Abstract:
A flip-chip semiconductor die assembly is provided with discrete projecting connective elements on the die and mating recessed contacts on the substrate, together forming a plurality of electrical, mechanical, and thermal connections between the die and substrate. The element and recess provide a self-aligning feature. Compared to the prior art, a less costly, simpler method for forming the element and recess is provided, which method requires fewer process steps, uses less raw materials, and generates less waste. The method allows for placement of fine-pitch connective elements and precise control of element size and height.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention pertains to a flip-chip semiconductor die assembly and, more particularly, to micro-size bumps and recessed contacts for self-aligned contact of the die to a substrate, and specifically to methods for forming the bumps and mating contacts. 
     2. State of the Art 
     As the complexity of integrated circuits on semiconductor dice has increased, semiconductor die manufacturers and assemblers have found a correspondingly increased need for improved input and output connections. A flip-chip arrangement is one conventional arrangement used to take advantage of its potentially higher number of arrayed input and output connections; that is, more such connections can be placed on the active surface of a die than with wire-bonding, TAB, or other conventional connection techniques. In a conventional wire bonded arrangement, the number of connections that can exist in a given surface area of a die is limited because of the diameter of the wire ball to be formed from the bond wire at the connection point or bond pad on the die surface, as well as by the number of wires which can be extended from bond pads to a lead frame or carrier substrate without shorting. The pitch, or nominal distance between the center of any two connection points, is generally limited to approximately 0.1 mm, although some arrangements have achieved a pitch of as low as 0.08 mm. Simply put, the wire balls are too bulky to allow a more dense array of connections, and potential mutual interference by the wires limits the usable patterns of such connections. 
     By contrast, a conventional solder-bumped flip-chip arrangement allows a high density of connections per given area of active surface on the die, and the least amount of die-to-carrier connection time because of the ability to effect all connections simultaneously. In a conventional flip-chip arrangement, solder bumps are formed or deposited on a semiconductor die, and the die (“chip”) is turned over, i.e. flipped, and then aligned with mirror-image solder bumps or bond pads on another die, or terminal pads of a die carrier or a printed circuit board or other, similar carrier substrate. By reflowing the solder after contact of the bumps with the pads or cooperating bumps on the mating component of the assembly being fabricated, a simultaneous electrical, mechanical, and thermal connection of each cooperating pair of contact points is achieved. Since bump size can, with some techniques, be smaller than wire ball size and bumps can, in some instances, be placed more accurately than wire balls, the potential density of bumps can exceed that of wire bonds, reaching a corresponding pitch of as low as 0.01 mm. Nevertheless, bump pitch is limited by the selection of bump size, bump shape, and bond pad metallization characteristics. As detailed below, when an improper combination of these elements is selected, the bumps may spread outward too far and form unwanted connections to other bond pads during reflow of the solder. 
     One of the first solder-bumped flip-chip arrangements was created using so-called Controlled Collapse Chip Connection (C4) technology. The technology involves, first, laying down a passivation layer on the surface of a semiconductor die which covers the bond pads where connections will be made between the die and a substrate. Next, holes are formed in the passivation layer over the bond pads and one or more layers of metallization are typically deposited over the exposed bond pads. Finally, solder bumps (typically of a tin/lead alloy, although other alloys are sometimes employed) are deposited on the metallized areas and a preliminary reflow performed so that the bumps take on a semi-spherical shape. Later, after alignment with terminal pads of conductive traces of a substrate, a final reflow will form the permanent die-to-substrate electrical connections. The metallization deposited on each bond pad must be limited in circumference to the approximate size of the hole through which it contacts the bond pad. However, the metallization may extend up the walls of the hole in the passivation layer through which the bond pad is exposed, and onto the top surface of the passivation layer, although obviously avoiding contact with neighboring bond pad metallization. 
