a representation of the bussed interconnection between a memory controller device and a multi-card memory arrangement of a memory subsystem of the prior art;

[0017] FIG. 1bis an enlarged, side elevational view of a vertical plated-through-hole attach connector and memory card of the prior art interconnection shown in FIG. 1a;

[0018] FIG. 2 is a side elevational view of a connector and memory card of the prior art;

[0019] FIG. 3ais a top view of a memory module in accordance with one embodiment of the present invention;

[0020] FIG. 3bis a cross sectional view of a multi-card configuration based on the memory module of FIG. 3a;

[0021] FIG. 3cis a cross sectional view of the multi-card configuration in accordance with an extension of the embodiment of FIG. 3b;

[0022] FIG. 4ais a cross sectional view of the multi-card configuration of FIG. 3bincluding a termination module;

[0023] FIG. 4bis a cross sectional view of the multi-card configuration of FIG. 3bincluding an additional module with other functionality; and

[0024] FIG. 5 shows a top view of an array of electrical contacts representative of the electrical connections of the interconnection arrays of the embodiments of the invention.

[0025] Generally speaking, the present invention is a family of specialized embodiments that represents improvements of the modules taught in the referenced copending U.S. patent applications. The memory module has granularity, upgradability, and a capacity of two gigabytes using 256 MB SDRAM or DDR SDRAM memory devices in CSPs in a volume of just 4.54 inches by 2.83 inches by 0.39 inch.

[0026] Each module includes a substrate, having contact pads and memory devices on its surfaces, and impedance-controlled transmission line signal paths to support high speed operation. The substrates may be conventional printed circuit cards preferably with CSP packaged memory devices along with other components attached directly to both sides of the substrates.

[0027] The inclusion of spaced, multiple area array interconnections allows a row of memory devices to be symmetrically mounted on each side of each of the area array interconnections, thereby reducing the interconnect lengths and facilitating matching of interconnect lengths. The footprints for the interconnections between the substrates and to the system board are the same to reduce part number and reliability and qualification testing. Short area array interconnections, including ball grid array (BGA), pin grid array (PGA), and land grid array (LGA) options, or interchangeable alternative connectors, provide interconnections between modules and the rest of the system. The distance between the spaced multiple area array interconnections is preferably chosen to ensure that the solder joints in the BGA interconnection option are reliable.

[0028] Referring first to FIG. 1a, there is shown a representation of a portion of a memory subsystem 10 of the prior art located on a system board 12, including a memory controller 14 and a multi-card memory arrangement with bussed interconnection therebetween. In this embodiment, memory subsystem 10 is based on DDR SDRAM technology, although many other technologies would also be applicable. Memory controller 14 is electrically connected to memory modules 16a-16d, each comprising a plurality of memory devices 28, through a plurality of bussed interconnections 18a-18d. In order to provide even higher density, memory devices 28 may be located on both sides (FIG. 1b) of memory modules 16a-16dand even stacked upon each other on either one or both sides. This type of stacking is costly, however, unreliable, and difficult to rework. Also, it is very difficult to cool such stacked devices.

[0029] Memory controller 14 connects to system board 12 through an array of BGA solder interconnections (not shown) located on the bottom surface of memory controller 14. Memory modules 16a-16dare approximately 5.25 inches long and 1.38 inches tall, with the top edge about 1.50 inches above the surface of the system board 12, and with a distance between them, “P,” of 0.5 inch. Modules 16a-16dinclude an array of contact pads 24 along their lower edge. Contact pads 24 provide electrical connection to system board 12 through an edge connector, which is not shown for purposes of clarity. A more detailed cross sectional view of a single connector 20 comprising mating contacts 22 and housing 26, and corresponding memory module 16ais shown in FIG. 1b.

[0030] Memory modules 16a-16dtypically are printed circuit structures comprising epoxy-glass-based materials (i.e., FR4) and including one or more conductive (i.e., signal, power and/or ground) layers therein. Due to stringent electrical specifications, the impedance of the signal traces must match the impedance of the corresponding traces on system board 12 within ten percent.

[0031] Assuming that each memory module 16a-16dhas 512 megabytes of memory (a quantity that is available today), the volumetric area required for the four modules 16a-16dis 5.25 inches by 1.80 inches by 1.50 inches, or about 13.90 cubic inches, and about 9.45 square inches of area on system board 12. Also, while the height of memory modules 16a-16djust fit in a 1U high enclosure, it is unlikely that these memory modules 16a-16dwill fit vertically in sub-1U high enclosures.

