Patent Publication Number: US-2021163283-A1

Title: Packaged environmental sensor

Description:
BACKGROUND 
     Technical Field 
     The present disclosure relates to a packaged environmental sensor. 
     Description of the Related Art 
     As is known, environmental sensors are sensors that enable detection of magnitudes of the external environment, such as pressure or humidity. The fields of use of environmental sensors are extremely varied. Among other applications, there has recently been a widespread use of environmental sensors in so-called wearable devices, such as watches, bracelets, and smartbands, which also enable detection of body parameters. 
     An environmental sensor normally comprises a sensor die, for example of a MEMS (MicroElectroMechanical System) type, and a control circuit, integrated in a separate control chip, also referred to as ASIC (Application-Specific Integrated Circuit) chip. The sensor die and the control chip are enclosed within a packaging structure (normally referred to as package), which comprises a supporting structure, normally of ceramic, and a lid. Defined in the supporting structure is a cavity, in which the sensor die and the control chip are housed. The sensor die is normally stacked on the control chip. The lid is arranged so as to close partially the cavity and has a detection port that enables coupling of the sensor die with the external environment to enable measurement of the magnitude to be detected. In other cases, the cavity can be defined between the supporting structure and the lid, for example thanks to the shape of the lid itself. The cavity is, moreover, as a rule, filled with a protective material, for example a potting gel, which has the property of transmitting the magnitude to be detected, at the same time preventing direct exposure of the sensor and of the control chip to the external environment, which may be harmful for the electronic components and the electrical connections, such as the bonding wires. Also in the case of devices commonly used as wearable devices, in fact, environmental sensors may be exposed to aggressive agents, for example chlorine in the water of a swimming pool, seawater, mineral oils or substances present on the skin, such as sweat or some perfumes. If, for the sensor die and for the control device, an acceptable level of protection can frequently be obtained with just the gel, there, however, arises the problem of sealing also the device in which the environmental sensor is incorporated. Between the environmental sensor and the casing of the device a gap is, in fact, present, which should be sealed to prevent entry of aggressive agents. The task is rendered problematical by the packaging structures of known environmental sensors. Known solutions use gaskets and a shell, which encloses both the sensor and the gaskets, keeping them pre-loaded. Other solutions use complex metal lids, with a dome-shaped portion that covers the control chip (having an area that is much greater than that of the sensor die) and is radiused to a substantially cylindrical neck. The neck has the dual function of guaranteeing coupling of the sensor die with the external environment and of defining a sealing surface for receiving the gasket (O-ring or the like). The solution, however, presents limits both because the lid is complex and costly to produce and on account of the problems that arise during the soldering step. In fact, the potting material has a coefficient of thermal expansion that is much greater than that of the material forming the lid. During soldering, the potting material may exert on the dome-shaped portion forces that tend to detach the lid from the supporting structure. The result is a substantial risk of failure or weakening of the connection between the lid and the supporting structure. 
     BRIEF SUMMARY 
     In various embodiments, the present disclosure provides a packaged environmental sensor that will enable the limitations described to be overcome or at least mitigated. 
     In at least one embodiment of the present disclosure, a packaged environmental sensor is provided that includes a supporting structure and a sensor die incorporating an environmental sensor and arranged on a first side of the supporting structure. A control chip is coupled to the sensor die and arranged on a second side of the supporting structure opposite to the first side. A lid is bonded to the first side of the supporting structure and open towards the outside in a direction opposite to the supporting structure, the sensor die being housed inside the lid. 
     In at least one embodiment, an electronic system is provided that includes a casing and a processing unit within the casing. A packaged environmental sensor is coupled to the processing unit. The packaged environmental sensor includes a supporting structure and a sensor die incorporating an environmental sensor and arranged on a first side of the supporting structure. A control chip is coupled to the sensor die and arranged on a second side of the supporting structure opposite to the first side. A lid is bonded to the first side of the supporting structure and open towards the outside in a direction opposite to the supporting structure, the sensor die being housed inside the lid. 
