WO2004100623A2 - Electronic system comprising slots for any possible excess fusible material, and corresponding assembly method - Google Patents

Abstract

The invention relates to an electronic system comprising at least one component which is mounted to a corresponding support using an assembly of elements which are made from a fusible material. One such system also comprises at least one slot, on at least one of the aforementioned components and/or the support, which is designed to receive any possible excess fusible material, such as to limit the total thickness of the system.

Description

 Electronic system with escape zones for any surplus of fusible material, and corresponding assembly process.

The field of the invention is that of electronic systems, and in particular radio communications. More specifically, the invention relates to the manufacture, and in particular the assembly of electronic components fitted to electronic systems. More precisely still, the invention relates to the transfer of components or radio communication modules to a printed circuit, in particular when there is a constraint of small thickness, for example for integration into small and / or small devices. thickness (mobile phone for example).

The invention applies in particular to the assembly technique BGA (“bail grid array”, in English, for “network of balls” in French) of electronic components inside such electronic systems and / or radio communication modules. , by means of fusible material.

The BGA assembly technique offers a number of advantages, including:

- low sensitivity to thermo-mechanical constraints;

- the low sensitivity to the potential lack of flatness of the daughter board and / or the motherboard on which the components must be assembled;

- the low interconnection pitch (distance between two connection points) required for the components;

- ease of reassembly of components; - its low cost.

This BGA type assembly technique is therefore widely used in the field of assembly of electronic components, in particular in the manufacture of radiocommunication systems which do not have real constraints of small thickness. However, in these systems, there is the major drawback of the relatively large thickness induced by the balls of fusible material between the components and the motherboard and / or daughter to which they are transferred, and the absence of precise control of the thickness resulting from the assembly. For illustrative purposes only, FIG. 1 thus represents the balls 11 of fusible material arranged on the underside of an electronic component or module 12, also called daughter card, and the transfer of the latter to a mother board 13, according to a classic BGA assembly process. Figures 2.a and 2.b show a sectional view of the component 12 before and after its assembly according to the BGA technique on a motherboard 13 by means of balls 11 of fusible material.

FIG. 2.b illustrates in particular the large thickness 21 induced by the balls 11, after their reflow, between the card 13 and the component 12, thickness which is incompatible when the height separating said card from said component to be transferred must be made minimum. This is due in particular to the fact that the balls have a relatively large volume, the quantity of material having to be sufficient to fill a possible deviation from flatness. In return, in the absence of such a gap, the balls impose an additional thickness.

There is no known simple and effective technique for overcoming this drawback, among the techniques known from the prior art. Indeed, this problem is simply considered by manufacturers or assemblers as inherent in the BGA assembly technique.

An approach known from the prior art in an attempt to reduce, if necessary, the height between the card (mother and / or daughter) and the component to be transferred onto the latter could consist, at first glance, of overwriting this component more strongly. on said card, during the reflow step.

However, such an approach according to the prior art has several drawbacks, the most important being:

- the significant risk of deterioration of the component; - The risk of short circuit potentially induced by the involuntary contact of the fusible material of the balls with each other and / or with one or more tracks of the circuit, during the reflow phase. This would in most cases lead to the disposal of the component and the card (mother or daughter).

However, in the case of the assembly of radiocommunication modules, complex and often very expensive to manufacture, such an approach according to the prior art is not acceptable, from the financial point of view.

The invention particularly aims to overcome these various drawbacks of one prior art.

More specifically, an objective of the invention is to provide an electronic system and its assembly method making it possible to achieve a minimum thickness between the component and / or module and the substrate on which it must be transferred, according to the technique of BGA assembly. Another object of the invention is to provide such a system and method which are inexpensive and easy to implement.

An additional objective of the invention consists in retaining all the known advantages of the BGA assembly technique, namely a low sensitivity to thermo-mechanical stresses, a low sensitivity to the potential lack of flatness of the daughter card and / or of the motherboard on which the components must be assembled, the need for a low interconnection pitch for the components and the ease of reassembly of the components.

