FR2867013A1 - Electronic module, e.g. electronic radiocommunication module, fabricating method e.g. for mobile telephone, involves forming fusible pads by placing set of solder balls on side of medium, where pads permit to flatten module on motherboard - Google Patents

Abstract

Electronic module fabricating method involves assembling an electronic component on a side of a printed circuit medium (4). Fusible pads (12) are formed by placing a set of solder balls on lands of another side (8) of the medium. The set of balls are fused and regrouped for forming the pads. The pads permit to flatten an electronic module on a motherboard by wicking at the level of the pads during assembling the module on the motherboard. Independent claims are also included for: (A) A method of assembling an electronic module (B) A motherboard of an electronic device.

Description

METHODS OF MANUFACTURING AND MOUNTING A MODULE

  ELECTRONIC APE TO BE MOUNTED ON A MOTHERBOARD, ASSOCIATED MOTHERBOARD.

GENERAL TECHNICAL FIELD.

  The present invention relates to a method of manufacturing and mounting an electronic module.

  More specifically, it relates to a method of manufacturing and mounting an electronic module providing RF (radio frequency), analog and digital processing functions, the module being intended for example for mobile terminal applications, modems or any electronic device. requiring these same functions.

  The invention also relates to a motherboard comprising such a module.

STATE OF THE ART.

  Current electronic modules used in mobile telephony generally consist of an epoxy card on which are reported surface mounted components covered with metal shields.

  In most cases, the components overlap both sides of the card.

  A large part of the surface is occupied for the connection of analog, digital and radio links (including the antenna).

  The decrease in the size of the terminals goes in the direction of an increasingly important integration of these functions.

  The evolution of the electronic architecture, in particular the integration of specific integrated circuits (or Application Specific Integrated Circuits (ASIC) according to the English terminology generally used by those skilled in the art - and the reduction in the number of components , allows to reduce the volumes of the modules.

  Although the dimensions of the modules significantly reduce, the number of connections continues to increase.

  There are now fine-pitch connectors that make it possible to ensure the electrical connections, but which appear much too bulky in surface and in thickness with respect to the surface occupied by the electronics.

  In addition, given the fineness of the step (0.4 mm), they remain very fragile and difficult to carry on the cards.

  Some electronic assemblies of the multi-chip module type - or MultiChip Modules (MCM) according to the English terminology - use the ball-type transfer principle - or Bali Grid Array (BGA) to obtain a gain on the size of the module. module.

  In these assemblies, the electronic components forming the chips are reported on a solder ball fused circuit board, and all the connections are placed under the module without increasing the surface area of the board.

  However, the epoxy materials used in the telecommunications field are inexpensive and the flatness defects of the equipped cards are important.

  Thus, the MCM modules also have non-fusible studs, balls or columns, which certainly give the assembly greater contact stiffness, but which do not make it possible to compensate for flatness defects induced by the printed circuit board material. epoxy. The studs, balls or columns do not completely compensate for the deformations of the components in the presence.

  The strength of the module can be mechanically reinforced when it is fixed on a motherboard by adding an adhesive at the periphery of the or under the component. This generates additional production costs and control of an additional manufacturing process. In addition, the module is not easily removable from the card in case of failure.

  Another method to improve flatness is to replace the epoxy printed circuit board with a ceramic printed circuit board.

  Although the lack of flatness is solved, the counterpart is the extra cost 30 material but also the significant coefficient of differential expansion with an epoxy motherboard for example.

  There are currently no very integrated, highly reliable and low cost macro components on the market.

PRESENTATION OF THE INVENTION

  The invention proposes to overcome at least one of the aforementioned drawbacks. One of the aims of the invention is to propose a method of manufacturing an electronic module that makes it possible to mount electronic components of the module on an inexpensive carrier.

  Another aim of the invention is to propose a method of manufacturing an electronic module that makes it possible to overcome the problems of flatness of the support of the electronic components.

  Another object of the invention is to provide a method of manufacturing an electronic module that allows an interconnection of electronic components that is both industrially reliable and highly integrated in surface and thickness.

