Abstract:
The invention relates to an assembly method for connecting two electronic components together, said components each having an assembly face, wherein the two assembly faces are moved together in what is known as an assembly direction X, and a given force F is applied to one and/or the other of the components, one and/or the other assembly face(s) having: —connection inserts made of rigid material having an elongate longitudinal shape in the assembly direction X; —connection tracks made of material having a hardness less than that of the inserts and having an elongate longitudinal shape transversely to the assembly direction X, wherein, in said method: —the inserts are aligned opposite corresponding tracks such that the inserts and the tracks form in pairs, after assembly, at least one approximately transverse intersection, —the force F is applied so as make the inserts penetrate into the tracks until the assembly is produced.

Description:
TECHNICAL FIELD 
       [0001]    The present invention relates to a flip-chip type process for assembling two electronic components, in which one of the two electronic components is flipped in order to enable front-to-front assembly or electrical connection. 
         [0002]    The invention allows electronic components to be assembled whatever their interconnect pitch and the gap between these components. 
         [0003]    The invention more particularly relates to assembly of a chip and a substrate, both of which may for example be made of silicon. 
         [0004]    The invention aims to decrease the force required to assemble electronic components. 
         [0005]    The invention is mainly applicable to any microelectronic devices requiring front-to-front interconnects having a very fine pitch. 
         [0006]    One particularly advantageous application of the invention is production of stacked three-dimensional 3D structures or of multispectral heterogeneous imagers. 
         [0007]    The expression “assembly of two electronic components” is understood, in the context of the invention, to mean either an assembly of two components made of different materials, or an assembly of two components made of the same material. In particular, an assembly according to the invention may be an assembly of an electronic chip and a substrate, both of which may possibly be made of silicon. 
         [0008]    The expression “interconnect pitch” is understood to mean the distance between two connecting tracks on a given electronic component. 
         [0009]    The expression “gap between components” is understood to mean the spacing between the two facing components, as defined by the interconnect height. 
       PRIOR ART 
       [0010]    The flip-chip technique is a well-known technique for mechanically and electrically interconnecting or assembling two components, such as a chip and a printed circuit board substrate. This technique is called the flip-chip technique because one of the components, in general the chip, which bears conductors, is flipped in order to bring the two components face to face in order to enable interconnection by bonding of the conductors and metal bumps forming contacts on the other component, in general a printed circuit board substrate. 
         [0011]    In this technique it is continuously being sought to decrease the gap between components and increase the number of connections. However, the three main categories of assembly technologies currently used in this technique, namely soldering, thermocompression and the use of adhesives such as ACFs (anisotropic conductive films), are each reaching their limit as regards decreasing gap size. 
         [0012]    In particular, low-temperature thermocompression obtained by forcing conductive inserts to penetrate into bumps, such as described in patent application WO 2006/054005, is limited by the very large force required if a very large number of connections are to be formed and by the feasibility of actual production of the inserts. 
         [0013]    Thus, to remedy the limits of this method, the Applicant has proposed, in patent application WO 2009/115686, to produce conductive inserts taking the form of blind tubes the bases of which are securely fastened to the surface of a component. Conductive inserts were the subject matter of improvement patent application EP 2 287 904, at least one area of the open end of an insert being left free so as to allow gas contained in the insert to escape during the insertion. Conductive inserts having various novel shapes were provided in this application, examples being an open bar and elements having star-shaped, cross-shaped or lobed cross sections, etc. 
         [0014]      FIGS. 1 ,  2  and  3  show assembly of two electronic components according to the aforementioned application WO 2009/115686. A hollow and open insert  10  taking the form of a blind tube of cylindrical cross section is securely fastened via its base to the surface of a substrate  11  of a first electronic component. A bump  20 , typically a solder bump, is securely fastened to the surface of a substrate  21  of a second electronic component  2 . The material of the bump  20  is less hard than that of the insert  10 . To assemble the two components  1 ,  2  by thermocompression, the insert  10  is aligned facing the bump  20  and then a substantially constant force F is applied in the assembly direction X shown by the arrow ( FIG. 1 ) until the assembly is obtained, i.e. until the insert  10  has been completely inserted into the bump ( FIGS. 2 and 2A ). 
         [0015]    Although satisfactory on the whole, especially as regards the feasibility of production of the inserts, the method in these patent applications could be improved further. 
         [0016]    In particular, there is a need to decrease the magnitude of the force to be applied to assemble two microelectronic components by thermocompression, especially when the interconnects between components require material(s) to be used that must not pass into the liquid state in packaging steps carried out after the actual assembly. 
         [0017]    It has been demonstrated in publications [1] and [2] that for a given insert produced in the form of a microtube, the required insertional force depends not only on the mechanical properties of the ductile material itself (hardness, plasticity, etc.) from which the pad is produced but also on the area of insertion, or in other words the area of intersection between the insert  10  and the pad  20  made of ductile material. 
         [0018]    Publication[3] moreover explains that the insertional force of a tube is directly proportional to the area of insertion Si of the tube into the pad, and therefore to its wall thickness and to the length L of its perimeter. Thus, in the example shown in  FIG. 3 , the area of insertion Si of an insert taking the form of a microtube of cylindrical cross section of radius R and of wall thickness e 1  is equal to the area of the cross section of the microtube  10  in a horizontal plane, i.e. in the plane orthogonal to the assembly direction X, namely Si=2*π*R*e 1 . The insertional force F is therefore equal to F=k*2*π*R*e 1 , where k is a constant. 
         [0019]    As regards the mechanical properties of the ductile material, when it is desired to carry out an insertion at room temperature, the fact that the insertional force is, to a first order approximation, inversely proportional both to the Young&#39;s modulus of the constituent metal of the connecting pad and to its elastic limit at room temperature, must be taken into account. Moreover, among known ductile metals, those having the lowest Young&#39;s modulus are those with melting points closest to room temperature. Table 1 below gives the list of ductile metals having both a low Young&#39;s modulus and a low melting point. 
         [0000]    
       
