Patent Publication Number: US-2022223486-A1

Title: Semiconductor packages with engagement surfaces

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     The present application claims priority to U.S. Provisional Patent Application No. 63/136,236, which was filed Jan. 12, 2021, is titled “Structured Layers Inside Packages And On Surfaces For Improved Mechanical Force Coupling,” and is hereby incorporated herein by reference in its entirety. 
    
    
     BACKGROUND 
     Force sensors are useful to detect one or more forces experienced by a member of interest. In some instances, a force sensor may be useful to detect stress, torque, compression, strain, tension, etc. experienced by the member of interest (e.g., a shaft, strut, beam). To facilitate the detection of these forces, the force sensor (or some component thereof) is mounted to the member so forces experienced by the member may be transferred to the force sensor during operations. 
     SUMMARY 
     Some examples described herein include a semiconductor package. In some examples, the semiconductor package includes a semiconductor die configured to detect a force. In addition, the semiconductor package includes a mold compound covering the semiconductor die. Further, the semiconductor package includes an engagement surface including a pattern of projections adapted to engage with a mounting surface on a member of interest. 
     In some example, the semiconductor package includes a die pad having a first side and a second side opposite the first side. In addition, the semiconductor package includes a semiconductor die mounted to the first side of the die pad, the semiconductor die being configured to detect a force. The second side of the die pad includes a pattern of projections that are adapted to engage a pattern of recesses in a mounting surface of a member of interest. 
     In some examples, the semiconductor package includes a die pad and a semiconductor die mounted to the die pad. In addition, the semiconductor package includes a mold compound having a first side and a second side opposite the first side. The compound covers the die pad and the semiconductor die, and the second side includes a pattern of projections that are adapted to engage with a mounting surface on a member of interest. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1A  is a side cross-sectional view of a force sensor for mounting to a member of interest according to some examples. 
         FIG. 1B  is a bottom view of a force sensor for mounting to a member of interest according to some examples. 
         FIG. 1C  is an enlarged cross-sectional view of an engagement surface of the force sensor and mounting surface of a member of interest according to some examples. 
         FIG. 2A  is a bottom view of a force sensor for mounting to a member of interest according to some examples. 
         FIG. 2B  is a bottom view of a force sensor for mounting to a member of interest according to some examples. 
         FIG. 2C  is a bottom view of a force sensor for mounting to a member of interest according to some examples. 
         FIG. 3  is a side cross-sectional view of a force sensor for mounting to a member of interest according to some examples. 
         FIG. 4A  is a side cross-sectional view of a force sensor for mounting to a member of interest according to some examples. 
         FIG. 4B  is a bottom view of a force sensor for mounting to a member of interest according to some examples. 
         FIG. 5A  is side cross-sectional view of a force sensor for mounting to a member of interest according to some examples. 
         FIG. 5B  is a side cross-sectional view of a force sensor for mounting to a member of interest according to some examples. 
         FIG. 6A  is side cross-sectional view of a force sensor for mounting to a member of interest according to some examples. 
         FIG. 6B  is side cross-sectional view of a force sensor for mounting to a member of interest according to some examples. 
         FIG. 7  is a side cross-sectional view of a force sensor for mounting to a member of interest according to some examples. 
     
    
    
     DETAILED DESCRIPTION 
     A force sensor may be mounted to a member of interest for detecting (e.g., directly, indirectly) forces within the member. The force sensor is mounted to the member of interest, and forces experienced by the member may be transferred to the force sensor via the mounting. Some mounting devices or techniques may dampen or absorb forces that are transferred from the member of interest thereby causing the force sensor to be less effective at detecting these forces during operations. Thus, mounting the force sensor to the member of interest may have a meaningful effect on the quality of data that may be obtained by the force sensor during operations. 
     In some instances, a force sensor may be useful for detecting forces in a particular direction along a surface of the member of interest. However, some mounting techniques may not allow a force sensor to adequately detect these targeted forces or force directions. Accordingly, examples described herein include force sensors that include projections on an engagement surface that are to engage with a mounting surface of a member of interest. The engagement of the projections with the mounting surface may amplify particular forces or force directions during operations. 
