METHOD FOR MOUNTING AN ELECTRONIC COMPONENT ONTO A SUBSTRATE BY MEANS OF SINTERING

Abstract

The method comprises the following successive steps: depositing sintering material (26) onto one of an electronic component (28) and a substrate (30); heating the material (26) so as to bring a temperature of the material to a preliminary exothermic peak, which precedes an exothermic sintering peak, without the temperature of the material reaching a maximum of the preliminary exothermic peak; fastening the other of the component (28) and the substrate (30) to the material (26) so that the material is interposed between the component and the substrate; and pressing the material (26) while hot so as to cause it to creep.

Claims

1. A method for mounting an electronic component onto a substrate the method comprising the following successive steps: depositing a sintering material onto one of an electronic component and a sub strategy; heating the sintering material so as to bring a temperature of the sintering material to a preliminary exothermic peak which precedes an exothermic sintering peak without the temperature of the sintering material reaching a maximum of the preliminary exothermic peak, fastening another of the electronic component and the substrate to the sintering material so that the sintering material is interposed between the electronic component and the substrate, and pressing the sintering material while hot so as to cause the sintering materials to creep.

2. The method according to claim 1, wherein the depositing takes place so that a sintering material thickness is comprised between 60 ?m and 140 ?m.

3. The method according to claim 1, wherein the depositing takes place by disposing the sintering material in boustrophedon shape.

4. The method according to claim 1, wherein the depositing takes place by forming with the sintering material beads in mutual contact.

5. The method according to claim 1, wherein the depositing takes place by depositing the sintering material on the substrate so that, after the pressing, the sintering material protrudes from edges of the electronic component.

6. The method according to claim 1, wherein the depositing takes place by depositing the sintering material on the electronic component set back from edges of the electronic component.

7. The method according to claim 1, wherein the temperature of the sintering material is between 140? C. and 150? C.

8. The method according to claim 1, wherein the heating takes place for a duration between 10 and 40 minutes.

9. The method according to claim 1, wherein, the pressing of the sintering material takes place between the first substrate and a second substrate.

10. The method according to claim 1, wherein the method comprises during the pressing: measuring a pressure of an arm resting on the sintering material and determining whether the pressure varies over a first predetermined amplitude, for a first predetermined duration; and/or measuring a position of the arm resting on the material and determining whether the position varies over a second predetermined amplitude for a second predetermined duration.

11. The method according to claim 1, further comprising sintering the sintering material.

12. The method according to claim 1, wherein one of the electronic component and the substrate comprises a contact surface with the sintering material after pressing the sintering material, wherein a product of a greater diagonal of the contact surface multiplied by a maximum thickness of the one of the electronic component and the substrate comprising the contact surface is less than or equal to 2.3.10.sup.?2 mm.sup.2.

13. A method for mounting an electronic component onto a substrate, the method comprising the following successive steps: heating a test sample of a sintering material by exposing the test sampled to an increasing temperature and measuring a temperature of the sintering material and detecting a first heating temperature value corresponding to a start of a preliminary exothermic peak which precedes an exothermic sintering peak and a second heating temperature value corresponding to a maximum of the preliminary exothermic peak; depositing a portion of the sintering material onto one of an electronic component and a substrate; heating the portion by exposing the portions to a temperature higher than the first value and lower than the second value; fastening another of the electronic component and the substrate to the portion so that the portion is interposed between the electronic component and the substrate; and pressing the portion while hot so as to cause the portions to creep.

14. An assembly for mounting an electronic component onto a substrate, the assembly comprising: a support, member configured to exert a pressure, heating means, and means configured to control: a deposit of a sintering material onto one of an electronic component and a substrate; a heating of the sintering material so as to bring a temperature of the sintering material to a preliminary exothermic peak which precedes an exothermic sintering peak without the temperature of the sintering material reaching a maximum of the preliminary exothermic peak; a fastening of another of the electronic component and the substrate to the sintering material so that the sintering material is interposed between the electronic component and the substrate; and a pressing of the sintering material while hot so as to cause it to creep.

