Patent Publication Number: US-11664475-B2

Title: Electronic device comprising a carrier substrate and an encapsulating cover mounted on the carrier substrate, and corresponding mounting process

Description:
PRIORITY CLAIM 
     This application claims the priority benefit of French Application for Patent No. 1912566, filed on Nov. 8, 2019, the content of which is hereby incorporated by reference in its entirety to the maximum extent allowable by law. 
     TECHNICAL FIELD 
     Embodiments and modes of implementation relate to microelectronics and, in particular, to the field of electronic packages comprising an encapsulating cover mounted on a carrier substrate. 
     BACKGROUND 
     A carrier substrate is typically used to support and connect an electronic chip and the encapsulating cover for enclosing the chip, and to hold a handleable device protected from exterior conditions. 
     The mounting of the encapsulating cover on the carrier substrate conventionally comprises placing an adhesive bead, for example on the periphery of the carrier substrate, and securely fastening the cover and the substrate by contact with the adhesive bead. 
     The thickness occupied by the adhesive bead (or BLT for Bound Line Thickness) may vary slightly from one embodiment to the next and the BLT thickness is limited in size in order to maintain a given precision in the vertical positioning of the cover on the substrate. For example, to maintain a precision in the vertical positioning of plus or minus 10 μm (microns), the BLT thickness of the adhesive bead is limited to 15 μm. 
     This being so, it may be desirable to benefit from a larger adhesive-bead thickness without decreasing the precision of the vertical positioning, and/or to benefit from a higher precision. 
     SUMMARY 
     According to one aspect, an electronic device is proposed, said device comprising a carrier substrate configured to carry at least one electronic chip and having a mounting front face and an encapsulating cover mounted on said front face of the carrier substrate through a mounting comprising at least one seating surface through which said cover and said substrate make contact, and at least one adhesive bead located elsewhere than the seating surface in order to securely fasten said cover and said carrier substrate. 
     Therefore, the precision of the vertical positioning is defined by the contact of the substrate and of the cover through the seating surfaces, and is not dependent on the penetration of the cover into the thickness of the adhesive bead. Thus, it is possible to provide a larger thickness of BLT adhesive bead, for example 30 μm, while benefiting from a very high precision in the vertical positioning, for example one higher than 10 μm. 
     According to one embodiment, the cover has at least one through-passage housing an optical system facing the through-passage. 
     Thus, the optical system may benefit from a better vertical precision and have needs in terms of precision in vertical positioning that are not constrained by the thickness of the BLT adhesive bead. 
     Therefore, the optical system may be more complex in order to achieve a higher performance than conventional systems, and, according to one embodiment, said at least one chip is mounted on said mounting front face and is housed in a chamber located in the cover and said optical system is configured to modify the properties of an incident light signal coming from outside the cover and directed towards a surface of said chip, the configuration of the optical system, i.e., the way in which it modifies the properties of the incident signal, varying as a function of the distance separating the surface of the chip from the optical system. 
     According to one embodiment, said at least one seating surface is located set back towards the interior with respect to an exterior lateral border of the carrier substrate and/or with respect to an exterior lateral border of the encapsulating cover, so that the adhesive bead fills a space located between the exterior lateral borders and the seating surface. 
     In other words, the adhesive bead is placed beforehand on the side of the seating surface in order to not be imprisoned between the portions making contact of the cover and of the carrier substrate. This allows a correspondingly higher precision. 
     According to one embodiment, said encapsulating cover has a rear edge to which said adhesive bead is fastened, and comprises at least one seating foot having a rear surface that protrudes from said rear edge, said seating surface comprising the rear surface of the seating foot making contact with the front face of the carrier substrate. 
     According to another embodiment, said mounting front face of the carrier substrate comprises at least one seating base having a front surface that protrudes from said mounting front face, said seating surface comprising the front surface of said seating base making contact with the encapsulating cover. 
     According to one embodiment, the device comprises a light-emitting electronic chip configured to emit a light signal and a light-receiving electronic chip configured to detect an incident light signal, the two chips being mounted on the mounting front face and housed in at least one chamber of the cover, the light-emitting chip and the light-receiving chip being configured to interact so as to measure a distance by time-of-flight of the light signal emitted then incident after a reflection. 
     Measurement by time-of-flight is a term used to designate the measurement of the time passed between the emission of a light signal and the reception of this signal after reflection or scatter of the signal from a distant surface. Multiplying the speed of light by this time returns the distance between the distant surface and the zone of the emission/reception. 
     According to another aspect, a process for fabricating an electronic device comprises mounting an encapsulating cover on a carrier substrate configured to carry at least one electronic chip and having a mounting front face, the mounting comprising bringing the cover into contact with the carrier substrate through at least one seating surface, and securely fastening the cover to the carrier substrate by means of at least one adhesive bead placed beforehand elsewhere than the seating surface. 