     Referring now to  FIG. 1A , a force sensor  100  according to some examples is shown mounted to a shaft  102  that is rotatable about a central or longitudinal axis  104 . The shaft  102  may be a rotating shaft of a pump, compressor, drivetrain or other mechanical system. The force sensor  100  is mounted to a mounting surface  106  which may include a planar or facetted surface that is defined on the otherwise curved outer surface  108  of shaft  102 . In some examples, the force sensor  100  may be mounted to another member of interest, such as, for instance, a beam, column, hinge, wing (or air foil), rotor blade, or any other mechanical or structural member that may experience forces during operations. 
     During operations, the force sensor  100  may detect, via the engagement with mounting surface  106 , the forces experienced by the shaft  102 . For instance, the shaft  102  may experience a torque about longitudinal axis  104 , axial stress (e.g., from tension or compression along longitudinal axis  104 ), bending stress, strain, etc. These various forces and stresses that may be experienced by the shaft  102  may be collectively and generally referred to herein as “forces.” The force sensor  100  may detect (e.g., directly or indirectly) any one or more of these forces during operations thereby allowing personnel to monitor the operating conditions of the shaft  102 . 
     The force sensor  100  is a semiconductor package that includes a semiconductor die  110 . Accordingly, the force sensor  100  may be referred to herein as a “semiconductor package.” The semiconductor die  110  has a device side  112  and non-device side  114  opposite the device side  112 . An active circuit  116  (or more simply “circuit  116 ”) is formed on the device side  112 . The non-device side  114  of semiconductor die  110  is secured to a die pad  118  via a die attach layer (not shown). 
     A mold compound  120  (e.g., a polymer or resin material) may cover the semiconductor die  110  and die pad  118 . The mold compound  120  may protect the semiconductor die  110  and die pad  118  from the outside environment (e.g., specifically from dust, liquid, light, contaminants in the outside environment), and may prevent undesired contact with conductive surfaces or members during operations. As referred to herein, the term “mold compound” includes a covering for a semiconductor die that is formed through any suitable process, such as a cavity molding operation, glob encapsulation, dam-and-fill type encapsulation, etc. The mold compound  120  may include a first side  122 , a second side  124  opposite first side  122 , and an outer perimeter  126  extending between the first side  122  and the second side  124  along an axis  128  that extends through (e.g., perpendicularly through) the sides  122 ,  124 . 
     The circuit  116  may be coupled to conductive terminals  130  via bond wires  132 . In some examples, the conductive terminals  130  may be so-called gull-wing leads. However, the force sensor  100  may include a quad flat no-lead (QFN) package and the conductive terminals  130  may be arranged and designed for inclusion therein. The conductive terminals  130  may be coupled to suitable connectors on a printed circuit board (PCB) (not shown) or other suitable device. The mold compound  120  may cover the bond wires  132  and a portion of the conductive terminals  130 . 
     Referring now to  FIGS. 1A-1C , the force sensor  100  may also include an engagement surface  134  that is defined by the die pad  118  and that is to engage with the mounting surface  106  of shaft  102 . More particular, the die pad  118  includes a first side  136  that is engaged with the semiconductor die  110  and a second side  138  opposite first side  136 . The second side  138  may be flush (or co-planar) with the second side  124  of mold compound  120 . The engagement surface  134  is defined on the second side  138 . In some examples, the engagement surface  134  includes a pattern of projections  140  that are to engage with mounting surface  106  during operations. 
     As best shown in  FIGS. 1B and 1C , the projections  140  may be parallel to one another. Also, the projections  140  may be spaced from one another along a plane that is aligned with a radius of axis  128 . Thus, the projections  140  may be radially spaced from one another with respect to axis  128 . The projections  140  and second side  138  of die pad  118  may be contained within (or bounded by) the outer perimeter  126  of mold compound  120 . 