     In at least one embodiment, a device is provided that includes a supporting structure having a first side and a second side opposite to the first side. The supporting structure defines a cavity extending into the supporting structure from the second side. An environmental sensor die is disposed on the first side of the supporting structure, and a control chip is disposed within the cavity and attached to the supporting structure. A lid is attached to the first side of the supporting structure and open towards the outside in a direction opposite to the supporting structure. The sensor die is housed between the lid and the supporting structure. 
    
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
       For a better understanding of the disclosure, some embodiments thereof will now be described, purely by way of non-limiting example and with reference to the attached drawings, wherein: 
         FIG. 1  is a simplified block diagram of an electronic system incorporating a packaged environmental sensor; 
         FIG. 2  is a side view, sectioned in a longitudinal plane, of the electronic system of  FIG. 1 ; 
         FIG. 3  is a front view, sectioned in the plane of trace of  FIG. 4 , of a packaged environmental sensor according to one embodiment of the present disclosure incorporated in the electronic system of  FIG. 1 ; 
         FIG. 4  is a plan view from beneath of the packaged environmental sensor of  FIG. 3 ; 
         FIG. 5  is a top view of the packaged environmental sensor of  FIG. 3 ; 
         FIG. 6  is a front view, sectioned in a transverse plane, of a packaged environmental sensor according to a different embodiment of the present disclosure that can be used in the electronic system of  FIG. 1 ; and 
         FIG. 7  is a front view, sectioned in a transverse plane, of a packaged environmental sensor according to a further embodiment of the present disclosure that can be used in the electronic system of  FIG. 1 . 
     
    
    
     DETAILED DESCRIPTION 
     With reference to  FIG. 1 , an electronic device is designated as a whole by the reference number  1  and may be an electronic device of any type, in particular, but not exclusively, a wearable device, such as a watch, a bracelet, or a smart band, a computer, such as a mainframe, a personal computer, a laptop, or a tablet, a smartphone, a digital musical player, a digital camera, or any other device adapted to process, store, transmit, or receive information. The electronic device  1  may be a general-purpose computer system or may be embedded in a device, an apparatus, or a further system. 
     The electronic device  1  comprises a processing unit  2 , data-storage media  3 , and a packaged environmental sensor  10 , and may moreover be provided with an input/output (I/O) device  5  (for example a keypad, a mouse, or a touchscreen), a wireless interface  6 , peripherals  7 . 1 , . . . ,  7 .N, and possibly further auxiliary devices, here not illustrated. The components of the electronic device  1  may be coupled in communication with one another directly and/or indirectly through a bus  8 . The electronic device  1  may moreover comprise a battery  9 . It should be noted that the scope of the present disclosure is not limited to embodiments that necessarily have one or all of the devices listed. 
     The processing unit  2  may comprise, for example, one or more microprocessors, microcontrollers, and the like, according to the design preferences. 
     The data-storage media  3  may comprise volatile memory devices and non-volatile memory devices of various kinds, for example SRAMs and/or DRAMs in the case of volatile memories, and solid-state memories, and magnetic and/or optical disks in the case of non-volatile memories. 
     With reference to  FIG. 2 , the electronic device  1  comprises a casing  11  that houses a printed-circuit board (PCB)  12  and at least part of the components. In particular, the packaged environmental sensor  10  is mounted on the PCB  12  either directly or via a socket  13 . In addition, the packaged environmental sensor  10  communicates with the outside world through an opening  15  in the casing  11 . The gap between the margin of the opening  15  and the packaged environmental sensor  10  is sealed by a gasket  16 , for example an O-ring. 
     The packaged environmental sensor  10  may be any sensor adapted to detect a parameter or magnitude of the external environment. By way of non-limiting example, the packaged environmental sensor  10  may be a pressure sensor, a humidity sensor, a temperature sensor, or some other environmental sensor. 
     As shown in  FIGS. 3-5 , the packaged environmental sensor  10  comprises a supporting structure  17 , a sensor die  18 , a control or ASIC chip  20 , and a collar-shaped lid  21 . The lid may be a collar, a ring, or the like. 
     The supporting structure  17  is, for example, of ceramic and has a first face  17   a , to which the sensor die  18  is fixed by a die-attach region  22 , and a second face  17   b  opposite to the first face  17   a.    