     According to one mode of implementation, the process further comprises the step of providing at least one through-passage in the cover and of housing, in the cover, an optical system facing the through-passage, before said mounting. 
     According to one mode of implementation, the process further comprises mounting said at least one chip on the mounting front face and housing said at least one chip in a chamber provided in the cover, said optical system being configured to modify the properties of an incident light signal coming from outside the cover and directed towards a surface of the chip in a way dependent on the distance separating the surface of the chip from the optical system. 
     According to one mode of implementation, the adhesive bead is placed so as to fill a space located between said at least one seating surface, which is located set back towards the interior with respect to an exterior lateral border of the carrier substrate and/or with respect to an exterior lateral border of the encapsulating cover. 
     According to one mode of implementation, the method further comprises producing, in said encapsulating cover, at least one seating foot having a rear surface that protrudes from a rear edge of said cover, said at least one seating surface, through which the cover is brought into contact with the carrier substrate, comprising said rear surface of the seating foot, and placing said adhesive bead on said rear edge of the cover. 
     According to one mode of implementation, the process further comprises producing, in the carrier substrate, at least one seating base having a front surface that protrudes from said mounting front face, said at least one seating surface, through which the cover is brought into contact with the carrier substrate, comprising said front surface of the seating base, and placing the adhesive bead on a portion of said mounting front face of the carrier substrate. 
     According to one mode of implementation, the process further comprises mounting a light-emitting chip configured to emit a light signal and a light-receiving chip configured to detect an incident light signal on the mounting front face and in at least one chamber of the cover, the light-emitting chip and the light-receiving chip being configured to interact so as to measure a distance by time-of-flight of the light signal emitted then incident after a reflection. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Other advantages and features of the invention will become apparent on examining the detailed description of completely non-limiting embodiments and modes of implementation and the appended drawings, in which: 
         FIG.  1    is a cross-sectional view of an example of an embodiment of an electronic device; 
         FIG.  2    shows a view of the bottom of an encapsulating cover; 
         FIG.  3    is a cross-sectional view of another example embodiment of an electronic device; 
         FIG.  4    shows a top view of a carrier substrate; and 
         FIG.  5    illustrates an example of an application of a mounting of an encapsulating cover on a carrier substrate. 
     
    
    
     DETAILED DESCRIPTION 
       FIG.  1    illustrates a cross-sectional view of an example of an embodiment of an electronic device DIS comprising a carrier substrate  100  having a mounting front face  101 , and an encapsulating cover  200  mounted on the mounting front face  101 . 
     The carrier substrate  100  is configured to carry at least one electronic integrated circuit (IC) chip  50  and, for example, to make electrical connections between terminals of the chip  50  and the exterior of the device DIS. 
     The chip  50  is mounted on the mounting front face  101  of the substrate  100 . The encapsulating cover  200  comprises a chamber  250  defining a free space for accommodating the chip  50 . 
     For example, the cover  200  has at least one through-passage  280  between the exterior of the cover  200  and the chamber  250 , and houses an optical system  285  facing the through-passage  280 . 
     The through-passage  280  of the cover  200  and the optical system  285  are aligned along a vertical optical axis, i.e., perpendicular to the mounting front face  101 , and centered on a surface  51  of the chip  50 , for example a photosensitive surface. 
     The optical system  285  may be configured to modify the properties of an incident light signal coming from outside the cover  200  and directed towards the photosensitive surface  51 , for example in order to focus the light on the surface  51  and to filter certain wavelengths. 
     As will become apparent below, the mounting of the cover  200  and of the substrate  100  allows the optical system  285  to be designed so that its effect depends on the distance Z separating the photosensitive surface  51  from the optical system  285 , in the direction of the optical axis. 
     Specifically, the cover  200  is mounted on the front face  101  of the carrier substrate  100  during a mounting phase offering a very high vertical precision. 
     The mounting phase comprises bringing the cover  200  into contact with the carrier substrate  100  through at least one seating surface  300 , and securely fastening the cover to the carrier substrate by means of at least one adhesive bead  150  placed beforehand elsewhere than on the seating surface  300 . 
     Thus, the thickness of the adhesive bead does not define the distance Z between the surface  51  of the chip  50  and the optical system  285 . 
     To define the distance Z between the surface  51  of the chip  50  and the optical system  285 , the encapsulating cover  200  comprises a plurality of separate seating protrusions (referred to in this implementation as “seating feet”)  210  that protrude from a rear edge  290  of the cover  200  and that are configured to be seated in contact with the front face  101  of the substrate  100 . 