     As is best shown in  FIG. 1C , in some examples the mounting surface  106  may have a pattern of recesses  142  that may be aligned with and that may receive the projections  140  on second side  138  during operations. The second side  138  and mounting surface  106  are shown separated from one another along axis  128  in  FIG. 1C , to better show the projections  140  and recesses  142 . The shape, size, and arrangement of the recesses  142  may be chosen to allow recesses  142  to align, engage, and interlock with projections  140  upon contact of second side  138  with mounting surface  106 . Thus, the recesses  142  may be spaced from one another along a plane that is aligned with a radius of axis  128 , and the recesses  142 , like projections  140 , may be radially spaced from one another with respect to axis  128 . 
     In some examples, the projections  140  may each include a crest  144  that is spaced (e.g., axially spaced with respect to axis  128 ) from second side  138 , and a pair of flanks  146  that extend from the crest  144  to the second side  138 . Likewise, each recess  142  may each include a root  148  that is inwardly spaced from mounting surface  106 , and a pair of flanks  150  that extend from the mounting surface  106  to the root  148 . In some examples, each projection  140  and each recess  142  may have a rectangular cross-section. Therefore, the crest  144  and root  148  of each projection  140  and recess  142 , respectively, may be a planar surface that is oriented radially relative to axis  128 . Also, each of the flanks  146  may extend perpendicularly (e.g., axially with respect to axis  128 ) to the crest  144  from the second side  138 , and each of the flanks  150  may extend perpendicularly (e.g., axially with respect to axis  128 ) to the root  148  from the mounting surface  106 . In some examples, the cross-sections of projections  140  and recesses  142  may have a variety of shapes, such as triangular, semicircular, oval, ovoid, truncated triangle, etc. 
     During operations, as engagement surface  134  of force sensor  100  is brought into contact with mounting surface  106 , the projections  140  are inserted within recesses  142 . In some examples, the engagement surface  134  is engaged with the mounting surface  106  via an adhesive or solder material. In some examples, the engagement surface  134  is welded (e.g., via ultrasonic welding) to the mounting surface  106 . 
     After force sensor  100  is secured to mounting surface  106 , forces experienced by the shaft  102  may be transferred to the circuit  116  via the engagement between the pattern of projections  140  on engagement surface  134  and mounting surface  106 . The semiconductor die  110  may be configured to detect the transferred forces. In particular, the circuit  116  of semiconductor die  110  may detect the transferred forces via piezoresistive changes caused in the circuit  116  by the forces. The circuit  116  may also produce an output signal that includes (or is indicative of) the detected force(s). In some examples, the force sensor  100  may include additional components (e.g., semiconductor dies, passive components such as antennas, capacitors, resistors, etc.) that may process the output from the circuit  116  and/or communicate the output from the circuit  116  to other electronic devices (e.g., computers, semiconductor packages). As is described in more detail below, the projections  140  on engagement surface  134  may facilitate a strong connection between the shaft  102  and force sensor  100  and may amplify forces in particular directions (e.g., such as a direction that is perpendicular to the projections  140  and recesses  142 ). 
     Referring now to  FIGS. 2A-2C , force sensors  200  that may each be the force sensor  100  of  FIG. 1A-1C  are shown according to some examples.  FIGS. 2A-2C  may be collectively referred to herein as “ FIG. 2 .” 
     In  FIGS. 2A-2C , the force sensors  200  may be semiconductor packages. Accordingly, the force sensors  200  may be referred to herein as “semiconductor packages.” The force sensors  200  each may include a die pad  202  that is flush (or co-planar) with a side  204  of a mold compound  206 . A semiconductor die (not shown) may be coupled to the die pad  202  and covered by the mold compound  206  as described above for force sensor  100 . The mold compound  206  includes an outer perimeter  208  that includes multiple sides  209 . Also, force sensors  200  may include multiple conductive terminals  210  that extend out of one or more sides  209  of the outer perimeter  208  of mold compound  206 . 
