Force sensor package and method of forming same

A low cost force sensor package and a method of forming such a package. The force sensor has a housing or package, such as a surface mount technology package (SMT), and a piezo resistive silicon die, or other force sensing element, carried on the housing or package. An actuator is operably coupled to the force sensing element for transferring force to the sensing element in response to receiving a force from an external source. The force sensing element is configured to sense the external force and generate an output signal representing the force. A signal conditioner is also carried on the housing for receiving the output signal. When the signal conditioner is electrically coupled to the force sensing element, the signal conditioner can condition the output signal and generate a conditioned output.

TECHNICAL FIELD

Embodiments are generally related to force sensors, and in particular, to discrete force sensors and methods of manufacturing such force sensors. Embodiments are additionally related to force sensors in the form of MEMS devices. Embodiments are also related to methods of packaging force sensors and force sensor packages formed thereby.

BACKGROUND OF THE INVENTION

Force sensors are configured to measure an external force applied to the sensors and provide output signals representative of the applied force.

Discrete force sensors, such as MEMS based force sensors, have applications in medical equipment control, such as kidney dialysis machines, drug delivery systems, hematology equipment, and medical instrumentation. Other applications of discrete force sensors include ultra-low displacement sensing in robotic end-effectors and variable tension equipment to name a few.

Miniature MEMS based force sensors are used for measuring very low force with good accuracy. Such force sensors employ a force sensing component, such as piezo resistive silicon (Si) die, in combination with an actuator which is arranged to press against the silicon die in response to an external force exerted against the actuator. The silicon die includes a flexible membrane which deflects in response to the actuator pressing on the silicon die. Deflection of the membrane causes piezo resistors disposed on the silicon die to stress and change resistance. Circuitry senses the change of resistance and determines the external force from this resistance change.

There is a need to provide force sensors which can be implemented in medical and other applications more efficiently and cost effectively.

BRIEF SUMMARY OF THE INVENTION

It is, therefore, one aspect to provide for an improved force sensor.

It is another aspect, to provide for a low cost force sensor package.

The aforementioned aspects of the invention and other objectives and advantages can now be achieved as described herein.

According to one aspect, a force sensor has a housing or package, which preferably is a surface mount technology package (SMT), and a force sensing element, which is preferably a piezo resistive silicon die, carried on the housing or package. An actuator is operably coupled to the force sensing element for transferring force to the sensing element in response to receiving a force from an external source. The force sensing element is configured to sense the external force and generate an output signal representing the force. A signal conditioner is also carried on the housing for receiving the output signal, whereby, when the signal conditioner is electrically coupled to the force sensing element, the signal conditioner can condition the output signal and generate a conditioned output.

A discrete, low cost force sensor package can be provided by arranging the signal conditioner on the same housing as the actuator and sensing element. Advantageously, a single package can be mounted on a circuit (not shown), such as a PCB, with a smaller surface area than the surface area required to mount existing force sensing and signal conditioning hardware separately so that less total surface area is occupied on the circuit.

The force sensing element can be isolated from the signal conditioner. Isolating the force sensing element from the signal conditioner enables the force sensing element to be maintained in a stable operating position. The force sensing element can be isolated from the signal conditioner by enclosing the sensing element in a cavity formed in the housing. The actuator can be configured on the housing to transfer force to the interior of the cavity.

The housing can define one other cavity in which the signal conditioner is enclosed. Enclosing the signal conditioner and force sensing die in respective cavities enables a robust sensor package to be provided.

The force sensing element can be a piezo resistive silicon die which is sandwiched between a pair of sealing members. The signal conditioner can be an ASIC, an instrumentation amplifier, or an operational amplifier, and optionally a temperature compensating circuit.

The force sensing element and signal conditioner can be electrically connected together internally or externally of the package. Preferably, one or more lead frames, carried on the housing, are electrically connected to the force sensing element and the signal conditioner so that the force sensing element and signal conditioner can be electrically connected via the lead frame to a printed circuit board. The force sensing element can be electrically connected to the signal conditioner via the printed circuit board.