     Thus, the precision of the vertical positioning of the optical system  285  with respect to the surface  51  of the chip  50  is defined by the precision of the fabrication of the cover  200  (and in particular the seating feet of the cover). Typically, the cover  200  is made from plastic by injection molding, which is a well-known and very reliable technique. 
     The rear edge  290  of the cover  200  is moreover provided to accommodate the adhesive bead  150 . 
     The seating surface  300  through which the cover  200  and the substrate  100  make contact is thus formed by rear surfaces  211  of the seating feet  210  of the cover  200 . Of course, the rear surface  211  of a seating foot  210  is the frontal surface of the protruding portion, i.e., the surface protruding parallel to the rear edge  290  of the cover  200 . 
     The seating surfaces  300  are advantageously located set back towards the interior with respect to the exterior lateral border  120  of the carrier substrate  100 , and also with respect to the exterior lateral border  220  of the encapsulating cover  200 . 
     Of course, the interior of the cover  200  is directed towards the chamber  250 . 
     Specifically, the exterior lateral borders  120 ,  220  of the substrate and of the cover, respectively, are not necessarily aligned in a common plane. Irrespectively, the seating surfaces  300  are advantageously located set back towards the interior with respect to each exterior lateral border  120 ,  220 , so as to leave an open space on the exterior and having surfaces  190 ,  290  to be bonded. 
     Thus, the open space on the exterior between the seating feet  210  and each of the exterior borders  120 ,  220  of the substrate  100  and of the cover  200  is provided to be filled by the adhesive bead  150 , in contact in particular with the rear edge  290  of the cover  200  and with one portion  190  of the front face  101  of the substrate  100 . 
     In this respect, reference is made to  FIG.  2   . 
       FIG.  2    shows a view of the bottom of the encapsulating cover  200 . The bottom of the encapsulating cover  200  in this example is the side of the cover  200  that makes contact with the front face  101  of the carrier substrate  100 . 
     In this example, the seating feet  210  have a surface  211  in the shape of an L that protrudes with respect to the rear edge  290 , so as to closely follow the shape of the ridges of the cover  200  at the intersection of two exterior faces  220 . The feet are nevertheless located set back towards the interior  222  with respect to said exterior faces  220  and ridges. 
     Provision is made for the amount by which the feet are set back towards the interior  222  to be sufficiently large to leave a space  122  between the seating surface  300  ( FIG.  1   ), i.e. the rear surface  211  of the feet  210 , and the exterior border  120  of the carrier substrate  100 , in order in particular to accommodate the adhesive bead  150  ( FIG.  1   ). 
     In the example illustrated in  FIG.  2   , the seating feet  210  are further positioned plumb with the interior faces of the cover  200  delineating the chamber  250  and closely follow the corners of the chamber  250 . 
     Of course, the seating feet  210  of the cover  200  may be positioned differently depending on the requirements. Advantageously, the seating feet  210  are nevertheless configured to leave a space allowing the adhesive bead  150  to be accommodated, without overflow onto the rear surfaces  211  of the feet  210 , i.e. the seating surface  300 . 
     In practice, an adhesive residue will possibly be present between the cover  200  and the substrate  100  on the seating surface  300 . Such an adhesive residue will have a negligible thickness with respect to the requirements in terms of vertical precision of the mounting. 
     The seating feet  210  of the cover  200  may also have any other shape, for example a square or rectangular shape, or even a cross shape. 
       FIGS.  3  and  4    illustrate another example embodiment of an electronic device DIS comprising an encapsulating cover  200  mounted on a carrier substrate  100  similar to the device described with reference to  FIG.  1   , and common elements have been referenced with the same references and will not be described in detail again. 
       FIG.  3    is a cross-sectional view of this other example embodiment and  FIG.  4    a top view of the carrier substrate  100  of this example. The top of the carrier substrate  100  is the side of the substrate  100  level with the mounting front face  101 , which will make contact with the rear edge  290  of the encapsulating cover  200 . 
     In this embodiment, to define the distance Z between the surface  51  of the chip  50  and the optical system  285 , the mounting front face  101  of the carrier substrate  100  comprises a plurality of separate seating protrusions (referred to in this embodiment as “seating bases”)  110  that protrude with respect to said mounting front face  101 . Said seating bases  110  are configured to be seated in contact with the rear edge  290  of the cover  200 . 
     A front surface  111  of the seating bases  110 , which protrudes from the front face  101  parallel to the front face  101 , forms the seating surface  300  in contact with the cover  200 . 
     The cover  200  does not necessarily comprise seating feet  210  as described above with reference to  FIGS.  1  and  2   . 