     One purpose of the metallization layer interconnecting the bump and the bond pad on the underlying active surface of the die is to provide improved solder adhesion to the bond pad. Another purpose is to control the contact area the bump will cover on the die surface by use of a very solder-wettable metal or alloy on the exposed surface of the metallization. The intent is to prevent the solder from spreading beyond the circumference of the deposited metallization. By controlling the contact area, the metallization partially controls the bump&#39;s height, since the bump will form a semi-sphere with a size somewhat dependent on the circumference of the metallized area on which it resides, as well as on the volume of the bump material. Understandably then, the metallized area is sometimes referred to as Ball-Limiting Metallurgy (BLM). If the volume of deposited solder becomes too large for a given contact area metallization, then the surface tension of the particular solder composition used will be insufficient to contain the molten solder in spherical form and the solder will overflow the metallization despite its presence. Even if the surface of the die is additionally coated with a low-surface tension material to inhibit spreading of the reflowed solder from the BLM, the effectiveness of such coatings is limited. The coating will probably help prevent incidental outflows from the BLM, unless a bump is too large and exceeds the surface tension of the molten solder. In that case, a low-surface tension coating will probably be insufficient to contain all of the escaping solder and avoid contact with another bond pad located nearby in a fine-pitch array. Thus, bump volume and pitch must be carefully considered and controlled to prevent defects in flip-chip connections. 
     The use of solder bumps to form connections between two dice, a die and a printed circuit board or other carrier substrate, or a carrier substrate and a higher-level package is well-known in the art. However, even though BLM is used on components of such assemblies carrying the solder bumps, the spacing or “pitch” of the bumps is limited by conventional technologies due to problems with preventing the bumps from flowing together during reflow of the solder. Many variations in the materials used in a C4 process and in the detailed process steps exist, since users have sought to match the technology to their particular applications, to meet reliability requirements, and to improve production efficiency and connection quality. The significant number of these variations is indicative of the complexity of conventional methods for forming solder bumps on dice and the number of problems inherent in the conventional methods. The complexity of forming adequate solder bump connections is further exemplified by the methods disclosed in U.S. Pat. Nos. 4,940,181; 5,477,086; 5,480,835; 5,492,235 and 5,505,367. The complexity of such methods contributes to the typical, relatively high cost of manufacturing solder-bumped dice, particularly as attempts are made to form smaller bumps with hopes to achieve a more densely-packed array of connections. 
     After forming bumps on a semiconductor die, the die must then typically be connected to another die, or to a printed circuit board or other carrier substrate. As indicated earlier, the die bumps are aligned with mirror-image terminal pads or solder bumps on the substrate to make the connection. Substrate bumps may generally be formed by the same methods used to form die bumps. However, the substrate bumps often possess a designed shape so as to facilitate aligning of the die bumps and making a reliable connection. In some instances, bumps of metals other than solder are employed, and connections are effected by means other than a reflow. In addition, metal-loaded polymer bumps have also been fabricated. See, for examples of the foregoing structures, U.S. Pat. Nos. 4,182,781; 5,246,880; 5,329,423; 5,346,857 (all non-solder metal bumps) and 5,508,228 (metallized compliant polymer bumps). Self-aligning connections may be desirable, and exemplary shapes previously designed for this approach are illustrated in U.S. Pat. Nos. 4,940,181; 5,019,673; 5,329,423 and 5,477,086. Absent designing a substrate bump or terminal pad with some sort of conforming shape wherein a projecting die bump can reside and, thus, self-align, highly accurate mechanical pre-reflow alignment of the die to the substrate must be achieved by another method. Thereafter, all the bumps on the die and cooperating contact areas on the substrate must be brought into relatively exact contact with one another and maintained in that position during both reflow and re-solidification of the solder. Because of the surface tension changes and capillary action that occurs during reflow and re-solidification of the solder bumps, maintaining a die in its proper position relative to a substrate may be more difficult than it first appears, particularly if some of the intended connection points are even slightly misaligned, which in turn tends to induce misalignment of other connection points due to the surface tension and capillary action of the solder material. 
     Given the added manufacturing cost of forming specially designed substrate bumps to match the shape of die bumps, some manufacturers have developed less costly self-alignment methods. For example, as disclosed in U.S. Pat. No. 3,811,186, spacers may be disposed between the die bumps such that the spacers nearly exactly occupy the gap between bumps. By using specifically-placed spacers of insulating material which may exceed the combined height of aligned die and substrate bumps, when the die and the substrate are brought into alignment and the assembly heated, the spacers will soften and reduce in height, permitting contact and fusion of the bumps in an aligned manner. When the assembly cools, the spacers will return to their normal height, elongating the fused, aligned bump connections. Attempts have also been made, as in U.S. Pat. No. 5,508,228, to eliminate the crucial need for self-alignment by using non-conductive adhesives surrounding the connection points to join metallized, compliant polymer bumps of a die to contact points on a substrate rather than relying on a solder bond. Additionally, bumps formed from conductive paste, as in U.S. Pat. No. 5,246,880, rather than solder, have been used to produce a higher aspect ratio bump than can be achieved with solder, and a bump of more precise and repeatable height. However, formation of a conductive paste bump according to the &#39;880 process will generally take longer than formation of a solder bump, since the bump is “built” in a plurality of layering steps, and the paste curing time exceeds solder re-solidification time. 
