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1. WO2001075970 - ELEMENT ET PROCEDES DE CONNEXION DE COMPOSANTS D'UN ENSEMBLE ELECTRONIQUE

Note: Texte fondé sur des processus automatiques de reconnaissance optique de caractères. Seule la version PDF a une valeur juridique

[ EN ]

ELEMENT AND METHOD FOR CONNECTING
CONSTITUENTS OF AN ELECTRONIC ASSEMBLY

The invention relates to an element and a method for connecting constituents of an electronic assembly with passive and/or active components such as integrated circuits.

In modern circuit board assemblies, packaging density and operating frequency as well as overall functionality of electronic systems is more and more increased. In these assemblies, heat dissipation becomes a challenge. In particular, the thermal coupling of Integrated Circuits (ICs) to the Printed Circuit Board or High Density Interconnect (PCB/HDI) is very important. To decrease the thermal resistance across an air gap between the IC and the PCB/HDI, special heat conducting and electrically insulating foils or pastes can be put underneath the IC. This technology has various disadvantages.

First, the placement has to be accurate, and this requires special equipment or costly manual handling procedures. Secondly, the thermally conduction material must be an excellent electrical insulator not causing problems with electrical shorts between the narrow spaced connections over the complete time of life. The use of such material together with modern array assemblies is almost not feasible, because the material would also form an insulation layer between the solder balls and the connection pads on the PCB/HDI.

Thermal connections are not only an issue concerning the connection of Integrated Circuits to PCB/HDIs, but also concerning connections between other constituents of a package. They may e.g. be an important subject in if, according to a Separation Of Functions (SOF) philosophy, a possibly flexible PCB or HDI is mounted on a hard element such as a core serving as heat sink.

It is therefore an object of the invention to provide an element and a method for connecting constituents of an assembly with integrated circuits which provide a satisfactory thermal connection between the constituents and overcome drawbacks of prior art thermal connections, and which especially allow the use of array assemblies. The element and the method should further enable an economical and flexible way to connect the constituents.

These objects are achieved by the invention as defined in the claims.

The invention is essentially characterized in that an interface element for connecting constituents of an electronic assembly is provided, which element comprises a layer of dielectric material with a regular array of openings forming through connections, and electrically conducting elements placed in the openings for electrically connecting components arranged on each side of the layer.

The through connections formed by the openings filled with conductor material are generally arranged in a regular pattern in order to make the interface element usable for a variety of different assemblies and different functions within assemblies. The electrically conducting elements are thus arranged within this foil in a given pitch. Accordingly, the interface element may be produced in a family of different versions, which match with the various types of packages. There, however, may be alternative, special cases for which an interface element with a specific structure of openings is fabricated to serve a particular purpose and be brought into line with the conductor pads etc. of the constituents to be connected.

The interface elements are usable for a variety of different packages or assemblies. In addition, identical or similar interface elements can also be used for different interfaces within the same assembly.

A first example of an application concerns the mounting of active and/or passive components on an electrical connecting element (e.g. a PCB/HDI). The interface element according to the invention can be placed between components and the electrical connecting element. For instance, the interface element may be sold already mounted onto the IC chips and does not have to be assembled during the ordinary population of a PCB/HDI substrate.

The concept of the interface element according to the invention used in this way is also well in line with state of the art array packages. Array packages often have standardized pitches of currently e.g. 1,25 mm, 1 mm, 0.8 mm or 0.5 mm.

If the interface element is placed between active or passive components and a PCB/HDI, the alignment of the interface element against the contact pads on the PCB/HDI may be critical. The thus required dimensional stability may be achieved by this interface element e.g. if the dielectric layer is a thin central foil which is perforated in the desired regular array pattern.

A second group of examples of applications of the interface element according to the invention concerns different parts of an electrical connecting element. The interface element may e.g. be placed between a foil (multi-) layer and a rigid substrate core serving as heat sink or between other parts of an electrical connecting element. Special examples of such parts, the implementation of which follows the SOF philosophy are e.g. power supply boards described in an application of the same applicant and based on the same priority application. If the interface element is placed between a rigid substrate core and a foil PCB/HDI, a simultaneous mechanical, electrical and thermal connection is enabled. In such a case, the pitch between the electrically conducting elements can in many cases be relatively large compared to an interface element used to connect active and/or passive components to a PCB/HDI.

