COMPOSITE SOLDER BALLS METALLISED ON THE SURFACE AND CALIBRATED FOR THE ASSEMBLY OF ELECTRONIC BOARDS

Abstract

The present invention relates to a method for manufacturing composite solder balls that are metallized on the surface and calibrated, these balls comprising a core consisting of a spherical support particle of diameter Do made of expanded polystyrene and having an intergranular porosity of at least 50%, and a shell covering said support particle and formed by a plurality of metallic surface layers. The present invention also relates to balls that can be obtained by the method according to the invention, as well as to the use thereof for the assembly of electronic boards.

Claims

1. Method for manufacturing composite solder balls that are metallized on the surface and calibrated, of diameter D.sub.f, said balls comprising: a core consisting of a spherical support particle of diameter D0, made of expanded polystyrene (EPS) and having an intergranular porosity of at least 50%, and a shell that covers said support particle and is formed by a plurality of metallic surface layers, comprising a copper coating of thickness E.sub.cu, at least one nickel layer of thickness E.sub.Ni, and a gold top coat of thickness E.sub.Au, said method being characterized in that it comprises the following steps: A) a first step of providing support particles, followed by B) a first step of granulometric sorting of the support particles consisting in a step of physical and/or mechanical separation of the support particles having a diameter Do such that D.sub.0=D.sub.f−2*(E.sub.Ni+E.sub.cu+E.sub.Au), it being possible for Do to be selected so as to be between 200 μm and 1000 μm; C) a step of activation treatment of the support particles thus selected, in order to obtain activated support particles; D) a first step of metallization of said activated support particles by means of autocatalytic chemical deposition of one or more layers of copper, said step being repeated until a copper coating of thickness E.sub.cu of copper of between 15 and 30 μm is obtained, in order to obtain copper-coated support particles; E) a second step of metallization of the copper-coated support particles by means of autocatalytic chemical deposition of at least one layer of nickel alloyed with phosphorous NiP having a mass percentage of from 7 to 10% phosphorous with respect to the total weight of said layer of NiP, said step being performed until a thickness ENi of the layer of chemical nickel of between 4 μm and 7 μm is obtained, in order to obtain support particles coated with chemical nickel; F) a third step of metallization of the support particles coated in chemical nickel consisting in a step of deposition of gold by means of a method of galvanic displacement implemented by immersion in an aqueous solution containing gold ions, so as to obtain composite solder balls that are metallized and coated on the surface with a gold top coat of a thickness E.sub.Au of between 0.05 μm and 0.12 μm; and G) a second step of granulometric sorting of the particles thus metallized in order to sort and select composite solder balls which are metallized on the surface and have a diameter D.sub.f, at a tolerance of +/−5%.

2. Method according to claim 1, wherein the step of granulometric sorting B) of the support particles consists in mechanical sieving in order to sort and select the support particles having a diameter Do selected so as to be between 200 μm and 1000 μm.

3. Method according to claim 2, wherein the mechanical sieving B) of the support particles is followed by heat treatment at a temperature of between 100° C. and 120° C., and preferably between 110° C. and 120° C., of the top slice of the particles thus treated, the size of which is greater than D.sub.0+5%.

4. Method according to claim 1, wherein the activation treatment of the support particles selected consists in depositing silver seeds on the surface thereof, by means of introduction into a bath of silver nitrate, or by depositing, on the surface thereof, a thin layer of copper of less than or equal to 1 μm, by means of physical vapor deposition (PVD).

5. Method according to claim 1, further comprising, between steps D) and E), a step of partial or total dissolution of the expanded polystyrene (EPS) making up the support particles, or a step of partial or total thermal decomposition of the expanded polystyrene (EPS) making up the support particles.

6. Use of composite solder balls that are metallized on the surface and calibrated, of diameter D.sub.f, obtained by the method as defined in claim 1, for the assembly of electronic boards.