     See also U.S. Pat. No. 5,445,994, assigned to the assignee of the present invention, for a disclosure of forming planar bond pad connectors by patterning a passivation layer with holes by using an overlying patterning layer of a dielectric material, etching holes down to bond pads on the die, filling the holes with a metal layer, and planarizing the metal layer to an endpoint within the patterning layer over the passivation layer. 
     Insofar as solder bumps become semi-spherical in shape when heated to a liquid state, an increase in the desired height of bumps will result in a decrease in the potential density of a bump array, as an increase in height of a semi-spherical bump necessarily results in an increased bump width. Taller, more slender bumps (the term “slender” indicating a bump height measurably more than the bump width, or an aspect ratio of height to width of greater than 1) of columnar or pillar configuration are desirable for two significant reasons. First, with relatively slender bumps for a given gap or clearance between the die and the substrate, more bumps can be disposed on the die without inadvertent lateral connection between bumps during solder reflow. Second, thermal expansion of the die and/or the substrate creates stresses which the solder bump connections will bear. A “fat” semi-spherical-shaped solder connection will not be able to endure as much flex and strain as a slender solder connection of equal height. Flex and strain capacity of solder connections becomes particularly important when a silicon semiconductor die has a significantly different coefficient of thermal expansion (CTE) than its corresponding carrier substrate. Under such circumstances, the substrate will typically have a larger CTE, and will thus expand and contract during heating and cooling cycles to a greater degree than the die, creating substantial stress in the solder connections. 
     To the inventors&#39; knowledge, those of ordinary skill in the art have failed to develop a relatively simple and cost-effective method for forming discrete connective elements, such as pillars or bumps, on a die, and mating recesses on a substrate. Furthermore, even the most simple methods in the art fail to yield both a self-aligning feature and a dense array of sufficiently slender connective elements. 
     SUMMARY OF THE INVENTION 
     According to the present invention, a semiconductor die and a substrate are provided with a number of discrete, projecting connective elements on either the die or the substrate, and a like number of respectively aligned mating recessed contacts in the other. When joined, the projecting elements and cooperating recessed contacts form a plurality of discrete electrical, mechanical, and thermal connections between the die and substrate. The structure of the projecting elements and mating recessed contacts is such that the die and substrate possess a self-aligning feature, similar to a plug-and-socket design. The projecting element or plug is preferably pillar-shaped and slender, with a relatively high aspect ratio, greater than 1. However, the present invention is not limited to a particular connective element aspect ratio (again, structure height÷structure width). An array of elements and recesses can be formed according to the invention with a pitch as small as or smaller than prior art flip-chip die-to-substrate connections because the connective element or “plug” need not possess a semi-spherical shape with its attendant closely-related height and width. 
     The present invention further includes a simple method, in comparison to the prior art, for forming a projecting connective element and mated recess using etch back technology, in one variation in combination with abrasive planarization. The simplicity of the method decreases the typical cost of forming flip-chip connections by decreasing the number of process steps and the volume of manufacturing materials required, such as etchants, electroplating solutions, and resist layers. Also, the present invention may be applied at the wafer-scale, multiple die, or single die level. As used herein “wafer scale” is not limited to traditional wafers but encompasses any semiconductive material layer on which a large plurality of discrete active devices may be fabricated, including without limitation silicon-on-insulator (SOI) and silicon-on-sapphire (SOS) structures. 
     In its basic form, the method of the invention involves patterning holes in the passivation layer of a die to expose underlying bond pads, depositing solder or conductive epoxy over the passivation layer and into the holes, removing the solder or epoxy layer to a level which re-exposes the passivation layer upper surface and substantially coplanar tops of the conductive material in the holes, and partially etching back the passivation layer to cause the conductive material to project as connective elements formed in the previously patterned holes and cause them to project from the remainder of the passivation layer. 