[0032] Referring now to FIG. 2, there is shown a side elevational view of a portion of a memory subsystem 30, displaying another embodiment of the prior art, located on a system board 12, including a memory controller 14 and a low profile, multi-card memory arrangement. In this embodiment, memory subsystem 30 is based on DDR SDRAM technology, although many other technologies would also be applicable. Again, memory controller 14 is electrically connected to memory modules 16a-16d, each comprising a plurality of memory devices 28, through a plurality of bussed interconnections 18a-18d. Again, in order to provide even higher density, memory devices 28 may be located on both sides of memory modules 16a-16dand even stacked upon each other on either one or both sides. This type of stacking is costly, as aforementioned, unreliable, and difficult to rework. Also, it is very difficult to cool such stacked devices.

[0033] Memory controller 14 connects to system board 12 through an array of BGA solder interconnections (not shown) located on the bottom surface of memory controller 14. Memory modules 16a-16dare still about 5.25 inches long, 1.38 inches wide, 0.35 inch tall, and a distance between them, “P′,” of 1.70 inch. Modules 16a-16dinclude an array of contact pads 24 along their lower edge. Contact pads 24 provide electrical connection to system board 12 through connectors 32. Each connector 32 further comprises mating contacts 32 and a housing 34, and corresponding memory modules 16a-16d.

[0034] Assuming again that each memory module 16a-16dhas 512 megabytes of memory, the volumetric area required for the four modules 16a-16dis 5.25 inches by 6.80 inches by 0.35 inches high, or about 12.50 cubic inches, and about 35.70 square inches of area on system board 12. Compared to the embodiment shown in FIGS. 1aand 1b, the cubic volume used is slightly reduced, but the amount of system board area used increases by over 350 percent, an increase unacceptable in many system designs.

[0035] Referring now to FIGS. 3aand 3b, there are shown a top view of a memory module 40 in accordance with one embodiment of the present invention, and a cross sectional view of a multi-card configuration 60 based on the memory module 40 of FIG. 3a, respectively.

[0036] In the embodiment of FIG. 3a, memory module 40 includes a substrate 42, a plurality of memory devices 48aand 48b, phase lock loops (PLLs) 44aand 44b, registers 46aand 46b, a configuration memory device 50, resistors 36, capacitors 38, and upper interconnection arrays 52aand 52b. Lower interconnection arrays 54aand 54band, optionally, additional memory devices 48aand 48bare located on the opposite side (FIG. 3b) of substrate 42.

[0037] In this embodiment, memory devices 48aand 48bare 256 MB SDRAM or DDR SDRAM memory devices in CSPs, although other packages, such as bare chip, thin, small-outline packages (TSOP), and chip on board (COB) may be used. The preferred use of packaged devices 48aand 48beliminates the issues associated with known good die (KGD). Although 256 MB devices are the largest devices available today, it should be understood that memory device capacity is certain to increase in the future, and the use of higher as well as lower capacity memory devices is well within the scope of this invention.

[0038] Examples of substrate 42 suitable for interconnection include printed circuit boards, circuit modules, etc. The term “printed circuit board” is meant to include but not be limited to a multilayered circuit structure including one or more conductive (i.e., signal, power and/or ground) layers therein. Such printed circuit boards, also known as printed wiring boards, are well known in the art and further description is not believed necessary. The term “circuit module” is meant to include a substrate or like member having various electrical components (e.g., semiconductor chips, conductive circuitry, etc.), which may form part thereof. Such modules are also well known in the art and further description is not believed necessary.

[0039] Substrate 42 may comprise a wide variety of dielectric materials. In one example it is made of epoxy-glass-based materials typically used in printed circuit board fabrication (e.g., FR4) and also includes one or more conductive layers therein. Due to stringent electrical specifications, the signal traces typically match the system impedance within a certain tolerance (e.g., ten percent). These materials are preferred because their CTE substantially matches the CTE of the surrounding structures, especially for applications including land grid array (LGA) connectors, and because of their relatively low cost. Other possible materials include polyimide and RO2800 (a trademark of Rogers Corporation). It should be understood by those skilled in the art that other materials may also be used without departing from the spirit of the invention.