15. The method according to claim 4, wherein the beads are in mutual contact over a heigh less than or equal to 10 ?m.

16. The method according to claim 4, wherein a diameter of each bead of the plurality of beads is between 100 ?m and 300 ?m.

17. The method according to claim 10, wherein the first predetermined amplitude is between 5% and 15% of the pressure.

18. The method according to claim 10, wherein the first predetermined duration is less than 30 seconds.

19. The method according to claim 10, wherein the second predetermined amplitude is between 30% and 40% of the position.

20. The method according to claim 10, wherein the second predetermined duration is less than 30 seconds.

Description

DESCRIPTION OF THE FIGURES

[0063] Embodiments of the invention will now be presented by way of nonlimiting examples in support of the drawings in which:

[0064] FIG. 1 is a graph showing the evolution of the mass of a sintering material (measured by thermogravimetry) marked on the ordinate axis on the left, as a function of the heating temperature applied thereto on the abscissa and the evolution of its energy (by a differential scanning calorimetry measurement) on the ordinate axis on the right, as a function of this same variable;

[0065] FIG. 2 is a graph of the thermodynamic stability ranges of silver and its oxide as a function of temperature and oxygen partial pressure;

[0066] FIG. 3 is a graph similar to that of FIG. 1 for a second sintering material;

[0067] FIG. 4 shows two steps of the method according to the invention implemented in an installation according to the invention;

[0068] FIG. 5 is a plan view of the sintering material as placed before drying;

[0069] FIG. 6 is a sectional view of the coil formed by the material of FIG. 5; and

[0070] FIG. 7 is a view of a sandwich structure made by means of the invention.

GENERAL PRESENTATION

[0071] In the method according to the invention, the following successive steps are carried out: [0072] depositing sintering material onto one of an electronic component and a substrate, [0073] heating the material so as to bring a temperature of the material to a preliminary exothermic peak which precedes an exothermic sintering peak without its temperature reaching a maximum of the preliminary exothermic peak, [0074] fastening the other of the component and the substrate to the material so that the material is interposed therebetween, and [0075] pressing the material while hot so as cause it to creep, before proceeding with its sintering.

[0076] The objective of the first heating step is to obtain a dried paste which can then be implemented under pressure to cause it to creep under a component to the chosen thickness. The activation of the material is obtained by drying the part coated with the material, alone, at a temperature comprised between 140 and 150? C. This drying can be done in air, under neutral gas or under vacuum with extraction.

[0077] This is a preliminary exothermic peak drying for a time not exceeding 40 minutes. Below this peak, it is not possible to apply pressure to the paste (because the paste pushes out under the action of pressure). Above this peak, in the subsequent creep step during sintering, the pressure or elevation of the arm serving to exert pressure are not able to vary by the desired amount and sintering is not effective. Most of the time in this case the chip is cracked.

[0078] Coupled analyzes by differential scanning calorimetry (DSC) and thermogravimetry (TGA) were carried out. They were carried out under air and under argon in order to determine what role the combustion of the solvents plays in their elimination.

[0079] Thus, FIG. 1 shows the evolution of the mass of the sintering material (measured by TGA) marked on the ordinate axis on the left, as a function of the heating temperature applied thereto on the abscissa. Similarly, it shows the evolution of its energy by measuring DSC on the ordinate axis on the right, as a function of this same variable. The DSC curve is expressed in ?V/mg and not in mW/mg. These coupled DSC/TGA analyzes relate here to a silver-based sinter paste in air. This is the paste marketed by the company Henkel under the name LOCTITE ABLESTIK SSP 2020. The crucibles used were made of alumina. The material is exposed to gradual heating with a temperature rise rate of 10? C./min.