     The die pad  202  (or an exposed side thereof) may define an engagement surface  212  of the force sensor  200  that is to engage with a mounting surface on a member of interest (e.g., mounting surface  106  on shaft  102  in  FIG. 1A ). The engagement surface  212  may include a pattern of projections  214  that may be similar to the projections  140  described above for force sensor  100 . 
     Referring specifically to  FIG. 2A , in some examples the outer perimeter  208  of the mold compound  206  may be generally rectangular in shape, and thus opposing sides  209  of the outer perimeter  208  may be parallel to one another. Also, the projections  214  may extend linearly in a direction that is perpendicular to two opposing sides  209  of outer perimeter  208  of mold compound  206 . Without being limited to this or any other theory, the orientation of the projections  214  in FIG.  2 A may provide additional sensitivity to force sensor  200  for forces that are directed along a direction that is perpendicular to the projections  214 . 
     Referring specifically to  FIGS. 2B and 2C , in some examples the projections  214  may extend across die pad  202  at a non-zero, nonparallel, and non-perpendicular angles to each of the sides  209 . Accordingly, in the examples of  FIGS. 2B and 2C , the projections  214  do not extend perpendicularly or parallel to any of the sides  209  of outer perimeter  208 . Without being limited to this or any other theory, by placing the projections  214  at an angle across the die pad  202 , the force sensor  200  may have sensitivity to forces directed along a pair of perpendicular or orthogonal directions across the side  204  of mold compound  206 . Accordingly, the force sensors  200  of  FIGS. 2B and 2C  may have multidirectional sensitivity in a plane extending parallel to the side  204  of mold compound  206 . 
     Referring now to  FIG. 3 , a force sensor  300  that may be the force sensor  100  of  FIG. 1  is shown according to some examples. The force sensor  300  is a semiconductor package that includes a semiconductor die  302 . Accordingly, the force sensor  300  may be referred to herein as a “semiconductor package.” The semiconductor die  302  has a device side  304  and non-device side  306  opposite the device side  304 . An active circuit  308  (or more simply “circuit  308 ”) is formed on the device side  304 . The non-device side  306  of semiconductor die  302  is secured to a die pad  309  via a die attach layer (not shown). 
     A mold compound  310  (e.g., a polymer or resin material) may cover the semiconductor die  302  and die pad  309 . The mold compound  310  may protect the semiconductor die  302  and die pad  309  from the outside environment (e.g., specifically from dust, liquid, light, contaminants in the outside environment), and may prevent undesired contact with conductive surfaces or members during operations. The mold compound  310  may include a first side  312 , a second side  314  opposite first side  312 , and an outer perimeter  316  extending between the first side  312  and the second side  314  along an axis  318  that extends through (e.g., perpendicularly through) the sides  312 ,  314 . Multiple conductive terminals  320  may extend out of the outer perimeter  316  of mold compound  310  and may be coupled to circuit  308  of semiconductor die  302  via bond wires  322 . 
     The force sensor  300  may also include an engagement surface  324  that is defined by the second side  314  of mold compound  310  that is to engage with the mounting surface  326  of member  328  of interest (e.g., shaft  102 ). More particularly, the die pad  309  is recessed into the mold compound  310 . Thus, die pad  309  is fully covered by mold compound  310 , and engagement surface  324  is defined by the second side  314  of mold compound  310 . 
     In some examples, the engagement surface  324  includes a pattern of projections  330  that are to engage with mounting surface  326  during operations. The projections  330  may be similar to the projections  140  described above. In some examples, the projections  330  may engage with similarly shaped recesses  332  that are defined on mounting surface  326  in a similar manner to that described above for projections  140  and recesses  142  ( FIG. 1C ). 
     During operations, the engagement surface  324  may be secured to the mounting surface  326  of member  328  of interest via adhesive, solder material, welding, or any other suitable manner. The interconnection between the projections  330  and recesses  332  may amplify forces in a particular direction (e.g., such as a direction that is applied in a direction that is perpendicular to the projections  330  and recesses  332 ). 
