EP1175275A1

EP1175275A1 - Method for producing weld points on a substrate and guide for implementing said method

Info

Publication number: EP1175275A1
Application number: EP00922746A
Authority: EP
Inventors: Eric Thomson-CSF Propriété Intellectuelle PILAT
Original assignee: Applied Utech
Current assignee: Applied Utech
Priority date: 1999-04-30
Filing date: 2000-04-26
Publication date: 2002-01-30
Also published as: US6708868B1; JP2002543603A; FR2792861A1; MY133204A; KR20020003243A; FR2792861B1; WO2000066312A1; TW473413B; HK1040660A1

Abstract

The invention concerns a method for moulding and soldering electrical connection studs on mounting lands (12) for electrical connection in circuits or electronic components. The method comprises a step which consists in injecting a conductive liquid alloy (48) into a guide open at one end located opposite the component mounting land. The guide is formed by two separable parts, a mould (16) and an injection matrix (18) with a shrunk portion of the guide at the interface of the parts, and in separating the guide parts while the alloy is still in liquid phase. The invention is useful for producing connection studs on substrates and electronic components.

Description

PRODUCTION PROCESS

OF WELDING PLOTS ON A SUBSTRATE

AND GUIDE FOR THE IMPLEMENTATION OF THE PROCESS

The present invention relates to a method of molding and soldering of electrical connection pads on reception areas for electrical connection of circuits or electronic components.

The electrical connections of electronic components, such as integrated circuits comprising a large number of connection points, are usually made by means of solder balls soldered onto metal receiving areas of the component substrate. These reception areas are located on the transfer face of the component on an electrical interconnection circuit. A known method of making the electrical connections of an integrated circuit comprises the following main steps: manufacturing the solder balls with the desired diameter; soaking the beads in a stream and depositing the beads on the substrate of the component; passage in a furnace of the component equipped with the balls in order to ensure brazing. The deposition of the balls on the substrate is carried out by a device such as a vacuum cleaner or sieve depositing the balls on the receiving areas of the component.

These devices are expensive and the production of balls and their storage are very expensive.

Another technique consists in producing and brazing beads from remelting of solder cream deposited by screen printing on the reception areas of the component. The cream is screen printed through two stencils superimposed above the substrate. The top stencil is only used to deposit the cream; the other stencil serves as a mold and remains in place until the cream has melted in a passing oven. Due to the presence of flux, the substrates and stencils must be cleaned.

Even if the cream is 3 to 5 times cheaper than a prefabricated ball, the need to use these stencils also makes it a very expensive technique.

In order to overcome the drawbacks of the prior art, the invention provides a method for producing balls or solder pads on an electrically conductive receiving area of an electronic component comprising an operation of injecting a conductive liquid alloy into a guide open at one end placed opposite the reception area of the component, characterized in that the guide is formed of two separable parts, a mold and an injection matrix , the mold and the injection matrix comprising passages, with a narrowing of the guide at the level of the separation of the parts, and the parts of the guide are separated while the alloy is liquid.

In the process for producing the solder pads according to the invention described below, the mold is the part in direct contact with the substrate of the component and the injection matrix the other part. In a first variant of the method according to the invention, the mold is removed from the component before the alloy solidifies. The molten metal present on the reception area of the component takes the form of a ball as it cools.

In another variant of the process, the mold is cooled below the liquidus point of the alloy so that the alloy solidifies in the mold after separation of the parts. The mold is separated from the component and a reflow of the alloy is carried out (optional) so that it takes the form of a ball.

The invention also relates to a guide for producing balls or solder pads on electrically conductive receiving areas of an electronic component, the guide being intended to contain a conductive liquid alloy and being open at one end, characterized in that it is formed of two separable parts comprising passages with a narrowing of the guide at the level of the separation of the parts.

In one embodiment of the guide, the two parts can be separated in the direction of injection of the liquid alloy into the guide.