Description

[0038] Other features and advantages of the invention will become clear from the following description, given by way of non-limiting example and with reference to the accompanying drawings, in which:

[0039] FIG. 1 is a schematic view of an example of a flexible ball known from the prior art, comprising contacts to be soldered for assembly thereof in integrated circuit housings,

[0040] FIG. 2 is a schematic view of an example of a flexible ball according to the invention,

[0041] FIG. 3 is also a schematic view of an example of a flexible ball according to the invention, but in which, for reasons of clarity, the surface layer of gold is not shown,

[0042] FIG. 4 is a photograph of an activated EPS ball (coated with a thin layer of silver), as obtained at the end of the step C) of activation of the method according to the invention,

[0043] FIG. 5 is a photograph of an activated EPS ball coated with a layer of copper, as obtained at the end of the step D) of metallization with copper of the method according to the invention,

[0044] FIG. 6 is a photograph of an activated EPS ball coated successively with layers of copper and of chemical nickel (NiP), as obtained at the end of the step E) of metallization with chemical nickel of the method according to the invention,

[0045] FIG. 7 is a photograph of a solder ball according to the invention, consisting in an activated EPS ball coated successively with layers of copper, chemical nickel (NiP) and gold, as obtained at the end of the step F) of metallization with chemical nickel of the method according to the invention,

[0046] FIG. 8 is an optical microscope image of a cross section of a ball according to the invention, showing the EPS support ball and the successive layers of copper, chemical nickel and gold, as obtained at the end of a first embodiment of the method according to the invention (EPS ball of approximately 655 μm diameter, and copper layer of 17 μm thickness),

[0047] FIG. 9 shows a first optical microscope image (A) of a cross section of a copper-coated EPS ball as obtained at the end of step D) of a second embodiment of the method according to the invention (EPS ball of 660 μm diameter and copper layer of 15 μm thickness), and a second optical microscope image (B) of a cross section of a copper-coated EPS ball as obtained at the end of step D) of a third embodiment of the method according to the invention (EPS ball of 630 μm diameter and copper layer of 31 μm thickness),

[0048] FIG. 10 shows a first optical microscope image A showing EPS support balls 10 having a size distribution Do of between 0.5 and 1 mm, and a second optical microscope image B showing EPS support balls 10 obtained after granulometric sorting selecting diameters of between 0.63 and 0.66 mm.

[0049] FIG. 11 shows metallized balls 1 according to the invention, having diameters of the order of 400 μm+/−5%,

[0050] FIG. 12 is a SEM image showing metallized balls 1 according to the invention prior to heat treatment at 360° C. (A), and a SEM image showing metallized balls 1 according to the invention having been subjected to subsequent heat treatment at 360° C. for 20 minutes (B) in order to verify the thermal resistance of the balls.

[0051] FIG. 13 is a SEM image showing balls having a dense polystyrene core which have been metallized in accordance with steps A to G of the method according to the invention and which have been subjected to subsequent heat treatment at 360° C. for 20 minutes (B) in order to verify the thermal resistance of the balls.

[0052] FIG. 1 is described in the presentation of the known prior art. It shows in particular flexible balls having contacts to be welded 1, 2, allowing for easy assembly thereof. FIG. 2 to FIG. 12 are described in greater detail in the following examples, which illustrate the invention without limiting the scope thereof.

EXAMPLES

[0053] Devices and Instrumentation [0054] Autocatalytic chemical deposition device (for the deposition of copper and of chemical nickel); [0055] Mechanical sieving device

[0056] Starting Products [0057] Expanded polystyrene (EPS) balls of diameter of between 595 μm and 665 μm; [0058] Expanded polystyrene (EPS) balls of diameter of the order of 400 μm, [0059] Chemical nickel (containing 7 to 10 wt. % P), [0060] Copper [0061] Silver nitrate, [0062] Gold ions.

[0063] Characterization: Morphological Analysis

[0064] The optical microscope observations are performed using an optical microscope, at the surface ([FIG. 4] to [FIG. 7]) and in cross section ([FIG. 8] and [FIG. 9]).

[0065] The observations of FIGS. 12 and 13 are obtained using a scanning electron microscope (SEM).

