WO2011159275A1

WO2011159275A1 - Moisture resistant mass flow sensor

Info

Publication number: WO2011159275A1
Application number: PCT/US2010/038571
Authority: WO
Inventors: Josh M. Fribley; Lamar F. Ricks; Mohammed Abdul Javvad Qasimi; Craig S. Becke
Original assignee: Honeywell International Inc.
Priority date: 2010-06-15
Filing date: 2010-06-15
Publication date: 2011-12-22

Abstract

A mass flow sensor apparatus may include an electrical substrate that includes a major surface and a plurality of electrical traces, a mass flow sensor die coupled to the major surface of the electrical substrate, and a unitary housing member coupled to the major surface of the electrical substrate. The unitary housing member may include an interior surface oriented to face the major surface of the electrical substrate and a plurality of projections that extend from the interior surface of the unitary housing member toward the major surface of the electrical substrate. The major surface of the electrical substrate, the interior surface of the unitary housing member, and the plurality of projections may define a first substantially enclosed cavity, a second substantially enclosed cavity that is fluidly isolated from the first substantially enclosed cavity, and an airflow cavity, which is fluidly isolated from the first substantially enclosed cavity and the second substantially enclosed cavity.

Description

MOISTURE RESISTANT MASS FLOW SENSOR

TECHNICAL FIELD

[0001] The disclosure relates to mass flow sensors.

BACKGROUND

[0002] Mass flow sensors may be utilized in many situations to accurately determine a flow rate of a fluid, such as a liquid or gas. For example, mass flow sensors may be utilized in manufacturing processes to measure and control flow rates of reactants in chemical reactions, in internal combustion engines to measure and control a ratio of air and fuel in the combustion gas, or in other processes in which a rate of fluid flow is important.

[0003] Another example of an apparatus which may utilize a mass flow sensor is a medical ventilator or respirator. Medical ventilators are used to provide supplemental oxygen to patients unable to breathe normally on their own accord. Medical ventilators may include a source of pressurized air or oxygen-rich air which is fluidly connected to the patient through a fluid conduit.

[0004] One type of mass flow sensor utilized in a medical ventilator is a thermal anemometer. In some examples, a thermal anemometer mass flow sensor is formed as a microelectromechanical system (MEMS). The MEMS may include, for example, a microbridge mass flow sensor. In a microbridge mass flow sensor, a current is provided through a conductive element across the microbridge, and the conductive element is heated proportionally to the current level. Flow of a fluid past the sensor cools a first set of resistive temperature sensors located on the microbridge upstream of the conductive element and heats a second set of resistive temperature sensors located on the microbridge downstream of the conductive element. The changing temperatures of the first and second sets of resistive temperature sensors cause a change in resistances of the respective temperature sensors. The MEMS mass flow sensor detects the mass flow rate of the fluid based on a voltage difference that results from the change in resistances of the resistive temperature sensors. SUMMARY

[0005] In general, the disclosure is directed to apparatuses, systems, and techniques for measuring fluid flow.

[0006] In one aspect, the disclosure is directed to a mass flow sensor apparatus that includes an electrical substrate that comprises a major surface having a first portion, a second portion and a third portion, and a plurality of electrical traces. According to this aspect of the disclosure, the mass flow sensor apparatus also includes a unitary housing member physically coupled to the major surface. The unitary housing member may include an interior surface oriented to face the major surface, and a plurality of projections that extend from the interior surface toward the major surface. The major surface, the interior surface, and the plurality of projections may define a first substantially enclosed cavity bounded by the first portion, a second substantially enclosed cavity that is fluidly isolated from the first substantially enclosed cavity and bounded by the second portion, and an airflow cavity that is fluidly isolated from the first substantially enclosed cavity and from the second substantially enclosed cavity and bounded by the third portion.

According to this aspect of the disclosure, the mass flow sensor apparatus further includes a mass flow sensor die physically coupled to the major surface. The mass flow sensor die may include (i) a sensing portion that is disposed within the airflow cavity and configured to sense air flow through the airflow cavity, and (ii) a wire bonding portion that is disposed within the second substantially enclosed cavity and electrically connected to the plurality of electrical traces by a plurality of wire bonds. The plurality of wire bonds may be encapsulated by an encapsulant.