[0080] Whether the experiment is carried out under air or under argon, the TGA curve shows that the loss of mass of this paste n? 1 (and therefore the evacuation of the solvents) takes place in two stages: a first major loss of mass, begins at 130? C. in both cases (first bump 2 visible in the figure) and a second, minor loss of mass, is observed above 250? C.

[0081] The major loss of mass coincides from 130? C. with two small peaks on the DSC curve. The first 6 is rather endothermic, the other 8 is exothermic. The small size of the peaks 6, 8 can be explained by the fact that, thermally, the two phenomena, which are more or less simultaneous, compensate each other.

[0082] Concerning the first peak 6, the loss of mass which is initialized at 130? C. is representative of the evaporation of the solvents and this is a datum provided by the manufacturer. It continues in practice up to 170.8? C. In this zone, the solvent remains present and clearly blocks the formation of silver oxide (Ag.sub.2O.sub.3) which could otherwise take place below 150? C. FIG. 2 indeed illustrates the domains of thermodynamic stability of silver and of its oxide as a function of temperature and oxygen partial pressure. It shows that, in an atmosphere composed of 20% oxygen, silver oxide is not thermodynamically stable above 150? C. Below this temperature, although thermodynamically possible, the silver oxidation reaction does not occur because it is kinetically blocked, which is the case in FIG. 1 thanks to the presence of an organic protective compound in the paste.

[0083] The first (and small) exothermic peak 8 which appears in FIG. 1 from 151.2? C. and which has its maximum at 170.8? C. is not mentioned either by the manufacturer or in the literature.

[0084] It is found on a similar analysis carried out under air on a sinter paste of another brand, which is also silver-based, and illustrated in FIG. 3. This is the paste marketed by the company Heraeus under the name Magic? DA295A. (Similar analyzes can be carried out on a paste marketed under the name Argomax? Alpha? by the company MacDermid Alpha Electronics Solutions and on a paste marketed under the name Quicksinter? by the company Indium Corporation). Here the first exothermic peak 8 starts at 192.4 and has its maximum at 198.7? C. Again, it is not mentioned by the manufacturer. In any case, only the evaporation of the solvents at 130? C. is mentioned by the manufacturers or the literature. However, between 130? C. and 170? C., other major phenomena are present.

[0085] Beyond 130? C. and below 170? C., this results in nonoxidized silver flakes having a size comprised between a few hundred nanometers and a few tens of microns and traces of solvents.

[0086] DSC coupled with TGA clearly shows that, in air, during major loss of mass, there is competition between an exothermic phenomenon and an endothermic phenomenon (combustion and evaporation). On the other hand, under argon, the main loss of mass corresponds to an endothermic peak. This means that, under neutral gas, the main loss of mass occurs more by evaporation than by decomposition of solvents.

[0087] However, this competition of the phenomena which appear in this zone, but potentially at different temperatures according to the pastes, gives rise to the exothermic peak 8 of the DSC curve which is the marker of the activation of the silver flakes for a subsequent creep or sintering.

[0088] It is observed that this exothermic peak 8 immediately precedes the main exothermic peak 10 associated with sintering.

[0089] If a temperature of 170? C. is exceeded, beyond the first exothermic peak 8 therefore, sintering begins to occur (the paste is made solid but not yet sufficiently densified), so that the paste is no longer conformable at the surfaces in presence by an application of pressure and temperature, for a creep or by need of adhesion.

[0090] Conversely, if the first heating step is interrupted before the start of the exothermic peak 8 with a minimum at 151.2? C. but a beginning of appearance of the phenomenon around 140? C. in FIG. 1 and around 192.4? C. in FIG. 3, the pressure applied during sintering does not allow controlled creep of the material. Indeed, one is then faced with an uncontrolled outpouring of the paste as if pressure was applied to a non-dried liquid paste. It is then not possible to obtain a paste which is conformable in surface (horizontal creep) and in thickness (vertical creep). To control this creep on the volume dedicated thereto, pressure must be applied to a joint which has undergone prior drying in accordance with the definition of this preliminary exothermic peak 8.