According to another aspect, a force sensor package has a package having an external surface, an opening formed in the external surface, and a force sensing element disposed in the interior of the package. An actuator is disposed in the opening for transferring force to the interior of the package in response to receiving a force from an external source. The force sensing element is configured to detect force and generate an output signal representing the force. A signal conditioning circuit is integrated in the package for receiving the output signal, whereby, when the signal conditioner is electrically coupled to the force sensing element, the signal conditioner can condition the output signal and provide a conditioned output.

The package can be a SMT package which can have a base and a cover which are configured to assemble with one another such that the cover and base define a cavity. The opening can be formed in the cover or base. The package can include one other cover which is configured to assemble with the base such that the one other cover and base define one other cavity in which the signal conditioner is enclosed.

The sensing element can be a piezo resistive silicon die. The actuator can be a spherical object, such as a metallic ball, retained in the opening. The signal conditioner can be an ASIC including temperature compensating circuitry and/or amplification circuit.

One or more lead frames can be carried on the package. The force sensing element and signal conditioner can be electrically coupled to the lead frame(s) for electrically connecting the force sensing element and the signal conditioner to a printed circuit board.

According to yet another aspect, a method of packaging a force sensor comprises providing a housing, arranging a force sensing element on the housing, operably coupling an actuator on the housing to the force sensing element for transferring a force to the force sensing element in response to receiving the force from an external source, and arranging a signal conditioner also on the housing for receiving the output signal, whereby, when the signal conditioner is electrically coupled to the force sensing element, the signal conditioner can condition the output signal and provide a conditioned output.

Preferably, the housing defines a cavity. The method step of arranging the force sensing element on the housing comprises enclosing the force sensing element in the cavity, and wherein the method step of operably coupling the actuator on the housing to the force sensing element comprises arranging the actuator on the housing to transfer the force to the interior of the cavity.

Preferably, the housing further defines one other cavity. The method step of arranging the signal conditioner on the housing comprises enclosing the signal conditioner in the one other cavity.

DETAILED DESCRIPTION OF THE INVENTION

The particular values and configurations discussed in these non-limiting examples can be varied and are cited merely to illustrate at least one embodiment of the present invention and are not intended to limit the scope of the invention.

Referring toFIG. 1of the accompanying drawings, which illustrates a plan view taken from above the force sensor package according to one embodiment, in conjunction withFIG. 4, which illustrates a cross-sectional view taken along line C-C ofFIG. 1, the force sensor package1generally includes a force sensing element11carried on a housing or package2and an actuator8, also carried on the housing, for receiving an external force. Actuator8, operably coupled to the sensing element11, exerts force against the sensing element in response to an external force being applied to the actuator. Also carried on the housing2is a signal conditioner13for conditioning the output signal. As best shown inFIG. 6, which illustrates a functional block diagram of the force sensor package incorporated in an electrical circuit in which the force sensing element and signal conditioner are electrically coupled together, the force sensing element11is configured to sense the force exerted against the actuator8and generate an output signal50representing the applied external force. When the signal conditioner13is electrically coupled to the force sensing element11, the signal conditioner13can condition the output signal50and generate a conditioned output signal51.

By arranging the signal conditioner13on the same housing2as the actuator8and sensing element11, a discrete, low cost force sensor package1can be provided. Advantageously, both the force sensing element and the signal conditioner can be incorporated into a single package which is entirely self contained. The force sensor package1can be mounted on a circuit (not shown), such as a PCB, with a smaller surface area than the surface area required to mount existing force sensing and signal conditioning hardware so that less total surface area is occupied on the circuit.

