Electronic component assembly and method of manufacture

Abstract

The invention relates to a microelectronic assembly (1) comprising a microelectronic component (10), a support (20) and a set of attachment members (30) fixing together the microelectronic component (10) on the support (20), each attachment member (30) comprising a conductive bead (303) fixed to the microelectronic component (10) by a first adhesion material (301) and to the support (20) by a second adhesion material (302), wherein, for at least one of the attachment members (30), a central area (331) of the surface of the conductive bead (303) considered along an axis transverse to the assembly (1) is bare or partially or totally covered by a layer of the first adhesion material (301) and / or a layer of the second adhesion material (302) having a respective thickness of at most 10 nm.

Description

[0001] Description

[0002] Title of the invention: Assembly of electronic components and manufacturing method

[0003] The invention lies in the field of electronic component assemblies, and relates in particular to assemblies of the system-in-package type.

[0004] Microelectronic devices such as semiconductor chips require a large number of electrical connections over a small area. For example, a complex semiconductor chip may have hundreds of contacts for signal input and output and for power supply, all over an area of ​​a few cm 2These contacts must be reliably connected to the corresponding contact pads on a substrate such as a chip carrier, circuit board or multi-chip module. The connection between the chip and the substrate must meet many, often conflicting, requirements and wishes.

[0005] In particular, it must be both inexpensive and highly reliable. It should require only a minimal surface area on the substrate. Ideally, the connected chip should be housed in an area of ​​the substrate approximately equal to that of the chip itself. The connections must be robust and able to withstand repeated changes in the temperature of the chip and substrate, as well as the associated differential thermal expansion and contraction of the chip and substrate. Indeed, since electronic components have a coefficient of expansion lower than the supporting substrate, relative displacements due to temperature variations occur. Thus, the connections require the use of a flexible mechanical interface that is resistant to displacement.

[0006] Additionally, connector systems should not impose unusual or costly requirements on the chip or substrate fabrication. It is desirable for the connector system to facilitate testing of the chip and the connector system itself before the chip is finally assembled to the substrate. Finally, three-dimensional system-in-package (SIP) assemblies require precise and repeatable control of board gaps.

[0007] All these considerations, taken together, represent a formidable technical challenge. These considerations are present, to a greater or lesser degree, in other connections used in microelectronic devices, such as, for example, connections between substrates or circuit boards. For this purpose, it is known to use bonding and / or soldering fixing techniques in combination or not with intermediate elements between the parts to be fixed.

[0008] Among adhesives, electrically conductive epoxy adhesives are traditionally used to attach electronic components to substrates in packaged systems. Epoxy adhesives are applied to solder pads on the substrate. The electronic components are then placed on the solder pads and the assembly is heated to cure the epoxy adhesive and create a strong bond between the electronic components and the substrate.

[0009] Adhesives can also be used to strengthen solder joint bonds, particularly in microelectronic components with a large chip on an organic substrate. For this purpose, an adhesive (called "underfill") is usually applied after the chip has been soldered to the substrate and is injected into the gap between the chip and the substrate using a syringe or an automated system. It is then cured by heating or exposure to UV light, creating a strong, durable layer between the chip and the substrate. However, such bonding makes it impossible to repair the assembly.

[0010] Solder assembly techniques using columns or balls are also known, where electronic components are attached to columns or balls fixed to a substrate. The metal columns are usually made of copper alloy, but can also be made of gold alloy for high-temperature applications. The columns are notably made of Pb90Sn10 to ensure assembly reliability. The balls can be made of a fusible alloy, ceramic, or comprise a rigid or flexible polymer core covered by a metal sphere, particularly copper. The assembly is achieved by soldering using a solder paste, which is placed on the columns or balls and heated to form solder joints between the columns and the solder pads on the substrate.However, the use of lead will no longer be possible when the European Union's RoHS (Restriction of Hazardous Substances) Directive exemptions for lead alloys used in solders for high-performance electronic components for space, military and medical applications are discontinued.

[0011] Document US6204455 describes the use of hollow balls inserted into openings in a dielectric sheet to prevent the solder from completely covering the ball and to preserve its flexibility at the level of the dielectric sheet. This results in better deformation and therefore better resistance to temperature variations. However, this assembly process has a high production cost and implementation difficulties. It also requires numerous manipulations of the balls prior to assembly.

