Abstract:
The details of a printed wiring board (PWB) sub-assembly and the method of producing the same are described. The sub-assembly comprises a printed circuit board electrically joined through a plurality of connections to one or more area array devices, such as modules or printed wiring boards. The sub-assembly can serve as a part of an original assembly. The sub-assembly can function as an after market item that can be readily substituted as a replacement for a failed component wherein the dimensional space between the printed circuit board and one or both of the area array devices must provide sufficient clearance for surface mounted devices.

Description:
FIELD OF THE INVENTION  
         [0001]    This invention relates to electronic sub-assemblies, and particularly to structures comprising a printed wiring board laminate that is electrically and mechanically connected through an interposer connector to an area array device, such as another board, a module or other active or passive device.  
         BACKGROUND OF THE INVENTION  
         [0002]    The mechanical and electrical core of a computer system comprises a plurality of printed wiring boards interconnected with other boards or modules and with other active or passive devices, such as diodes, semiconductors, capacitors and resistors. The success or failure of such computer systems is dependent upon, among other factors, their ability to operate without mechanical breakdowns and electrical failures.  
           [0003]    One type of computer system is a microprocessor called a network server. A principal function of a network server is to achieve organized channels of communication between a plurality of personal computers. It also serves to house many programs that it shares with the personal computers. Thus, the reliable operation of the server is critical to the operation of an entire network of such personal computers.  
           [0004]    The interconnection between the mating surfaces of a printed wiring board and a module or other active or passive device may be provided through an interposer. An interposer is a structure that provides electrical contact between two such devices (hereinafter referred to as area array devices). These area array devices typically are approximately parallel. The electrical connection between an area array device and a printed wiring board is achieved by pressing the device and the wiring board together with the thin conductive interposer between the two. The interposer can include compressible conductive elastomers, coil springs or leaf springs to establish the electrical connection between the wiring board and the other device.  
           [0005]    The need for area array connectors is growing due to the increase in inputs and outputs on area array modules. An area array connector is a type of high-density, low inductance socket available from a number of suppliers. These connectors may be any one of a number of different types. One such socket uses compressible ‘fuzz buttons’. These are compressible wadded wires described, for example, in the following patents: U.S. Pat. No. 5,552,752; U.S. Pat. No. 5,146,453 and U.S. Pat. No. 5,631,446. These are small, irregularly wound and inter-twined pads or balls that are made of gold plated beryllium copper wool or gold plated molybdenum wire. These wadded wire balls are compressed in holes in the interposer, which is a thin sheet of insulative material that separates the printed wiring board from the area array device. The holes are arranged in a pattern that matches a pattern of conductive pads on the surfaces of the printed wiring board and the area array device in contact with the interposer. In the assembly process, the ‘fuzz buttons&#39; are compressed between the conductive pads, thereby providing electrical connections between the pairs of pads on the two surfaces that are separated by the interposer. Among the other types of connectors are metal filled elastomers, such as those sold by Tyco Inc. (formerly Thomas &amp; Betts) as Metal Particle Interconnect Elastomers. Metal springs are also used. These metal springs generally are leaf springs having a number of geometries, such as C-shaped or V-shaped. Soldered connections complicate the disassembly or separation of the printed wiring board from the area array modules if repair or replacement is required.  
           [0006]    A limitation of this type of interconnect technology is the inherent failure rate that can be obtained while placing electrical contacts in series. The intrinsic failure rate of electrical contacts in series can be approximated by:  
           [0007]    Failure rate=n·IFR, wherein  
           [0008]    Failure Rate equals the contacts in series  
           [0009]    n=number of contacts in series  
           [0010]    IFR=intrinsic failure of each contact interface.  
           [0011]    Therefore, addition of more electrical contacts in series typically increases the failure rate of an electrical circuit.  
           [0012]    Land grid area array interposers have been described in which an interposer is designed for electrically connecting an electrical device to a printed circuit board wherein the interposer has been fabricated to provide adequate stand-off between the electrical device and the printed circuit board to provide space for active and/or passive devices.  
           [0013]    To provide a connection between a printed wiring board and another area array device (which may be a second printed wiring board) using a land grid array (LGA) connector, a space of about 3 mm is needed to allow clearance for any active or passive devices, such as diodes, capacitors and capacitors that are mounted on the surface of the board. An option is to request the vendors of the connector to devise a custom connector that provides the needed height. As is well recognized, the customization of any piece of hardware or software can add appreciably to the overall product development schedule, as well as the cost of assembly and/or use.  