Other characteristics and advantages of the invention will appear on reading the description of embodiments of the guide and variants of the molding and brazing method according to the invention of the solder pads on a receiving area of the component in which: - Figures 1, 2, 3, 4 and 5 show different embodiments and variants of these embodiments of the guide according to the invention.

- Figure 6 shows a device for implementing the method of producing the solder pads according to the invention using the guide. - Figures 7, 8, 9 and 10 show different phases of a first variant of the molding and brazing process according to the invention of solder pads on a component.

- Figures 11, 12, 13 and 14 show different phases of a second variant of the molding and brazing process according to the invention.

FIG. 1 represents a guide 10 comprising several identical passages for carrying out the molding and soldering of electrical connection pads on receiving pads 12 for electrical connection of an integrated circuit 14. The guide comprises a mold 16 and a matrix injection 18 each having two parallel main faces, a substrate face 20 and an internal mold face 22 for the mold, and an internal face 24 and an external face 26 for the injection matrix.

The guide respectively comprises first passages 28 in the mold and second passages 30 in the injection matrix, each of the first passages being aligned coaxially along an axis XX 'with one of the respective second passages facing it. The axis XX 'is substantially perpendicular to the main faces of the guide.

The distribution of the passages in the guide is the same as that of the metal areas 12 for receiving the integrated circuit 14 so that each of the metal areas of the integrated circuit in contact with the substrate face 20 of the mold 16 is located opposite 'a passage from the guide.

In a first embodiment of the guide, the first and second passages are of frustoconical shape, the small diameters of the frustoconical passages facing each other at the level of the partitions of the two parts of the guide so that, when these faces (22, 24) are in contact, the passage in the guide comprises a narrowing or a recess of the diameter of the guide at the level of the separation of the parts.

In a first variant of the first embodiment of the guide, shown in FIG. 1, the openings of smaller diameter, of the first and second frustoconical passages respectively on the faces of the mold and of the injection matrix in contact have substantially the same diameter d1.

In a second variant of this first embodiment (FIG. 2), the opening of the first passage 28 of the mold facing the injection matrix is of a diameter d2 greater than the diameter d3 of the opening of the second passage 30 of the injection matrix facing the mold.

In a third variant of the first embodiment (FIG. 3), the opening of the first passage (28) of the mold facing the injection matrix has a diameter greater than the opening of the second passage (30) of the matrix d injection facing the mold. In addition, the opening on the side of the internal face 24 of the second passage 30 of the injection matrix 18 has a rim 29 penetrating, when the mold and the injection matrix are in contact, in the first frustoconical passage 28 of the mold. In this third variant, the flange 29 around the opening of the injection matrix can be produced by machining part of the thickness of its internal surface 24 facing the mold, for example using a laser.

In a second embodiment of the guide (Figure 4), the first passage 28 of the mold is of approximately semi-spherical shape, the largest opening being located on the substrate face 20 of the mold 16 and a small opening on the internal face 22 of the mold , the second passage 30 being of frustoconical shape, the smallest opening of which is on the internal face 24 of the injection matrix 18 facing the small opening of the first semi-spherical passage.

In a third embodiment of the guide (FIG. 5), the first passage 28 in the mold is of frustoconical shape, the smallest diameter of the first passage facing the injection matrix and the second passage in said injection matrix being of cylindrical shape with a very small diameter compared to the smallest diameter of the first passage 28 in the mold 16.

The implementation of the method according to the invention for producing the solder pads on the substrate of a component is ensured by the guide in two separable parts. To this end, a device 40, represented by a block diagram in FIG. 6, produces the injection into the passages of the guide 10 of the liquid alloy for molding the electrical connections on the component 14.

The component 14 is pressed by a piece 42 pushed by an elastic element 44 against the guide 10 so that the receiving areas 12 of the component are opposite the first passages 28 of the mold.