[0012] There is therefore a need for an assembly of electronic components that allows excellent performance in resisting relative movements of these components as well as temperature variations, while reducing production costs. There is also a need for an assembly method that is not specific to the use of glue or solder, and which allows repair of the assembly if necessary.

[0013] To this end, the invention relates to a microelectronic assembly comprising a microelectronic component, a support and a set of attachment members fixing the microelectronic component to each other on the support, each attachment member comprising a conductive ball fixed to the microelectronic component by a first adhesion material and to the support by a second adhesion material, where for at least one of the attachment members a central zone of the surface of the conductive ball considered along an axis transverse to the assembly is bare or partially or totally covered by a layer of the first adhesion material and / or a layer of the second adhesion material with a respective thickness of at most 10 nm.

[0014] The microelectronic assembly according to the invention advantageously uses little adhesion material, thus reducing production costs, while increasing the stress resistance of the attachment members. Thus, the reduced quantity of adhesion material used compared to the prior art results in a low coverage, or even no coverage, of the balls at their central zone, providing flexibility to the attachment members and thus guaranteeing a limitation of the mechanical stresses of the assembly. In particular, the central third of each ball is weakly covered, or even bare. In addition, the distribution of the adhesion materials at the level of the balls is obtained without fusible alloy and without the aid of a holding device, such as a dielectric sheet, simplifying the production of the assembly. Finally, the assembly according to the invention can be easily disassembled without degradation of the components, allowing their repair or replacement.

[0015] According to one embodiment of the invention, the microelectronic component forms the upper part of the assembly and the support forms the lower part of the assembly, in which for at least one of the attachment members: at least 95% of the volume of the first adhesion material is located between the microelectronic component and the upper third of the height of the conductive ball considered along an axis transverse to the assembly, and at least 95% of the volume of the second adhesion material is located between the support and the lower third of the height of the conductive ball considered along an axis transverse to the assembly.

[0016] According to one embodiment of the invention, for at least one of the attachment members, the first adhesion material comprises a constriction arranged opposite the top of the conductive ball and the second adhesion material comprises a constriction arranged opposite the base of the conductive ball, the top and the base of the conductive ball being considered along an axis transverse to the assembly.

[0017] According to one embodiment of the invention, the microelectronic component forms the upper part of the assembly and the support forms the lower part of the assembly, in which for at least one of the attachment members the volume of the first and second adhesion materials corresponds respectively to that of a truncated cone comprising a recess. In particular, said truncated cone has a base having a diameter equal to the diameter of the conductive ball and for a height one third of the diameter of said conductive ball and said recess having the shape of a spherical cap having a height one third of the diameter of the conductive ball.

[0018] According to one embodiment of the invention, for at least one of the attachment members, the conductive ball comprises a hollow copper sphere covered with a layer of nickel, preferably a layer of medium nickel phosphorus, more preferably the phosphorus content is less than 15%.

[0019] In particular, the copper sphere has a thickness of less than 100 pm, in particular less than or equal to 50 pm, in particular still 15 to 20 pm.

[0020] In particular, the nickel layer has a thickness of less than 10 pm, preferably 4 to 7 pm.

[0021] According to one embodiment of the invention for at least one of the attachment members, the conductive ball comprises a coating in a material chosen from the group consisting of gold, silver, platinum, palladium and an alloy of several of these materials.

[0022] The invention also relates to a method for attaching a microelectronic component to a support using an attachment member of a microelectronic assembly as defined above, the microelectronic component and the support each comprising a plurality of connection pads, the method comprising the following steps: a) a first deposition of a first adhesion material on each pad of the microelectronic component and of a second adhesion material on each corresponding pad of the support, b) a second deposition of a conductive ball on the second adhesion material of each corresponding pad of the support, c) a matching of the pads of the microelectronic component with the corresponding pads of the support, and a contacting of the first adhesion materials of each pad of the microelectronic component with the conductive balls,d) suitable heating and then suitable cooling of the assembly in order to obtain the formation of the attachment members by shaping and hardening the first and second adhesion materials, thus causing the microelectronic component to be fixed to the support, where during the first deposition step a), for at least one of the attachment members the volume deposited of the first and second adhesion materials on the pads is adapted so that during the heating step d) a central zone of the surface of the conductive ball considered along an axis transverse to the assembly is bare or partially or totally covered by a layer of the first adhesion material and / or a layer of the second adhesion material with a respective thickness of at most 10 nm.,