         BRIEF DESCRIPTION OF THE INVENTION  
         [0014]    The present invention relates to an electronic sub-assembly and its method of manufacture. The sub-assembly comprises a printed wiring board (PWB) having a first surface including a plurality of vias terminating at electrical contacts at said surface. These contacts generally are in the form of contact pads. A first area array device having a surface, including a plurality of electrical contacts, is spaced from the PWB. A plurality of connectors electrically couple the electrical contacts on the surface of the PWB to the electrical contacts on the surface of the first area array device. The printed wiring board typically has a second surface that includes a plurality of electrical contacts that are coupled to the second surface of the PWB by a plurality of connectors. The vias extend through the PWB to form electrically conductive plated through holes.  
           [0015]    In one embodiment, an interposer separates the first surface of the printed wiring board from the surface of the first area array device. A second interposer may also separate the second surface of the PWB from the second area array device. Each of the interposers includes the connectors that couple said printed circuit board to the respective area array device. The area array devices may be spaced a fixed minimum distance from the printed wiring board, preferably a distance of about 3 mm. The sub-assembly preferably uses connectors that are selected from the group consisting of compressible wadded wire contacts, metal springs, filled conductive elastomers, solder balls and hard solder balls.  
           [0016]    For ease of repair and replacement, the connectors are compressible wadded wire having a first end soldered to said contact pads and a second end compressed against the electrical contacts on the surface of the area array device. This allows the area array device to be easily detached from the sub-assembly.  
           [0017]    Means are also provided for maintaining a fixed minimum spacing between the PWB and each area array device  
           [0018]    The invention also relates to a printed wiring board sub-assembly. The printed wiring board has a first surface containing one or more vias extending from the first surface into the board, and a conductive contact pad on the surface of the printed wiring board electrically joined to each of said vias. An interposer structure has a first surface facing the first surface of the printed wiring board, and a second surface. The structure includes a pattern of holes extending therethrough that correspond to the holes in the printed wiring board. Wadded wire electrical contacts are pressed into each hole in the pattern. Each of the electrical contacts has a first end that is soldered or is pasted to a contact pad on the first surface of the printed wiring board. The second end of the wadded wire contact is electrically coupled by compressive contact to a conductive lead on a surface of a module or another printed wiring board.  
           [0019]    At least some of the vias typically extend through the board to form plated through holes that terminate at a second board surface. A second interposer structure can be joined to contact pads on the second surface in a manner similar to that of the first interposer to provide an interconnect to a second area array device.  
           [0020]    In yet another embodiment, the invention relates to an electronic sub-assembly that comprises a PWB spaced a fixed minimum distance from an area array device, and the method of making the same. The printed wiring board has a first surface including a plurality of vias extending from the surface into the board. A plurality of electrical contact pads are connected to each of the vias, at least some of the contact pads being offset from the corresponding vias. The area array device has a surface including a plurality of electrical contacts. The surface of the PWB and the surface of the area array device are spaced a fixed minimum distance, preferably about 3 mm, from one another. A plurality of connectors electrically couple the contact pads on the surface of the printed circuit board to the electrical contacts on the surface of the area array device. The connectors are selected from the group consisting of compressible wadded wire contacts, metal springs, filled conductive elastomers, solder balls and hard solder balls. The hard solder balls typically are composed of copper balls coated with a layer containing tin. If normal solder balls are used, they may be embedded in an underfill material filling the space between the printed wiring board and the area array device. The wadded wire connectors, filled elastomers and metal springs typically are used by placing them in holes extending through the interposer between the surface of the PWB and the corresponding surface of the area array device. The interposer may also comprise a pair of plastic caps, each cap including a lip extending over an edge of the PWB.  