Additionally, the mass flow sensor apparatus may include an integrated circuit that is disposed within the first substantially enclosed cavity and electrically coupled to the mass flow sensor die via at least one of the plurality of electrical traces.

[0007] In another aspect, the disclosure is directed to a method that includes attaching a mass flow sensor die to a major surface of an electrical substrate. In this aspect of the disclosure, the mass flow sensor die includes a sensing portion and a wire bonding portion, and the wire bonding portion includes a plurality of die wire bond pads. Additionally, the plurality of die wire bond pads may be electrically coupled to respective ones of a plurality of electrical substrate wire bond pads on the major surface of the electrical substrate by respective wire bonds. The method further includes dispensing an adhesive along at least a portion of a plurality of projections extending from an interior surface of a unitary housing member, a predetermined path on a major surface of the electrical substrate. The method additionally includes coupling the unitary housing member to the electrical substrate by positioning in substantial alignment with the plurality of projections extending from the interior surface of the unitary housing member a predetermined path on the major surface of the electrical substrate and contacting the

predetermined path and the adhesive. According to this aspect of the invention, the predetermined path defines a first portion, a second portion, and a third portion of the major surface of the electrical substrate. In some embodiments, the sensing portion of the mass flow sensor is disposed on the first portion of the electrical substrate and the wire bonding portion of the mass flow sensor and the electrical substrate wire bond pads are disposed on the second portion of the major surface of the electrical substrate. The interior surface of the unitary housing member, the plurality of projections, and the major surface of the electrical substrate may define a first substantially enclosed cavity, a second substantially enclosed cavity that is fluidly isolated from the first substantially enclosed cavity, and an airflow cavity. An integrated circuit may be disposed within the first substantially enclosed cavity and electrically connected to at least one electrical trace of the electrical substrate, and the at least one electrical trace and at least one of the bond wires may electrically connect the mass flow sensor die and the integrated circuit. In some examples, the third portion of the major surface of the electrical substrate bounds the first substantially enclosed cavity. Further according to this aspect of the disclosure, the wire bonding portion of the mass flow sensor die is disposed within the second substantially enclosed cavity, and the second portion of the major surface of the electrical substrate bounds the second substantially enclosed cavity. Additionally, the sensing portion of the mass flow sensor die is disposed within the airflow cavity and is configured to sense air flow through the airflow cavity, the first portion of the major surface of the electrical substrate bounds the airflow cavity, and the airflow cavity is fluidly isolated from the first substantially enclosed cavity and from the second substantially enclosed cavity.

[0008] The details of one or more embodiments are set forth in the accompanying drawings and the description below. Other features, objects, and advantages of the disclosure will be apparent from the description and drawings, and from the claims.

BRIEF DESCRIPTION OF DRAWINGS

[0009] FIG. 1 is a perspective view of an example of a mass flow sensor apparatus.

[0010] FIG. 2 is a plan view of an example of a mass flow sensor apparatus as viewed from a back of the mass flow sensor apparatus.

[0011] FIG. 3 is a plan view of an example of a mass flow sensor apparatus as viewed from above a major surface of a unitary housing member.

[0012] FIG. 4 is a plan view of an example of a printed circuit board of a mass flow sensor apparatus.

[0013] FIG. 5 is section diagram of an example of a mass flow sensor apparatus.

[0014] FIG. 6 is section diagram of an example of a mass flow sensor apparatus.

[0015] FIG. 7 is a plan view of an example of a mass flow sensor apparatus and shows greater detail of a mass flow sensor die.

[0016] FIG. 8 is a flow diagram of an example of a technique for manufacturing a mass flow sensor apparatus.