  Another object of the invention is to provide a method of manufacturing an electronic module that allows a large margin of safety during the transfer of electronic components and ensure a plating of the module on a motherboard.

  For this purpose, the invention provides a method of manufacturing an electronic module, comprising the steps of: - surface mounting at least one electronic component on only one first face of a printed circuit support; and - arranging on a second face of the printed circuit support a matrix of solder balls capable of forming electrical shifts, so that the support forms a ball housing adapted to be mounted on a motherboard, characterized in that comprises a blasting step consisting of forming fusible reinforcing pads by arranging a set of solder balls on reception pads of the second face of the support to enhance the mechanical strength of the support, a step of melting and grouping said assembly beads to form said pads, the solder balls of the matrix remaining in individual form, said pads being able to press the module on the motherboard by capillarity at said pads when mounting the module on the motherboard.

  The invention is advantageously completed by the following steps, taken alone or in any of their technically possible combination, the steps of: - arranging the set of beads capable of forming the reinforcing pads 5 simultaneously with the provisioning step other solder balls of the matrix; - Have, on each reception range of each stud, a set of balls whose total volume before merging to form the pads, is between 2/3 and 3/3 of the total volume of the right prism based on the reception range of said pad and for height the distance between the second face of the support and the motherboard once the module mounted on the motherboard; placing the reinforcing fuse blocks on the periphery of the second face of the support in order to reinforce the mechanical strength of the support; - Lay the fusible reinforcing pads in corners of the support; - Have on the second side of the support solder balls in excess relative to the electrical reports necessary for the operation of each electronic component, to enhance the mechanical strength of the support; define each reception range inside an unvarnished zone on the second face of the support and in which the balls capable of forming the reinforcing pads are arranged.

  The invention also relates to a method of mounting a module obtained by a method according to the invention.

  In particular, the invention proposes a method of mounting an electronic module having at least one electronic component at the surface on only one first face of a printed circuit support and comprising, on a second face of the support, solder balls capable of forming electrical transfers in matrix and fusible reinforcing pads, said pads being arranged by a manufacturing method according to the invention, the module being mounted on a motherboard, characterized in that it comprises a step consisting in: - bringing in reflowing said reinforcing pads; - Reflow the balls of the matrix, said balls remaining in the form of individual beads, - plate and position the module on the motherboard, the plating being obtained by attractive capillarity forces at said pads, and the positioning at a nominal distance from the motherboard being obtained by repulsive capillary forces at the balls of the matrix.

  The invention also relates to a motherboard comprising such a module.

PRESENTATION OF THE FIGURES.

  Other features, objects and advantages of the invention will emerge from the description which follows, which is purely illustrative and nonlimiting, and which should be read with reference to the accompanying drawings in which: - Figure 1 shows schematically a perspective view from above a module obtained by a possible method according to the invention; Figures 2 and 3 show schematically a bottom perspective view of an advantageous embodiment of a module obtained by a possible method according to the invention; FIG. 4 schematically represents a profile view of an advantageous embodiment of a module obtained by a possible method according to the invention; - Figure 5 shows a detail of a range of bead reception capable of forming reinforcing pads; and - Figure 6 schematically shows the forces present during assembly of a module according to the invention on a motherboard.

DETAILED DESCRIPTION.

  A manufacturing method according to the invention of a module makes it possible to meet the requirements of integration, reliability and low cost.

  FIG. 1 schematically represents a perspective view of a module 1 obtained by a possible method according to the invention.

  According to one possible method, at least one electronic component 2 is mounted on the surface at only a first face 3 of a printed circuit support 4.

  The Young's modulus of the support 4 is for example less than 150 GPa.

  In a very preferential manner, the printed circuit support 4 is made of epoxy material.

  It thus comprises a flexural strength, in the longitudinal direction, of between 400 and 460 GPa, with typical values between 415 and 442 GPa. It has a flexural strength in the radial direction of between 300 and 460 GPa, with typical values between 345 and 435 GPa.

  For example, the epoxy printed circuit support 4 is of a thickness of 1 mm, for example with eight superimposed layers of conductors.

  The layers 6 are conventionally interconnected by metallized through holes, buried and vias laser drilled.