           [0021]    In yet another embodiment, an electronic sub-assembly comprises a printed wiring board having first and second generally planar surfaces and a plurality of vias terminating in electrical contact pads on the surfaces. A first area array device has a generally planar surface facing the first generally planar surface of the printed wiring board. The surface of the array device has a plurality of electrical contacts thereon. A plurality of connectors electrically couple the electrical contacts on the first surface of the printed circuit board to the electrical contacts on the surface of the first area array device. These connectors can be wadded wire contacts, metal springs, filled conductive elastomers, solder balls or hard balls or combinations thereof. A first interposer separates the first surface of the printed wiring board from the surface of the first area array device a fixed minimum distance of about 3 mm. The interposer contains a pattern of holes corresponding in location to the contacts on the first surface of the printed wiring board and the connectors are positioned in these holes. The sub-assembly further includes a second area array device having a surface including a plurality of electrical contacts for electrically coupling the second surface of the printed circuit board to the second area array device. A second interposer separates the second surface of the printed wiring board from the surface of the second area array device. As with the first interposer, the second interposer contains a pattern of holes. These holes correspond in location to the contacts on the second surface of the printed wiring board. A plurality of connectors are positioned in the pattern of holes in the second interposer for electrically coupling the electrical contacts on the second surface of the printed circuit board to the electrical contacts on the surface of the second area array device. At least some of the electrical contacts on one or both of the generally planar surfaces of the printed wiring board comprise contact pads that are offset from the corresponding vias in the printed wiring board to which the contact pads are electrically connected. At least some of the connectors in the holes of said interposers have a first end soldered or pasted to contact pads on a surface of the printed wiring board, while the second end is pressed against the electrical contacts on the generally planar surface of the corresponding area array device without soldering or pasting. The invention also contemplates that the second interposer may separate the second surface of the printed wiring board a fixed minimum distance of about 3 mm from the surface of the second area array device. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0022]    [0022]FIG. 1 is a circuit board assembly using an interposer with wadded wire electrical contacts compressed between two circuitized substrates according to the teachings of the present invention;  
         [0023]    [0023]FIG. 2 shows a similar assembly with two interposers with wadded wire connections between three circuitized substrates;  
         [0024]    [0024]FIG. 3 shows a circuit board assembly using wadded wire contacts, and an underfill material;  
         [0025]    [0025]FIG. 4 shows another circuit board assembly using a stand-off interposer;  
         [0026]    [0026]FIG. 5 is a planar view of the interposer and printed circuit board of FIG. 4;  
         [0027]    [0027]FIG. 6 shows another circuit board assembly with C-spring connectors and a ball grid array using hard electrically conductive balls; and  
         [0028]    [0028]FIG. 7 shows another circuit board assembly using conductive rubber contact pads.  
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0029]    Land grid area array interposers have been described in which an interposer is designed and fabricated for electrically connecting an electrical device to a printed circuit board wherein the interposer has been specifically designed to provide adequate stand-off between an electrical device and a printed wiring board to provide space for active and/or passive devices mounted thereto.  
         [0030]    Turning to the drawings for greater detail, FIG. 1 shows a circuit board assembly comprising a printed wiring board  110  electrically connected to an area array device  112 , such as a printed circuit board or module. The printed wiring board  110  includes a plurality of vias  120  extending from the surface  124  into the board. Conductive pads  122  are soldered, pasted or plated to the openings of each via  120  on the top surface  116  of the PWB  110 , and pads  126  are soldered, pasted or plated to the bottom surface  124  of the PWB  110 .  
         [0031]    An interposer sheet  130  is provided with holes  132  positionally corresponding to the vias  120  on the bottom surface  124  of the PWB  110 . Each hole  132  is filled with wadded wire  134 . One end of the wadded wires in the interposer sheet  130  is placed in contact with the contact pads  126  on the bottom surface  124  of the PWB and is soldered thereto.  
         [0032]    The area array device  112  contains electrical contacts or interfaces  148  on the board surface facing the PWB  110 . The contacts are positioned to abut the other end of the wadded wire contacts  134 . The wadded wire contact is compressed to the plated through hole  120  to form an electrical contact from the pads  148  on the module  112  to the plated through hole  120  using a conventional clamping device of the type commonly employed in the art. For purposes of illustration, the clamping pressure in FIG. 1 is shown by opposing arrows  100  and is capable of evenly distributing clamping pressure to the mating surfaces of the PWB  110 , the area array device  112 , and the two surfaces of the interposer sheet  130 . The clamping means  100  can, for example, comprise several C-clamps placed around the edges of the mating surfaces sandwiched between backing plates (not shown). Instead, a plurality of bolts can extend through holes in the various layers of the sandwich and through backing plates, and can be secured by nuts to provide the clamping pressure.  
         [0033]    In accordance with one embodiment of the invention, the solder is then reflowed to form the electrical connection between the contact pad  126  and the wadded wire  134  in the interposer  112 . This then comprises the interposer sub-assembly  144 . However, the interface between the wadded wire and the contact pad  148  is not soldered and, thus, remains separable, relying only on the clamping pressure to maintain electrical contact. This mechanical connection then facilitates the disassembly of the wiring board  110  and the area array device  112 .  