[0023] According to one embodiment of the invention, for at least one of the attachment members, the volume deposited of the first and second adhesion materials on the pads corresponds to that of a truncated cone comprising a recess, preferably said truncated cone has a base having a diameter equal to the diameter of the conductive ball and for a height one third of the diameter of said conductive ball and said recess having the shape of a spherical cap having for a height one third of the diameter of the conductive ball.

[0024] Brief description of the figures

[0025] The invention will be better understood on reading the following description, given solely by way of example and with reference to the appended drawings in which:

[0026] Figure 1 represents a side view of a microelectronic assembly according to one embodiment of the invention.

[0027] Figure 2 shows a detailed side view of a fastener of the microelectronic assembly of Figure 1. The edges of a conductive ball of the fastener are shown in dotted lines when hidden. Figure 3 shows a sectional view of the fastener of Figure 2.

[0028] Figure 4 shows opposite the volume of an adhesion material deposited for the manufacture of a fastening member of Figure 2 (bottom) and a sectional view of a conductive ball of the fastening member of Figure 2 (top). For clarity, the base of the adhesion material is shown in gray.

[0029] Figure 5 represents three steps (A, B and C) of a manufacturing process of the microelectronic assembly of Figure 1.

[0030] Detailed description of the invention

[0031] The following embodiments are examples. Although the description refers to one or more embodiments, this does not necessarily mean that each reference relates to the same embodiment, or that the features apply only to a single embodiment. Simply features of different embodiments may also be combined and / or interchanged to provide other embodiments.

[0032] The invention relates to a microelectronic assembly 1 comprising a microelectronic component 10, a support 20 and a set of attachment members 30 fixing the microelectronic component 10 and the support 20 together, as shown in Figure 1.

[0033] The microelectronic component 10 forms the upper part of the assembly 1. The microelectronic component corresponds in particular to a chip, a microprocessor, a dynamic random access memory, a microelectromechanical system (“MEMS” in English) or even a flash memory.

[0034] The support 20 forms the lower part of the assembly 1. The support 20 is in particular a printed circuit, a ceramic substrate, an organic substrate from additive manufacturing or any other support for conductive tracks.

[0035] The microelectronic component 10 and the support 20 each comprise connection pads 11, 21 in order to establish electrical connections between these elements. The attachment members 30 are arranged so as to connect the pads 11, 21 together. Thus, each attachment member 30 has a dual function of fixing the microelectronic component 10 to the support 20 and of electrical connectivity between these two elements 10, 20. It is therefore essential that each attachment member resists mechanical and thermal adverse effects in order to avoid, on the one hand, a separation of the microelectronic component 10 from the support 20, and on the other hand, the loss of electrical connections between these elements which can render the microelectronic assembly 1 non-functional.

[0036] As shown in Figure 2, each attachment member 30 comprises a first adhesion material 301, a second adhesion material 302 and between the two a conductive ball 303.

[0037] The first and second adhesion materials 301, 302 are notably chosen from solders, in particular tin-based solders, and conductive adhesives, in particular epoxy resin-based adhesives.

[0038] The first adhesion material 301 attaches the electronic component 10 to the conductive ball 303, and the second adhesion material 302 attaches the support 20 to the conductive ball 303.

[0039] At the level of the microelectronic component 10 and the support 20, the adhesion materials 301, 302 are arranged at the level of the connection pads 11, 21.

[0040] The first adhesion material 301 thus comprises an attachment surface 310 with the electronic component 10.

[0041] Similarly, the second adhesion material 302 comprises an attachment surface 320 with the support 20.

[0042] The invention involves a restricted use of quantity of each of the adhesion materials 301, 302 during the manufacture of the assembly 1, described in detail later.

