Abstract:
A compliant, high-density land grid array connector and the process of making such a connector. The process includes the steps of: (a) forming holes in a supporting substrate; (b) forming threaded sidewalls by tapping the holes; (c) plating the threaded sidewalls to form bellows-like structures; and (d) etching a surface of the supporting substrate after the plating to leave portions of the bellows-like structures protruding past a surface of the substrate. The resulting connector includes a substrate having bellows-like contacts extending from one or both sides for resiliently engaging pads such as those of an LGA module. As an alternative, the holes may be formed as blind holes. Ends of the bellows-like contacts may be roughened. The connector may also be formed by casting the substrate in a mold box having screw-like mandrels followed by steps of mandrel removal, hole plating and surface etching.

Description:
FIELD OF THE INVENTION 
     The present invention relates, in general, to an electrical connector and, more specifically, to a compliant, high-density land grid array (LGA) connector for connecting an LGA module to a supporting substrate. 
     BACKGROUND OF THE INVENTION 
     The current design trend for connectors used in the computer field is to provide both high area density and high reliability connections between various circuit devices within a computer. Typically, integrated circuit chips are attached to a chip carrier, thermally conductive module chip carrier, circuit card, or board by solder bonding, brazing, controlled collapse chip connection (C 4 ), wire lead bonding, metal bump bonding, tape automated bonding, or the like. High reliability for such connections is essential due to potential end product failure should poor connections of these devices occur. 
     There are two levels of interconnections that are typically required for electronic modules used for integrated circuit applications. The first level of interconnection is between the electronic module or substrate and the integrated circuit or chip. 
     The second level of interconnection is between the electronic module and a printed circuit board (PCB) or card. This is usually accomplished by a soldering process, relying on a lead metal alloy to provide both electrical and mechanical connection to the PCB or card. 
     For many applications the area density of the connections between the module and the PCB is low and the size of the module is only slightly larger than the chip. For these applications solder interconnections are usually acceptable. When modules become larger than about 32 mm as is the case of multi-chip modules (MCMs) which have more than one integrated circuit device and many inputs/outputs (I/Os), the reliability of solder joints for second level interconnection is compromised. 
     Connecting electrical terminations between the surfaces of two components, such as a ceramic chip carrier module and a glass-epoxy printed circuit board by solder requires a significant amount of compliance in the connector. This requirement is due to the non-planarity and irregularities inherent in the surfaces of the board and the module and, more importantly, their different coefficients of thermal expansion. 
     A solder joint is established by the strategic placement of solder balls, columns or the like between circuit elements (e.g., pads), and reflowing the solder to effect interconnection. During the cool down of the joined assembly from the reflow temperature, the solder connections at the outer most positions (typically the corners of the connection array), undergo shear stress due to differences between coefficients of thermal expansion of the module and the PCB. 
     Moreover, during use of the completed device, ambient heating or device generated heat causes uneven expansion of the board and the module, thereby leading to further stressing of the solder joint. The connector must accommodate this stress without severing the mechanical and electrical connection between the module and PCB. It should be noted that as the module size gets larger, the size of the interconnection array usually also increases. The mismatch due to differing coefficients of thermal expansion and the resultant shear stress in the solder also increases. For example, as the interconnection array size increases to greater than about 32 mm (1.25 inches), the stress on the outermost interconnections increases. This can sometimes be mitigated by using long solder connections between the module and the PCB as is practiced in solder column or wire column interconnection. These types of interconnects are often fragile and easily damaged during handling. 
     LGAs are electrical contacts located on an electrical device, which are used to provide electrical connections to the wiring within the device. The LGA typically consists of an array of metal pads, deposited through thick or thin film deposition techniques, and usually coated by plating with gold to provide non-oxidizing and highly conductive surfaces. Connections are made to these pads by metal or metallized connectors that physically contact the pads without the use of solders. The contacts between the LGA pads and the connectors are maintained by a force exerted by compressing the device with the LGA pads against the connectors. These connectors are typically arranged in an array within a socket so that a large number of electrical contacts can be made. A major advantage of LGA socket connectors is that they can be used to allow replacement of the electrical device quickly without the need to unsolder, remove, and resolder the device. 
     A variety of structures are used to connect the LGA pads on an electrical device to the metallized connectors. The connector used for LGAs is often made to connect two mating LGA pads positioned on each side of each connector. In this configuration the connector array is often referred to as an interposer since it is placed between opposing LGA features. Typically, interconnection features use a plurality of contacts held in a polymer laminate on centers complimentary to pads of components and PCB circuits. These contacts range from those requiring an extremely low closure force, such as those made of a conductive gel, to those requiring an intermediate force which are formed of fine conductive wire termed “fuzz” buttons and, for more rigorous applications, a type of coil spring known as a “canted” coil spring. The housings for the different applications clamp the contacts against the pads and are, accordingly, of different constructions, dependent upon the environment, vibration, and stress of the connectors. 