DETAILED DESCRIPTION

[0017] FIGS. 1-7 illustrate various views of a mass flow sensor apparatus 10 according to one embodiment of the disclosure. In particular, FIG. 1 is a perspective view of mass flow sensor apparatus 10. Mass flow sensor apparatus 10 includes a unitary housing member 12 and an electrical substrate 14. In some embodiments, unitary housing member 12 is formed of a single piece of material. For example, unitary housing member 12 may be formed of a single, molded piece of plastic. Unitary housing member 12 may facilitate manufacture of mass flow sensor apparatus 10, as compared to a mass flow sensor apparatus 10 that includes a nonunitary housing member (i.e., a housing member that includes two or more separate portions). In some embodiments, manufacture of mass flow sensor apparatus 10 may be simpler or less expensive than manufacture of a mass flow sensor apparatus that includes a housing member having multiple portions.

[0018] Electrical substrate 14 may comprise a suitable substrate that includes electrical traces for electrically connecting various components of mass flow sensor apparatus 10. For example, electrical substrate 14 may include a printed circuit board (PCB), a ceramic, such as a thick film network (TFN), or a flexible circuit. In some embodiments, electrical substrate 14 may include a multilayer PCB, TFN, or flexible circuit, in which electrical traces may be formed on multiple layers within the PCB, TFN, or flexible circuit. In some examples, electrical traces may be formed on at least one of first major surface 36 or second major surface 38 of electrical substrate 14.

[0019] Unitary housing member 12 includes a major exterior surface 16, which is oriented substantially away from electrical substrate 14. Unitary housing member 12 defines a first aperture 18 and a second aperture 20 in major exterior surface 16. First aperture 18 and second aperture 20 extend through unitary housing member 12 to a second interior surface 96 (FIG. 6) of unitary housing member 12, where second interior surface 96 is oriented toward electrical substrate 14. First aperture 18 and second aperture 20 can facilitate fluidic communication between an exterior of mass flow sensor apparatus 10 and a first substantially enclosed cavity 90 (FIG. 6) of mass flow sensor apparatus 10, which is defined by a second major surface 38 of electrical substrate 14 and unitary housing member 12. In this way, first aperture 18 and second aperture 20 facilitate filling of the internal cavity with an encapsulant 86 (FIG. 6), such as silicone, after assembly of electrical substrate 14 and unitary housing member 12 to form mass flow sensor apparatus 10, as will be described in further detail below.

[0020] Unitary housing member 12 also includes a first external projection 24 that defines a first orifice 28 in fluidic communication with airflow cavity 88 (FIG. 6) of mass flow sensor apparatus 10, which is defined by second major surface 38 of electrical substrate 14 and integral housing member 12. Further, integral housing member 12 includes a second external projection 22 that defines a second orifice 26, also in fluidic communication with airflow cavity 88 of mass flow sensor 10. As illustrated in FIG. 2, first orifice 28 may be fluidly connected to a fluid source and function as an inlet orifice through which the fluid enters air flow cavity 88 (FIG. 6). Second orifice 26 may function as an outlet orifice from which the fluid exits air flow cavity 88 into a subsequent flow channel. In other embodiments, first orifice 28 may be configured as an outlet orifice and second orifice 26 may be configured as an inlet orifice. In some embodiments, at least one of first external projection 24 and second external projection 22 may be configured to couple to a conduit which defines a fluid flow path. For example, first external projection 24 may be coupled to a flexible tube that is connected to a fluid source. The conduit and first external projection 24 may be affixed to each other using friction fit, an adhesive, or another mechanical fixation mechanism, such as a clamp, clip, or the like.

[0021] Although first external projection 24 and second external projection 22 are illustrated in FIG. 1 as extending from major exterior surface 16, in other embodiments, projections 24 and 22 may extend from other surfaces of unitary housing member 12, such as, for example, one or more of walls 72, 74, 76, or 78.

[0022] FIG. 2 is a plan view mass flow sensor apparatus 10 when viewed from a back of the apparatus 10. As shown in FIG. 2, unitary housing member 12 includes a first clip 32 and a second clip 34, which facilitate mechanical coupling between electrical substrate 14 and unitary housing member 12. As shown in FIGS. 2 and 3, first clip 32 and second clip 34 are configured to be inserted in a first notch 42 defined by electrical substrate 14 and a second notch 44 defined by electrical substrate 14, respectively. When inserted, first clip 32 engages with electrical substrate 14 proximate to first notch 42 and second clip 34 engages with electrical substrate 14 proximate to second notch 44. Together, first clip 32 and second clip 34 mechanically couple unitary housing member 12 to electrical substrate 14.