         
               
               
             
               
               
               
               
               
               
             
               
               
               
               
               
               
             
           
               
                   
                 TABLE 1 
               
             
             
               
                   
                   
               
               
                   
                 Metal 
               
             
          
           
               
                   
                 Indium 
                 Tin 
                 Lead 
                 Zinc 
                 Aluminum 
               
               
                   
                 (In) 
                 (Sn) 
                 (Pb) 
                 (Zn) 
                 (Al) 
               
               
                   
                   
               
             
          
           
               
                 Young&#39;s 
                 11 
                 49.9 
                 16.1 
                 104 
                 70 
               
               
                 modulus 
               
               
                 (GPa) 
               
               
                 Melting point 
                 156 
                 231 
                 327 
                 419 
                 660 
               
               
                 (° C.) 
               
               
                   
               
             
          
         
       
     
         [0020]    In addition to the above considerations regarding ductile metals, when an electronic chip and a wafer, or a chip and another chip, are to be assembled by insertional thermocompression, the connections should be formed with materials that will not pass into the liquid state in the packaging steps carried out after the insertion. These packaging steps employ temperatures of up to 230° C. This temperature of 230° C. is that required to solder the electronic components to a printed circuit board using an AgCuSn solder. 
         [0021]    Thus, on the basis of table 1, only aluminum is able to respect all of the technical constraints. Specifically, indium has a melting point below 230° C., tin is liable to create whisker-type defects, and lead is prohibited. Zinc (Zn) would be suitable but it remains less advantageous than aluminum (Al), the latter having a high Young&#39;s modulus. 
         [0022]    Nonetheless, aluminum still has a substantial Young&#39;s modulus, which has the following drawbacks:
       it complexifies inserting machine design, all the more so when there are a large number of connections: by way of example, producing 106 connections in accordance with patent application WO 2009/115686 requires an insertional force of 5000 N to be applied;   it means that substantial lengths of time must be dedicated to the insertion, these lengths of time being proportional to the required insertional force; and   it leads to a risk that active layers underlying the connecting pads will be destroyed, because of the force exerted on the inserts during the insertion.       
 