[0091] During an experiment, the preliminary exothermic peak 8 must be determined in air because it does not appear in the DSC under neutral gas or under vacuum.

[0092] On the basis of these elements, the invention is implemented as follows by way of example.

FIRST EMBODIMENT

[0093] A first embodiment of the invention will be described.

[0094] The method is implemented in an installation 20 like that of FIG. 4. It comprises: [0095] a support or frame 22, [0096] a member such as an arm 24 mounted to move relative to the support capable of exerting pressure, [0097] a plate 21 fastened to the lower end of the arm and extending opposite the support, [0098] heating means 27, and [0099] control means 29 capable of controlling the execution of the method.

Material Deposition

[0100] In a first step, a sintering material 26 is deposited onto one of an electronic component 28 and a substrate 30, for example the substrate 30.

[0101] The deposition step takes place here by disposing the boustrophedon-shaped material following a continuous coil as illustrated in FIG. 5, by forming with the material beads in mutual contact as illustrated in FIG. 6, for example over a maximum height R of 10 ?m. The beads have for example a diameter B comprised between 100 and 300 ?m. The deposition step takes place in this case such that a material thickness is comprised between 60 ?m and 140 ?m. The material is deposited on the substrate so that, at the end of the method, the material protrudes from the edges of the component 28, in particular over a distance comprised between 10 and 150 ?m from the edges of the component.

[0102] The diameter of the paste coils depends on the final thickness to be reached at the end of sintering. It does not condition the steps of the method because, once dried in accordance with the preliminary exothermic peak 8, the paste 26 conforms to the need, allowing a total flattening of the coils (no cavity is observed under the components) to form a single rectangular parallelepiped of defined surface and thickness. It is thus possible to obtain variable bonding layer thicknesses comprised between 1 and 10 mm.

[0103] The paste is chosen from commercial references.

[0104] The sintering material is spread by automatic or manual dispensing at room temperature. It is in fact preferred to place it by distribution rather than by screen printing because its application by screen printing does not allow to limit the rises over several tens of microns in height during the demolding of the screen printing frame. Moreover, the distribution gives very good control of the rate of cavities at the end of sintering, in particular a rate of less than 1% on the surface of the component.

Activation of the Material by Prior Drying

[0105] Next, the material 26 is heated so as to place the temperature of the material in the increasing phase of the preliminary exothermic peak 8 which precedes the exothermic sintering peak 10 without the temperature of the material reaching the maximum of the preliminary exothermic peak 8 (170.8? C. in FIG. 1).

[0106] This first heating step takes place in this case for a duration comprised between and 40 minutes at a temperature comprised between 140? C. and 150? C.

[0107] This heating takes place either in air or in an environment comprising at least 90% nitrogen, optionally under a pressure below atmospheric pressure.

Fastening

[0108] Then, the component 28 is fastened to the material 26 so that the material is interposed between the component 28 and the substrate 30.

[0109] If the substrate is intended for an assembly with a component and another substrate (which is the case of a 3D sandwich structure like that of FIG. 7), the component is aligned with the material then pressed thereon at ambient temperature between 0.4 and 2 MPa to allow its adhesion to the substrate for manual or automatic handling of the assembly formed by the substrate and the component. The assembly can thus be turned over, moved and handled to allow subsequent operations in order to obtain the 3D structure.

[0110] In other cases, the component 28 is simply contacted with the material 26 on the substrate and then the actual sintering step is carried out.

Creep

[0111] Then, indeed, in the installation 20 of FIG. 4, pressure is applied to the material 26 and it is heated so as to cause it to creep. The pressure is obtained as illustrated in FIG. 4 by pressing the material by means of the arm 24 between the component 28 and the support 22, against the substrate 30. Then, this causes the paste to creep.