[0040] PLLs 44aand 44bare used to control and synchronize the timing against a known system clock for memory devices 48aand 48b. Registers 46aand 46bare used to buffer and latch the state of the address and control buses against a known system clock. Configuration memory device 50 is used to store configuration information about the module 40 for use by the system. In this embodiment device 50 is an electrically erasable programmable read-only memory (EEPROM) device. Resistors 36 may be placed in series in the various electrical nets to dampen reflections. Capacitors 38 are strategically located, especially near memory devices 48aand 48b, and function as decoupling capacitors. Both resistors 36 and capacitors 38 are implemented as surface mount devices in this embodiment but may be implemented in other form factors such as embedded components.

[0041] A significant contribution to the advantages of the present invention is derived from the locations of the footprint of upper interconnection arrays 52aand 52band mating lower interconnection arrays 54aand 54b(FIG. 3b), which may be implemented in many ways. These short area array interconnections may use BGA, PGA, or LGA options, or interchangeable alternative connectors as better shown in FIG. 3b. The specific choice of connectors for interconnection arrays 52a-52band mating 54a-54bis design dependent and may vary depending on a specific set of requirements. Fox example, the PGA and LGA options are demountable and are therefore useful for applications requiring field upgradability. The LGA option may require an alignment and clamping mechanism. Implementations of these items are covered in one of the referenced copending U.S. patent applications.

[0042] The separated interconnection arrays 52aand 52bon the top surface and 54aand 54bon the bottom surface allow a row of memory devices 48aand 48bto be symmetrically mounted on each side of each respective interconnection cluster, which provides the shortest possible electrical path from the memory devices 48aand 48bto system board 12 (FIG. 3b) and facilitates the matching of interconnect length. From a mechanical point of view, the distance between the interconnection arrays 52a-52band 54a-54bis chosen to be wide enough to support the required quantity of memory devices 48aand 48b, but narrow enough to ensure that the solder joints in the BGA interconnection option are reliable.

[0043] Component positioning on memory module 40 provides another benefit over the prior art. Components are positioned on memory module 40 to allow the module 40 to support multiple (two, in this case) channels 40aand 40bof memory. This capability allows a single memory module 40, with memory devices 48aand 48bon both sides (assuming memory devices 48aand 48beach have a capacity of 256 MB) to support up to 1 GB of memory (512 MB per channel), with a granularity of 256 MB per channel.

[0044] Depending on how interconnection arrays 54a-54bare wired on the system board 12 (FIG. 3b), the memory devices 48aand 48bmay be configured to operate in either a single channel or as multiple independent channels. In one example, this option allows the operation of two 64-bit memory channels (40aand 40b) or a single 128-bit memory channel. A 128-bit memory channel has twice the throughput of a 64-bit channel when operated at the same frequency. A higher throughput is important in many industries required to run real-time applications (e.g., gaming, video graphics, speech processing, and networking applications). Increasing throughput through widening the bus is often much easier to implement and less expensive compared to methods such as doubling the clock frequency of the memory subsystem, reducing latency in bus cycles, implementing complex multi-symbol modulation schemes or pulse code modulation (PCM) type approaches.

[0045] For applications requiring less memory, a single channel 40aor 40bcan be populated and therefore implemented. For this type of application, since only a single interconnection array pair 52a-54aor 52b-54bis needed but mechanical stability of the overall memory module 40 is desired, to reduce costs the other interconnection array location may be populated by a spacer of similar dimensions as the interconnection array pair. For applications requiring finer granularity, half of the full quantity of memory devices 48aor 48bon a given channel 40aand/or 40bcan be populated to reduce the granularity to 256 MB.

[0046] System electrical performance can be further enhanced by including additional functionality, such as termination components to the module 40, without significantly increasing the cost and size of the module 40. This is taught in one of the referenced copending U.S. patent applications. Also, heatspreaders or equivalent thermal conduction devices may be placed in contact with memory devices 48aand 48bto provide improved thermal management if required. This is shown in FIG. 3c.

[0047] Another example of additional functionality is the inclusion of decoders (not shown) that may be used to perform functions such as generating extra chip selects for referencing additional memory channels on module 40.

[0048] A third example of additional functionality is the inclusion of field programmable components (not shown), which may be used to perform functions such as changing the values of the termination components. The field programmable components may include a field programmable gate array (FPGA), whose outputs control solid state switches to switch in resistive, capacitive, or inductive blocks to establish a termination scheme that provides optimized performance. Some connections on the FPGA may be dedicated to a standard PC bus interface such as I2C, to make the terminations soft programmable.