         [0034]    Referring now to FIG. 2, an interposer sub-assembly  244  comprises an inner PWB  210  containing a plurality of vias  220  extending between the surfaces of the PWB to form plated through holes (PTH)s)  218 . These PTHs join conductive pads  222  on the upper surface  216  of the PWB  210  to corresponding conductive pads  226  on the bottom surface  224  of the PWB  210 . The upper pads  222  contact fuzz buttons  234  pressed into holes  242  in interposer  240 . The fuzz buttons, in turn, make electrical contact with contact pads  246  on the upper area array device  214 . The pads  226  on the lower surface  224  of the PWB  210  are electrically and mechanically coupled to a lower area array device  212  through fuzz buttons of wadded wire  234  compressed into holes  232  in the interposer sheet  230 . The electrical connections from wadded wire  234  to the conductive pad  226  and the wadded wire  234  to conductive pad  222  is augmented with solder, conductive paste, or other permanent connection to improve control reliability or improve handling by reducing the number of loose pieces. On the other hand, mechanically coupling the sub-assembly  244  to the area array devices without solder or paste facilitates disassembly for repair and replacement. Another advantage of this system is increased design flexibility with minimum inventory. Modifications can be made in the thickness of the printed circuit board  210  to allow the same interposers  230 ,  240  to accommodate multiple standoff heights and/or component heights on devices  212 ,  214 .  
         [0035]    It should be understood that the invention can also be practiced by replacing the fuzz buttons  234  with springs, such as C-springs made by Intercon Corporation, or by the use of electrically conductive polymers or solder balls or other means for effecting an electrical connection between the PWB and the devices that are spaced from the PWB.  
         [0036]    The present invention also relates to the use of a standard height connector to get a fixed minimum space between the printed wiring board and the area array device. A 3 mm gap is adequate for many standard devices used in the industry today. The PWB is a laminate structure that can be soldered to a daughter card using established processes. The contact pads have a dog-bone structure on both the top and the bottom surfaces of the board, and these pads are electrically joined to the plated through holes. The plated through holes can be as small as 0.010″ for an interposer thickness up to 0.110″. For greater thickness, larger PTHs are required.  
         [0037]    Turning now to FIG. 3, a PWB  310  is shown between a lower area array device  312  and an upper area array device  314 . The PWB  310  is shown as a laminated structure comprising conductive metal layers  328  embedded in a prepeg. The PWB contains a plurality of vias extending through the PWB to form plated through holes  318  joining the upper surface  316  of the PWB with its lower surface  324 . The PTHs  318  are joined to conductive pads  322  on the upper surface  316  and to corresponding conductive pads  326  on the bottom surface  324 . The pads  322  are offset from the vias  318  in a so-called dog bone configuration. This permits the use of enlarged pads to establish electrical contact between the two surfaces of the PWB and the corresponding surfaces of the modules. The upper pads  322  contact solder balls  360  which, in turn, make electrical contact with the upper area array device  314 . The solder balls are supported by use of an underfill material  370  in accordance with well established practices. The underfill serves to reduce the creep of the solder balls under compression load and improves the fatigue life of the solder balls due to thermal strains. The pads  326  on the bottom surface  324  of the PCB are electrically connected to the lower area array device  312  through fuzz buttons  334  compressed into holes  332  in the interposer  330  in the manner previously described. As can be seen from the drawing, with the use of the dog bone configuration for the pads, it is not necessary to have an exact alignment between the solder balls, the plated through holes and the fuzz buttons. It should be understood that solder balls, metal springs or other connectors can be substituted for the fuzz buttons in the interposer.  
         [0038]    To avoid the need to use an underfill material to control movement of the solder balls, a plastic comb structure as shown in FIGS. 4 and 5 may be used. This structure provides a physical limit between a PWB  410  such as a mother board and a mezzanine device  414 . This plastic comb structure comprises a pair of plastic caps  480  to prevent the collapse of the solder balls  460  between the device  414  and the printed wiring board  410  when the components are clamped together. Each cap includes a lip  482  extending downward over the edge of the PWB. The lips serve to accurately position the interposer with respect to the PWB and to restrict relative movement between these two components. The caps include a plurality of semi-circular cutouts  484  corresponding to the outer row of solder balls  460 . The thickness of the caps is at least as great as the diameter of the solder balls. Thus, when the assembly is clamped together, the balls are not flattened or damaged by the compressive forces. Furthermore, the caps  480  allow the area array device  414  to be removed for purposes of rework or replacement.  