[0112] During this step, the pressure of the arm 24 resting on the material 26 is measured and it is determined whether the pressure varies over a predetermined amplitude, for example comprised between 5% and 15%, for a predetermined duration, for example less than 30 seconds. Creep is acceptable in this case when the measured pressure has varied between 5 to 15% of its original value then stabilizes in less than 30 seconds.

[0113] In addition, a position of the arm 24 resting on the material 26 is measured and it is determined whether its position varies over a predetermined amplitude for a predetermined duration. The position measured here is the distance from the plate 21 to the face of the substrate 30 receiving the material 26. The variation in position therefore corresponds to the variation in thickness of the paste. It is considered in this case that the creep is acceptable when this distance has varied between 30 and 40% of the distance before the start of sintering (for example it goes from 100 ?m to a value between 60 and 70 ?m) in a time which does not exceed 30 seconds. The variation in distance h has been illustrated in FIG. 4.

[0114] These steps are carried out using conventional servo-control means.

[0115] Once this step is completed, the material is ready to be sintered in a subsequent heating step so that the component is firmly fastened to the substrate.

[0116] The invention is applicable in particular when the component 28 is called thin component, namely has a contact surface with the material, at the end of the method, such that

D*E=<2.3.Math.10.sup.?2 mm.sup.2

where: [0117] D designates a greater diagonal of the contact surface, and [0118] E indicates a maximum thickness of the element.

[0119] The invention is also applicable to the production of a sandwich or 3D structure like that of FIG. 7. This structure here comprises two substrates 30 between which are interposed two components 28 of dimensions different from each other. In particular, the two components 28 differ in their height. A layer of sintering material 26 is interposed between the first component 28, on the left in the figure, and the lower substrate, and another layer 28 is interposed between this same component and the upper substrate 30. Two other layers of sintering material 26 are likewise interposed between the right component 28 and the respective substrates 30. The arrangement may comprise a greater number of components interposed between the two substrates in this way. Or else, a larger number of components can be stacked with each other in the same stack with the interposition of a layer of sintering material between two components, between the two substrates.

[0120] To achieve the arrangement of FIG. 7, the two lower layers of sintering material 26 are disposed on the lower substrate 30 then the step of drying this material is carried out.

[0121] Then the two components 28 are installed and the other two layers of sintering material are applied thereon. A new drying step is then carried out.

[0122] The upper substrate 30 is then applied. For this purpose, the components are pressed between the two substrates, in particular at room temperature and in an environment under a gas pressure comprised between 0.4 and 2 MPa.

[0123] The simultaneous creep of all the layers of material 26 is then carried out. For this purpose, a silicone mattress is placed under the plate and all the components are pressed, for example under 250? C./10 MPa. During this last step, the pressure applied by the arm allows to maintain parallelism between the two faces of the substrates facing each other and therefore to compensate for the differences in height between the components 28.

[0124] The material is then sintered.

SECOND EMBODIMENT

[0125] In this second embodiment, a method for testing a sintering material is first implemented.

[0126] A test sample of the sintering material is heated by exposing it to an increasing temperature.

[0127] During heating, a temperature of the material is measured. A first heating temperature value corresponding to a start (151.2? C. in FIG. 1) of a preliminary exothermic peak 8 which precedes an exothermic sintering peak 10 is detected. This value is a local minimum. A second heating temperature value corresponding to a maximum of the preliminary exothermic peak 8 (170.8? C. in FIG. 1) is also detected. These tests are carried out for example like the aforementioned analyses.

[0128] Next, a method is implemented for mounting an electronic component onto a substrate in a manner similar to the first embodiment. The method this time comprises the following successive steps: [0129] depositing a portion of the material 26 onto one of an electronic component 28 and a substrate 30, [0130] heating the portion 26 by exposing it to a temperature higher than the first value and lower than the second value, [0131] fastening the other of the component and the substrate to the portion so that the portion is interposed between the component and the substrate, and [0132] pressing the portion while hot so as to cause it to creep.

[0133] The sintering is then carried out.

[0134] Many modifications may be made to the invention without departing from the scope thereof.