Another objective of the invention is to provide a system and a method eliminating the significant risks of deterioration of the component and of potential short circuits which can be induced by an involuntary contacting of the balls of fusible material with one or more tracks of the circuit. .

These objectives, as well as others which will appear subsequently, are achieved using an electronic system comprising at least one component mounted on a corresponding support, using a set of elements of fusible material. Such a system advantageously comprises, on at least one of the components and / or on the corresponding support, at least one escape zone designed to receive a possible surplus of fusible material, so as to limit the overall thickness of the system. It effectively overcomes the initial constraints related to the height of the balls which usually penalizes the thickness of the daughter card and / or the motherboard. In fact, the system according to the invention advantageously makes it possible to evacuate the surplus of the balls of fusible material after assembly of the daughter card on the mother board.

Advantageously, the escape zone or zones are covered with a wettable material. It is in fact a question of using the capillary properties of the soldering cream, or of any other fusible material, on a wettable surface to evacuate the excess solder.

Preferably, at least one of the escape zones defines a track ending in a dead end.

Advantageously, the track or tracks end on an enlarged section portion.

Advantageously, an escape zone is associated respectively with each of the elements of fusible material. This is indeed to avoid any possibility of short circuit after assembly of the daughter card on the mother board.

Also advantageously, the same escape zone is associated with at least two of the elements of material to be welded, corresponding to electrical connections at the same potential. In certain particular embodiments, it may indeed be advantageous to take advantage of the surplus of discharged fusible material to make certain connection tracks, or else certain other electrical connections. The elements to be welded and / or the escape zones are preferably designed so that the surplus material is directed towards the escape zones under the effect of the capillarity applied to the component (s).

Advantageously, the escape zones belong to the group comprising: - wells of predetermined diameter and depth; - grooves of predetermined shape and depth;

- electrical tracks of predetermined shape and length;

- the internal cavities made in at least one of the components and / or in the support; - through vias.

Different geometric shapes in two and / or three dimensions (2D or

3D) can indeed be envisaged according to the mechanical and / or electrical constraints required at the time of assembly, or else according to the constraints related to more or less large spaces available on the motherboard and / or daughter card.

Preferably, at least one of the escape zones is extended by a connection zone covered with varnish. Such a technique in fact makes it possible to prevent the surplus of solder material recovered from spreading more widely over the substrate of the motherboard and / or daughterboard. The invention also relates to a support intended to receive at least one component in an electronic system as described above. Such a support advantageously comprises at least one escape zone designed to receive a possible surplus of material to be welded during the assembly of one or more components on it, according to the BGA technique or other, so as to limit the overall thickness of the electronic system thus obtained.

The invention also relates to a component intended to be mounted on a support in an electronic system such as that described above. Such a component advantageously comprises at least one escape zone designed to receive a possible surplus of material to be welded during the assembly of this component on the aforementioned support, according to the BGA technique or other, so as to limit the overall thickness. of the electronic system thus obtained.

Advantageously, such a component forms a module grouping in the same entity at least two components interconnected with one another.

The invention also relates to a method for producing an electronic system such as that described above, in which at least one component is mounted on a corresponding support, as described above, using a set of elements of material to be welded, according to the BGA technique or the like. In such a process, at least one of the components and / or on the support is provided with at least one escape zone designed to receive a possible surplus of material to be welded, so as to limit the overall thickness of the system obtained.

Preferably, when at least one of the escape zones ends in a hollow housing, this housing is produced using a laser or any other chemical and / or mechanical tool enabling such housing to be produced. hollow. Advantageously, such a method comprises the following steps:

- positioning of the material to be welded on the support or on the component or components;

- heating, so as to melt the material elements;

- evacuation of any excess melted material towards the escape area (s).

