Force sensor

A force sensor is disclosed. In one illustrative embodiment, the force sensor may include a sensing die mounted to a substrate and an actuation assembly for transmitting an external force to the sensing die. The sensing die may include a diaphragm and one or more sensing elements (e.g. piezoresistive elements) positioned on the diaphragm. The actuation assembly may include a spherical member or object (e.g. ball bearing) in contact with the diaphragm, a pin member having a first end coupled to the spherical object and a second end coupled to a button member. The actuator assembly may transmit a force applied to the button member to the diaphragm via the pin member and spherical member. In some cases, the front side of the sensing die may be mounted to the substrate with an adhesive, such as a pattern of conductive adhesive and nonconductive adhesive to electrically connect one or more bond pads of the sensing die to one or more bond pads of the substrate without the need for wire bonds. The force sensor may also include signal conditioning circuitry.

FIELD

The present disclosure relates generally to sensors, and more particularly, to force sensors for sensing a force applied to the sensors.

BACKGROUND

Force sensors are often used to sense an external force applied to the sensors and provide an output signal representative of the applied force. Such sensors can be used in a wide variety of applications, including medical applications. Example medical applications include use in medical equipment control of, for example, kidney dialysis machines, drug delivery systems, hematology equipment, infusion pumps, entrial feeders, ventilation equipment, as well as other medical equipment. Force sensors are also commonly used in non-medical applications.

SUMMARY

The present disclosure relates generally to sensors, and more particularly, to force sensors for sensing a force applied to the sensors. In one illustrative embodiment, a force sensor includes a sensing die mounted to a substrate, and an actuation assembly for transmitting an external force to the sensing die. The sensing die may include a diaphragm and one or more sensing elements (e.g. piezoresistive elements) positioned on the diaphragm. The actuation assembly may include a spherical or other shaped member or object, such as a ball bearing, in contact with the diaphragm, a pin member having a first end coupled to the spherical object and a second end coupled to a button member. The actuator assembly may be configured to transmit a force applied to the button member to the diaphragm via the pin member and spherical (or other shaped) member. In some cases, only part of the outside surface of the spherical member is spherical in shape. Also, in some cases, the force sensor may also include a housing member positioned on the substrate. The housing member may define a cavity around the sensing die and include an opening sized to receive the pin member therethrough.

In some embodiments, the front side of the sensing die (side with the sensing elements formed thereon) may be mounted toward the substrate, sometimes using an adhesive. In some instances, the adhesive may include a pattern of electrically conductive adhesive and nonconductive adhesive to selectively electrically connect bond pads of the sensing die to bond pads on the substrate. In this instance, wire bonds may not be needed to electrically connect the sensing die to the substrate, which may help increase the reliability and/or durability of the force sensor.

In some cases, the force sensor may also include signal conditioning circuitry mounted on the substrate in electrical communication with the sensing die. The signal conditioning circuitry may be configured to receive one or more electrical signals from the sensing die, and condition the signals to provide a conditioned output signal from the force sensor.

An illustrative method of manufacturing a force sensor is also disclosed. An illustrative method may include flip chip mounting a front side of a sensing die to a first side of a substrate, wherein the sensing die includes a diaphragm and one or more sensing elements. The method may also include providing an actuation assembly in contact with a back side of the diaphragm (e.g. the side of the sensing die that was etched to form the diaphragm), wherein the actuation assembly is configured to transmit an external force to the diaphragm. The actuation assembly may include a spherical member in contact with the back side of diaphragm, a pin member having a first end engaging the spherical member, and a button member engaging a second end of the pin member. In some cases, the method may also include positioning a housing member on the first side of the substrate to define a first cavity around the sensing die and the spherical member, wherein the pin member is configured to extend out through an opening in the housing member. The method may also include mounting signal conditioning circuitry to the substrate in electrical communication with the sensing die, wherein the signal conditioning circuitry may be configured to receive an electrical signal from the sensing die and condition the signal to provide a conditioned output signal from the force sensor.

DESCRIPTION

The following description should be read with reference to the drawings wherein like reference numerals indicate like elements throughout the several views. The detailed description and drawings show several embodiments which are meant to be illustrative of the disclosure.