     The semiconductor die  302  may be configured to detect forces experienced by the member  328  of interest via the connection between the projections  330  on engagement surface  324  and the recesses  332  on the mounting surface  326 . Specifically, the circuit  308  of semiconductor die  302  may detect the forces via piezoresistive changes and may produce an output signal that includes (or is indicative of) the detected forces as described above. The force sensor  300  may include additional components for communicating and/or processing the output from circuit  308  during operations as described above. 
     Referring now to  FIGS. 4A and 4B , a force sensor  400  that may be the force sensor  100  of  FIG. 1  is shown according to some examples. The force sensor  400  is a semiconductor package that includes a semiconductor die  402 . Accordingly, the force sensor  400  may be referred to herein as a “semiconductor package.” The semiconductor die  402  has a device side  404  and non-device side  406  opposite the device side  404 . An active circuit  408  (or more simply “circuit  408 ”) is formed on the device side  404 . 
     The non-device side  406  defines an engagement surface  409  that is to engage with a mounting surface  410  of a member  412  of interest (e.g., shaft  102  in  FIG. 1 ). In some examples, the engagement surface  409  includes a pattern of projections  414  that are to engage with mounting surface  410  during operations. 
     As best shown in  FIG. 4B , the projections  414  are arranged in multiple rows  416  and columns  418  across the non-device side  406  of semiconductor die  402 . In some examples, the projections  414  may be shaped as truncated pyramids. However, in other examples the projections  414  may have other shapes such as rectangular parallelepiped, spherical, semispherical, etc. The projections  414  may be formed on non-device side  406  via a sputtering and plating process. 
     A number of passive devices  420  are coupled to the circuit  408  along device side  404 . The passive devices  420  may be coupled to circuit  408  via solder members  422  (which may be referred to as “solder bumps”). In some examples, the passive devices  420  may include capacitors, inductors, antennas, coils and/or other components that may perform a function (or functions) either independently of or along with circuit  408 . In some examples, the passive devices  420  may include an antenna and a filter that are coupled to circuit  408  and that are configured to receive and/or send wireless electronic signals to other devices (e.g., computers, semiconductor chip packages) either directly or via a network. Specifically, during operations, the antenna, formed or defined by the passive devices  420 , may transmit output signals of the force sensor  400  that may include, or be indicative of, forces detected by the force sensor  400 . 
     Referring specifically to  FIG. 4A , during operations, the engagement surface  409  may be coupled to the mounting surface  410 . In particular, the projections  414  of engagement surface  409  may engage and/or mesh with a set of projections  424  positioned along the mounting surface  410 . In some examples, the projections  424  on mounting surface  410  may be similarly shaped as the projections  414  on engagement surface  409 . The projections  424  may be arranged to be positioned between adjacent projections  414  on engagement surface  409 . Accordingly, the projections  424  may be interleaved between projections  424  upon securing engagement surface  409  to mounting surface  410 . Without being limited to this or any other theory, the interleaving of the projections  424 ,  414  may allow forces to transfer from the member  412  of interest to the force sensor  400  along a plane that passes through the projections  424 ,  414 . 
     In some examples, the force sensor  400  may be secured to the member  412  of interest via solder material  426  (e.g., a metallic material that may be melted and re-solidified to bond two objects or members together). The solder material  426  may form a bond between the engagement surface  409  and the mounting surface  410  and between the interleaved projections  414 ,  424 . Accordingly, during operations, forces experienced by the member  412  of interest may be transferred from the mounting surface  410  to the force sensor  400  via the projections  424 , solder material  426  and projections  414 . The engagement of projections  414 ,  424  via solder material  426  may provide a secure contact between the force sensor  4300  and the member  412  while allowing force detection sensitivity in multiple directions. 