     Although not a most preferred embodiment, at this point in the process, a functional conductive element arrangement exists wherein the die may be flipped over, the projecting elements placed in contact with a mating arrangement of substrate terminal pads and attached thereto by reflowing the element (if solder) or curing the element (if conductive epoxy), or using a conductive adhesive to bond the projecting element to the terminal. Also, the projecting elements could be formed on the substrate instead of the die, and contact made with bond pads of the die. 
     However, the invention preferably involves patterning holes in a passivation layer deposited on a substrate to expose terminals thereon, depositing solder or conductive epoxy on the passivation layer and into the holes over the terminal surfaces, removing the solder or epoxy to a level which re-exposes the upper surface of the passivation layer, and partially etching back the solder or conductive epoxy in the holes over the terminal surface to form recessed contacts. Solder or epoxy pads will thus remain on the terminals at the bottom of the recesses to facilitate electrical, mechanical, and thermal connection to projecting connective elements formed on a die. 
     It will be understood that the term “conductive epoxy”, as used herein, includes both inherently-conductive epoxy compounds as well as epoxies filled with discrete conductive particles, and that the term “epoxy” is employed as a generic term encompassing all suitable adhesives, including thermosetting adhesives, thermoplastic adhesives, “B” stage adhesives employing a pre-cure step, and so-called “snap cure” adhesives. 
     The features and advantages of the present invention mentioned above, as well as others, will be readily understood by reading the following description in conjunction with the accompanying figures, wherein like reference numerals have been applied to designate like elements throughout the several views. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a cross-sectional view of one preferred embodiment of a flip-chip semiconductor die assembly according to the present invention; 
     FIG. 2 is a cross-sectional view of either a die or a substrate at the beginning of step one of the method for forming the die or the substrate in the assembly of FIG. 1; 
     FIG. 3 is a cross-sectional view of either a die or a substrate at the end of step two of the method for forming the die or the substrate in the assembly of FIG. 1; 
     FIG. 4 is a cross-sectional view of either a die or a substrate at the end of step three of the method for forming the die or the substrate in the assembly of FIG. 1; 
     FIG. 5 is a cross-sectional view of a connective element at the end of step four of the method for forming the element in the assembly of FIG. 1; 
     FIG. 6 is a cross-sectional view of a mating recess at the end of step four of the method for forming the recess in the assembly of FIG. 1; 
     FIG. 7 is a schematic, side partial cross-sectional view of a multichip module according to the present invention; and 
     FIG. 8 is a schematic of a computer system including devices formed according to the present invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     A first embodiment of a flip-chip semiconductor die, multiple die or wafer assembly  10  with a self-aligned contact to its carrier substrate  16  is shown in FIG.  1 . The semiconductor die  12  is provided with at least one discrete projecting connective element  14  of solder or conductive epoxy and the carrier substrate  16  is provided with at least one mating recess  18 , wherein is deposited a connective or contact pad  20 , also of solder or conductive epoxy, depending on the material of projecting connective element  14 . The substructure  22  upon which the bond pad  24  and passivation layer  26  is formed is shown generically in FIG. 1 for both the semiconductor die  12  and carrier substrate  16 , it being understood that die substructure  22  comprises a substrate of semiconductive material carrying integrated circuit devices on its active surface, while carrier substrate substructure  22  may comprise a semiconductor material, ceramic, glass fiber, molybdenum, or other suitable substrate material known in the art. The passivation layer  26  is also shown generically for both the semiconductor die  12  and carrier substrate  16 , except that an overlayer  28  of low surface tension material such as a polyimide, which has critical surface tension at 44 dynes/cm, is shown on the passivation layer  26  of the carrier substrate  16 . As discussed earlier, providing a proper overlayer  28  is one measure which will help control unwanted solder spread during the connecting reflow. 
     In practice, the substructure  22  would be multi-layered. For example, as shown in FIG. 2, the substructure  22  of an exemplary die could comprise a silicon or other semiconductor base layer  30  with integrated circuit (IC) features  32  formed thereon and an inter-layer dielectric (ILD) coating  34  formed on the IC features. In practice, the passivation layer  26  could also be multi-layered and consist of single or multiple layers selected from the group comprising silicon dioxide, silicon-based glass such as phosphorous silicate glass (PSG) or borophosphorous silicate glass (BPSG), silicon nitride, polyimides, photoresists, and other conventional passivation materials. Myriad combinations of layers in the substructure and passivation of a die or substrate are envisioned within the present invention since it is useful in forming connections between a wide variety of devices, such as two dice, a die and an adapter-type carrier, a die and a conventional planar substrate such as a printed circuit board, and a die carrier. Additionally, forming the projecting connective element  14  on the carrier substrate  16  rather than the semiconductor die  12  with the mating recess  18  in the semiconductor die  12  is envisioned within the present invention. 