         [0026]    The inventor was therefore confronted with the problem of decreasing the forces of insertion into a given ductile material, more particularly into aluminum. 
         [0027]    The general aim of the invention is to provide an improvement of the assembly method disclosed in the aforementioned patent applications WO 2009/115686 and EP 2 287 904 and that at least partially mitigates the aforementioned drawbacks of the prior art. 
         [0028]    One particular aim of the invention is to provide a solution that decreases the insertional force required to assemble two microelectronic components using a low-temperature thermocompression method, and that prevents any risk of electrical short-circuits between the conductive inserts of a component and the connecting tracks of the other component. 
       SUMMARY OF THE INVENTION 
       [0029]    To do this, the subject of the invention is a flip-chip type process for assembling two electronic components, said components each comprising what is referred to as an assembly side, in which the two assembly sides are brought nearer to each other in what is referred to as an assembly direction X and a given force F is applied to one and/or other of the components, one and/or other of the assembly sides comprising:
       connecting inserts made of a rigid material having an elongate longitudinal shape in the assembly direction X; and   connecting tracks made of a material of lower hardness than that of the inserts and of elongate longitudinal shape transversely to the assembly direction X, process in which:   the inserts are aligned facing the corresponding tracks so that the inserts and the tracks form, pairwise, after assembly, at least one substantially transversal intersection; and   then, the given force F is applied in order to make the inserts penetrate into the tracks until the assembly of the two components is obtained.       
 
         [0034]    According to one preferred embodiment, the connecting inserts of a component have a unitary wall thickness e 1  and a unitary wall length L 1  whereas the connecting tracks of a component have a unitary thickness e 2 , the unitary thicknesses e 1  and e 2  being the smallest dimensions considered transversely to the assembly direction X, and the length L 1  being the largest wall dimension considered transversely to the assembly direction X, 
         [0035]    process in which, before the inserts have been aligned facing the corresponding tracks, the dimensions e 1 , e 2  and L 1  are defined so that each area of insertion Si between a given insert and track is substantially equal to a multiple N of the product of the unitary thicknesses N=n*e 1 *e 2  where n is an integer, this multiple N being very much lower than the cross-sectional area Si of an insert considered transversely to the assembly direction S 1 =L 1 *e 1 . 
         [0036]    The expression “very much lower than the cross-sectional area S 1  of an insert” is understood to mean only a very small portion of the cross-sectional area of an insert makes contact with the ductile connecting material during the insertion, this very small portion moreover being enough to ensure the mechanical rigidity of each connection produced. Advantageously, the area of insertion Si is smaller by at least 40% than the cross-sectional area S 1 . 
         [0037]    An insert according to the invention may comprise one or more walls. Thus, the expression “unitary wall length L 1 ” is understood to mean the perimeter bounded by these one or more walls. For example, for a star-shaped wall, the unitary length is the sum of all the lengths of the arms of the star, which make up the perimeter. 
         [0038]    A connecting track according to the invention consists of a continuous metal deposit having any possible shape. Thus, a single insert may be inserted into a plurality of portions of a given track, and therefore the area of insertion Si to be considered according to the invention comprises all the intersections between a thickness of a track and an insert. 
         [0039]    In other words, the invention essentially consists in very substantially decreasing the cross section of the ductile material of a component that each insert of the other component penetrates when they are assembled by connection therebetween. The invention is simple to implement since all that needs to be done is produce very thin tracks of ductile material; there is no need to modify the shape or the dimensions or the hard material that makes up the inserts or the process used to produce them known from the aforementioned patent applications WO 2009/115686 and EP 2 287 904. 
         [0040]    By virtue of the invention, it is possible to decrease considerably the forces of insertion to be applied to form the connections. As a corollary, the need for insertion machines of complex design is thus avoided and the lengths of time required to implement the actual insertion steps is considerably decreased. 
         [0041]    The unitary wall thickness e 1  of the inserts may be comprised between 0.1 and 1 μm and preferably between 0.1 and 0.5 μm. 
         [0042]    The unitary track thickness e 2  may be comprised between 0.05 and 2 μm and preferably between 0.1 and 1 μm. 
         [0043]    According to one variant embodiment, the constituent material of the tracks is preferably a ductile metallic material chosen from aluminum Al, indium In, gold Au, tin Sn, lead Pb, bismuth, antimony Sb, zinc Zn, an aluminum-copper alloy AlCu and the alloys SnAgCu, SnAg, AgCu and SnCu. The tracks made of ductile metallic material may advantageously be produced by additive or subtractive pattern transfer or by electrolysis of the metal or alloy. 
         [0044]    According to one alternative variant embodiment, the constituent material of the tracks may be a hard metallic material chosen from copper Cu, titanium Ti, titanium nitride TiN, tungsten W, tungsten nitride WN, molybdenum Mo, gold Au, chromium Cr, nickel Ni and platinum Pt. The connecting tracks made of hard metallic material may be produced by additive or subtractive pattern transfer. Tracks made of Au, Cu or Ni may be produced by electrolytic growth. 
         [0045]    The tracks made of hard metallic material are advantageously produced using the same production technique as the inserts. 
         [0046]    The inserts according to the invention are preferably blind micro-tubes the base of which is securely fastened to one of the components. The connecting inserts and if needs be the connecting tracks are thus advantageously fabricated as described in patent application WO 2009/115686 or in patent application EP 2 287 904. When the connecting tracks are produced using the technique described in these applications, they may have a very small thickness e 2 , advantageously of about 0.1 μm or even 0.05 μm, thereby allowing the forces required to insert the inserts therein to be even more substantially decreased. Generally, the conductive inserts according to the invention may be any shape: micro-tube, tip, open bar, an element having star-shaped, cross-shaped or lobed cross section, etc. 
         [0047]    According to one advantageous variant, in order to better distribute mechanical stresses during the insertion, the connecting track portions that are to be penetrated by a given tube take the form of arms that are at least three in number and distributed symmetrically about a point of symmetry, preferably four arms distributed at 90° from each other. 
         [0048]    The force F applied per insert may be very low, preferably lower than 5 mN, preferably lower than 0.8 mN and typically equal to 0.5 mN. 
         [0049]    Advantageously, the alignment and the application of the force F are carried out at room temperature. 
         [0050]    The gap between the two components corresponding to the height H is preferably comprised between a ratio p/20 and a ratio equal to p/2, p being the interconnect pitch between two connecting tracks of a component. 
         [0051]    The interconnect pitch p between two connecting tracks of a component may be 50 μm or finer. 
         [0052]    The gap between the two components corresponding to the height H is advantageously smaller than 20 μm and typically equal to 1 μm. 
         [0053]    According to one advantageous embodiment, one of the components is a chip and the other component is a printed circuit board substrate. 
     