In the illustrative embodiment of the force sensor package1, the force sensing element11comprises piezo-resistive silicon force sensor die11, such as a micro machined piezo-resistive Si-sensor die having a resistive bridge network as is known in the art. However, other types of pressure sensitive devices, such as a piezo resistive MEMS device or other force sensitive components, can be employed instead of a piezo-resistive silicon die. Also, in the illustrative embodiment, the actuator8comprises a spherical object8, such as a stainless steel ball or other type of metallic ball, disposed in an opening43formed in the external surface of the housing2and in contact with the force sensing element11. However, other types of actuators8can be readily utilized in the force sensor package1, such as for example, slidably mounted plungers or shafts or point contact type components other than spherical objects.

The housing2of the force sensor package1of the illustrative embodiment is a surface mount technology package of elastomeric construction. However, the housing2need not be a SMT package and can be any type of housing made from plastic or other material, preferably having a high melting point, which is suitable for carrying the actuator, sensing element and signal conditioner and electrically isolating the components as required. An example of a piezoresistive pressure transducer having an elastomeric construction is disclosed in U.S. Pat. No. 5,184,107 which is entitled “Piezoresistive pressure transducer with a conductive elastomeric seal”, issued to Maurer on Feb. 2, 1993, and which is incorporated herein by reference.

Referring additionally toFIG. 5, which illustrates an exploded perspective view of the force sensor package ofFIG. 1, the housing2includes first and second cavities21,29in which the force sensing element11and signal conditioner13are respectively enclosed together with their associated components. The housing cavities21,29can be formed in the housing in a variety of manners. Enclosing the signal conditioner13and force sensing die11in separate cavities enables a robust sensor package to be provided.

As best shown inFIG. 2, which illustrates a cross-sectional view taken along line B-B ofFIG. 1, andFIG. 5, the housing2includes a first cover4and base3which are configured to assemble with one another such that when the first cover is attached to the base, the first cover and base define the first cavity21.

A recess19, formed in the upperside of the base3, has a bottom surface25on which is disposed the signal conditioner die13. The signal conditioner13is electrically coupled to a lead frame34, carried on the base3, for mounting to contact pads of a PCB. Lead frame34is preferably insert molded in housing2and includes individual leads or pins14for surface mounting to corresponding conductive pads of a PCB. Bond wires15electrically connect the signal conditioner13to portions16of leads14which lead portions are embedded in the recess bottom surface25and extend outwardly through the base to the exterior.

First cover4has a complimentary recess18formed in its underside and is ultrasonically welded or heat sealed to the base3covering the base recess19so that the base recess19together with the first cover recess18form the cavity21in which signal conditioner13, associated bond wires15and lead portions16are sealed. Those skilled in the art would understand that the first cover4can be sealed to the base by methods other than ultrasonic welding or heat sealing. If required, alignment pins can be incorporated into the base3to enable the package1to be positioned accurately on a PCB.

In the illustrative embodiment, the signal conditioner13preferably comprises an ASIC. The ASIC may be configured as a digital amplifier with a built in temperature sensor for compensating temperature induced changes of the force sensor die output signal50caused by temperature affecting the operating characteristics of the force sensor package components. A differential sensor signal conditioner ZMD31050, supplied by ZMD America, Inc. Sensor ICs Business Unit 201 Old Country Road, Suite 204 Melville, N.Y. 11747 U.S.A., is an example of such an ASIC.

ASIC signal conditioner13is shown inFIG. 7which illustrates a block diagram of the electrical components of the force sensing die11electrically coupled to the ASIC13. The differential output from the resistive bridge39of the sensor die11is pre-amplified by the programmable gain amplifier30(PGA). A multiplexer31(MUX), coupled to the PGA30, transmits the signals from the bridge39to the analog-to-digital converter32(ADC). A temperature sensor36is configured to communicate with the MUX31. A calibration microcontroller (CMC)44corrects the digital signal based on calibration rules and data located in memory ROM33and EEPROM35. A digital-to-analog converter (DAC) converts the calibrated signal from the CMC back to analog providing an output51in analog format and a serial interface (SI)37provides output51in digital format. The ASIC circuitry is therefore configured to provide a digital output or an analogue output as indicated inFIG. 7. Alternatively, other types of signal conditioners can be employed such as an instrumentation amplifier or an operational amplifier.