FIG. 1is a cross-sectional view of an illustrative embodiment of a force sensor10. In the illustrative embodiment, the force sensor10includes a sense element or sensing die20mounted to a package substrate12using an adhesive24, solder or the like. In some embodiments, the sensing die20may be a micromechanical sensor element fabricated using a silicon wafer and suitable fabrication techniques. In this example, the sensing die20may have one or more sensing elements, such as piezoresistive sensing components, and/or other circuitry (e.g. trim circuitry, signal conditioning circuitry, etc.) formed using suitable fabrication or printing techniques. In some cases, the sensing die20may include a sensing diaphragm22including the one or more sensing elements formed thereon for sensing a deflection of the sensing diaphragm22. In some cases, the sensing diaphragm22may be fabricated by back-side etching a silicon die, however, any suitable process may be used as desired.

When provided, the piezoresistive components may be configured to have an electrical resistance that varies according to an applied mechanical stress (e.g. deflection of sensing diaphragm22). In some cases, the piezoresistive components may include a silicon piezoresistive material; however, other it is contemplated that non-silicon materials may be used if desired. The piezoresistive components may be connected in a Wheatstone bridge configuration (full or half bridge). It is to be understood that the piezoresistive components are only one example of a sensing element that can be used, and it is contemplated that any other suitable sensing elements may be used, as desired.

In the illustrative embodiment, the package substrate12may include a ceramic material, however, it is contemplated that other suitable materials may be used as desired (e.g. PCB). In some cases, the sensing die20may be mounted to the substrate12using an adhesive24such as a silicone, RTV, a silicone-epoxy, a soft epoxy, or a regular or hard epoxy. In some embodiments, the adhesive24may include a conductive adhesive, a nonconductive adhesive, or a combination of conductive and nonconductive adhesives. When provided, the combination of conductive and nonconductive adhesive may be provided in a pattern to electrically connect bond pads of the sensing die20to bond pads on the substrate12. In any event, it is contemplated that the sensing die20may be mounted to the substrate12using any other suitable bonding mechanism (e.g. solder, eutectic, etc.).

As shown inFIG. 1, the sensing die20may be directly mounted to the substrate12with adhesive24with no intervening isolation layer(s) or substrate provided therebetween, but this is not required. In some instances, an isolation layer or glass substrate (not shown) may be provided in force sensor10between the sensing die20and the substrate12, if desired. In some embodiments, the sensing die20may include a silicon material and the package substrate12may include an alumina ceramic, which may have similar temperature expansion coefficients. The sensing die20and package substrate12, however, may be made of other suitable materials, if desired.

In some cases, the sensing die20may be mounted over an opening26in the package substrate12that is sized to expose the sensing diaphragm22to the bottom side of the package substrate12. In this instance, the back side of force sensor10may have a reference pressure that can be transmitted to the sensing diaphragm22via opening24, but this is not required.

In the illustrative embodiment ofFIG. 1, the sensing die20is flip chip mounted to the substrate12, using an adhesive24as a bonding material. In other words, the top side of the sensing die20(i.e. the side of the sensing die20with the sensing elements) is shown facing downward (in the orientation shown inFIG. 1) toward to the top side (in the orientation shown inFIG. 1) of the substrate12, and mounted thereto with adhesive24. In this example, the adhesive24(e.g. a combination of conductive and nonconductive adhesive) may be configured to electrically connect bond pads of the sensing die20to one or more bond pads or trace conductors on the substrate12without the need for wire bonds. In another example, the sensing die20may be flip chip mounted to the substrate12using bump bonds, a solder ball grid array, or any other suitable technique, as desired. In some cases, it is contemplated that the back-side of the sensing die20(i.e. the side of the sensing die20etched to form the diaphragm22) may be mounted to the substrate12with adhesive24. In this instance, wire bonds (not shown) may be provided to electrically connect bond pads on the top side of the sensing die20to bond pads on the substrate12, if desired.

In the illustrative embodiment, the force sensor10may also include an actuation assembly for transmitting an external force to the sensing die20. As shown inFIG. 1, the actuation assembly may include a spherical (or other shaped) object40, an extender41, and a button44. It can be appreciated that other types of actuators may be utilized such as, for example, slidable mounted plungers or shafts, point contact type components other than spherical objects, “T” shaped transfer mechanisms, in accordance with alternative embodiments. In some cases, only part of the outside surface of the object40may be spherical in shape.