The device 40 essentially comprises a closed tank 46 comprising an alloy 48 in fusion. The alloy can be put under pressure by a gas 50 coming from a tank 52 containing the gas under pressure. During a molding of the electrical connections on the component 14, the pressurized molten alloy 48 fills, through a conduit 54, a cavity 56 comprising an opening 58 encompassing all the passages of the guide 10. The component 14 is maintained by pressing on the guide, which is itself pressed against the opening 58 of the cavity 56, closing this cavity. The face of the component 14 comprising the receiving areas is pressed against the substrate face 20 of the mold and the external face 26 of the injection matrix is pressed against the face of the cavity 56 comprising the opening 58.

The alloy 48 molten in the cavity 56 is injected under pressure into the guide 10 rapidly filling, through the second passages of the injection matrix, the first passages 28 of the mold and wetting the receiving areas 12 of the component 14 .

The component reception areas are assumed not to be oxidized or polluted by organic matter; they are wettable by the molten alloy; otherwise, an additional cleaning step will be required beforehand to prepare the substrates accordingly.

In the first variant, represented by FIGS. 7 to 10, of the method of molding and soldering of electrical connection pads on pads for receiving the substrate of an integrated circuit package (component 14), the method comprises at least the following steps:

- First step (Figure 7); injection of the alloy: during the injection of the liquid alloy 48 under pressure into the passages of the guide, the mold 16 is maintained at a temperature lower than that of the injection matrix 18, but higher than the liquidus threshold of alloy 48. - Second step (see Figure 8); mold / injection matrix separation: the injection pressure drops or even reverses, the liquid alloy 48 withdraws into the injection matrix 18. The liquid alloy filling the mold 16 remains because the mold is cooler than the injection matrix, the receiving area 12, on which the alloy has wetted, has a larger surface area than the mold hole on the injection matrix side. The tension which will retain the liquid alloy is therefore greater than that which tends to draw it into the injection matrix. Then the mold is separated from the injection matrix allowing an inert gas Gz or even reducing to come to protect the still liquid alloy against oxidation. This same gas is also injected into the passages (or nozzles) of the injection matrix to keep it very clean for the next cycle. - Third step (see Figure 9); component (or substrate) / mold separation: before the alloy solidifies, component 14 is separated from the mold 16; despite the misalignments, the alloy has wetted over a sufficient area of the reception area 12 so that the liquid alloy 48 remains attached to the substrate of the component and not to the mold. The mold material is chosen (316L stainless steel with chemical deburring, or graphite, or Teflon, or treated silicon for example) so as to minimize the alloy / mold surface tension.

The liquid alloy 48 is always in a neutral or even reducing gaseous environment (Gz).

- Fourth step (Figure 10); solidification: the molten alloy no longer undergoing mechanical stress takes the form of a spherical cap 60 because it is in this configuration that the surface tensions are reduced to a minimum. As it cools, the alloy freezes in this form. Since there was no need to use a flux, there is no need to clean the substrate of component 14.

In the second variant, the method comprises at least the following steps:

- First step (Figure 11); During the injection of the liquid alloy 48 under pressure into the passages of the guide, the mold 16 is at a temperature below the liquidus threshold of the alloy, but sufficiently high to allow the wetting of the reception areas 12 and the filling passages.

- Second step (see Figure 12); solidification of the alloy in the mold, separation of the mold / injection matrix: the mold 16 is kept at a temperature below the liquidus threshold of the alloy, so that the latter solidifies rapidly in the first passages 28 of the mold. The injection pressure drops, the liquid alloy 48 withdraws into the injection matrix. The mold is separated from the injection matrix allowing a neutral gas Gz see reducing to saturate the atmosphere under the alloy and in the passages of the injection matrix, so that it is very clean for the next cycle .

- Third step (see Figure 13); component (or substrate) / mold separation: the shape of the first passage (or cavity) of the mold open to the maximum on the substrate side, a mold material having a coefficient of expansion lower than that of the alloy and a surface treatment of the first passage of chemical deburring type mold, facilitates demolding. The solidified studs 62 have substantially the shape of the first passages 28 in the mold.