     An alternative connection method may involve the placement of connections directly on a surface by solder or electrically conductive epoxy. The solder or epoxy connects only one end of the connector to a PCB, for instance. The other end of the connector may be available to provide contact to an LGA pad and module, substrate or a different PCB. 
     The most widely used form of LGA socket (in this case, an array of connectors that contact opposing LGA pads) is the fuzz button connector, which is a polymer sheet that a very fine diameter molybdenum wire with numerous coatings, the last of which is predominantly gold. The connector is entangled and compressed, much like a steel wool pad, and then press-fit into the holes of the polymer sheet. Each connector is discretely formed and there is a tendency for a connector to fall out of the hole. Considerable force is required to maintain the contact between each connector and the LGA pad. There is also a very limited contact area between the fine diameter wire and the LGA pads. The connector size presents a problem because the pitch between pads needed for increasing Input/Output (I/O) density on a substrate requires very small, but accurately placed, connectors. The processes of handling and repairing small, discrete spring connectors approaching 15 to 20 mils in diameter is difficult at best. Consequently, an integrated connector array is required. 
     Specific examples that teach the use of discrete springs to effect interconnection between pads and metallized connectors may be seen in U.S. Pat. No. 5,139,427 issued to Boyd et al. on Aug. 18, 1992, and in a sales brochure by Servometer Corporation of Cedar Grove, N.J. The sales brochure teaches the use of discrete bellows for electrical connections. These bellows are manufactured from electrodeposited nickel and gold and are designed to provide force repeatability. Each bellows is a discretely formed cylindrical structure ranging in diameter from 0.037 inches to 0.125 inches outside diameter. At one end of the bellows, a contact having a convex conical tip is formed. At the other end, a contact having a convex conical receptacle is formed. Each bellows is discretely mounted on a substrate surface by inserting the receptacle end onto a pin and press fitting it into place. Alternatively, the receptacle end may be spot welded to a pin or soldered into place. 
     Discrete bellows used for an electrical array may have the same problems as the discrete fuzz buttons. The use of discrete bellows in high-density I/O applications may not be feasible. The processes of handling and repairing discrete small springs approaching 15 to 20 mils in diameter is impractical and very expensive. Therefore, a need exists for a process and structure which increases reliability and decreases the complexity of fabrication of the connection between an area array package and a supporting substrate. 
     SUMMARY OF THE INVENTION 
     To meet this and other needs, and in view of its purposes, the present invention is directed to a compliant, high-density LGA connector formed on a supporting substrate and its process of fabrication. The process forms a structure having a plurality of contacts on the surface of a substrate, which may connect to complementary LGA pads on a printed circuit board, circuit module, chip carrier, or the like. The contacts may be made en-masse and, therefore, are suitable for high-density LGA connections. 
     In one embodiment, the invention includes forming holes in a supporting substrate. Next, threaded sidewalls are formed by tapping the holes. The threaded sidewalls are plated to form bellows-like structures. A surface of the supporting substrate is etched after the plating. The process includes roughening the surface prior to etching. Tapping of the holes is accomplished with a tap preferably having rounded convoluted edges. The substrate may be a printed circuit board, a circuit module, or a chip carrier. In another embodiment, both surfaces of the supporting substrate are etched to expose the bellows-like structures on both sides of the substrate. In yet another embodiment, the threaded holes are made in a die having screw-like mandrels. 
     It is understood that the foregoing general description and the following detailed description are exemplary, but are not restrictive, of the invention. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     The invention is best understood from the following detailed description when read in connection with the accompanying drawing. It is emphasized that, according to common practice, the various features of the drawing are not to scale. On the contrary, the dimensions of the various features are arbitrarily expanded or reduced for clarity. Included in the drawing are the following figures: 
     FIGS. 1 a ,  1   b ,  1   c , and  1   d  are enlarged perspective views of a supporting substrate at various stages of fabrication in accordance with the present invention; 
     FIG. 2 is an enlarged cross-sectional view of the supporting substrate of FIG. 1 at a stage ready to be connected to an LGA module; 
     FIG. 3 is a perspective view of an unassembled microelectronic circuit package showing the supporting substrate of FIG. 2 ready to be connected to an LGA module; 
     FIG. 4 is a cross-sectional view of the assembled microelectronic circuit package of FIG. 3; 
     FIGS. 5 a ,  5   b , and  5   c  are enlarged perspective views of a supporting substrate at various stages of fabrication in accordance with another embodiment of the present invention; 
     FIGS. 6 a  and  6   b  are perspective views of a mold box having threaded screws protruding from its underside for forming a supporting substrate in accordance with yet another embodiment of the present invention; and 
     FIG. 7 is a cross-sectional view of a bellows-like structure in accordance with the present invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The present invention is directed to a compliant, high-density LGA connector formed on a supporting substrate and to its process of fabrication. The present invention is used to join semiconductor chips and circuit modules, such as LGA modules, to a substrate, such as a printed circuit board, a microelectronic circuit card, or any organic or ceramic chip carrier or circuit card. The present invention may also be used to join circuit boards or circuit cards together. 