     The cover  200  may nevertheless comprise shoulders  215  directed towards the interior of the chamber  250  and configured to closely follow the entire front surface  111  of the seating bases  110 . 
     Analogously to the arrangement presented above with reference to  FIG.  2   , the seating surfaces  300 , i.e., the frontal surfaces  111  of the seating bases  110 , are located set back towards the interior  122 ,  222  with respect to the exterior lateral borders  120 ,  220  of the carrier substrate  100  and of the encapsulating cover  200 . 
     Thus, a free surface  190  open towards the exterior is provided to accommodate the adhesive bead  150  between the seating bases  210  and the exterior borders  120  of the substrate  100 . 
       FIG.  5    illustrates an example of an application of a mounting of an encapsulating cover  200  on a carrier substrate  100  such as described above with reference to  FIGS.  1  to  4   , in a time-of-flight device TOF. 
     Elements common with the examples described above have been referenced with the same references and will not be described again in detail here. 
     The TOF device comprises a light-receiving chip  50  configured to detect an incident light signal, said chip being mounted on the mounting front face  101  of the substrate  100 . 
     A light-emitting electronic integrated circuit (IC) chip  60  configured to emit a light signal is also mounted on the mounting front face  101  of the substrate  100 . 
     The light-receiving electronic chip  50  and the light-emitting chip  60  are housed in a first chamber  250  and second chamber  260  that are provided in the cover  200 . 
     The cover  200  has a through-passage  280  between the exterior of the cover  200  and the first chamber  250 , and a complex optical system comprising three elements is housed facing the through-passage  280 . 
     The through-passage  280  and the elements of the complex optical system are aligned along a vertical optical axis, i.e. perpendicular to the mounting front face  101 , and centered on a first photosensitive surface  51  of the light-receiving chip  50 . 
     The cover  200  has a second through-passage  270  between the exterior of the cover  200  and the second chamber  260 , and an optical element  271 , for example a protecting window or a filter, is housed facing the second through-passage  270 . 
     The light-emitting chip  60  and the light-receiving chip  50  are configured to interact so as to measure a distance by time-of-flight. 
     The light-emitting chip  60  is housed in the second chamber  260  of the cover and comprises an emitting zone  61  configured to emit an output light signal for example having a wavelength in the infrared. The light-emitting chip  60  may be configured to emit, in the emitting zone  61 , the light signal in all directions, or to emit a laser signal. The output signal passes through the optical element  271  to the exterior of the cover  200 . 
     A reference photosensitive surface  52  of the light-receiving chip  50  is located in the second chamber  260  of the cover and immediately detects the output signal emitted by the light-emitting chip  60 , so as to define an emission time of the signal. 
     The output signal is intended to be reflected or scattered from an element outside the cover  200 , and the reflected signal is collected by the complex optical system as an incident light signal coming from outside the cover  200 . 
     The complex optical system is configured to modify the properties of the incident light signal, and in particular to direct it towards the detecting photosensitive surface  51  of the light-receiving chip  50 . 
     The light-receiving chip  50  thus detects a reception time of the reflected signal, and the time passed between the emission time and the reception time is directly proportional to the distance separating the device TOF from the exterior object. 
     An opaque wall  255  is provided in the cover  200  between the first chamber  250  and the second chamber  260  in order not to bias the detection by the signal emitted by the emitting surface  61 . An opaque adhesive bead  151  may fill a potential opening between the wall  255  and the light-receiving chip  50 . 
     The reference photosensitive surface  52  may, for example, be that of a single-photon avalanche diode (SPAD), and the detecting photosensitive surface may comprise those of an array of such SPADs, for example in order to obtain a map of the measured distances. 
     The complex optical system may, for example, comprise an aspherical first lens  281 , a second lens  282  and an optical filter  283  configured to be selectively transparent for a given wavelength range, typically the range comprising the wavelength of the signal emitted by the light-emitting chip  60 , the infrared for example. 
     The complex optical system may have a vertical positioning sensitivity of about 10 μm over the distance Z between an element  282 , or a frame of reference of the complex optical system, and the photosensitive surface  51 . 
     Furthermore, because of the presence of the seating feet  210  in the cover  200  making contact with the mounting front face  101  on the seating surface  300 , the thickness Z 0  of the adhesive bead  150  has no or few variations and the mounting is compatible with the constraints of the optical system. 
     Moreover, the invention is not limited to these embodiments and modes of implementation because it encompasses all the variants thereof; for example, it will be recalled that the shapes and positions of the seating feet  210  and of the seating bases  110  may vary, and the device could comprise both seating feet  210  as described with reference to  FIGS.  1  and  2   , and simultaneously seating bases  110  as described with reference to  FIGS.  3  and  4   .