- Fourth step (see Figure 14); reflow of the solidified pads 62: this step is necessary to obtain connections in the form of balls 64 perfectly positioned relative to their receiving range 12. This operation can be carried out collectively in an oven in a neutral gaseous environment of nitrogen type. The alloy undergoing no mechanical stress takes a regular spherical shape corresponding to the configuration of minimum surface tension. This reflow does not require a flow, or else a slightly active flow and it is not necessary to clean the substrate after reflow.

These methods according to the invention have the advantage of using solid tin in the form of a bar at a cost much lower than the cost of the soldering cream or of the beads used in the methods of the prior art, without take into account the storage aspect which is much less sensitive. In the first and second variants of the method for producing the solder pads, it is possible to improve the rupture of the solder between the two parts of the guide at the time of their separation. To this end, the guide is made to vibrate when the parts are separated so that this rupture always takes place in the same place at the level of the narrowing of the guide. This ensures very good reproducibility of the volume of the solder pads.

Another advantage of the method according to the invention lies in the fact that it makes it possible to form and braze the connection on the reception area in a single step with possibly a reflow in an oven while the other principles of the prior art require in addition to a screen printing and / or ball placement machine, a pass-through oven, or even a cleaning machine. The associated machine will in principle be half as expensive to produce. Finally, the tools associated with each product are much less expensive.

Claims

1. Method for producing balls or solder pads (60, 62, 64) on an electrically conductive reception area (12) of an electronic component (14), comprising a liquid alloy injection operation ( 48) conductor in a guide (10) open at one end placed opposite the reception area of the component, characterized in that the guide is formed of two separable parts, a mold (16) and an injection matrix ( 18), the mold and the injection matrix comprising passages (28, 30), with a narrowing of the guide at the level of the separation of the parts, and the parts of the guide are separated while the alloy is liquid.

2. Method according to claim 1, characterized in that, the mold (16) is separated from the component before solidification of the alloy, the molten metal present on the reception area of the component taking the form of a ball as it cools .

3. Method according to claim 1, characterized in that, the mold (16) is cooled below the liquidus point of the alloy so that the alloy solidifies in the mold after separation of the parts, the mold is separated of the component and a reflow of the alloy is carried out (optional) so that it takes the form of a ball.

4. Method according to claim 2, characterized in that it comprises the following steps:

- positioning and holding by pressure of the component on the mold, then injection of the liquid alloy (48) under pressure into the guide (10) rapidly filling the first passages (28) of the mold and wetting the reception areas (12) of the component (14), the mold being at a temperature lower than that of the injection matrix (18), but higher than the liquidus threshold of the alloy (48).

- withdrawal of the liquid alloy in the injection matrix (18) followed by the separation of the mold (16) from the injection matrix, the liquid alloy filling the first passages of the mold remaining in the mold, the mold being colder than the injection matrix and the receiving area, on which the alloy has wetted, having a larger surface area than the mold hole on the injection matrix side.

separation of the component from the mold (16) before the alloy (48) solidifies, the alloy having wetted over a sufficient surface of the reception area (12) so that the liquid alloy (48) remains attached to the component substrate and not to the mold.

- cooling of the alloy producing its solidification in the form of a spherical cap (60).

5. Method according to claim 4, characterized in that the withdrawal of the alloy from the injection matrix is carried out by inversion of the injection pressure of the liquid alloy in the guide.

6. Method according to claim 4, characterized in that the withdrawal of the alloy from the injection matrix is carried out by a drop in the injection pressure of the liquid alloy in the guide.

7. Method for producing solder pads on a substrate according to claim 3, characterized in that it comprises the following steps:

positioning and maintaining by pressing the component on the mold, then injecting the liquid alloy (48) under pressure into the guide (10) rapidly filling the first passages (28) of the mold and wetting the reception areas (12) of the substrate, the mold being at a temperature below the liquidus threshold of the alloy, but sufficiently high to allow the wetting of the reception areas and the filling of the passages.