     The preferred embodiment for the process of fabricating the compliant, high-density LGA connector is shown in FIGS. 1 a - 1   d . In the first step, shown in FIG. 1 a , a plurality of through-holes  12   a ,  12   b , and  12   c  are drilled or mechanically punched into or through supporting substrate  10 . Supporting substrate  10  may be, for example, an insulator used as a printed circuit board or a chip carrier. Although only three through-holes are shown in FIG. 1 a , it will be appreciated that typically an array of holes may be drilled or mechanically punched into supporting substrate  10 . Furthermore, the holes need not be drilled completely through substrate  10 , as long as the depth of each hole is sufficient to allow placement of the desired number of threads in the subsequently explained tapping step. The array of holes is formed on centers which are complementary to the pads of the LGA module to be connected. The holes are preferably formed in an array on a large support substrate which can subsequently be cut into smaller arrays for individual LGA connector applications. Thus, gang drilling, punching, and threading may be used to improve production efficiency. 
     In the second step, shown in FIG. 1 b , threaded sidewalls  14   a ,  14   b , and  14   c  are made by threading through-holes  12   a ,  12   b , and  12   c , respectively, with tap  18 . Preferably, threaded sidewalls  14   a ,  14   b , and  14   c  may extend continuously from the top surface  26  to the bottom surface  28  of the substrate. For applications where protruding bellows are desired on only one side, blind holes may also be made. These may be used to connect to electrical wiring buried within substrate  10  (not shown) and to surface pads (not shown). 
     Next, as shown in FIG. 1 c , the inside of the threaded holes are plated with a metal having a high yield strength and a low elastic modulus to form metal plated structures  15   a ,  15   b , and  15   c . As the threaded holes are plated with the metal, the radial outer edges of the plated structures  15   a ,  15   b , and  15   c  conform to the radial contours of the threaded sidewalls. The plating metal may be chosen from the group including nickel, copper, cobalt, rhodium, and molybdenum. Plating may be done by conventional mechanisms using any number of commercially available plating baths. It is desirable to control the plated thickness to balance the effects of the mechanical and electrical properties of the deposited metal. It is also desirable to achieve a thickness that may permit the metal to elastically deform during stressing conditions, but have high resistance to permanent deformation. As shown in FIG. 1 c , the thickness of supporting substrate  10  is represented by reference number  19 . If desired, a temporary mask may be applied over surfaces  26  and  28  of substrate  10  prior to drilling, threading and plating to prevent plating of these surfaces. Alternately, the top and bottom surfaces may be plated as the threaded holes are plated. This plating may then be removed in the following step. 
     The next step is shown in FIG. 1 d . At least one surface of the supporting substrate is etched by a chemical process, so that the metal plated threaded hole structures are exposed and protrude above the etched surface. The etching of the supporting substrate may be done in a conventional manner by using acid, alkaline, or other solvent solutions. The concentration, temperature, and etching time may be determined by the etching rate and durability differences between the plated metal and the substrate materials. Alternatively, etching processes such as reactive-ion etching (RIE) or RF generated plasma etching may be performed. The removal of material from supporting substrate  10  by the etching process leaves supporting substrate  10  with a reduced thickness represented by reference number  20  in FIG. 1 d . Clearly, thickness  20  is less than thickness  19 . 
     The exposed metal structures form bellows  16   a ,  16   b , and  16   c , each having a bellow-like appearance due to the radial contours of the threaded sidewalls. The bellows, shown in a cross-sectional view in FIG. 2, may now be used for making electrical contact to other conductive surfaces, for example, LGA pads  22   a ,  22   b , and  22   c  of LGA module  21 . 
     As an additional step (not shown), the exposed top surfaces  17   a ,  17   b , and  17   c  of bellows  16   a ,  16   b , and  16   c  may be roughened to provide better electrical connection with LGA pads  22   a ,  22   b , and  22   c . This step may be done prior to etching substrate  10 , by roughening the top surface  26  of the substrate with either sandpaper, abrasive pads, or the like. In this manner, connections may be formed between the top surfaces of the bellows and the LGA surface pads of a mating module with minimum resistance and capacitive coupling. 
     Another step (not shown) may include plating the bellows  16   a ,  16   b , and  16   c  with gold so that the exposed surfaces are prevented from tarnishing. If gold plating is performed, then it should preferably be performed after roughening the top surface of the substrate. 