The interface element may comprise a flexible and at least slightly compressible dielectric material and thus be an 'interface mat' or a 'Mechanical, Electrical and Thermal Interface Mat (METIM)'.

The interface element should preferably stick to the constituents between which it forms an interface. To this end, one of the following three possibilities may be chosen for the dielectric material:

The dielectric material by be a foil plus an adhesive layer. As foil material, any dielectric material, preferably with high thermal conductivity, can be used.

- The dielectric layer may, as an alternative be made of a thermoplastic foil material which has the property of being itself an adhesive. Examples for such materials comprise the increasingly important Liquid Crystal Polymers (LCPs) with their excellent characteristics including their heat resistance.

The dielectric material may also be a preferably fiber reinforced duroplastic material such as fiber reinforced epoxy resin, a polyimide, etc.. The adhesion is then achieved by curing the material when the assembly is mounted. The curable material is e.g. highly filled with a powder of inorganic material with high thermal conductivity, like Aluminum Oxide, Aluminum Nitride or hexagonal Boron Nitride.

The electrically conducting elements provide for the electrical interconnection and also for a good portion of the thermal extraction by helping to conduct heat across the bonding film. These elements may further act as a stand-off. Placed between an active and/or passive component and an PCB/HDI they may e.g. provide a certain compliance between the component and the PCB/HDI, which helps to reduce stress on the solder joints, built up by a mismatch of the CTE of the component and the board itself. In addition, the resin matrix replaces an under-fill, which is usually necessary to reduce the stress also and to increase the reliability to an acceptable level.

The electrically conducting elements may comprise a solder material surface or be entirely made of solder material. If the interface element is mounted at a somewhat elevated temperature, e.g. a temperature around 150°C-400°C, the solder material therefore may form a reliable electrical connection between the electrically conducting elements and the joints to be contacted. If the solder material sticks to the elements it has to contact electrically it also contributes to the sticking effect of the entire interface element.

The shape of the material defined by the shape of the openings is for instance a ball shape or a cylinder shape. If the openings are cylinder shape, the 'vertical' positioning of the conducting elements, i.e. the dimensioning and positioning in the - 0 -

direction perpendicular to the foil sheet plane, can be adapted to requirements and to the flexibility of dielectric material.

The electrically conducting elements may be glued into the openings by the adhesive material. However, depending on the dielectric and conducting materials and on the shape of the openings, such gluing may be unnecessary.

As already mentioned, the interface element can be fabricated as a standard, off-the-shelf item. For this reason, it can be used for a variety of packages or assemblies. It can therefore be produced in large quantities and is apart from its technical advantages, also a progress from the economical point of view.

Ideally, interface element (or the METIM) can be disconnected from the constituents by a shock cooling process. Its use therefore provides a contribution to the ecologically and economically favorable recycling concept.

In the following, examples of the invention are described with reference to drawings. In the drawings:

- Figure 1 shows a view onto a schematically drawn interface mat,

Figure 2 represents a cross section through part of the embodiment of Figure 1

Figure 3 shows an analogous cross section a part of an other embodimentd of an interface mat, Figures 4a and 4b represent cross sections through variations of the embodiment of figure 3,

Figure 5 very schematically shows the principle of an exemplary fabrication method of an interface element, and

Figure 6 - also schematically - shows a batch of produced interface elements.

The interface element 1 of Figure 1 is a mechanical, electrical, thermal interface mat (METIM). It comprises a dielectric layer 2 and openings within this dielectric layers with electrically conducting elements 3 arranged in a given pitch. The thickness of the layer and thus of the entire interface element is preferably between 15 μm and 200 μm and for instance between 25 μm and 100 μm. The distance between two neighboring electrically conducting elements (the pitch) is preferably between 0.05 mm and 5 mm. If the interface element is to be used for connecting circuit components to a PCB/HDI, the pitch may be adapted to standardized pitches of array packages, currently e.g. 1.25 mm, 1 mm, 0.8 mm, and 0.5 mm, in the future probably less. The pitch generally may be adapted to the function of the interface element. As an example, if the interface element is to be placed between components of an electrical connecting element, i.e. between a hard core and a foil like HDI substrate or between other components of the same connecting element, the pitch may be chosen to be bigger. As an example, it may then be in the range of 0.5 - 5 mm, e.g. 2 mm.