     The solder material  426  may be bonded to the engagement surface  409  (including the projections  414 ,  424 ) by any suitable manner. For instance, localized heat may be applied to melt the solder material  426  and allow it to flow between the projections  414 ,  424 . In some example, the solder material  426  may be placed between the engagement surface  409  and the mounting surface  410 , and then the force sensor  400 , member  412 , and solder material  426  may be placed in an environment having an elevated temperature (e.g., an oven, chamber). The elevated heat of the surrounding environment may cause the solder material  426  to melt and flow between the projections  414 ,  424 . 
     The semiconductor die  402  may be configured to detect forces experienced by the member  412  of interest via the connection between the projections  414  and  424  on engagement surface  324  and mounting surface  410 , respectively. Specifically, the circuit  408  of semiconductor die  402  may detect the forces via piezoresistive changes and may produce an output signal that includes (or is indicative of) the detected forces as described above. The passive components  420  may then communicate and/or process the output from circuit  408  during operations as described above. 
     In some examples, the semiconductor die  402  may be mounted to a die pad. Accordingly, the die pad may define the engagement surface  409  having the projections  414  in some examples. 
     Referring now to  FIGS. 5A and 5B , force sensors  500  that may be the force sensor  100  of  FIG. 1  are shown according to some examples. The force sensors  500  are semiconductor packages that each include a semiconductor die  502 . Accordingly, the force sensors  500  may be referred to herein as “semiconductor packages.” The semiconductor die  502  has a device side  504  and non-device side  506  opposite the device side  504 . An active circuit  508  (or more simply “circuit  508 ”) is formed on the device side  504 . 
     The non-device side  506  defines an engagement surface  509  that is to engage with a mounting surface of a member of interest (e.g., mounting surface  106  of shaft  102  in  FIG. 1 ). In some examples, the engagement surface  509  includes a pattern of projections  514  that are similar to the projections  414  described above for force sensor  400 . 
     A number of passive devices  520  are coupled to the circuit  508  of semiconductor die  502 . In the example of  FIG. 5A , the passive devices  520  may be coupled to circuit  508  via solder members  522  (which may be referred to as “solder bumps”). In the example of  FIG. 5B , the passive devices  520  are coupled to circuit  508  via a redistribution layer  524 . In particular, the redistribution layer  524  is a conductive member that selectively couples the passive devices  520  to the circuit  508  (or particular parts thereof). The passive devices  520  may be coupled to the redistribution layer  524  via multiple conductive members  526  that may be engaged (e.g., soldered) to the redistribution layer  524 , and in turn, the redistribution layer  524  is coupled to the circuit  508  via solder members  522 . 
     In some examples, the passive devices  520  may be similar to the passive devices  420  described above for force sensor  400 . Thus, during operations, the passive devices  520  may perform the same function(s) described above for the passive devices  420 . 
     The force sensors  500  both also include a mold compound  530  (e.g., a polymer or resin material) may cover the semiconductor die  502 . The mold compound  530  may protect the semiconductor die  502  from the outside environment (e.g., specifically from dust, liquid, light, contaminants in the outside environment), and may prevent undesired contact with conductive surfaces or members during operations. The mold compound  530  may include a first side  532 , a second side  534  opposite first side  532 , and an outer perimeter  536  extending between the first side  532  and the second side  534  along an axis  538  that extends through (e.g., perpendicularly through) the sides  532 ,  534 . In  FIG. 5A , the second side  534  of mold compound  530  is engaged with device side  504  of semiconductor die  502 . In  FIG. 5B , the second side  534  of mold compound  530  is flush (or coplanar) with the non-device side  506  of semiconductor die  502 . 
     During operations, the engagement surface  509  may be secured to a mounting surface of a member of interest (e.g., mounting surface  106  on shaft  102  in  FIG. 1 ). In some examples, the engagement surface  509  may be engaged with a mounting surface of a member of interest via a solder material (e.g., solder material  426  described above). Also, in some examples the projections  514  may be interleaved with projections (e.g., projections  424  described above) on the mounting surface in a similar manner to that described above for force sensor  500 . During operations, forces experienced by the member of interest may be transferred to the force sensor  500  via the projections  514  on engagement surface  509 . 