- Maintaining the mold (16) at a temperature below the liquidus threshold of the alloy, so that it solidifies rapidly in the first passages of the mold;

- removal of the liquid alloy in the injection matrix followed by the separation of the mold from the injection matrix;

- Separation of the mold component showing solder pads (62), welded on the receiving pads, the shape of the first passages (28) of the mold (16).

8. Method according to claim 7, characterized in that the withdrawal of the alloy from the injection matrix is carried out by a drop in the injection pressure of the liquid alloy in the guide.

9. Method according to one of claims 7 or 8, characterized in that one carries out a reflow of the solder pads (62) allowing connections to be obtained in the form of balls (64) perfectly positioned with respect to their range of reception.

10. Method according to claim 9, characterized in that the reflow of the studs is carried out collectively in an oven with a neutral environment of nitrogen type.

11. Method according to one of claims 1 to 10, characterized in that the guide (10) is vibrated at the time of the separation of the parts so that the rupture of the weld between the two parts of the guide takes place at same location of the shrinking of the guide, ensuring very good reproducibility of the volume of the solder pads.

¹ 12. Method according to one of claims 4 to 11, characterized in that an inert gas makes it possible to saturate the atmosphere under the alloy and in second passages of the injection matrix.

13. Guide for producing balls or solder pads on electrically conductive receiving areas of an electronic component, the guide being intended to contain a conductive liquid alloy and being open at one end, characterized in that it is formed by two separable parts (16, 18) having passages (28, 30) with a narrowing of the guide at the level of the separation of the parts.

14. Guide according to claim 13, characterized in that the two parts are separable in the direction of injection of the molten alloy.

15. Guide according to one of claims 13 or 14, characterized in that it comprises a mold (16) and an injection matrix (18) each having two parallel main faces, a substrate face (20) and a face internal mold (22) for the mold and an internal face (24) and an external face (26) for the injection matrix, the mold (16) and the injection matrix (18) respectively comprise first passages (28 ) in the mold and second passages (30) in the injection matrix, each of the first passages being aligned coaxially along an axis XX 'with one of the respective second passages facing it, the axis XX' being substantially perpendicular to the faces main guide.

16. Guide according to claim 15, characterized in that the first

(28) and second passages (30) are of frustoconical shape, the small diameters of the frustoconical passages facing each other at the level of the separations of the two parts of the guide so that, when these faces (22, 24) are in contact, the passage in the guide comprises a narrowing or a recess of the diameter of the guide at the level of the separation of the parts.

17. Guide according to claim 16, characterized in that the openings of smaller diameter, first and second passages located on the faces of the mold and the injection matrix in contact, have the same diameter (d1).

18. Guide according to claim 16, characterized in that the opening of the first passage of the mold facing the injection matrix is of diameter

(d2) greater than the diameter (d3) of the opening of the second passage of the injection matrix facing the mold.

19. Guide according to claim 16, characterized in that the opening of the first passage of the mold facing the injection matrix is of greater diameter than the opening of the second passage of the injection matrix facing the mold, a rim (28) of the opening on the side of the internal face (24) of the second passage (30) of the injection matrix (18) penetrating when the mold and the injection matrix are in contact, in the first frustoconical passage of the mold.

20. Guide according to claim 15, characterized in that the first passage (28) of the mold is of semi-spherical shape, the largest opening being located on the substrate face (20) of the mold and a small opening on the internal face. of the mold (22), the second passage (30) being of frustoconical shape, the smallest opening of which is on the internal face (24) of the injection matrix facing the small opening of the first semi-spherical passage.

21. Guide according to claim 15, characterized in that the first passage (28) in the mold is of frustoconical shape, the smallest diameter of the first passage facing the injection matrix and the second passage in said matrix injection being of cylindrical shape with a very small diameter compared to the smallest diameter of the first passage in the mold.

22. Guide according to one of claims 15 to 21, characterized in that the mold is made of a material chosen from either 316L stainless steel with chemical deburring, or graphite, or Teflon, or silicon.