     Lid-attach regions  23   a - 23   c , for example, of a metal solder paste, are arranged on the first face  17   a  of the supporting structure  17  around the sensor die  18 . In one embodiment, the lid-attach regions  23   a - 23   c  extend along respective closed continuous paths. In particular, the lid-attach regions  23   a - 23   c  are in a geometrical relationship of similarity with one another, and one of the lid-attach regions  23   a - 23   c  is conformable to a margin of the lid  21  bonded to the supporting structure  17 . Furthermore, the lid-attach regions  23   a - 23   c  are nested within one another without mutual contact. For instance, the lid-attach regions  23   a - 23   c  may be three circular or elliptical concentric rings and are separate from one another. It is understood, however, that the shape and number of the lid-attach regions  23   a - 23   c  may be freely selected according to the design preferences, for example so as to correspond to the shape and dimensions of the lids available or in any case used. For instance, the lid-attach regions  23   a - 23   c  may have a polygonal shape. In turn, the dimensions of the lids are selected according to the dimensions of the gaskets  16 . 
     The sensor die  18  contains an environmental sensor  19 . In particular, the sensor die  18  may contain a MEMS detection structure, for example a membrane structure that may function as pressure sensor or sensor for detecting some other environmental magnitude, such as temperature or humidity. 
     A cavity  24  is formed in the second face  17   b  of the supporting structure  17  and houses the control chip  20 . In one embodiment, a step  25  is formed around one or more sides of the control chip  20 . Consequently, in practice, the control chip  20  is housed in a recess on the bottom of the cavity  24 , and therefore on the opposite side of the supporting structure  17  with respect to the sensor die  18 . The control chip  20  may comprise control circuits for driving the MEMS structure and for conversion of the physical magnitudes transduced into useful signals. 
     The sensor die  18  and the control chip are coupled together by bonding wires  29  between contact pads  30  of the sensor die  18  and contact pads  31  on the first face  17   a  of the supporting structure  17 , connection lines  32  embedded in the supporting structure  17  between the contact pads  30  and contact pads  34  on the step  25  of the supporting structure  17  around the control chip  20 , and bonding wires  35  between the contact pads  34  and contact pads  37  of the control chip  20 . Further contact pads  37 , bonding wires  35 , contact pads  34 , and connection lines  32  connect the control chip to conductive solder pads  40 , with which the supporting structure  17  is bonded to a PCB  41 . In turn, the PCB  41  is mechanically and electrically coupled to the socket  13  (or directly to the PCB  12  in the absence of the socket  13 ). Alternatively, the supporting structure  17  can be soldered directly to the PCB  12 . 
     A protective structure  45 , for example a glob-top structure, fills at least part of the cavity  24  and incorporates or covers the control chip  20  and the bonding wires  35 . 
     The lid  21  is defined by an annular wall and is bonded to one of the lid-attach regions  23   a - 23   c  (in the example of  FIG. 3 , to the intermediate fixing region  23   b , with respect to which the margin of the lid  21  is conformable). In one embodiment, the lid  21  has a frustoconical shape, and its generatrix G forms (externally) an angle α greater than 80° with a plane parallel to the first face  17   a  of the supporting structure  17 . Alternatively, the lid  21  may be cylindrical. The gasket  16  that seals the gap between the margin of the opening  15  of the casing  11  and the packaged environmental sensor  10  is arranged around the lid  21 . 
     The lid  21  is open on both of its bases. The small base  21   a  of the lid  21  is bonded to the first face  17   a  of the supporting structure  17  and has a profile conformable to one of the lid-attach regions  23   a - 23   c , in particular to the intermediate lid-attach region  23   b . The large base  21   b  of the lid  21  is open towards the outside in a direction opposite to the supporting structure  17 . The height of the lid  21  is such that the sensor die  18  is completely contained within the lid  21 . 
     Moreover, the lid  21  is at least in part filled with a protective layer  43  that completely englobes the sensor die  18  and the bonding wires. The protective layer  43  is of a material that has the characteristics of coupling the sensor die  18  to the environmental magnitude to be measured and, at the same time, protects the sensor die  18 , the bonding wires  29 , and the pads  31  from aggressive external agents. For instance, the protective layer  43  may a silicone potting gel. 