[0043] This limited quantity of material results in a weak, total or partial covering, or an absence of covering, of the conductive ball 303 by the first and / or the second material 301, 302 at the level of a central zone 331. Thus, as can be seen in FIG. 2, the conductive ball 303 can be partially covered by the two adhesion materials 301, 302.

[0044] The central zone 331 corresponds in particular to the central third of the conductive ball 303. In the embodiment shown, the central third 331 of the conductive ball 303 is bare, that is to say that it is not covered by the first or the second adhesion material 301, 302. The central third 331 of the conductive ball 303 is the surface extending between the upper third and the lower third of the ball 303 according to its height H. By "height of the conductive ball", is meant the diameter of the conductive ball 303 considered along an axis transverse to the assembly 1. According to an advantageous embodiment of the invention, the conductive ball 303 is covered at most up to the upper third of its height H by the first adhesion material 301, and at most up to the lower third of its height H by the second adhesion material 302.

[0045] According to an alternative embodiment, the ball 303 is covered at least up to the upper third of its height H by the first adhesion material 301, and / or at least up to the lower third of its height H by the second adhesion material 302. In particular, the ball 303 is entirely covered by the first and second adhesion materials 301, 302. In particular, the covering of each of the materials 301, 302 stops, in particular substantially, at the equator of the ball 303 considered according to its height H. In these embodiments, the respective thickness of the adhesion materials 301, 302 at the central third 331 is thin, i.e. less than 10 nm, in particular less than 7 nm, for example less than 5 nm.Thus, at least 95% of the volume of the first adhesion material 301 may be located between the microelectronic component 10 and the upper third of the height H of the ball 303, and at least 95% of the volume of the second adhesion material 302 may be located between the support 20 and the lower third of the height H of the conductive ball 303. In particular, this is at least 97% of their respective volumes, in particular at least 99%. The presence of an overlap at the central third of the conductive ball 303 despite a restricted quantity of adhesion material 10, 20 used may be due in particular to a capillary effect of the adhesion materials 301, 302 along the surface of the ball 303 during the manufacture of the assembly 1, as will be seen later.

[0046] The limited quantity of adhesion material 10, 20 in the assembly 1 can also give a particular shape to the first and second adhesion materials 301, 302. Thus, according to one embodiment of the invention, the first adhesion material 301 comprises a constriction 311 arranged opposite the top 332 of the conductive ball 303 and the second adhesion material 301 comprises a constriction 321 arranged opposite the base 333 of the conductive ball 303. The top 332 of the ball 303 is considered to be the highest point according to the height H. The base 333 of the ball is considered to be the lowest point according to the height H. These constrictions 311, 321 can in particular be a consequence of the capillary effect of the adhesion materials 301, 302 along the surface of the conductive ball. 303 during the manufacture of assembly 1.

[0047] According to a particular embodiment of the invention, at least one of the attachment members 30 comprises a first adhesion material 301 and a second adhesion material 302 whose respective volume corresponds to that of a truncated cone 340 comprising a recess 341, as shown in FIG. 3. Such a volume makes it possible to optimize the adhesion of each adhesion material 301, 302 to the conductive ball 303 as well as to the microelectronic element 10 and to the support 20 respectively, while providing good thermal and mechanical resistance properties. The recess 341 corresponds to the location of the conductive ball 303 in the adhesion material 301, 302. The diameter d B of the base 342 of the truncated cone 340 is notably chosen in a range going from a diameter less than 5% of the diameter D of the conductive ball 303 to 10% of the diameter D. In particular, the diameter d B is equal to the diameter D. The height h cof the truncated cone 340 is notably less than or equal to one third H 1 / 3 of the height H of the conductive ball 303, and in particular greater than or equal to a quarter of the height H. The diameter d s of the truncated summit 343 of the cone 38 is notably less than or equal to the diameter d C s of the base of a spherical cap having a height of one third of that of the conductive ball 303, and in particular greater than or equal to the diameter of the base of a spherical cap having a height of one quarter of that of the conductive ball 303. The diameter d s is less than the diameter d B The recess 341 has in particular a volume less than or equal to that of a spherical cap having a height one third of that of the conductive ball 303, and in particular greater than or equal to the volume of a spherical cap having a height one quarter of that of the conductive ball 303. Of course, the volume of the recess 341 is limited by the dimensions of the height hc of the truncated cone 340 and the diameter d s from its peak 343.