In the illustrative embodiment shown inFIG. 1, the spherical object40may include stainless steel or other suitable metal. In some instances, the spherical object40may be a ball bearing. However, it is contemplated that other generally spherical elements may be used, if desired, including polymer based objects. As shown inFIG. 1, the spherical object40may be in contact with the sensing diaphragm22of the sensing die20. The extender41, or pin, may be configured to include a first end42and a second end43, with a length extending therebetween. The first end42of the extender41may engage, and in some cases may be secured or attached to, the spherical object40in any suitable manner including, for example, soldering or adhesively securing end42to the spherical object40, press fitting end42into a hole formed in the spherical object40, or any other suitable manner of securing end42to the spherical object40. In some cases, the first end42of extender41may be generally conical shaped or otherwise pointed and may engage an opening in the spherical object41. However, it is contemplated that end42may be flat or have another shape, as desired. While spherical object40and extender41are shown as separate members, it is contemplated that the spherical object40and the extender41may be integrally formed.

As shown inFIG. 1, button44may be engaged or otherwise secured to the second end43of extender41. For example, the second end43of the extender41, which may be generally conical shaped, may be inserted into an opening47of the button44and secured therein in any suitable manner. However, it is contemplated that end42may be flat or have another shape, as desired. When generally conical shaped, the button44may be able to swivel with respect to the second end43of the extender41, which may help increase the repeatability of the force sensor10regardless of the orientation of the applied force. This may also be present when the first end42of the extender41is spherical shaped. Also, for example, the second end43of pin41may be secured using an adhesive, solder, welding, press fitting, or any other suitable technique. As shown, the button44may have a surface48opposite the sensing die20that may be configured to receive a force. In some cases, surface48may be generally flat or planar, however, it is contemplated that in other embodiments, surface48may be curved (e.g. concave or convex) or may be formed to include other surface features for engaging a force transmitting object. For example, surface48of button44may be contoured to engage a flexible tube, such as tubes used in the medical industry.

In the illustrative embodiment ofFIG. 1, a protective housing14, or cover, of the force sensor10may be provided. The protective housing14may define a cavity generally shown at30for housing the sensing die20. As illustrated, the protective housing14is disposed on a top side (in the orientation shown inFIG. 1) of the substrate12. With such a configuration, the protective housing14may help protect the sensing element of sensing die20. In some cases, the protective housing14may be formed from, for example, plastic, polyamide, ceramic, or any other suitable material. Although not shown, it is contemplated that a bottom protective housing (not shown) may be provided on the bottom surface of the substrate12, if desired. When so provided, the bottom protective housing may define a cavity defining a reference pressure for the sensing diaphragm, or may include a pressure opening for exposing the sensing element (e.g. bottom side of sensing diaphragm22) to a second input pressure. In some cases, two protective housings14may be attached to the substrate12with the same or substantially the same “footprint” on each side, but this is not required.

In the illustrative embodiment, the protective housing14may be configured to include an opening36for the extender41and an inward protrusion body45defining a chamber for holding the actuation assembly, namely the spherical object40and extender41, in place. As shown, body45is formed as part of the protective housing14, however, it may be separately formed, as desired. Body45may be precisely formed with respect to the dimensions of spherical object40so as to maintain a relatively constant point of contact between spherical object40and sensing diaphragm22. Although not necessary, the body45may extend all the down to the substrate12to help isolate the sensing element20.

In the illustrative embodiment ofFIG. 1, the protective housing14may be attached to the package substrate12using a suitable adhesive or any other suitable bonding mechanism (e.g. solder, eutectic, etc.). As shown inFIG. 1, the protective housing14may define an opening36which provides access to the pressure sensing die22from the environment external to the protective housing. The opening may be sized according to the extender41to maintain the spherical object40and extender41in a generally upright (in the orientation shown inFIG. 1) orientation.

Although not shown, the sensor assembly10may include one or more electrical leads on the substrate12that can be electrically connected to the pressure sensing die20for receiving one or more signals corresponding to the pressure or force sensed by the sensing die20(e.g. sensing diaphragm22). In some cases, the one or more electrical leads may include metal, however, any suitable material may be used, as desired, such as conductive polymers.