  It is recalled that the epoxy supports 4 generally comprise numerous flatness defects. For example, the standard flatness defect on an epoxy card is of the order of 0.7%, ie 0.35 mm for a 50 mm diagonal component.

  Of course, the module advantageously comprises more than one component 2.

  The module obtained by a possible method according to the invention is advantageously but not limitatively an electronic radiocommunication module.

  For this purpose, it integrates all the means 7 capable of performing the analog and digital radio functions of a multiband mobile, in particular according to the protocols of the global system for mobile communications (GSM), and / or the DCS system (Digital Communication System ), and / or PCS Personal Communications Services.

  All the means 2 and 7 are surface-mounted on the upper face 3 of the support 4, and only on the upper face 3.

  The means 2 and 7 are advantageously mounted on the face 3 of the support 4 by a transfer by ball housing - or Bali Grid Array (BGA) according to the English terminology - known to those skilled in the art.

  The module 1 also comprises a shield consisting of a welded belt and a cover, not shown in the figures.

  As shown in FIG. 2, according to a possible method of the invention, a matrix 9 of solder balls 10 is provided on a second face 8 of the printed circuit support 4 capable of forming electrical reports for the components 2 and 7, so that the support 4 forms a ball housing adapted to be mounted on a motherboard.

  It is recalled that the ball bearing or BGA transfer solution makes it possible to place all the connections under the module 1, namely on the lower face 8, opposite to the face 3, without increasing the surface area of the module 1.

  One of the functions of the balls 10 is to ensure electrical continuity between the components 2 and 7 on the one hand, and a motherboard on the other. Thus, the number of balls 10 is a function of the number of inputs / outputs defined by the circuit diagram of the module.

  It is also understood that one of the other functions of the balls 10 is to provide the transfer link with a motherboard. The process of transfer by BGA on a motherboard is very reliable, and the extra thickness given by the balls after reflow for attachment to a motherboard remains acceptable for applications to the mobile phone, for example.

  The fusible beads 10 crash under the effect of the weight of the module 1 during the passage in the oven. This physical phenomenon allows a report of module 1 on all types of motherboards, thick or thin.

  In addition, the balls 10 make it possible to ensure the mechanical strength of the support 4.

  The module 1 is advantageously of the multi-chip or MCM type. It has the distinction of having a large size and a weight higher than conventional BGA components. It is indeed for example a rectangular dimension 33 mm x 38 mm. It has a thickness of 3 mm overall, that is to say without the balls 10, and its mass is about 9 g fully equipped.

  In addition, the number of functional connections is greater than 160.

  It is therefore necessary to adapt the connection system. The number of balls 10 and the pitch 11 between the balls 10 is thus also chosen as a function of the weight and the surface of the support 4, and not only as a function of the number of inputs / outputs defined by the electrical diagram of the module 1.

  On the second face 8 of the support 4, there are then solder balls 10 which are larger than the electrical ratios necessary for the operation of each electronic component 2 or 7, to reinforce the mechanical strength of the support 4.

  Thus, in addition to the electrical connection, the balls make it possible to obtain a high mechanical strength.

  A diameter of balls 10 is also provided sufficient to absorb the flatness defects of the support 4.

  Indeed, the load per ball 10 varies depending on its diameter. The larger its diameter, the higher the permissible load.

  The residual distance between the support 4 and a motherboard after assembly is obviously a function of the diameter of the balls. This factor can be taken into account to optimize the thickness of the product.

  The number and the diameter of the balls 10 are therefore the result of a compromise.

  In view of the above elements, an exemplary embodiment of a module according to the invention is the arrangement of 442 balls of diameter 635 pm in the free state and distributed over a matrix at pitch 11 of 1.27 mm.

  The balls used are fusible and preformed balls compatible with lead-free and lead-free technologies. For example, the balls 10 are one formed of a tin / lead alloy.

  The method of manufacturing the blasting of the module can be for example by screen printing or pick in place.

  Whatever the solution adopted, the balls 10 will be deposited on the support 4 on reception areas not shown in the figures, and previously coated flux.

  As shown in Figures 2 and 3, the matrix 9 of balls 10 is grouped in the center of the support 4, in the area of least stress.