[0048] We will now turn to Figure 4, which allows a better view of the conductive ball 303. The conductive ball 303 is capable of conducting an electric current. The conductive ball 303 of a fastening member 30 comprises in particular a hollow sphere 334 made of copper covered with a layer 335 of nickel, as shown in Figure 4, which makes it possible to obtain a barrier to the diffusion of copper in the adhesion materials 301, 302. The conductive ball 303 thus has an empty core 336 so that its deformation properties are increased compared to the use of a solid or hollow sphere with a core in particular made of ceramic, metal or even polymer. The deformation properties of the conductive ball 303 coupled with the limited quantity of adhesion material 301, 302 makes it possible to increase tenfold the deformation properties of the fastening members 30 and therefore their capacity to resist mechanical and thermal forces.

[0049] According to a preferred embodiment of the invention, the thickness of the copper sphere 335 is less than or equal to 100 μm, in particular less than or equal to 70 μm, particularly less than or equal to 50 μm, in particular less than or equal to 30 μm, and in particular still from 12 to 20 μm. Reducing the thickness of the copper sphere 335 makes it possible to increase the deformation capacities of the conductive ball, and therefore the resistance of the attachment member to mechanical and thermal forces. The nickel layer 334 of the conductive ball 303 may in particular be a medium nickel phosphorus layer. In particular, the phosphorus content in the medium nickel phosphorus layer is less than or equal to 15% by weight of the medium nickel phosphorus layer, in particular less than or equal to 10%, more particularly from 7 to 10%.

[0050] According to a preferred embodiment of the invention, the thickness of the nickel layer is less than or equal to 10 μm, in particular 4 to 7 μm, making it possible to obtain the desired diffusion barrier effect.

[0051] According to one embodiment of the invention, the conductive ball 303 of an attachment member 30 may comprise a coating in a material chosen from the group consisting of gold, silver, platinum, palladium and an alloy of several of these materials. Such a coating makes it possible to obtain good adhesion properties to the adhesion materials 301, 302. The coating is in particular a flash coating. The thickness of the coating is in particular less than 0.2 μm.

[0052] Figure 4 also shows the connection pad 11 of the microelectronic element 10. Here the connection pad 11 is shown embedded in the microelectronic element 10, but any pad arrangement can be used.

[0053] We will now turn to Figure 5 which represents different steps of a method of fixing a microelectronic component 10 to a support 20 using attachment members 30 in order to obtain an assembly as previously described.

[0054] A first step, represented in FIG. 5A, concerns a first deposition of a first adhesion material 301 on each pad 11 of the microelectronic component 10 and of a second adhesion material 302 on each corresponding pad 21 C of the support 20. During this step, the first and second adhesion materials 301, 302 are in particular in pasty form.

[0055] The support 20 comprises so-called “corresponding” pads 21 C. Indeed, the support may comprise more connection pads 21 than the microelectronic component 10, in particular for fixing other microelectronic components. Thus, among all the pads 21 of the support 20, all or some of these pads 21 form the corresponding pads 21 C. The pads 11 of the microelectronic component and the corresponding pads 21 C of the support 20 are intended to be placed opposite each other and connected in a conductive manner to each other so as to form electrical bridges between the microelectronic component 10 and the support 20. A pad 11 of the microelectronic component 10 and its corresponding pad 21 C of the support 20 are intended to be fixed to each other by an attachment member 30.For at least one attachment member 30, the deposited volume of adhesion material 301, 302 is adapted so that a central zone 331 of the surface of the conductive ball 303 is bare or at least partially covered with a thin layer of the first and / or second adhesion material 301, 302. In particular, the deposited volume corresponds to that of a truncated cone 340 comprising a recess 341, as described previously. It is important to note that the shape of the adhesion materials 301, 302 deposited during this first step does not necessarily correspond to a truncated cone 340 comprising a recess 341. Indeed, during the heating step which will follow, the adhesion materials 301, 302 will liquefy before taking their final shape and setting. Thus, it is the volume, and therefore the quantity, of adhesion material 301, 302 which is important during this step. The deposited volume is adapted to obtain the desired final shape.