     As mentioned, the dimensions of the lid  21  are in relation with the dimensions of the opening  15  to be sealed and of the gasket  16 . 
     In the embodiment of  FIG. 6 , a packaged environmental sensor  100  is identical to the packaged environmental sensor  10  already described, except for the dimensions of the lid, which is here designated by the number  121 . In this case, the lid  121  has dimensions compatible with those of the outermost lid-attach region  23   a  and, through the latter, is bonded to the first face  17   a  of the supporting structure  17 . 
       FIG. 7 , where elements already described are designated by the same reference numbers, represents a packaged environmental sensor  200  according to a different embodiment of the disclosure. The packaged environmental sensor  200  comprises a supporting structure  217 , the sensor die  18 , and the control chip  20 . In this case, the control chip  20  is bonded to the supporting structure  217  using the flip-chip technique. The cavity  224 , which is obtained in the second face  217   b  of the supporting structure  217  and houses the control chip  20 , does not need steps around the control chip  20  itself. Moreover, connection lines  232  connect the contact pads  31  on the first face  217   a  of the supporting structure  217  to a first set of contact pads  37  of the control chip  20 , and a second set of contact pads  37  to solder pads  240  of the supporting structure  217 . Formed on the first face  217   a  of the supporting structure  217  are lid-attach regions  223   a - 223   c , also in this case three having a circular shape and being concentric with respect to one another. The lid  221  is bonded to the innermost lid-attach region  223   c.    
     The packaged environmental sensor described affords several advantages. In the first place, the control chip, which is located on the opposite side of the supporting structure with respect to the sensor die, is enclosed in a space that, in use, may be conveniently sealed with the gasket between the collar and the margin of the opening in the casing of the electronic device, where the packaged environmental sensor is installed. Consequently, the packaged environmental sensor is never exposed to the physically and/or chemically aggressive external environment. Therefore, not only is the control circuit in intrinsically safer conditions as compared to known environmental sensors, but there is also greater freedom of choice of the protective material in which the control circuit is embedded. Furthermore, the control chip is isolated from the external light so as to remain immune therefrom even though its surface may be sensitive to light. 
     The shape and dimensions of the lid are regardless of the dimensions of the control chip, which has an area that is typically five to six times larger than that of the sensor die. The control chip can be conveniently housed in a cavity of the supporting structure of ceramic, and the dimensions of the lid may be conveniently chosen on the basis of the structural and dimensional characteristics of just the sensor die. 
     The shape of the lid may be extremely simple because no particular connectors are necessary, as instead happens in some known environmental sensors especially between areas that protect the control chip, which are extensive and have a rectangular shape, and the circular neck for coupling to the O-ring that functions as gasket. 
     The shape, dimensions, and number of the lid-attach regions may be selected with the highest flexibility according to the design preferences. It is thus possible to produce on a vast scale a large number of examples of the supporting structure itself, to the advantage of manufacturing costs. Given the presence of a plurality of lid-attach regions, the same type of supporting structure can be used with lids and gaskets of various size, without any need for costly interventions of customization. 
     During the soldering steps, the shape of the lid, whether cylindrical or frustoconical with the small base bonded to the supporting structure, enables accommodation of the higher degree of thermal expansion of the potting material as compared to that of the material forming the lid, without causing damage to the structure itself. As represented by the arrows in  FIG. 2 , in particular, the forces generated by the different coefficients of thermal expansion in a direction parallel to the first face of the supporting structure are discharged on the side walls, which can withstand the load without any risk. In a direction perpendicular to the first face of the supporting structure, instead, the potting material can expand freely outwards without exerting forces that tend to detach the lid from the supporting structure. 
     Finally, it is evident that modifications and variations may be made to the packaged environmental sensor described, without thereby departing from the scope of the present disclosure. 
     The various embodiments described above can be combined to provide further embodiments. These and other changes can be made to the embodiments in light of the above-detailed description. In general, in the following claims, the terms used should not be construed to limit the claims to the specific embodiments disclosed in the specification and the claims, but should be construed to include all possible embodiments along with the full scope of equivalents to which such claims are entitled. Accordingly, the claims are not limited by the disclosure.