    
     
       DETAILED DESCRIPTION 
         [0054]    Other advantages and features of the invention will become more clearly apparent on reading the detailed description of the invention given by way of nonlimiting illustration with reference to the following figures, in which: 
           [0055]      FIG. 1  is a schematic cross-sectional view of two electronic components level with a connecting insert and pad according to the prior art, before their assembly; 
           [0056]      FIG. 2  is a schematic cross-sectional view of two electronic components level with a connecting insert and pad according to the prior art, once they have been assembled; 
           [0057]      FIG. 3  is a top view of  FIG. 2 ; 
           [0058]      FIG. 4  is a schematic top view of two electronic components level with a connecting insert and track according to one embodiment of the invention, once they have been assembled; 
           [0059]      FIG. 5  is a schematic top view of two electronic components level with a connecting insert and track according to another embodiment of the invention, once they have been assembled; 
           [0060]      FIGS. 6 and 6A  are top and cross-sectional views, respectively, of two electronic components level with a connecting insert and track, showing their assembly in detail; and 
           [0061]      FIGS. 7 to 10  are schematic top views of two electronic components level with a connecting insert and track according to four other embodiments of the invention, once they have been assembled. 
       
    
    
       [0062]    For the sake of clarity, elements of electronic components according to the prior art and of electronic components according to the invention that are equivalent are designated by identical references in all of  FIGS. 1 to 10 . 
         [0063]    It will be noted that the various elements, in particular the connecting tracks, according to the invention are shown only for the sake of illustration and that they are not to scale. 
         [0064]      FIGS. 1 to 3 , which relate to an insertional assembly according to the prior art, have already been described in the preamble. They are not described in detail here. 
         [0065]      FIGS. 4 to 10  show a connecting insert  10  inserted into a connecting track  20 , each belonging to one of two electronic components  1 ,  2 , such as electronic chips hybridized by means of a pressing tool brought to bear against the top component. 
         [0066]    The component  1 , which is the flipped component, comprises a substrate  11  to which conductive inserts  10  taking the form of blind tubes are securely fastened via their bases, the inserts all having a height h. The choice of the height h of the inserts advantageously depends on the minimum pitch p between the interconnects to be produced. Thus, the height h is preferably at least about p/20 in order to accommodate for non-planarities between the components  1 , 2  to be assembled. Preferably, the height h is at most about p/2 in order to limit what are referred to as buckling effects subsequently. 
         [0067]    To produce these conductive inserts  10 , the process described in patent application WO 2009/115686 is advantageously used. Each insert  10  has a wall length L 1  and a unitary wall thickness e 1 . Here, the expression “unitary thickness e 1 ” is understood to mean the average dimension, or average width, of the wall of the insert in a direction transverse to the longitudinal direction of the latter. The thickness, length and height directions form locally an orthogonal coordinate system. The unitary thickness e 1  of a tube  10  is for example equal to 0.2 μm. Each insert tube  10  may have any cross-sectional shape, as depicted in  FIG. 4 . It may be a question of a tube with a square cross section ( FIG. 5 ), a circular cross section ( FIGS. 7 to 10 ), etc. 