[0023] Although the embodiment of mass flow sensor apparatus 10 shown in the FIG. 2 illustrates a unitary housing member 12 that includes two clips 32 and 34, in other embodiments, unitary housing member 12 may include more than two clips or fewer than two clips. In some embodiments, unitary housing member 12 may not include any clips and may be coupled to electrical substrate 14 using other means, such as an adhesive.

[0024] In some embodiments, as illustrated in FIG. 3, electrical substrate 14 defines a number of notches 42 and 44 corresponding to the number of clips that unitary housing member 12 includes. Thus, in some embodiments in which unitary housing member 12 includes more than two or fewer than two clips, electrical substrate 14 defines a corresponding number of notches 42 and 44. In some embodiments, electrical substrate 14 may not define a number of notches 42 and 44 corresponding to the number of clips included with unitary housing member 12. For example, at least one clip 42, 44 may engage with an edge of electrical substrate 14 which is substantially linear (i.e., which defines a substantially straight line). In some embodiments, electrical substrate 14 does not define any notches 42 and 44, and all clips included in unitary housing member 12 may engage with a substantially linear edge of electrical substrate 14.

[0025] FIG. 2 also illustrates a plurality of electrical contacts 30 extending from electrical substrate 14. The plurality of electrical contacts 30 can facilitate electrical connection between circuitry within mass flow sensor apparatus 10 and external circuitry.

[0026] As shown in FIGS. 1 and 2, when unitary housing member 12 is coupled to electrical substrate 14 to form mass flow sensor apparatus 10, outer surfaces of unitary housing member 12 and electrical substrate 14 define outer surfaces of mass flow sensor apparatus 10. In the illustrated embodiments, the outer surfaces of mass flow sensor apparatus 10 include a first major surface 36 of electrical substrate 14 (FIG. 2), major exterior surface 16 of unitary housing member 12, walls 72, 74, 76, and 78 of unitary housing member 12, and first exterior projection 24 and second exterior projection 22. In other embodiments, the outer surfaces of mass flow sensor apparatus 10 may be defined by more or fewer surfaces of electrical substrate 14 and unitary housing member 12.

[0027] In some embodiments, mass flow sensor apparatus 10 may be utilized in a medical ventilator or respirator, in which mass flow sensor apparatus 10 is used to detect a mass flow rate of oxygen or air. In some implementations, electrical substrate 14 may not need to be fully enclosed and substantially isolated from the surrounding environment. For example, first major surface 36 of electrical substrate 14 may define an outer surface of mass flow sensor apparatus 10. In some embodiments, this may simplify a manufacturing process of mass flow sensor apparatus 10 compared to a mass flow sensor that includes a housing that encloses substantially all of an electrical substrate. Similarly, the materials used in mass flow sensor apparatus 10 and the manufacture of mass flow sensor apparatus 10 may be less expensive, due to a reduced amount of materials utilized in unitary housing member 12 and simplified assembly of electrical substrate 14 and unitary housing member 12 to form apparatus 10.

[0028] FIG. 3 is a plan diagram showing major exterior surface 16 of unitary housing member 12 as viewed from above major exterior surface 16. FIG. 3 also illustrates as dashed lines features formed on second major surface 38 of electrical substrate 14. FIG. 4 illustrates a plan view of second major surface 38 of electrical substrate 14 with unitary housing member 12 removed. Initially, FIGS. 3 and 4 illustrate first notch 42 and second notch 44 defined by electrical substrate 14 along a perimeter of electrical substrate 14.