Also advantageously, the method further comprises at least two preliminary steps:

- a first step of defining the geometry of the escape zones as a function of possible positioning constraints on the support;

a second step of identifying and preparing the zones which can also serve as a means of electrical connection after evacuation of any surplus molten material.

Other characteristics and advantages of the invention will appear more clearly on reading the following description of a preferred embodiment, given by way of simple illustrative and nonlimiting example, and of the appended drawings, among which:

- Figure 1, described in the preamble, illustrates an electronic module (or component) transferred according to a conventional BGA assembly on a motherboard; - Figures 2.a and 2.b, also described in the preamble, respectively illustrate the steps before and after reflow of the assembly of a component comprising balls of fusible material, according to the BGA assembly technique known from prior art;

- Figures 3 and 4 illustrate an embodiment of the thin BGA assembly according to the invention, respectively before and after reflow.

- Figure 5 shows a top view of an example of BGA assembly areas 51 connected to exhaust means 52 of the reservoir type. The invention therefore relates to the addition, on the printed circuit (motherboard and / or daughter) and / or the electronic component and / or the radiocommunication module, of escape means making it possible to evacuate the possible surplus of material fuse during the reflow phase, so as to minimize the height between the component (or module) and the motherboard.

More specifically, FIG. 3 illustrates, before reflow, an electronic and / or radiocommunication component 31 comprising balls 32 of fusible material, the assembly being ready to be assembled according to the method according to the invention on a motherboard 33 comprising transfer and connection zones 34 connected to escape means 35 of the well type. These wells 35 are further covered with a thin metallic layer 36 giving them the wettability properties necessary for the invention.

FIG. 4 shows, still according to a preferred embodiment of the invention, the same elements as those of FIG. 3 (electronic and / or radiocommunication component 31, motherboard 33), but once assembled according to the method according to the invention. This figure also perfectly highlights the minimum height 43 thus obtained between said component 31 and said motherboard 33, the fusible material of the BGA balls after reflow being then present in an amount simply sufficient to guarantee the properties of the electrical connections between the zones. component 41 and the areas of connection 34 of the motherboard. The figure also illustrates the fact that according to the invention, the surplus 42 of fusible material from the BGA balls is (are) evacuated towards the wells 35 forming escape means.

The general principle of the invention is therefore based on a new and inventive approach consisting in using in a combined way escape means on the motherboard and / or daughter and the capillarity properties of the BGA balls of fusible material on a wettable surface. , to allow the evacuation of a possible surplus of fusible material during the reflow phase, and thus reduce as much as possible the height separating the motherboard and / or daughter and the assembled component, after said reflow phase.

We note that we have planned to associate an escape area with each connection, therefore with each ball. Of course, the same escape zone can be shared, in the case where the balls are connected to the same element (for example, ground or a supply voltage, or reference). The additional advantage of being able to share the same loophole zone between each connection zone is all the more important for certain types of electronic systems requiring the passage of high power signals, in particular of the radiofrequency (RF) type, several BGA balls. can be assigned to the same signal. According to the preferred embodiment of the invention, the escape means are provided on the motherboard and / or daughter and are in the form of simple dead ends intended only for the recovery of the possible surplus of fusible material after reflow.

Other embodiments are also possible for variants of the invention. These other possible embodiments of the invention relate in particular to:

- the geometric shape of said escape zones, in two or three dimensions, belonging to the group comprising:

• wells of predetermined diameter and depth; "grooves of predetermined shape and depth; "the electrical tracks of predetermined shape and length;" the internal cavities made in at least one of said components and / or in said support; "through vias.

the use of the surplus of fusible material contained in said escape means as an electrical connection element and / or other component (s), said escape means then no longer being considered as dead ends , but as fully-fledged elements of the electronic and / or radiocommunication system under consideration.