         [0068]    The component  2  for its part comprises a substrate  21  on which connecting tracks  20  of the same height H have been produced. The choice of the height H of the tracks  20  advantageously depends on the minimum pitch p between the interconnects to be produced. Thus, the height h is preferably at least about p/20 in order to accommodate for non-planarities between the chips to be assembled and the height H is at most equal to p/2 in order to allow for complete insertion of a tube  10  of maximum height h. The height H of the tracks  20  is calculated so that said height is larger than that h of the inserts  10 , in order to prevent the hard metal of the inserts  10  from making contact with the circuit(s) under the tracks  20  during the insertion. Each track  20  has a unitary thickness e 2 . The unitary thickness e 2  is for example equal to 1 μm. Here, the expression “unitary thickness e 2 ” is understood to mean the average dimension, or average width, of the wall of the track in a direction transverse to the longitudinal direction of the latter. The thickness, length and height directions form locally an orthogonal coordinate system. 
         [0069]    The tracks  20  take the form of linear vertical features. Each track  20  may take the form of a single elongate strip ( FIGS. 4 and 9 ), a tube, for example with a square or rectangular cross section ( FIG. 10 ), or a cross ( FIGS. 5 ,  7  and  8 ) with its arms connected by a via. 
         [0070]    According to one variant embodiment, the constituent material of the tracks  20  is a ductile metallic material chosen from aluminum Al, indium In, gold Au, tin Sn, lead Pb, bismuth, antimony Sb, an aluminum-copper alloy AlCu and the alloys SnAgCu, SnAg, AgCu and SnCu. The tracks made of ductile metallic material may be produced by additive or subtractive pattern transfer or by electrolysis of the metal or alloy. 
         [0071]    According to one alternative variant embodiment, the constituent material of the tracks is a hard metallic material chosen from copper Cu, titanium Ti, titanium nitride TiN, tungsten W, tungsten nitride WN, molybdenum Mo, gold Au, chromium Cr, nickel Ni and platinum Pt. The connecting tracks made of hard metallic material may be produced by additive or subtractive pattern transfer. Tracks made of Au, Cu or Ni may be produced by electrolytic growth. 
         [0072]    According to the invention, the dimensions e 1 , e 2  and L 1  are defined by calculation so that each area of insertion Si, Si 1 +Si 2 +Si 3 , etc. between a given insert and track is substantially equal to a multiple N of the product of the unitary thicknesses N=n*e 1 *e 2  where n is an integer, this multiple N being very much lower than the cross-sectional area S 1  of an insert considered transversely to the assembly direction S 1 =L 1 *e 1 . 
         [0073]    Thus, by considerably decreasing the cross section of insertion comparatively to that during an insertion of the complete cross section of a tube according to the prior art, as shown in  FIG. 3 , the constant force of insertion that it is necessary to apply between an insert  10  and a connecting track  20  is very substantially decreased. 
         [0074]    In other words, according to the invention, the cross section of insertion between a track and an insert is minimized while keeping it large enough to obtain the mechanical rigidity desired for the interconnecting contact. Depending on circuit layout, tracks may be produced in a number of possible configurations and therefore the number of unitary wall thicknesses e 2  intersected by a given insert  10  may be relatively large. Thus, a given insert  10  may be inserted into a single thickness e 2  ( FIGS. 4 and 9 ), into two thicknesses e 2  ( FIG. 10 ), four thicknesses e 2  ( FIGS. 5 and 8 ), eight thicknesses e 2  ( FIG. 7 ), etc. 
         [0075]    The various steps of the assembly process according to the invention will now be described. 
         [0076]    Step  1 : the two components  1 ,  2  are aligned and brought together so that each insert  10  faces one portion of a connecting  20 . 
         [0077]    Step  2 : a force F is applied in the assembly direction X orthogonal to the sides of the substrates bearing the inserts  10  and tracks  20 . The force F is applied using a pressing tool brought to bear against the top component  1  and leads to insertion of the inserts  10  into the tracks  20 . Si designates the cross-sectional area intersected by each insert  10  and is substantially equal to a multiple N of the product of the unitary thicknesses N=n*e 1 *e 2 . The cross-sectional area Si is very small compared to the applied force, the stress generated is very high and each track  20  is thus plastically deformed. The insertion of each insert  10  takes place via plastic deformation of each corresponding track  20 . 