     In FIG. 1, the wafer assembly  10  is shown with the semiconductor die  12  in alignment with the carrier substrate  16  preliminary to a permanent electrical, mechanical, and thermal connection being formed therebetween. That is, a complete connection could be effected by reflowing (solder) or curing (epoxy) the projecting connective element  14  and contact pad  20  or by using an interposed conductive adhesive to join the projecting connective element  14  and contact pad  20 . The element and pad may be formed from a conventional Pb/Sn solder of a composition suited to the particular flip-chip assembly or from a thermoplastic-type or thermosetting-type of conductive or conductor-filled epoxy which is capable of reflow to effect connection. A so-called “B stage” epoxy, which is partially curable after deposition to maintain its shape, and then subsequently fully cured or set to effect an adhesive bond, may also be employed. Also, as previously mentioned, a “snap” cure adhesive with a cure time measured in seconds is also contemplated as usable. Conductive fillers for epoxy known in the art include silver particles and polystyrene balls coated with nickel. It is preferred that the conductive filler particles be of silver and employed in a high percentage by weight to maximize conductivity, minimize impedance and increase the viscosity of the conductive epoxy for enhanced definition of the connective elements. It is preferred that the connection elements, if formed of epoxy, comprise at least fifty percent (50%) by weight silver conductive filler material. It is also preferred that the filler particles be of relatively small size in comparison to those employed in the prior art, for example in the size range of about 0.05 to 0.1 μm, to provide more inter-particle conductive surface area contact and effect a better electrical connection between the bond pads and the terminals. Alternatively, the element and pad may be formed from a non-reflowable, non-adhesive conductive material, provided that another conductive or conductor-filled adhesive interposed between the element and pad is used to effect the connection. While use of a recessed contact pad  20  is preferred, the present invention will still operate in its absence to effect a direct connection between the projecting connective element  14  and its associated bond pad  24 . 
     The present invention includes a simple four-step method for forming the projecting connective element on a die or substrate as discussed above, or for forming the mating recessed contact pad discussed above on the other mating component (such as a carrier substrate) of the proposed assembly. The process begins with a die or substrate as shown in FIG. 2 wherein the semiconductor base layer  30 , IC features  32 , if any, ILD coating  34 , if any, bond pad or terminal pad  24 , and passivation layer  26  are already provided. The passivation layer  26 , preferably of a thickness of 4 kA to 2 μm, is patterned using conventional methods to form an aperture or hole  27  therethrough to expose the bond pad  24 . As shown in FIG. 3 in broken lines, an optional intermediate conductive layer  29  may be deposited by techniques known in the art prior to conductive layer  36  to improve adhesion of conductive layer  36  to bond pad or terminal pad  24  and/or to provide a stop layer for subsequent chemical mechanical planarization (CMP) as discussed below. Intermediate conductive layer  29  may be applied before or after formation of hole  27 , as desired, depending upon whether enhanced adhesion of the material of passivation layer  26  to bond pad or terminal pad  24  is desired. Next, a solder or conductive epoxy stop layer  36  is deposited on the passivation layer  26  and into hole  27 , as shown in FIG. 3, preferably using conventional methods such as sputtering, chemical vapor deposition (CVD) or wave soldering techniques in the case of solder, and spin coating in the case of a conductive epoxy. Alternatively, other less-preferred conventional methods, such as spray coating or roll-on coating, may be used to deposit the conductive layer  36 . In the case of a solder or other heat-softenable conductive material stop layer  36 , a pre-reflow heating may optionally be effected to eliminate voids in the stop layer and ensure complete filling of hole  27 . In the next step, as shown in FIG. 4, the conductive layer  36  (and optional layer  29 , if employed) is substantially uniformly removed to re-expose the passivation layer  26  by using conventional methods such as abrasive or “mechanical” planarization, “chemical-mechanical” planarization (CMP), or plasma etching. Upon removal of the upper portion of the conductive layer  36  to the level of the upper surface of the passivation layer  26 , a conductive solder or epoxy accumulation  38  will remain in the now-filled aperture or hole  27  previously created, with the top of the conductive accumulation  38  being substantially co-planar with the upper surface of passivation layer  26 . Forming a projecting connective element  14  involves partially etching back the passivation layer  26  to the exclusion of the solder or epoxy accumulation  38  using conventional wet or dry etching methods, thus forming the structure shown in FIG.  5 . If a mating recess  18  with a contact pad  20  is to be formed instead of a projecting connective element  14 , step four involves partially etching back the conductive accumulation  38  to the exclusion of the passivation layer  26  using conventional wet or dry etching methods, thus forming the structure shown in FIG.  6 . By partially etching back the conductive accumulation  38 , a conductive contact pad  20  will remain at the bottom of the mating recess  18  to aid ultimately in effecting connection of the die to the substrate. 