     The semiconductor die  502  may be configured to detect forces experienced by the member of interest via the connection therebetween. Specifically, the circuit  508  of semiconductor die  502  may detect the forces via piezoresistive changes and may produce an output signal that includes (or is indicative of) the detected forces as described above. The passive components  520  may then communicate and/or process the output from circuit  508  during operations as described above. 
     Referring now to  FIGS. 6A and 6B , force sensors  600  that may be the force sensor  100  of  FIG. 1  are shown according to some examples. The force sensors  600  are semiconductor packages that each include a semiconductor die  602 . Accordingly, the force sensors  600  may each be referred to herein as a “semiconductor package.” The semiconductor die  602  of each force sensor  600  has a device side  604  and non-device side  606  opposite the device side  604 . An active circuit  608  (or more simply “circuit  608 ”) is formed on the device side  604 . The non-device side  606  of semiconductor die  602  is secured to a die pad  609  via a solder paste (not shown). 
     The force sensors  600  may also include an engagement surface  610  that is defined by the die pad  609  that is to engage with a mounting surface  612  of a member  614  of interest. More particularly, the die pad  609  includes a first side  616  that is engaged with the semiconductor die  602  and a second side  618  opposite first side  616 . The engagement surface  610  is defined on the second side  618 . In some examples, the engagement surface  610  includes a pattern of projections  620  that are similar to the projections  140  of force sensor  100  describe above. 
     During operations, the projections  620  may engage with the mounting surface  612  so as to maintain a spacing D between the second side  618  of die pad  609  and the mounting surface  612 . Referring specifically to  FIG. 6A , in some examples the mounting surface  612  may be a substantially planar and the spacing D may be defined by a length of the projections  620  from the second side  618 . Referring specifically to  FIG. 6B , in some examples the mounting surface  612  may include a number of recesses  622  that are to receive the projections  620  therein (e.g., in a similar manner to the recesses  142  on mounting surface  106  described above and shown in  FIGS. 1A and 1C ), and the spacing D may be defined as the difference between the length of projections  614  from second side  618  and the depth of recesses  622  from mounting surface  612 . In either case, the spacing D may be chosen to provide sufficient clearance between the second side  618  and mounting surface  612  to receive adhesive, solder, etc. therein. In some examples, the spacing D may be chosen to elevate the force sensor  600  above the mounting surface  612  to prevent forces from transferring to the force sensor  600  from the member  614  of interest via pathways other than through engagement surface  610  (including projections  612 ). In some examples, the spacing D may range from a few micrometers to a few millimeters. In some examples, the spacing D may allow projections  620  to dampen (or buffer) forces over a magnitude, to reduce damage to the force sensor  600  or components thereof. 
     The semiconductor die  602  may be configured to detect forces experienced by the member  614  of interest via the connection therebetween. Specifically, the circuit  608  of semiconductor die  602  may detect the forces via piezoresistive changes and may produce an output signal that includes (or is indicative of) the detected forces as described above. 
     Referring now to  FIG. 7 , a force sensor  700  that may be the force sensor  100  of  FIG. 1A-1C  is shown according to some examples. The force sensor  700  is a semiconductor package that includes a semiconductor die  710 . Accordingly, the force sensor  700  may be referred to herein as a “semiconductor package.” The semiconductor die  710  has a device side  712  and non-device side  714  opposite the device side  712 . An active circuit  716  (or more simply “circuit  716 ”) is formed on the device side  712 . The circuit  716  may be coupled to conductive terminals  718  via bond wires  720  in a similar manner to that described above for conductive terminals  130  and bond wires  132  of force sensor  100  ( FIG. 1A ). 
     A mold compound  722  (e.g., a polymer or resin material) may cover the semiconductor die  710  and die pad  718 . The mold compound  722  may protect the semiconductor die  710  and die pad  718  from the outside environment (e.g., specifically from dust, liquid, light, contaminants in the outside environment), and may prevent undesired contact with conductive surfaces or members during operations. 