In operation, when a current is applied to the piezoresistive sensing elements (e.g. to the Wheatstone bridge configuration of the piezoresistive sensing elements), an electrical output signal may be generated that is proportional to the degree of deflection of the diaphragm22or force applied to the force sensor10. The actuation assembly may be configured to transmit an external force to the sensing diaphragm22, thereby deflecting the sensing diaphragm22and, changing the resistance of the piezoresistive sensing elements. In some instances, the point of contact between the sensing diaphragm22and the spherical object40will determine to some extent the amount of output electrical signal, with differing points of contact producing different output signals for the same applied force. By restricting the movement of the spherical object40with the body45of the housing, and hence the point of contact on the sensing diaphragm22, increased repeatability of the output electrical signal for a given applied external force can be achieved.

In some applications, by detecting a force, the force sensor10may be used to determine the rate of flow of a medium through a tube. For example, the force sensor ofFIG. 1can be used to sense the amount of pressure a medium exerts on the interior walls of a tube, and may output an electrical signal that corresponds to the pressure exerted. The amount of pressure exerted on the inner walls of the tube may correlate to the rate of flow of the medium through the tube. As such, the electrical output of the force sensor10can be converted into the flow rate of the medium through the tube.

FIG. 2is a cross-sectional view of an illustrative force sensor110including signal conditioning circuitry127. Force sensor110may be similar to force sensor10shown inFIG. 1, but further includes signal conditioning circuitry127mounted on substrate12. As shown inFIG. 2, the signal conditioning circuitry127may be provided on a separate die or other electronics, and may be mounted in cavity130formed by protective housing116. In some cases, the signal conditioning circuitry127may include a microprocessor, a microcontroller, and/or an ASIC (Application Specific Integrated Circuit). In some cases, signal conditioning circuitry127may be mounted to the substrate112using an adhesive131or any other suitable bonding mechanism (e.g. solder, eutectic, etc.). As shown, signal conditioning circuitry127may be secured to the substrate12adjacent to the sensing die20. The signal conditioning circuitry127may be electrically connected to sensing die20via trace conductors on the substrate12, and in some cases, via bond wires129. Trace conductors on the substrate12may be connected to connectors, leads, bond pads or terminals (not shown) of the force sensor110. In some cases, it is contemplated that signal conditioning circuitry127may be electrically connected to the sensing die20in other ways, including direct die-to-die wire bonds, if desired.

When provided, the signal conditioning circuitry127may include circuitry that receives an output signal from the sensing die20, and may generate in response an output signal whose magnitude is representative of a magnitude of the force applied to the sensing die20. The signal conditioning circuitry127may condition the output signal of the sensing die to correct for repeatable variations, such as offset, sensitivity, non-linearity, temperature effects, and/or other variations. The signal conditioning circuitry127may condition the output signal to compensate for temperature-dependent variations in the electrical characteristic and/or to account for a nonlinear relationship between changes in the electrical characteristic and corresponding changes in the magnitude of the force.

FIG. 3is a cross-sectional view of another illustrative embodiment of a force sensor210including signal conditioning circuitry127. Force sensor210may be similar to force sensor110shown inFIG. 2in many respects, but force sensor210includes a housing (e.g. protective housing216) that defines a first cavity240and a second cavity240for housing the sensing die20and signal conditioning circuitry127, respectively. In the illustrative embodiment, the protective housing216may include one or more protrusions245and246that are configured to engage and/or seal to substrate12to isolate cavities230and240from one another. With such a configuration, the signal conditioning circuitry127may be physically isolated from the sensing die20to, in some cases, help protect the signal conditioning circuitry from becoming contaminated and/or provide a more robust sensor package.

In the illustrative embodiment, protective housing216may be formed or otherwise manufactured to include a recess formed in its underside to define cavity230for housing the signal conditioning circuitry127. The protective housing216may also be formed or otherwise manufactured to have a recess and/or protrusions defining cavity240for housing the sensing die20and actuation assembly. The protrusions, such as245and246, may extend to the substrate12and be bonded, secured, and/or sealed thereto. It is contemplated that other manners or techniques for forming cavities230and240may be used, as desired.

FIG. 4is a schematic diagram of an illustrative adhesive pattern that may be used for bonding the sensing die20to the substrate12. As shown inFIG. 4, the sensing die20may be configured to include one or more bond pads412for electrical connection to the conductive traces or bond pads on the substrate12. The bond pads412may be electrically connected to the sensing elements (e.g. piezoresistors) and/or other circuitry (e.g. trim, amplification, etc.) on the sensing die20. As shown, there are four bond pads412, however, this is merely illustrative and it is contemplated that any suitable number of bond pads412may be used, as desired.