  However, given the size of the module 1, the balls 10 installed at the periphery of the die 9 are highly stressed and can break shear stresses. On the type of module 1 targeted by the invention, the zones of weakness are in the corners outside the circle 13 of reliability.

  This phenomenon appears in particular when the balls 10 are too small, poorly positioned on the module 1, and even when the mechanical rigidity of the assembly receiving the module 1 is insufficient.

  It is for this reason that a step is advantageously provided to have the reinforcing pads 12 on the periphery of the second face 8 of the support 4, to reinforce the mechanical strength of the support and to protect the connection of the balls 10 The studs 12 are in particular visible in FIGS.

  In the case of a support 4 of rectangular shape, the pads 12 of mechanical reinforcement are preferably placed in the corners of the support 4. The module thus comprises four studs 12.

  The pads 12 have several functions and advantages.

  First, they reinforce the mechanical strength of module 1 after transfer to a motherboard.

  Then, they increase the circle 13 of reliability so as to protect the connection of the balls 10.

  In addition, they ensure the mechanical contact regardless of the tolerances of the diameter of the balls 10 and the unevenness of the module 1.

  Finally, they delete the sticking operation of module 1 on a motherboard. The module 1 is easily removable from a motherboard by reflow of the pads 12 for a troubleshooting operation for example.

  As shown in Figure 4, the pads 12 have a particular shape and are at the same height 13 as the solder balls 10. There is no problem caused by a difference in height during the report of module 1 on a motherboard.

  According to a particular step of a method according to the invention, the reinforcing fuse pads 12 are formed by means of a set of solder balls 14 shown in FIG. 5.

  According to a preferred implementation of the method, the balls 14 are arranged capable of forming the reinforcing pads 12 simultaneously with the step of disposing of the other solder balls 10.

  Preferably also, the step of producing the reception range 5 15 of the balls 14 of each pad 12 of reinforcement is identical and performed simultaneously with the method of producing the reception range of the fusible balls 10 of the matrix 9.

  Advantageously, the fusible balls 14 of the pads 12 are identical to the fusible balls 10.

  Advantageously, the transfer of the fusible beads 14 is performed by screen printing in the reception area 15 surrounded by a varnish 16 of savings.

  There is no additional material input by a preform for example, and therefore no additional cost generated by the realization of the pads 12 of reinforcement outside the material price of the balls 14, which is negligible in front of the total cost of module 1.

  The number of balls 14 needed to form a reinforcing pad 12 depends on the free height of each ball 10 after transfer.

  The density and the position of the balls 14 are a function of the production parameters of the ball depositing stencil 14.

  A number of balls 14 is thus calculated in order to obtain a height of the stud 12 equal to the height of the ball 10 reported.

  Given the high volume of solder to be provided on the stud 12, the number of balls 14 is important. The placement step 21 of the balls 14 on the reception range 15 is minimized.

  As shown in Figure 5, small protuberances 22 indentation in the varnish 16 at the periphery of the beach 15 and at the corners to position the number of balls 14 sufficient.

  During soldering, the balls 14 are reflowed and by capillary effect are grouped in the center of the range 15 so as to create the volume of the pad 12 and the desired solder height.

  In our example, the host range of the pad is 4.83 mm squared. 49 beads 14, which have a diameter of 635 μm, are placed on a matrix at a pitch of 890 μm.

  The reception area 15 may, however, be of any shape.

  During the transfer of the module 1 on a motherboard, the balls 10 and the pads 12 are reflowed. Surface tension forces are then exerted on the solders composed of the balls 10 and the pads 12. The surface tension forces maintain the position of the module 1 until cooling and solidification of the solders.

  As shown in FIG. 6, there is a balance between the forces exerted on the balls 10, on the studs 12 and the module 1.

  The balls 10 are put under compression under the action of the weight 190 of the module 1 and they exert a force 17 of repulsion capillarity of the module. The reinforcing pads 12 by their shape, in turn, exert a force 18 of capillarity of attraction of the module. The studs 12 thus promote the physical connection of the balls 10 melting on the home ranges 19 of the motherboard 20, while the balls 10 provides a nominal distance between the module 1 and the motherboard 20. This phenomenon of forces of Attractive capillarity at the level of the studs 12 appears because the solder volume present on the pads 12 pads is such that after postponement, the brazed joint has a hyperboloid shape that creates attraction forces 18.