     The completed structure of the microelectronic circuit package  30  is shown in FIGS. 3 and 4. As shown, FIG. 3 is a perspective view of LGA pad  21  about to be connected with supporting substrate  10 . LGA pads  22   a ,  22   b , and  22   c  are placed on top of bellows  16   a ,  16   b , and  16   c  and held in place without solder by a conventional fastening structure providing a compressive force on the connections (not shown). FIG. 4 depicts a cross-sectional view of the microelectronic circuit package  30  after having been assembled together. In the embodiment shown, substrate  10  may be a printed circuit board, for example, having vias  23  electrically contacting bellows  16   a ,  16   b , and  16   c  and power planes  25 . 
     The protruding bellows, as a connector array, are particularly suited for LGA connections because each bellow may be made compressible without permanent deformation. Adequate elastic deflection (greater than 3 mils) may result under reasonable loads (50 g) in order to compensate for non-planarity of the substrate. Calculations show that the deflection does not exceed the yield point of plated nickel when used as the connector material and, moreover, the deflection is repeatable well beyond the anticipated duty of the connector when subjected to cyclic testing. 
     Another embodiment of the present invention is shown in FIGS. 5 a ,  5   b , and  5   c , which illustrate the fabrication process of another compliant, high-density LGA connector. A multilayer structure  40  is used as the starting point for making the LGA connector. As shown, support carrier  44  is sandwiched between top layer  42  and bottom layer  46 . The multilayer structure may be used to provide an array of connectors for contacting LGA pads on both sides of the array. Thus, the outer two layers (top layer  42  and bottom layer  46 ) may be chosen from material which is more easily removable than the inner, permanent support layer of support carrier  44 . 
     The steps of the fabrication process for multilayer structure  40  proceed in a sequence similar to that described above. First, multilayer structure  40  is provided as shown in FIG. Sa. Multilayer structure  40  is drilled or tapped to make an array of holes on centers complementary to the LGA pads (not shown). After drilling, the holes are tapped to produce threaded sidewalls  48  (FIG. 5 b ). The holes are then plated to form the bellows and the exposed surfaces of multilayer structure  40  are roughened (not shown). Next, the structure is etched in a conventional manner, until only the inner layer (support carrier  44 ) remains. In this manner, support carrier  44  remains with bellows  50  protruding on both of its surfaces, as shown in FIG. 5 c . The support carrier may be chosen from a number of plateable, stiff polymers such as polyimides, polyamides, epoxies, and the like. Such materials are typically used as metallization supports for printed circuit boards and chip carriers in the electronics industry. 
     It will be appreciated that the bellows  50  may be made to protrude on one side only, or on both sides, of support carrier  44 . If the LGA connector is made on one side only, the other side may be used for solderable pads that allow the connector array to be soldered in place, thus helping alignment. 
     In another embodiment, the process may start with a structure having only two layers. For example, the multilayer structure  40  may only include top layer  42  and inner layer  44 , without bottom layer  46 . The top layer is then etched, resulting in bellows  50  protruding on one side only. The other side may be used for soldering to a surface pad. 
     It will also be appreciated that the threaded sidewalls of the supporting substrate may be formed in a manner other than by punching, drilling, and tapping. The threaded sidewalls may be formed by a casting or molding technique using mandrels. The mandrels may consist of an array of protruding threaded screws, as shown in FIG. 6 a . Mold box  60  is shown having an array of threaded screws  62   a ,  62   b ,  62   c , and  62   d , which protrude above the bottom surface  64  of mold box  60 . The array of threaded screws may be attached to a gear train (not shown) at the bottom of the mold box to allow the screws to be unscrewed from the mold box. After the screws are in place, a polymer may be poured or injected into mold box  60  to form polymer sheet  66 , as shown in FIG. 6 b . The polymer sheet is cured to form a sturdy carrier for the connector. The threaded screws are then removed and polymer sheet  66  is now ready for plating and etching in the same manner described above. 
     Lastly, FIG. 7 shows an enlarged cross-sectional view of a single bellows  70 . In its preferred shape, bellows  70  is comprised of rounded convoluted edges formed from a threaded screw having round edges. As shown, convoluted outer edge  72  and convoluted inner edge  74  are both rounded in shape. The rounded edges provide the greatest fatigue resistance for multiple compressions along the length of the exposed connector. The convolution pitch  76  may vary and is a function of the required fatigue resistance and the length of the exposed connector. 
     Although illustrated and described herein with reference to certain specific embodiments, the present invention is nevertheless not intended to be limited to the details shown. Rather, various modifications may be made in the details within the scope and range of equivalents of the claims and without departing from the spirit of the invention. It will be understood, for example, that the present invention is not limited to only the LGA connector described. Rather, the invention may be extended to be used as an interposer between a package and a printed circuit board or between any two substrates that may repeatedly be assembled and unassembled.