In the illustrative embodiment, the adhesive may include a conductive adhesive416and nonconductive adhesive414patterned to electrically and mechanically attach the sensing die20to the substrate12(e.g. conductive traces). As shown inFIG. 4, the conductive adhesive416and the nonconductive adhesive414may be configured in an alternating pattern on the sensing die20. In this example, the conductive adhesive416may be at least partially applied over each or the one or more bond pads412to electrically connect the bond pads412to the bond pads of the substrate12. The nonconductive adhesive414may be applied between adjacent conductive adhesives416to provide electrical isolation between the bond pads. In the illustrated example, with four bond pads412being shown, there may be four applications of conductive adhesive416(one for each bond pad412), each separated by nonconductive adhesive414. This, however, is just one example and it is contemplated that other adhesive patterns and number of applications of conductive and nonconductive adhesive may be used.

In the illustrative embodiment, it is contemplated that any suitable conductive adhesive and nonconductive adhesive may be used. One example nonconductive adhesive is RTV6424, which is available from Momentive Performance Materials Inc. of Waterford, N.Y. One example conductive adhesive is SDC5000, which is available from Momentive Performance Materials Inc. of Waterford, N.Y. These are just examples, and it is contemplated that any other suitable conductive and nonconductive adhesive may be used.

In some embodiments, the conductive and nonconductive adhesives416and414may be applied at any suitable thickness to mechanically attach the sensing die20to the substrate12. For example, the conductive adhesive416and the nonconductive adhesive414may have a thickness in the range of about 0.01 millimeters to about 1.0 millimeters, about 0.05 millimeters to about 0.75 millimeters, about 0.05 millimeters to about 0.5 millimeters, about 0.10 millimeters to about 0.25 millimeters, or any other range of thicknesses, as desired.

WhileFIG. 4is shown with the conductive adhesive416and the nonconductive adhesive414applied to the sensing die20, it is to be understood that the conductive adhesive416and the nonconductive adhesive414may be applied to the substrate12and/or the sensing die20, if desired. Further, the illustrative pattern of conductive adhesive416and the nonconductive adhesive414is shown prior to the sensing die20mounted to the substrate12or, in other words, how the conductive adhesive416and the nonconductive adhesive414may be initially applied. Although not explicitly shown inFIG. 4, in some cases, the non-conductive adhesive and the conductive adhesive overlap one another, or otherwise collectively provide a continuous adhesive pattern around a perimeter of the sensing die20. This may help provide a seal between the sensing die20and the substrate12all the way around the perimeter of the sensing die20, if desired.

FIGS. 5 and 6are cross-sectional schematic views of other illustrative force sensors. As shown inFIG. 5, force sensor510may be similar to force sensor10, but may have an extender541having a first end542that may be generally conical shaped or otherwise pointed. Generally conical shaped first end542may engage a corresponding opening543in spherical object541.

As shown inFIG. 6, force sensor610may be similar to the foregoing force sensors, but may have the button644directly engaged to spherical object640without any extender. In this embodiment, button member644may include a protrusion645having a generally conical or pointed end that is configured to engage an opening646in the spherical object640. However, it is contemplated that protrusion645may be flat or have another shape, as desired. Moreover, it is contemplated that the button644may engage the spherical object640in any suitable manner including, for example, soldering, adhesively securing, or engaging protrusion643to the spherical object640, press fitting protrusion645into a hole formed in the spherical object640, or any other suitable manner of button member644to spherical object640with or without protrusion645.

Although not shown, it is contemplated that force sensors510and610may include any other feature of the foregoing force sensors. For example, force sensors510and610may include signal conditioning circuitry, if desired.

Having thus described the preferred embodiments of the present disclosure, those of skill in the art will readily appreciate that yet other embodiments may be made and used within the scope of the claims hereto attached. Numerous advantages of the disclosure covered by this document have been set forth in the foregoing description. It will be understood, however, that this disclosure is, in many respect, only illustrative. Changes may be made in details, particularly in matters of shape, size, and arrangement of parts without exceeding the scope of the disclosure. The disclosure's scope is, of course, defined in the language in which the appended claims are expressed.