  The plating forces 18 are preponderant and greatly contribute to the quality of the mechanical connection of the module 1 on its motherboard 20, in the sense that the balls 10 are highly compressed.

  This ensures in all cases the contact between the balls 10 and the home ranges 19 of the motherboard 20, even with a significant flatness of printed circuits. The flatness defects of the support 4 are naturally compensated by this phenomenon.

  So that the balls 14 of the pads 12 merge and create attractive forces between the module 1 and the motherboard 20, it is necessary that the walls of the pads have a concave hyperboloid shape. In this case, the capillary forces are attractive. The pad wets the welcome area 15.

  The volume of the balls 14 intended to form a stud 12 must not be too small, in which case the balls 14 can not merge with each other by establishing a bridge between the motherboard 20 and the module 1, nor too large, in which case the walls of the stud 12 are curved and the forces are repulsive and no longer attractive.

  The total volume of the balls 14 placed on a reception range must be between 2/3 and 3/3 of the volume of the right prism resting on the reception area 15 and delimited by the motherboard 20 and the support 4 after postponement.

  For example, if spherical beads with a diameter of 635 μm are placed on the range, a total volume of 6.56 mm 3 is obtained.

  The volume of the right prism based on the reception range 15 and 10 delimited by the motherboard 20 and the support 4 (base 4.84 mm side for a height of 350 pm) is 8.17 mm3.

  The pad 12 after brazing has a hyperbola shape, whose volume can be approximated to twice the volume of a pyramid whose base has a surface of the range 15 - a square of 4.83 mm side - and a height 350 pm - the distance between module and motherboard after transfer.

  The volume for a pyramid is 2.72 mm 3 (S x H x 1/3), ie 5.44 mm 3 for the pad 12 approximated to the two pyramids, ie 2/3 of the volume of the right prism resting on the reception area 15 and delimited by the motherboard 20 and support 4.

  The volume of the stud 12 constituted by the balls 14 (6.56 mm 3) is greater than the volume of the two pyramids.

  Thus, the volume 12 of the stud is well between 2/3 and 3/3 of the volume of the right prism based on the reception area 15 and defined by the motherboard 20 and the support 4.

  Indeed, 2 / 3x (8.17 mm3) = 5.44 mm3 <6.56 mm3 8.17 mm3).

  It will be recalled that the standard flatness defect on an epoxy card is of the order of 0.7%, ie 0.35 mm for a 50 mm diagonal component.

  A ball 10 of 635 μm in free diameter has a height of 550 μm carried on the support 4 of the module 1. After transfer to the motherboard 20, the height of the ball 10 is 350 μm, ie a compression 0.2 mm.

  The deposition of solder cream is of the order of 150 pm for this type of module.

  The sum of solder paste thickness and bead deformation gives 0.35 mm, which corresponds to the flatness tolerance typical of large series printed circuits.

  However, if the pads 12 of reinforcement are not present, the compression of the balls is reduced to 0.1 mm. In this case, the mechanical connection of the balls 10 with the motherboard 20 may not be ensured.

  It would then be necessary in this case to define a flatness tolerance of the support 4 of the module 1 and the motherboard 20 lower - of the order of 0.5% - with the consequence of an increase in costs.

  It is also recalled that the pads 12 described in the present description are preferably square (for example 4.838 mm × 4.838 mm). They can, however, have various geometric shapes of the rectangle, round or any other shape.

  As the plating forces 18 are induced by the surface tension and capillarity forces present in the solders in the liquid state, it will be relatively easy to control and modify them by changing the dimensions or shapes of the pads 12.

  Reinforcing pads may also be positioned in the center of the matrix 9 of balls 10. In this case, they will mainly have a function of plating the module and not mechanical reinforcement, since the areas of weakness are peripheral.

  The postponement of module 1 on a motherboard 20 is performed on conventional laying machines thus allowing great control of the manufacturing processes.