[0056] This first step can in particular be carried out using a first screen printing screen comprising openings adapted to the position of the pads 11 of the microelectronic component 10 and a second screen printing screen comprising openings adapted to the position of the pads 21 of the support 20. Typically the adhesion materials 301, 302 are deposited in the different openings of the respective screen printing screen, then the screens are removed, leaving the desired volume of adhesion material 301, 302 on the pads 11, 21.

[0057] In a second step, conductive balls 303 are deposited on the second adhesion material 302 of the corresponding pads 21 C of the support 20.

[0058] This second step can be carried out for example using a sieve or using a surface-mounted component deposition machine. Thus, this step of arranging the conductive balls 303 can advantageously be carried out using conventional means allowing optimization of production costs.

[0059] A third step is then carried out of matching the pads 11 of the microelectronic component 10 with the corresponding pads 21 C of the support 20, and of bringing the first adhesion materials 301 of each pad 11 of the microelectronic component 10 into contact with the conductive balls 303. The result obtained is shown in FIG. 5B. It can be seen that each ball 303 is sandwiched between a first and a second adhesion material 301, 302. At this stage, the fixing of the microelectronic component 10 on the support 20 is not yet operational.

[0060] To obtain it, a fourth step is carried out combining suitable heating and cooling. The heating and cooling values ​​and times suitable for each adhesion material are known to those skilled in the art. This step makes it possible to firstly obtain the liquefaction of the first and second adhesion materials 301, 302, then their subsequent hardening. The assembly 1 shown in FIG. 5C is thus obtained. The shape of the first and second adhesion materials is approximately that of a truncated cone comprising a recess, deformed by the capillary effects appearing during the heating step.

[0061] During heating, the capillary effects will cause the conductive balls 303 to be coated with the first and second materials 301, 302. This coating may be partial or total, as seen previously.

[0062] During cooling, the second adhesion materials 302 will fix the conductive ball 303 to the support 20 and the first adhesion materials 301 will fix the conductive ball 303 to the microelectronic component 101, thus forming the different attachment members 30.

[0063] Example: Stress Simulation

[0064] In this example, the inventors compared simulations carried out with Ansys® software between different types of attachment member connecting a microelectronic component to a support. A first comparative attachment member (1) was in the form of a Sn63Pb37 solder ball. A second comparative attachment member (2) comprised a hollow copper ball with a shell thickness of 25 μm and a large quantity of Sn63Pb37 solder, equivalent for each pad to the volume of a right circular cylinder 12 μm high including a recess corresponding to half the volume of the hollow ball. A fastening member according to the invention (3) comprised a hollow copper ball with a shell thickness of 25 pm and a small amount of Sn63Pb37 solder, equivalent for each pad to the volume of a truncated cone with a base having a diameter of 25 pm and a height of 8 pm as well as a recess having the shape of a spherical cap having a height of 8 pm.

[0065] The results obtained are reported in the following table: The results show that the use of a thin thickness of the ball shell coupled with a smaller quantity of solder makes it possible to reduce the stresses at the solder joints by an improved deformation of the ball which recovers more stresses, and therefore a better resistance to thermal expansions and contractions of the attachment member.

Claims

Claims 1. Microelectronic assembly (1) comprising a microelectronic component (10), a support (20) and a set of attachment members (30) fixing the microelectronic component (10) to the support (20) together, each attachment member (30) comprising a conductive ball (303) fixed to the microelectronic component (10) by a first adhesion material (301) and to the support (20) by a second adhesion material (302), characterized in that for at least one of the attachment members (20) a central zone (331) of the surface of the conductive ball (303) considered along an axis transverse to the assembly (1) is bare or partially or totally covered by a layer of the first adhesion material (301) and / or a layer of the second adhesion material (302) with a respective thickness of at most 10 nm.