Example 1

[0066] Manufacture of Solder Balls According to the Invention, in Accordance with a First Embodiment of the Method According to the Invention,

[0067] Solder balls according to the invention are developed from expanded polystyrene (EPS) balls according to a first embodiment of the method according to the invention. The manufacturing is split according to the following steps: [0068] 1) A first mechanical sieving of EPS balls 10 having a size distribution of between 0.5 and 1 mm is performed, in order to keep just the balls of a diameter of the order of 630 μm ([FIG. 10]); [0069] 2) Then, the surface of the EPS grains is activated in order to allow for good adherence of the subsequent metallization treatment. The activated ball 10′ is obtained by depositing silver seeds on the surface of the balls. The operation is performed in a bath, by reduction of the silver nitrate. At the end of this treatment, a ball 10′ is obtained as shown in [FIG. 4]; [0070] 3) The third step consists in performing the metallization treatment using copper. A layer of copper 110 of 30 μm thickness Ecu is obtained by means of autocatalytic chemical deposition (cf. [FIG. 2], [FIG. 3]). At the end of this treatment, a ball 10″ is obtained as shown in [FIG. 5] (surface view) or in [FIG. 9], part B (cross-sectional view); [0071] The fourth step consists in depositing a layer of chemical nickel (NiP) 111 of 5 μm thickness on the layer of copper 110 (cf. [FIG. 2], [FIG. 3]). Said coating is also implemented by means of autocatalytic chemical deposition. At the end of this treatment, a ball 10′″ is obtained as shown in [FIG. 6]; [0072] The last metallization layer is a top coat 112 of gold (cf. [FIG. 2]). The deposition is obtained by galvanic displacement following immersion of the chemical nickel-coated balls 10′″ in a solution containing gold ions. A layer of gold is thus deposited, the thickness E.sub.AU of which is between 0.05 μm and 0.12 μm (cf. [FIG. 2]). At the end of this treatment, a solder ball 1 is obtained as shown in [FIG. 7]; [0073] The final step is granulometric sorting of the balls in order to keep only the balls according to the invention 1 of diameter D.sub.f of the order of 700 μm+/−5%.

Example 2

[0074] Manufacture of Solder Balls According to the Invention, in Accordance with a Second Embodiment of the Method According to the Invention

[0075] Solder balls according to the invention are formed in the same way as in example 1, having support balls of 660 μm diameter and a deposition of copper of the order of 15 to 17 mm thickness (cf. FIGS. 8 and 9A).

Example 3

[0076] Manufacture of Solder Balls According to the Invention, in Accordance with a Third Embodiment of the Method According to the Invention

[0077] Solder balls according to the invention are formed in the same way as in examples 1 and 2, in order to obtain balls of diameter D.sub.f of the order of 400 μm+/−5% (cf. [FIG. 12]).

Example 4

[0078] Effect of Heat Treatment after the Step of Sieving

[0079] The SEM analyses ([FIG. 12]) show balls that are metallized and calibrated according to the invention, before (12a) and after heat treatment at 360° C. (12b). It is noted that the balls have not undergone deterioration/modifications following the heat treatment, demonstrating their temperature stability.

Example 5 (Comparative)

[0080] In order to establish a comparative analysis of the weight of the balls 3, batches of balls (of diameter Do 700 μm+/−5%) were formed according to steps A to G of the method according to the present invention (Cu/NiP/Au metallization), then subjected to subsequent heat treatment at 360° C. for 20 minutes (B) in order to verify the thermal resistance of the balls. In order to achieve this, 3 different types of support balls were used: [0081] a first batch formed based on expanded polystyrene balls (exhibiting approximately 95% porosity), [0082] a second batch formed based on crosslinked polystyrene balls (dense), and [0083] a third batch formed based on metal copper balls (dense).

[0084] For each batch, 1000 balls were removed and weighed. The following results were obtained, summarized in table 1 below:

TABLE-US-00001 TABLE 1 Support balls Weight for 1000 metallized balls (g) Expanded polystyrene 0.225 Dense polystyrene 0.390 Dense copper 2.100

[0085] The balls formed according to the present invention based on expanded polystyrene (first batch) have a mass reduction factor of between 1.7 and 9.4 compared with the current solutions (dense core made of organic or metal material).

[0086] On the other hand, it has been noted that the thermal stability of the balls is improved by using an EPS core (balls according to the invention, of the first batch) compared with the balls obtained on the basis of a dense core (balls of the second and third batches). Indeed, it is noted that, from 360° C., the balls formed based on dense polystyrene (balls of the second and third batches) exhibit a behavior defect, with the appearance of “droplets” of organic material on the surface of the balls (FIG. 1). The phenomenon is not observed in the balls according to the invention formed on the basis of expanded polystyrene (balls of the first batch).