The invention is based in fact on the fact that the escape zones, whether made in two dimensions (2D) or in three dimensions (3D), are covered with a wettable material, which facilitates reception and guiding of the molten material (the characteristics of the wettable material are of course chosen to facilitate this operation, without involving more material than necessary, so that the electrical connection remains ensured in good conditions).

This approach is therefore opposed to the conventional technique, which provides that the printed circuits, and in particular the tracks, are covered with a varnish on which the solder does not set.

It should also be noted that the two approaches are not incompatible. A track can be essentially varnished, with the exception of a specific portion having a wettable surface according to the invention. This approach combining the concept of conventional runway and loopholes according to the invention is also interesting in the case of vias, which the excess material will fill.

It should also be noted that these loopholes according to the invention can be provided on the printed circuit (motherboard) and / or on the daughterboard, and / or directly on the components or modules.

Claims

1. Electronic system comprising at least one component mounted on a corresponding support using a set of elements of fusible material, characterized in that it comprises on at least one of said components and / or on said support at least an escape zone designed to receive a possible surplus of fusible material, so as to limit the overall thickness of said system. 2. Electronic system according to claim 1, characterized in that the one or more escape zones are covered with a wettable material. 3. Electronic system according to any one of claims 1 and 2, characterized in that at least one of said escape zones defines a track ending in a dead end. 4. Electronic system according to claim 3, characterized in that the said track or tracks terminate on a portion of enlarged section. 5. Electronic system according to any one of claims 1 to 4, characterized in that an escape zone is associated respectively with each of said elements of fusible material. 6. Electronic system according to any one of claims 1 to 5, characterized in that the same escape zone is associated with at least two of said elements of material to be welded, corresponding to electrical connections at the same potential. 7. Electronic system according to any one of claims 1 to 6, characterized in that said elements to be welded and / or said loopholes are designed so that said surplus material is directed towards said loopholes under l effect of the capillarity applied to the said component or components. 8. Electronic system according to any one of claims 1 to 7, characterized in that the one or more escape zones belong to the group comprising: - wells of predetermined diameter and depth; - grooves of predetermined shape and depth; - electrical tracks of predetermined shape and length; - the internal cavities made in at least one of said components and / or in said support; - through vias. 9. Electronic system according to any one of claims 1 to 8, characterized in that at least one of said escape zones is extended by a connection zone covered with varnish. 10. Support provided for receiving at least one component in an electronic system according to any one of claims 1 to 9, characterized in that it comprises at least one escape zone designed to receive a possible surplus of material to be welded during the assembly of one or more components on said support, according to the BGA or other technique, so as to limit the overall thickness of said electronic system. 11. Component intended to be mounted on a support in an electronic system according to any one of claims 1 to 9, characterized in that it comprises at least one escape zone designed to receive a possible surplus of material to be welded during of the assembly of this component on said support, according to the BGA technique or other, so as to limit the overall thickness of said electronic system. 12. Component according to claim 11, characterized in that it forms a module grouping in the same entity at least two components interconnected with each other. 13. Method for producing an electronic system according to any one of claims 1 to 9, according to which at least one component is mounted on a corresponding support using a set of elements of material to be welded, according to the BGA or other technique, characterized in that at least one of said components and / or on said support is provided with at least one escape zone designed to receive a possible surplus of material to be welded, so as to limit the thickness overall of said system. 14. Method according to claim 13, characterized in that when at least one of said escape zones ends in a hollow housing, said housing is produced using a laser or any other chemical and / or mechanical tool enabling such housing to be produced. 15. Method according to claim 13, characterized in that it comprises the following steps: - placing of said elements of material to be welded on said support or on said component or components; - Heating, so as to melt said material elements; evacuation of any surplus molten material towards the said escape area or areas. 16. Method according to any one of claims 13 to 15, characterized in that it further comprises at least two preliminary steps: a first step of defining the geometry of said loopholes according to possible positioning constraints on said support; a second step of identifying and preparing said zones which can also serve as a means of electrical connection after evacuation of any surplus melt.