     The force sensor  700  may also include a first engagement surface  724  that is defined by the die pad  718  and that is to engage with a mounting surface of a member of interest (e.g., shaft  102  in  FIG. 1A ). The first engagement surface  724  may be similar to the engagement surface  134  described above for force sensor  100 . Thus, the first engagement surface  724  may include a pattern of projections  726  that are to engage with a mounting surface on a member of interest during operations in a similar manner to that described above for projections  140 . 
     In addition, force sensor  700  may include a second engagement surface  728  on the die pad  718  on a side of the die pad  718  that is opposite from the first engagement surface  724 . The second engagement surface  728  may include a pattern of projections  730  that are similar to the projections  140  described above ( FIGS. 1A-1C ). The projections  730  may engage and interlock with a pattern of recesses  732  formed on the non-device side  714  of semiconductor die  710  in a similar manner to the engagement described above for projections  140  and recesses  142  ( FIG. 1C ). In some examples, the projections  730  may be formed on the non-device side  714  of semiconductor die  710  and the recesses  732  may be formed on the die pad  718 . 
     During operations, the second engagement surface  728  may enhance force transfer from the die pad  718  to the semiconductor die  718  (and ultimately to circuit  716 ). As described above, the engaged projections  730  and recesses  732  may facilitate a strong connection between the semiconductor die  710  and die pad  718  and may amplify forces in particular directions (e.g., such as a direction that is perpendicular to the projections  730  and recesses  732 ). 
     The examples described above include force sensors that include patterned projections that are to engage with a mounting surface of a member of interest and amplify particular forces or force directions during operations. Thus, the projections formed on the engagement surface of the example force sensors described herein may enhance the connection of the force sensor to the member of interest and the sensitivity of the force sensor for detecting forces of interest during operations. 
     While examples described herein have included semiconductor packages that function as force sensors (e.g., forces sensors  100 ,  200 ,  300 ,  400 ,  500 ,  600 ,  700 ), some examples described herein may include semiconductor packages that provide additional and/or different functionality (e.g., other than force sensing). Thus, generally speaking, examples described herein may include semiconductor packages having engagement surfaces as described herein that may be mounted to a suitable member or surface. 
     In this description, the term “couple” may cover connections, communications or signal paths that enable a functional relationship consistent with this description. For example, if device A provides a signal to control device B to perform an action, then: (a) in a first example, device A is directly coupled to device B; or (b) in a second example, device A is indirectly coupled to device B through intervening component C if intervening component C does not substantially alter the functional relationship between device A and device B, so device B is controlled by device A via the control signal provided by device A. 
     A device that is “configured to” perform a task or function may be configured (e.g., programmed and/or hardwired) at a time of manufacturing by a manufacturer to perform the function and/or may be configurable (or reconfigurable) by a user after manufacturing to perform the function and/or other additional or alternative functions. The configuring may be through firmware and/or software programming of the device, through a construction and/or layout of hardware components and interconnections of the device, or a combination thereof. 
     A circuit or device that is described herein as including certain components may instead be adapted to be coupled to those components to form the described circuitry or device. For example, a structure described as including one or more semiconductor elements (such as transistors), one or more passive elements (such as resistors, capacitors, and/or inductors), and/or one or more sources (such as voltage and/or current sources) may instead include only the semiconductor elements within a single physical device (e.g., a semiconductor die and/or integrated circuit (IC) package) and may be adapted to be coupled to at least some of the passive elements and/or the sources to form the described structure either at a time of manufacture or after a time of manufacture by an end-user and/or a third-party. 
     While certain components may be described herein as being of a particular process technology, these components may be exchanged for components of other process technologies. 
     Unless otherwise stated, “about,” “approximately,” or “substantially” preceding a value means +/−10 percent of the stated value. Modifications are possible in the described examples, and other examples are possible within the scope of the claims.