[0049] A field programmable component may alternatively be employed to switch the module operation type from DDR to SDR, for example. Field programmable switches may also be used to deactivate the inverting net of all differential clocks that are not used in SDR operation, as well as to switch in extra components as needed. Other components that may be added include clock synthesizers, skew control blocks, FIFOs, and thermal shutdown or thermal monitoring integrated circuits, which may be installed at strategic hot points on module 40. A thermal shutdown device may be used to disable a power supply until conditions improve. This improves the reliability of memory devices 48aand 48bon module 40.

[0050] Compared to the prior art memory modules 16a-16dof memory subsystem 10 (FIG. 1a), the inventive memory modules 40 offer improved signal integrity, due to the fact that the modules 40 have a reduced stub effect. Each electrical net in the prior art memory modules 16a-16dhas a stub length up to 1.5 inches long. A stub is any net connecting parallel to the net or controlled transmission line of interest. It may include components. Unterminated stubs often are the result of used connectivity pathways for one or more components that are not populated in a given assembly, and can result in composite reflections that are twice the level of the initial signal. A stub degrades performance due to factors such as the timing relationships of the reflections exiting the stub, how that compares with the propagation delays to the other components on the bus, and the length of duration of the bus cycle. In short the design performance degradation associated with stubs tends to worsen with increasing frequency, longer stub lengths, more stubs, and greater spacing between stubs.

[0051] It should be understood by those skilled in the art that the various components of the invention may consist of alternate materials, instead of or in addition to the particular ones described in the disclosed embodiments, without departing from the spirit of the invention.

[0052] Referring now again to FIG. 3b, there is shown a cross sectional view of a multi-card configuration 60 based on the memory module 40 of FIG. 3a. In one example of this embodiment, multi-card configuration 60, which includes two memory modules 41aand 41b, has a capacity of two gigabytes of memory in a volume of just 4.54 inches by 2.83 inches by 0.39 inch, or about 5.17 cubic inches, and requires only 1.8 inches by 2.5 inches, or 4.5 square inches of area on system board 12. Compared to prior art embodiments, the volumetric requirement of the invention is reduced by over 200 percent. The area of the system board 12 required for multi-card configuration 60 is reduced by 50 percent compared to the prior art embodiment in FIG. 1a(which cannot support sub-1U high enclosures), and almost 800 percent compared to the prior art embodiment in FIG. 2, while allowing even higher memory expansion capability through additional stacking. Also, the additional amount of system board 12 real estate required for printed circuit traces to wire memory controller 14 to memory modules 16a-16din FIGS. 1a-2 is significantly greater than for wiring memory controller 14 to memory modules 41aand 41bof multi-card configuration 60 in the present invention, for additional system board 12 real estate savings.

[0053] Lower interconnection arrays 54aand 54bon the lower module 41aare provided to allow electrical interconnection to a memory controller 14 on system board 12 through a mating connectors 52aand 52b. Upper interconnection arrays 52aand 52bon the lower module 41amate with lower interconnection arrays 54aand 54bon the upper module 41bto extend the address and control buses from the memory controller 14. Upper interconnection arrays 52aand 52bon the upper module 41bprovides for the stacking of additional memory modules 40 (FIG. 3a) in the future. Maintaining uniform footprints for the interconnection between memory modules as well as to system board 12 reduces the proliferation of different memory module 40 (FIG. 3a) part numbers, and minimizes reliability and qualification testing. The substrates 42 are designed so that the modules 41aand 41bare positionally independent within the stack. In other words, the lower module 41aand upper module 41bmay be interchanged within multi-card configuration 60 and still function properly.

[0054] The positioning of memory devices 48aand 48bon stackable modules 41aand 41brelative to memory controller 14 (FIGS. 1aand 2) provides several improvements compared to the prior art of FIGS. 1a-2. This is taught in one of the referenced copending U.S. patent applications. The improvements include: reduced propagation delays and hence potentially higher operating frequencies; reduced settling times and periods for the ringing cycle established and associated with impedance mismatches between the bus and termination in a given net, which can also reduce inter-symbol interference (ISI); potentially less intra-bus skew be variation, leading to better timing margins; and simplified and reduced system board 12 routing. Reducing the spacing between modules 41aand 41bcompared to the prior art examples shown hereinabove provides a performance improvement since reflections off stubs will exist longer on the bus if the spacing between stubs is longer.