     It is contemplated that the present invention will prove suitable for the fabrication of multi-chip modules (MCMs), including SIMMs, DIMMs, TRIMMs and other memory cards, as well as motherboards and other MCMs including dice having other functions, including microprocessors and logic dice, as well as application-specific integrated circuits (ASICs). A schematic, side elevation of an MCM  100  including three dice  102  connected to a substrate  104  according to mating connections  106  of the present invention is depicted in FIG.  7 . Likewise, as depicted in FIG. 8, an MCM such as an exemplary DIMM  100  including dice  102  may be associated with a separate microprocessor  150  in combination with an input device  200  and an output device  202 , such as is the case in a personal computer. 
     The connective element and mating recessed contact pad of the present invention, and the described method for forming them, offer significant advantages over conventional flip-chip solder bumps and solder bumping methods. Most notably, the present invention allows the formation of a dense, precisely dimensionally-controlled (as to height, width and pitch) array of connections at low cost. First, an array of elements or recesses on a die or substrate may be formed at the wafer-scale level such that, once processing of the wafer is complete, the dice may be separated with their connective structures (projecting or recessed) already in place. Understandably, while conventional screen printing methods may also be used to deposit discrete solder bumps on dice at the wafer-scale level, such screen-printed solder bumps are, however, limited to a semi-spherical shape and to a comparatively large pitch because of the required first reflow and the precision limits of screen-printing. Very small bumps are difficult to deposit with precision using screen-printing technology because of the inherently poor resolution. Electroplating methods may be used to overcome the precision limits of screen printing, but cannot presently be conducted at the wafer-scale level; to electroplate solder bumps, the wafer must first be separated into dice. Electroplating, moreover, adds additional time, process steps and materials beyond that required by the present invention, and thus adds to manufacturing and waste disposal costs. The present invention allows highly precise placing and sizing of flip-chip connections without all the etchants, electroplating solutions, and resist layers needed to electroplate solder bumps. Specifically, the present invention uses only a single mask in the four step process (to pattern the passivation layer with holes for connective elements) and is suitable for the use of relatively rapid and simple abrasive planarization and dry etchback techniques, rather than more complex, slow and costly electroplating and wet etching. 
     It is also notable that the present invention provides a self-aligning feature and well-controlled height of the connective element, the latter due to the use of the previously-maintained abrasive or CMP planarizing techniques to remove the conductive material in a precisely-controlled manner. Pillar-shaped, high aspect ratio conductive connective elements of almost any desired height or geometric cross-section may be formed to easily align with mating recesses on a substrate. Uniform heights and uniform cross sections for all of the projecting and recessed connective elements in mating arrays ensure complete electrical, mechanical and thermal connection of each opposing pair of pads on the die and substrate. Absent such uniformity, some prior art connective elements may form a complete connection to their cooperating pads, while others may not with no particular predictability. 
     The well-controlled element height also ensures that any desired vertical gap width is maintained between the die and substrate. When connective elements on the die are formed from solder or other reflowable material, it may be desirable to provide insulative spacers of some type on either die or substrate, as shown in broken lines  40  in FIG. 1, to guarantee desired vertical spacing. That is, upon reflow, a pillar will most likely deform to fill the mating recess and correspondingly collapse somewhat. By providing spacers  40  in areas of the die/substrate interface not populated with connecting elements and recessed contacts, collapse of one component toward another is prevented even if the connective elements are of limited compressive strength. 
     The present invention has been disclosed in terms of certain preferred embodiments as illustrated and described herein. However, those of ordinary skill in the art will recognize and appreciate that it is not so limited, and that many additions, deletions and modifications to, and combinations of, the disclosed embodiments may be effected without departing from the scope of the invention as hereinafter claimed.