         [0039]    The PWB  410  contains several conductive layers  428  laminated therein, and a plurality of plated through holes  418  extending therethrough. These holes  418  connect with conductive pads  426  on the bottom surface  424  of the PWB  410 . An interposer sheet  430  contacts the bottom surface  424  and contains a plurality of holes  432  into which wadded wire fuzz buttons  434  are pressed. The dog-bone shape of the pads  422 ,  426  permits the solder balls  460  and wadded wire  434  to make electrical contact through the conductive PTHs, even though the solder balls and the wadded wire are not located directly above or below the plated through holes. The caps  480  typically are added after the PWB  410  and the top module  414  are soldered together and cleaned. Thus, even if the two members creep toward one another, there is a set limit, as determined by the thickness of the cap, that will keep them apart and will avoid an electrical short between the solder balls.  
         [0040]    [0040]FIG. 6 shows an arrangement wherein the deformable solder balls are replaced by hard electrically conductive balls composed of, for example, copper cores plated with a surface layer of tin or a tin alloy. The sub-assembly  644  comprises a printed wiring board  610 , an array of hard electrically conductive solder balls  662 , and an upper device  614  as another printed wiring board or a module. The PWB  610  contains a plurality of plated through holes  618  connected to a plurality of conductive pads  622  arranged in a dog-bone pattern on the upper surface  616  and to another set of offset pads  626  on its bottom surface  624 . As previously noted, this dog-bone arrangement of the pads eliminates the necessity of placing the hard solder balls directly over the PTHs and the C-springs directly beneath the PTHs. The hard balls  662  complete the electrical contacts between the pads  622  and the module  614 . The hard balls  662  resist collapse when the sub-assembly  644 , the upper device  614  and the lower device  612  are clamped together. With this arrangement of hard solder balls, no underfill material or plastic caps are needed to keep the solder balls from being collapsed under pressure. The sub-assembly  644  is soldered and cleaned, and is then ready to be connected to a lower device  612  via the land grid array connector. For this, an interposer sheet  630  contains a pattern of holes  632  containing C-springs  664 . This construction allows a thick interposer sub-assembly to be constructed with internal ground and power planes to reduce the noise and inductance that are inherent in long electrical leads. As previously described, the C-springs can be replaced with other types of electrical connectors, such a wadded wire, without departing from the present invention..  
         [0041]    The electrical connectors through the interposer may be selected from a number of different types. Among these are metal filled polymers. Others are compressible wadded wires commonly referred to as fuzz buttons. Metal springs, as previously described, may also be used.  
         [0042]    The interposer is generally planar and is made from plastic or similar material having good mechanical strength and dimensional stability. It usually is an insulator made from plastic material, such as a polyphenylsulfide resin known as Ryton R-4 sold by Phillips 66 Corporation, or a liquid crystal polymer, such as VECTRA E130 i available from Hoechst Celanese Corporation. The interposer serves to electrically and mechanically isolate the area array devices from the printed wiring board.    
         [0043]    Turning now to FIG. 7, another embodiment is shown wherein a thick interposer  730  includes a plurality of holes extending therethrough, filled with a conductive paste  790 , such as a cured silver filled elastomeric compound. Each hole is filled with a carefully controlled excess of the compound which is then cured or hardened. Upon hardening, the excess forms a contact button  792  on the top surface of the interposer and another contact button  794  on the bottom surface. The interposer is clamped between an upper area array device  714  and a lower area array device  712  to form a completed assembly. The various contact points on the two area array devices are not shown in the drawing. The interposer  730  and the contact buttons  792 ,  794  serve to maintain a minimum space of about 3 mm, for example, between the two area array devices.  
         [0044]    The costs associated with the constructing and qualifying of specialized interposers is reduced by following the teachings of the present invention. Further, variations in size and thickness from one interposer to the other are reduced. The invention utilizes the advantages of metal-to-metal contact throughout the system. It allows the use of solder joints in compression to improve their reliability.  
         [0045]    The specific details and operation of the sub-assembly described herein as well as the details of the various passive and active devices that are used here are known to persons of ordinary skill in the art. Accordingly, these details do not comprise a part of the present invention, except to the extent that they and their operation have been modified to become part of the present invention.  
         [0046]    While the invention has been described in combination with embodiments thereof, it is evident that many alternatives, modifications, and variations will be apparent to those skilled in the art in light of the foregoing teachings. Accordingly, the invention is intended to embrace all such alternatives, modifications and variations as fall within the spirit and scope of the appended claims.