         [0078]    Step  3 : the force F is applied until the entire height h of the inserts  10  has been inserted into the connecting tracks  20 . 
         [0079]    Step  4 : the pressing tool is released and retracted. The two components  1 ,  2  are assembled (hybridized), an electrical connection having been established between each connecting track  20  and each conductive insert  10 . 
         [0080]      FIG. 6A  shows in detail an assembly level with an insert  10  and the corresponding track  20 , obtained using the assembly process described above. 
         [0081]    The insertional force required according to the invention is proportional to the cross section of insertion common to each insert  10  and each track  20 . Thus, for example, by choosing a track  20  of unitary width e 2  very much smaller than the circular perimetric cross section of a tube of radius R, insertion of a tube  10  of radius R into a single track  20  according to the invention ( FIG. 9 ) requires much less insertional force than an insertion, according to the prior art, of the entire circumference of the same tube ( FIG. 3 ), in a ratio equal to e 2 /2*π*R. 
         [0082]    Specifically, an insertion according to  FIG. 9  involves a cross section equal to e 1 *e 2  whereas an insertion according to  FIG. 3  involves a cross section equal to e 1 *2*π*R. 
         [0083]    In configurations in which the interconnects are subjected to substantial thermomechanical solicitations, the insert  10  may be inserted into two track portions that are symmetrical about a point. Thus, it is possible for example to choose for an insert  10  and a track  20  to intersect a number of times symmetrically about the centre of the insert ( FIGS. 5 ,  7  and  8 ), i.e. so that they counterbalance. 
         [0084]    The number of track portions  20  to be intercepted by a tube  10  may be multiplied, especially in order to allow tracks  20  of very small unitary thickness e 2  (typically of about the unitary thickness of the wall e 1  of an insert) to be used. Thus by way of example, one tube may be inserted into eight unitary wall thicknesses as shown in  FIG. 7 . Thus, it is possible for the thicknesses e 1  and e 2  to be very small, typically equal to 0.1 μm, the cross section of insertion, equal to 8*e 1 *e 2 , then being very small, typically equal to 8*0.1*0.1, i.e. to 0.08 μm 2 , and likewise the required insertional force. 
         [0085]    By way of example, the case where the connections to be produced have a pitch equal to 10 μm and where a tube  10  of radius R=2.5 μm is used is considered. 
         [0086]    In the prior art, such a tube  10  requires an insertional force F 1  equal to 4 mN if its entire circumference is to be inserted into an aluminum connection pad having a diameter equal to 7 μm. 
         [0087]    According to the invention, in order to decrease considerably the insertional force, an aluminum track  20  having a symmetric cross shape ( FIG. 8 ) and an aluminum track  20  taking the form of an elongate strip ( FIG. 9 ) are produced with unitary thicknesses e 2  equal to 1 μm. It will be noted that in the example shown, the cross  20  is connected to a via aperture  22  of diameter equal to 2 μm, this connection taking the form of a round pad  23  of diameter equal to 3 μm surmounted by four arms  20  of unitary width e 2 . 
         [0088]    Preferably, in order to better distribute mechanical stresses during the insertion, the track portions  20  to be intercepted by a tube  10  take the form of arms that are at least three in number and distributed symmetrically about a point of symmetry. It may therefore for example be a question of three arms distributed at 120° to each other, of four arms distributed at 90° from each other so as to form a symmetric cross ( FIG. 8 ), or of eight arms grouped pairwise, one group being distributed at 90° to another so as to again form a symmetric cross ( FIG. 7 ). Thus, an isostatic mechanical connection is obtained between the inserts and tracks in every direction. 
         [0089]    For a given insertion depth, the insertional forces to be applied to the cross in  FIG. 8  and to the strip in  FIG. 9 , respectively, are, comparatively to the force F 1  according to the prior art, equal to: 
         [0000]        F 2= F 1*4* e/ 2π R , i.e. equal to 0.25* F 1;
 