Referring toFIG. 5in conjunction withFIG. 2, which illustrates a cross-sectional view taken along line A-A ofFIG. 1, also formed in the base upperside, adjacent and spaced from recess19, is another recess22having sidewalls27which protrude upwardly beyond the base upper surface26. Recess22is dimensioned so that the piezo-resistive Si sensor die11together with an elastomeric sealing plate10and an elastomeric conductive plate12can be accommodated in the recess22with the sensor die11sandwiched between the sealing and conductive plates10,12as shown inFIG. 2. The sealing plate10has a throughhole28in which the actuator8, in this case metallic ball8, is disposed in contact with the piezo-resistive Si sensor11.

A second cover9is securely attached in position, in cooperation with the base3, covering recess22such that the second cover9and base3form another cavity29in which the sensing die11, sealing plate10and conductor plate12and actuator8are retained in their operating positions by the second cover. The second cover9is fitted to the base3by means of longitudinal lugs or protrusions40, which are arranged to engage mating recesses41formed in the top of sidewalls27, and subsequently ultrasonically bonded or heat welded to the base. The sealing and conductive plates10,12ensure that the sensor die11is sealed within the housing2.

By isolating the forces sensing die11from the signal conditioner13, the stack of elastomeric sealing plate10, force sensing element11and elastomeric conductive plate12can be maintained in a constrained position ensuring that there is no relative movement in the stack elements.

The sensor die11is electrically coupled to the lead frame34, which extends through the housing to the cavity29, via elastomeric conductive seal12. The actuator ball8protrudes through the opening43formed in the second cover9so that an external force applied to the external surface of the second cover is concentrated through the ball8directly to the sensor die11. The metallic ball8provides high mechanical stability and is adaptable to a variety of applications.

By encapsulating both the signal conditioner13, the force sensing die11and actuator8within the same housing3, a discrete, self-contained low cost force sensor package1can be produced. Furthermore, utilizing a single package1enables the cost of installing the signal conditioner13and the force sensing die11, together with the actuator8, on a PCB, or other external circuit, to be reduced and secondary operations associated with installation to be eliminated. Also, employing a single housing2, particularly a SMT package, provides a miniature force sensor that can be mounted on a PCB with a smaller pad dimension ensuring less space is occupied on the PCB. Furthermore, the force sensor package1can directly provide an amplified and/or temperature compensated force sensing reading which would be advantageous in medical sensing applications and other sensing applications.

In the illustrative embodiment of the force sensor package shown in the accompanying figures, the sensor die11and signal conditioner13are located in separate cavities21,29isolated from one another and electrically connected to respective leads14of the lead frame34so that the sensor die11and signal conditioner13can be electrically coupled together externally via the PCB circuitry to which the lead frame34is mounted. Alternatively, however, the signal conditioner13and sensor die11can be electrically connected together internally by means of the lead frame34itself which could be insert molded into the housing such that the frame interconnects cavities21and29.

A method of packaging a force sensor according to one embodiment will now be described with reference to the force sensor package of the illustrative embodiment shown inFIGS. 1-5. Initially, the base3and first and second covers4,9, formed by plastic molding techniques known in the art, are provided. Thereafter, the lead frame34is embedded in the base3by means of insert molding techniques.

The signal conditioner13, which is preferably an ASIC13die, is located in base recess19and bonded to the recess bottom surface25. Preferably, the ASIC13is located in the recess19with at least part of the ASIC die situated in direct thermal contact with the lead frame34so that the temperature sensor in the ASIC can more efficiently sense temperature changes of the package1for compensation purposes.