Figure 2 represents a schematic cross section through the embodiment of the interface element of Fig. 1. The electrically conducting elements according to this embodiment are solder balls comprising a copper core 5 and a solder outer covering - o -

7. Due to the copper, the thermal and electrical conductivity of the solder balls, and thus the thermal conductivity of the entire interface element, are enhanced. The interface element - or interface mat - comprises an alignment core 9 being a thin central foil which is perforated in the desired array pattern. The alignment core 9 is for instance a foil of curable inorganic material, like epoxy resin, polyimide, etc. or of a thermoplastic material. The material may be filled with a powder of inorganic material with high thermal conductivity such as aluminum oxide, aluminum nitride or hexagonal boron nitride. The alignment core 9 is on both sides covered by a layer 11 of a curable adhesive. The curable adhesive serves for fixing the constituents to be connected to the mat, but also to fix the electrically conducting elements.

In Figure 3, a cross section through a further embodiment of the interface element is shown. The interface mat 21 comprises a layer 22 of a dielectric material which is perforated in a pattern of cylindrical openings. The material of the layer 22 may be a curable organic material, possibly filled with inorganic material with a high thermal conductivity. The curable organic material serves as alignment core and as adhesive for detachably fixing the constituents to each other at the same time. As an alternative, the layer material may also be a thermoplastic material, e.g. a liquid crystal polymer (LCP), or possibly a mixture of electrically insulating materials comprising a thermoplastic component. The properties of LCPs, as examples, makes this group of materials highly for being used for an interface mat. These materials, upon being heated, tend to be slightly adhesive, so that the use of a separate adhesive layer is not required. In addition, these materials are slightly elastically deformable, so that, if the constituents are pressed into the mat, a mechanical connection is created. Also other properties of the LCPs (heat conductivity, heat resistance, etc.) make them highly suited for the purpose. The electrically conducting elements 23 are e.g. copper columns which may on both faces be covered with a solder layer 24. They, however, may be made of any other electrically conducting material or combination of electrically conducting materials, e.g. they may be entirely made of solder material.

In Figures 4a and 4b, variations of the dimensioning and positioning of the cylindrical electrically conducting elements 23', 23" in the direction perpendicular to the foil sheet plane are shown. The column 23' being a. copper columns with solder coverings 24' protrudes on one side from the layer surface. If a constituent is pressed onto the mat, the column is displaced, deformed and/ore fused with an element to be contacted. The column 23" has a height which is slightly less than the layer thickness. It contacts the elements to be contacted only when the mat is compressed. The set-up of Figure 4b is preferably chosen if the column is relatively hard and not deformable, e.g. if it is a copper column.

Of course, also combinations of the above set-ups or different set-ups can be used. Especially, also electrically connecting element shapes that differ from the above described shapes can be used, e.g. elliptical or truncated elliptical shapes.

With reference to Figures 5 and 6, features of a possible manufacturing process of one embodiment of the invention are shortly described. The device 41 for manufacturing interface elements comprises a cylindrical drum 43. The drum comprises a surface of an acid resistant polished material, e.g. of a surface of polished high-grade steel. During the manufacturing process the drum 43 is partially immersed in an electrolyte liquid 44. The device further comprises a copper anode 45 which in the figure is only schematically shown. Power supply means 47 serve for generating a plating voltage between the anode and the drum 43. A flexible dielectric layer 49 slightly sticking to the drum 43, namely a LCP layer or a layer of an other appropriate polymer material, can be guided and transported by rotating the drum in the direction indicated by the arrow. The layer comprises a regular pattern of cylindrical openings 51. If a voltage is applied between anode and drum, the cylindrical openings are slowly filled with copper, forming copper columns as electrically conducting elements. Due to the polished drum surface, the dielectric layer may easily be removed from the drum. The copper columns in the more or less completely filled holes may in a subsequent step be provided with solder coverings.

As can be seen in Figure 6, the LCP layer 49 comprises preferably a pattern of a plurality of substrates 53 for interface elements, each substrate comprising a pattern of openings 51. By finally separating the different substrates from each other, a plurality of interface mats can be produced in one step.