         [0000]        F 3= F 1* e/ 2π R , i.e. equal to 0.06* F 1.
 
         [0090]    Thus, a very substantial decrease in the insertional force required is obtained with a track  20  according to the invention comparatively to a complete circumferential insertion into a ductile pad made of the same material according to the prior art. 
         [0091]    Theoretical required insertional forces have been compared by varying the tube diameter of the inserts  10  and the unitary thickness of the tracks  20  with the area of insertion Si. 
         [0092]    The results, between a configuration with insertion according to the prior art ( FIG. 3 ), an insertion according to the invention with a single elongate track  20  ( FIG. 9 ) and an insertion according to the invention with a track  20  taking the form of a tube with a rectangular cross-sectional area ( FIG. 10 ) are given in the following tables 2 to 4, respectively. 
         [0093]    It will be noted that the pads according to the prior art and the tracks  20  according to invention are made of aluminum and that the insertional force calculated at constant pressure for insertion of the entirety of the circumference of a tube  10  into a pad according to the prior art is equal to 5 mN. The force required to obtain the same pressure was then calculated comparatively for the two configurations according to the invention. 
         [0000]    
       
         
               
             
               
               
               
               
               
             
           
               
                 TABLE 2 
               
             
             
               
                   
               
               
                 (TUBE 10/CONNECTING PAD 20 ASSEMBLY 
               
               
                 ACCORDING TO FIG. 3) 
               
             
          
           
               
                 Wall 
                   
                 Cross section of 
                 Pressure 
                   
               
               
                 thickness 
                 Tube diameter 
                 insertion S 
                 equal to 
                 Force F 
               
               
                 e1 (μm) 
                 10 
                 (μm 2 ) 
                 F/S (Gpa) 
                 (mN) 
               
               
                   
               
               
                 0.2 
                 4 
                 2.512 
                 1.99 
                 5 
               
               
                   
               
             
          
         
       
     
         [0000]    
       
         
               
             
               
               
               
               
               
             
           
               
                 TABLE 3 
               
             
             
               
                   
               
               
                 (TUBE 10/CONNECTING TRACK 20 
               
               
                 ASSEMBLY ACCORDING TO FIG. 9) 
               
             
          
           
               
                 Wall 
                 Track thickness 
                 Cross section 
                 Force 
                 Insertional force 
               
               
                 thickness 
                 equal to e2 
                 of insertion Si 
                 F′ 
                 reduced by (%) 
               