Electrical contacts/pins (not shown) on the ASIC13can be then electrically connected to respective embedded portions16using bond wires15which are conductively bonded at one end to the ASIC contacts and at the other end to the respective lead portions16. Conductive bonding can be performed by known reflow soldering techniques at a maximum peak reflow temperature of the order of 250° C. or by other techniques apparent to those skilled in the art. The first cover4is ultrasonically bonded or heat sealed to the upper surface26of the base3such that the first cover4and recess19form the cavity21in which the ASIC13and wire bonding15are sealed.

The conductive sealing plate12, force sensing die11and sealing plate10are mounted in the other base recess22so that the force sensing die11is held sandwiched between the plates12,10. The actuator ball8is then mounted in the sealing plate throughhole28in contact with the force sensing die11. Thereafter, the second cover9is aligned with the sidewall27, the second cover protrusions40are press fit into the sidewall mating recesses41and the second cover ultrasonically bonded to the base such that the second cover9is securely attached to the top of the sidewall27and thereby retains the force sensing die11together with the conductive sealing and sealing plates12,10and ball8in their operating positions in the cavity29defined by the second cover9and recess22.

Preferably, the sensor die and signal conditioner are formed at the wafer level using conventional semiconductor and micro electro mechanical system (MEMS) equipment and fabrication techniques. The sensor dies and signal conditioners are singulated using known dicing or sawing techniques and then assembled in diced form on the bases together with the sealing plates. Following wire bonding by reflow soldering, the sensor dies, signal conditioners and associated components are sealed in the packages by attaching the covers thereby forming the final force sensor assembly.

A method of operating the force sensor package according to one embodiment will now be described with reference to the accompanying drawings and, in particular,FIGS. 6 & 7.

Initially, the force sensor package is connected to an external control circuit (not shown) and the signal conditioner and force sensing element are electrically connected together via the control circuit. In the case of the force sensor package shown inFIG. 1, the package1is incorporated into the control circuit by surface mounting the package on an external PCB forming part of a control circuit. The force sensor die11and ASIC13are electrically connected together via the PCB so that the sensor die output signal50can be fed via the PCB to the ASIC. An external power supply (not shown) supplies a power voltage Vs to the sensor and ASIC dies via the appropriate leads14.

Actuator8exerts a force against the force sensor die11in response to an external force being applied to the external surface of the second cover9. As indicated inFIGS. 6 & 7, the force sensor die11generates a sensor output signal50representing the applied force and the output signal is then fed to the signal conditioner13which conditions the sensor signal. In the example of the force sensor package ofFIG. 1, the actuator8is a metallic ball which transfers the force directly to the sensor die11causing deflection of the sensor die bridge resistors which deflection causes a change in resistance delta R of the bridge. This change of resistance causes the sensor output voltage Vo to change. Signal conditioner13receives the change in output signal voltage and accordingly conditions the change in sensor signal. The ASIC preferably includes temperature compensation and amplification of the sensor signal.

The embodiments and examples set forth herein are presented to best explain the present invention and its practical application and to thereby enable those skilled in the art to make and utilize the invention. Those skilled in the art, however, will recognize that the foregoing description and examples have been presented for the purpose of illustration and example only. Other variations and modifications of the present invention will be apparent to those of skill in the art, and it is the intent of the appended claims that such variations and modifications be covered.

For example, in the illustrative embodiments, separate covers can assemble with the base to form respective signal conditioner and force sensing cavities, however, a single cover could be configured to cooperate with the base to form both cavities. Furthermore, the signal conditioner may be isolated from the force sensing die and associated elements without enclosing the signal conditioner in a cavity.

The description as set forth is not intended to be exhaustive or to limit the scope of the invention. Many modifications and variations are possible in light of the above teaching without departing from the scope of the following claims. It is contemplated that the use of the present invention can involve components having different characteristics. It is intended that the scope of the present invention be defined by the claims appended hereto, giving full cognizance to equivalents in all respects.