  Soldering in a conventional reflow oven makes it possible to remel the balls 10 and the fusible pads 12 thus creating a physical connection of the module 1 on the card 20 visible in FIG. 6.

  The shape and the realization of the reception areas 19 and ground plane of the motherboard 20 are similar to those of the module 1.

  Reception ranges of the same dimensions are recommended to obtain an equivalent mechanical resistance on the module 1 and on the card 20.

  A sighting system makes it possible to install the module 1 with a precision identical to that of an electronic component mounted on a traditional surface.

  The module is easily removable from the motherboard 20 by melting the pads 12 and balls 10.

Claims (10)

CLAIMS.   A method of manufacturing an electronic module (1), comprising the steps of: - surface mounting at least one electronic component (2) on only a first face (3) of a support (4) of printed circuit ; and - arranging on a second face (8) of the printed circuit support (4) a matrix (9) of solder balls (10) capable of forming electrical shifts, so that the support (4) forms a ball casing adapted to be mounted on a motherboard (20), characterized in that it comprises - a blasting step of forming fusible pads (12) of reinforcement by arranging a set of solder balls (14) on the pads of receiving (15) the second face (8) of the support (4) to enhance the mechanical strength of the support (4), - a step of melting and grouping said set of balls (14) to form said pads (12) , the solder balls (10) of the matrix (9) remaining in individual form, said studs (12) being able to press the module (1) on the motherboard (20) by capillarity at said pads (12) when mounting the module (1) on the motherboard.   2. Method according to claim 1, characterized in that it comprises a step consisting in arranging the set of balls (14) capable of forming the pads (12) of reinforcement simultaneously with the step of arranging the other balls (10). ) of the matrix (9).   3. Method according to one of claims 1 or 2, characterized in that it comprises a step consisting in arranging, on each reception range (15) of each stud (12), a set of balls (14) of which the total volume, before their melting to form the studs (12), is between 2/3 and 3/3 of the total volume of the right prism based on the reception range (15) of said stud (12) and for height the distance between the second face (8) of the support (4) and the motherboard (20) once the module (1) mounted on the motherboard.   4. Method according to one of claims 1 to 3, characterized in that it comprises a step of arranging the pads (12) reinforcing fuses on the periphery of the second face (8) of the support (4) to strengthen the mechanical strength of the support (4).   5. Method according to claim 4, characterized in that it comprises a step of arranging the fusible pads (12) of reinforcement in the corners of the support (4).   6. Method according to one of claims 1 to 5, characterized in that it comprises a step of arranging on the second face (8) of the support (4) balls (10) solder in excess compared to reports electrical components necessary for the operation of each electronic component (2, 7), to enhance the mechanical strength of the support (4).   7. Method according to one of claims 1 to 6, characterized in that it comprises a step of defining each range (15) home within an unvarnished area on the second side (8) of the support (4) and in which the balls (14) are arranged able to form the pads (12) of reinforcement.   8. A method of mounting an electronic module (1) having on the surface at least one electronic component (2) on only one first face (3) of a support (4) of printed circuit and having on a second face (8). ) of the support of the solder balls (10, 14) capable of forming electrical matrix bearings (9) and reinforcing fuse blocks (12), said pads (12) being arranged by a method according to one of claims 1 at 7, the module (1) being mounted on a motherboard (20), characterized in that it comprises a step consisting in: - remelting said reinforcing pads (12); - Reflow the balls (10) of the matrix (9), said balls (10) remaining in the form of individual balls, - plate and position the module (1) on the motherboard (20), the plating being obtained by attractive capillary forces at said pads (12), and the positioning at a nominal distance of the mother board (20) being obtained by repulsive capillary forces at the balls (10) of the matrix (9).   9. Motherboard (20) of an electronic device comprising at least one electronic module (1) having on the surface at least one electronic component (2) on only one first face (3) of a support (4) of printed circuit and welding balls (10) and reinforcing pads (12), characterized in that the module (1) is mounted on the motherboard (20) according to claim 8.   10. Motherboard (20) according to claim 9, characterized in that the pads (12) comprise a hyperboloid form.