[0029] Second major surface 38 is oriented toward unitary housing member 12 (that is, second major surface 38 faces unitary housing member 12) and forms an interior surface of mass flow sensor apparatus 10. As illustrated in FIG. 3, attached to second major surface 38 is a mass flow sensor die 46, which generally includes a sensing portion 48 and a wire bonding portion 50. In some

embodiments, mass flow sensor die 46 may be adhered to second major surface 38. Mass flow sensor die 46 may be adhered to surface 38 using, for example, an epoxy resin or other suitable adhesive.

[0030] In the depicted example, second major surface 38 includes at least three portions. A first major portion 58 of second major surface 38, together with a first interior surface 94 of unitary housing member 12 and second projection 102, defines airflow cavity 88. As described briefly above, airflow cavity 88 is in fluidic communication with first orifice 28 and second orifice 26. Air or another fluid may flow via first orifice 28 and through airflow cavity 88. In the depicted example, sensing portion 48 of mass flow sensor die 46 is disposed within airflow cavity 88, allowing the mass flow sensor die 46 to sense a mass flow rate of air or another fluid flowing through airflow cavity 88. The air or other fluid then flows out second orifice 26, perhaps to subsequent portions of the fluid flow path.

[0031] A second major portion 56 of second major surface 38, together with a second interior surface 96, first projection 100, and second projection 102 of unitary housing member 12, defines a first substantially enclosed cavity 90. Wire bonding portion 50 of mass flow sensor die 46 may be disposed within first substantially enclosed cavity 90, and may be separated from airflow cavity 88 by first projection 100 (FIG. 6). In this way, wire bonding portion 50 may be substantially fluidly isolated from airflow cavity 88, which may reduce or substantially eliminate exposure of wire bonding portion 50 to the fluid flowing through airflow cavity 88.

[0032] In some embodiments, second projection 102 may extend toward a surface of mass flow sensor die 46 between wire bonding portion 50 and sensing portion 48, but may not contact the surface of mass flow sensor die 16. In some cases, this may allow room for mass flow sensor die 46 and/or unitary housing 12 to expand and/or contract due to changes in temperature without contact between the surface of mass flow sensor die 16 and first projection 100. In such cases, encapsulant 86 may extend at least partially into the gap between first projection 100 and the surface of mass flow sensor die 46 to substantially fluidly isolate first substantially enclosed cavity 90 from airflow cavity 88.

[0033] FIG. 3 also illustrates first aperture 18 and second aperture 20, which are defined by major exterior surface 16 of integral housing member 12. As shown in FIGS. 3 and 5, first aperture 18 and second aperture 20 are positioned so that they open into first substantially enclosed cavity 90. First aperture 18 and second aperture 20 may facilitate dispensing of an encapsulant 86 (FIG. 6) into first substantially enclosed cavity 90 after unitary housing member 12 is coupled to electrical substrate 14. In some embodiments, encapsulant 86 is dispensed into first substantially enclosed cavity 90 through first aperture 18 while second aperture 20 allows venting of air from first substantially enclosed cavity 90. In other embodiments, first aperture 18 may be a vent and encapsulant 86 may be dispensed through second aperture 20. In other embodiments, major exterior surface 16 of integral housing member 12 may define a single aperture opening into first substantially enclosed cavity 90, or may define more than two apertures that open into first substantially enclosed cavity 90.

[0034] A third major portion 66 of second major surface 38, together with a third interior surface 98, first projection 100, second projection 102, and a third projection 104 of unitary housing member 12, define a second substantially enclosed cavity 92. Second substantially enclosed cavity 92 may contain signal conditioning circuitry, which can perform initial processing of signals generated by mass flow sensor die 46. For example, the signal conditioning circuitry may include at least one application specific integrated circuit (ASIC) 106 (FIG. 6), another processing unit, such as a general purpose processor, a field programmable gate array (FPGA), or passive electrical components, such as resistors, inductors, or capacitors. The signal conditioning circuitry may be electrically coupled to mass flow sensor die 46 via at least one electrical trace carried by electrical substrate 14. Second substantially enclosed cavity 92 may be substantially fluidly isolated from first substantially enclosed cavity 90 and airflow cavity 88. In some embodiments, the signal conditioning circuitry is electrically coupled to at least one of plurality of electrical contacts 30 to provide electrical communication between the signal conditioning circuitry, mass flow sensor die 46, and electrical circuitry outside mass flow sensor apparatus 10.