               
                 e1 (μm) 
                 (μm) 
                 (μm 2 ) 
                 (mN) 
                 100-F′/F 
               
               
                   
               
               
                 0.2 
                 2 
                 0.4 
                 0.80 
                 84% 
               
               
                   
               
             
          
         
       
     
         [0000]    
       
         
               
             
               
               
               
               
               
             
           
               
                 TABLE 4 
               
             
             
               
                   
               
               
                 (TUBE 10/CONNECTING TRACK 20 ASSEMBLY 
               
               
                 ACCORDING TO FIG. 10) 
               
             
          
           
               
                 Wall 
                 Track thickness 
                 Cross section 
                 Force 
                 Insertional force 
               
               
                 thickness 
                 equal to 2*e2 
                 of insertion Si 
                 F′ 
                 reduced by (%) 
               
               
                 e1 (μm) 
                 (μm) 
                 (μm 2 ) 
                 (mN) 
                 100-F′/F 
               
               
                   
               
               
                 0.2 
                 0.2 
                 0.08 
                 0.1592 
                 97% 
               
               
                   
               
             
          
         
       
     
         [0094]    From tables 2 to 4 it will be clear that the insertional force per insert may be considerably decreased by virtue of the invention, by 84 to 97% in this example. 
         [0095]    In conclusion, comparatively to prior-art thermocompression assembly processes such as described in the aforementioned patent applications WO 2009/115686 and EP 2 287 904, the invention allows the constant insertional force to be considerably decreased for a given ductile material. 
         [0096]    One beneficial advantage of the invention is that it allows the number of points hybridized at constant insertional force to be multiplied for a given ductile material. 
         [0097]    According to the invention, it is possible to produce a stack of two assemblies each obtained using the reduced-insertional-force assembly process described above. 
         [0098]    The invention is broadly applicable to any microelectronic devices intended to operate at a high operating temperature and requiring front-to-front interconnects having a very fine pitch. 
         [0099]    One particularly advantageous application of the invention is the production of three-dimensional 3D structures or multispectral heterogeneous imagers. 
         [0100]    Many other applications may be envisioned for the invention and more particularly for:
       large heterogeneous detector arrays with large numbers of insertional connections (cooled IRCMOS arrays, X-ray detector arrays, etc.);   temperature-sensitive arrays that are hybridized “cold” (i.e. at room temperature); and   arrays sensitive to mechanical stresses.       
 
         [0104]    It is for example possible to produce such arrays by providing ductile aluminum tracks according to the numerical example given above: the same force of about 0.5 mN may be applied to produce a connection according to the invention in an aluminum track  20  such as shown in  FIG. 9  as for a connection according to the prior art in an indium pad  20  as shown in  FIG. 3 . However, it is better to apply a small force to insert an insert into an aluminum track, because it is much easier and less expensive to produce a track  20  made of aluminum as shown in  FIG. 9  by a subtractive photolithography technique (etching) than an indium pad  20  as shown in  FIG. 3  by an additive photolithography technique (lift-off) or by electrolysis. 
         [0105]    Other variants and improvements may be provided without however departing from the scope of the invention. 
         [0106]    The invention is not limited to the examples described above; in particular features of the illustrated examples may be combined together in variants that are not illustrated. 
       CITED REFERENCES 
       [0000]    
       
         [1]: B. Goubault de Brugière, F. Marion, M. Fendler et al. “ Micro tube insertion into indium, copper and other materials for  3 D applications .” Proc 60th Electronic Components and Technology Conf, Las Vegas, Nev., 2010 p 1757; 
         [2]: B. Goubault de Brugière, F. Marion, M. Fendler et al “ A  10  μm pitch interconnection technology using micro tube insertion into Al - Cu for  3 D applications.”,  Proc 61st Electronic Components and Technology Conf, Orlando, Fla., 2011 p 1400; 
         [3]: D. Saint-Patrice, F. Marion, M. Fendler et al. “ New Reflow Soldering and Tip in Buried Box  ( TB 2)  Techniques For Ultrafine Pitch Megapixels Imaging Array ,” Proc 58th Electronic Components and Technology Conf, Orlando, Fla., 2008 p 46-53.