2. Microelectronic assembly according to claim 1, where the microelectronic component (10) forms the upper part of the assembly (1) and the support (20) forms the lower part of the assembly (1), in which for at least one of the attachment members (30): at least 95% of the volume of the first adhesion material (301) is located between the microelectronic component (10) and the upper third of the height (H) of the conductive ball (301) considered along an axis transverse to the assembly (1), and at least 95% of the volume of the second adhesion material (302) is located between the support (20) and the lower third of the height (H) of the conductive ball (303) considered along an axis transverse to the assembly (1).

3. Microelectronic assembly according to claim 1 or claim 2, characterized in that for at least one of the attachment members (30), the first adhesion material (301) comprises a constriction (311) arranged opposite the top (332) of the conductive ball (303) and the second adhesion material (301) comprises a constriction (321) arranged opposite the base (333) of the conductive ball (303), the top (332) and the base (333) of the conductive ball (303) being considered along an axis transverse to the assembly (1).

4. Microelectronic assembly according to one of claims 1 to 3, where the microelectronic component (10) forms the upper part of the assembly (1) and the support (20) forms the lower part of the assembly (1), in which for at least one of the attachment members (30) the volume of the first and second adhesion material (301, 302) corresponds respectively to that of a truncated cone (340) comprising a recess (341), preferably said truncated cone (340) has a base (342) having a diameter (d B ) equal to the diameter (D) of the conductive ball (303) and for height (h c ) one third of the diameter (D) of said conductive ball (303) and said recess (341) having the shape of a spherical cap having a height of one third of the diameter (D) of the conductive ball (303).

5. Microelectronic assembly according to one of claims 1 to 4, in which for at least one of the attachment members (30), the conductive ball (303) comprises a hollow sphere (334) made of copper covered with a layer (335) of nickel, preferably a layer (335) of medium nickel phosphorus, more preferably the phosphorus content is less than 15%.

6. Microelectronic assembly according to claim 5, characterized in that the copper sphere (334) has a thickness of less than 100 pm, preferably less than or equal to 50 pm, more preferably 15 to 20 pm.

7. Microelectronic assembly according to claim 5 or claim 6, in which the nickel layer (335) has a thickness of less than 10 pm, preferably 4 to 7 pm.

8. Microelectronic assembly according to one of claims 1 to 7, in which for at least one of the attachment members (30), the conductive ball (303) comprises a coating in a material chosen from the group consisting of gold, silver, platinum, palladium and an alloy of several of these materials.

9. Method for fixing a microelectronic component (10) to a support (20) using an attachment member (30) of a microelectronic assembly (1) according to one of claims 1 to 8, the microelectronic component (10) and the support (20) each comprising a plurality of connection pads (11, 21), the method comprising the following steps: a) a first deposition of a first adhesion material (301) on each pad of the microelectronic component (10) and of a second adhesion material (302) on each corresponding pad (21 C) of the support (20), b) a second deposition of a conductive ball (303) on the second adhesion material (302) of each corresponding pad (21 C) of the support (20), c) matching the pads (11) of the microelectronic component (10) with the corresponding pads (21 C) of the support (20), and bringing the first adhesion materials (301) of each pad (11) of the microelectronic component (10) into contact with the conductive balls (303), d) suitable heating then suitable cooling of the assembly in order to obtain the formation of the attachment members (30) by shaping and hardening the first and second adhesion materials (301, 302), thus causing the microelectronic component (10) to be fixed to the support (20), where during the first deposition step a), for at least one of the attachment members (30) the deposited volume of the first and second adhesion materials (301, 302) on the pads (11,21 C) is adapted so that during the heating step d) a central zone (331) of the surface of the conductive ball (303) considered along an axis transverse to the assembly (1) is bare or partially or totally covered by a layer of the first adhesion material (301) and / or a layer of the second adhesion material, (302) with a respective thickness of at most 10 nm.

10. Fixing method according to claim 9, wherein for at least one of the attachment members (30), the volume deposited of the first and second adhesion materials (301, 302) on the pads (11, 21 C) corresponds to that of a truncated cone (340) comprising a recess (341), preferably said truncated cone (340) has a base (342) having a diameter (d B ) equal to the diameter (D) of the conductive ball (303) and for height (h c) one third of the diameter (D) of said conductive ball (303) and said recess (341) having the shape of a spherical cap having a height of one third of the diameter (D) of the conductive ball (303).

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