[0055] Conventional memory modules 16a-16d(FIGS. 1a-2) have a length of about 5.25 inches, which may result in lower frequencies for system board 12 resonance, for the resonant modes established along that axis. This in turn can lead to greater bounce of the power planes for signals whose edge frequencies (one divided by the rise or fall time) are close to the system board 12 resonant modes. The inventive modules 40 are relatively square in shape, with shorter X and Y axes, which may shift the frequency of the resonant modes higher than frequencies of interest. Also, by designing slits in the power planes (for instance between banks of memory devices), the resonant frequencies can be moved even higher.

[0056] Referring now to FIG. 3c, there is shown a cross sectional view of a multi-card configuration 70 in accordance with an extension of the embodiment of FIG. 3b, further including thermal management structures 72.

[0057] The natural cooling efficiency of a module 40 is low due to the lack of an effective thermal transfer medium from the die or package of memory devices 48aand 48bto the air, and the lack of a short air channel in the direction of air flow (i.e., parallel to system board 12). The thermal problem is exacerbated by the relatively large size of today's memory devices 48aand 48band the proximity to other heat generating devices 48aand 48bin such a dense module 40. The thermal management structures 72 of the inventive modules 40 are designed to optimize both thermal conduction and radiation, thus allowing maximum circuit density without heat build-up, which could degrade memory device 48aand 48bperformance and reliability.

[0058] Thermal management structures 72 are intended to sink heat away from memory devices 48aand 48b. Such structures 72 may be stand alone elements (e.g., heatsinks) or they may provide a low resistance thermal path to another surface such as the outer enclosure of a device (e.g., a laptop computer), which may include thermally conductive material.

[0059] Thermal management structures 72 may be implemented in many ways. Structures 72 may be as simple as a layer of thermally conductive material, such as aluminum, attached or retained to memory devices 48aand 48bby thermally enhanced compounds or clamps. Structures 72 may be more complex and include elements such as fins (not shown) to augment cooling. Other methods may include the use of conformal pouches of liquid thermal transfer material, thin heat pipes, and thermoelectric devices. Even other methods of solving thermal issues will be obvious to those skilled in the art.

[0060] System electrical performance can be further enhanced, and significant system board real estate can be saved by including additional functionality through the inclusion of additional stacked modules. The much lower profile of memory modules 40 allows the stacking of these modules with additional functionality even in sub-1U high applications.

[0061] Referring now to FIG. 4a, there is shown a cross-sectional view of a multi-card configuration 80 based on the memory module 40 of FIG. 3a. In this embodiment, multi-card configuration 80, which includes two memory modules 40, further includes a termination module 82. Termination module 82 comprises a plurality of components 84, which typically requires a large number of bulk capacitors, ferrite bead inductors, switching regulators, decoupling capacitors and termination components. The termination components may be passive components such as resistors and/or capacitors, but they may also include active filter-type components.

[0062] Referring now to FIG. 4b, there is shown a cross-sectional view of a multi-card configuration 90 based on the memory module 40 of FIG. 3a. In this embodiment, multi-card configuration 90, which includes two memory modules 40, further includes an additional module 92. Module 92 comprises a memory controller 94 that may service the memory modules 40 below it. Module 92 may also use interconnection arrays 52a-52band 54a-54bto establish electrical interconnection to system board 12.

[0063] Referring now to FIG. 5, there is shown an array of electrical contacts 100 representative of the electrical connections that make up upper interconnection arrays 52aand 52b, and lower interconnection arrays 54aand 54b. Array of electrical contacts 100 includes inner contacts 102 and outer contacts 104. For applications in which arrays 52a-52band 54a-54bare implemented as BGA or LGA interconnections, high frequency electrical radiation may be significantly reduced by assigning the majority of signal connections to inner contacts 102, and assigning primarily ground and reference voltage connections to outer contacts 104, thereby providing a level of shielding. Electromagnetic comparability issues may be further reduced by placing a ground ring (not shown) around each module 40 (FIG. 3a).

[0064] Since other modifications and changes varied to fit particular operating requirements and environments will be apparent to those skilled in the art, this invention is not considered limited to the example chosen for purposes of this disclosure and covers all changes and modifications which do not constitute departures from the true spirit and scope of this invention.

[0065] Having thus described the invention, what is desired to be protected by Letters Patent is presented in the subsequently appended claims.