[0035] As FIGS. 3 and 4 illustrate, first major portion 58, second major portion 56, and third major portion 66 of second major surface 38 of electrical substrate 14 may be defined by a first adhesive channel 54, a second adhesive channel 68, and a third adhesive channel 70. In some embodiments, channels 54, 68, and 70 may comprise depressions in second major surface 38 of electrical substrate 14 or ridges on second major surface 38 of electrical substrate 14. In other

embodiments, channels 54, 68, and 70 may be substantially coplanar with second major surface 38 of electrical substrate 14, and may simply be defined locations along which the adhesive can be dispensed.

[0036] Together, first adhesive channel 54, second adhesive channel 68, and third adhesive channel 70 define a substantially continuous interface along which first projection 100, second projection 102, and third projection 104 of unitary housing member 12 may contact second major surface 38 of electrical substrate 14. An adhesive may be dispensed along first adhesive channel 54, second adhesive channel 68, and third adhesive channel 70 prior to coupling unitary housing member 12 to electrical substrate 14. The adhesive then may couple unitary housing member 12 and electrical substrate 14, and may also contribute to fluid isolation among airflow cavity 88, first substantially enclosed cavity 90, and second substantially enclosed cavity 92 by providing a seal between electrical substrate 14 and the respective protrusions 100, 102, and 104. In some embodiments, the adhesive may include an epoxy or another adhesive which is capable of adhering the materials from which electrical substrate 14 and integral housing member 12 are formed.

[0037] As shown in FIG. 4, wire bonding portion 50 of mass flow sensor die 46 includes a plurality of die wire bond pads 52. Electrical substrate 14 includes a corresponding number of electrical substrate wire bond pads 64, which are located on second major portion 56 of second major surface 38 of electrical substrate 14, within first substantially enclosed cavity 90. Electrical connection between mass flow sensor die 46 and electrical substrate 14 is made via a plurality of wire bonds 80, each wire bond connecting a respective one of the plurality of electrical substrate wire bond pads 64 to a respective one of die wire bond pads 52. In some embodiments, at least one of electrical substrate wire bond pads 64 may be connected to ground, and at least one other of electrical substrate wire bond pads 64 may be connected to a voltage source. The electrical source and electrical ground may be used to provide a voltage bias to the mass flow sensor located on sensing portion 48 of mass flow sensor die 46 (e.g., microbridge mass airflow sensor 60 shown in FIG. 7). Electrical substrate wire bond pads 64 are electrically connected to electrical traces 40 carried by electrical substrate 14 (FIG. 7). In embodiments in which electrical substrate 14 is a single-layer electrical substrate, electrical traces 40 may be formed on second major surface 38. In other embodiments, electrical substrate 14 may be a multi-layer electrical substrate, and electrical traces 40 may be formed on second major surface 38, on an inner layer of electrical substrate 14, or a combination thereof. Electrical traces 40 electrically connect electrical substrate wire bond pads 64 to signal conditioning circuitry within second substantially enclosed cavity 92, such as ASIC 106. In this way, mass flow sensor die 46 is electrically connected to signal conditioning circuitry within second substantially enclosed cavity 92, while being located in a separate, substantially fluidly isolated, first substantially enclosed cavity 90.

[0038] Because the signal conditioning circuitry is located in second substantially enclosed cavity 92, which is substantially fluidly isolated from both first substantially enclosed cavity 90 and airflow cavity 88, the signal conditioning circuitry may not need to be encapsulated in an encapsulant. In some

embodiments, this may reduce an amount of encapsulant used, and may reduce material costs for mass flow sensor apparatus 10. Similarly, not encapsulating the signal conditioning circuitry may simplify and reduce the cost of manufacturing for mass flow sensor apparatus 10 compared to an apparatus 10 in which the signal conditioning circuitry is encapsulated by an encapsulant. In other embodiments, the signal conditioning circuitry may be encapsulated with an encapsulant within second substantially isolated cavity 92.

[0039] FIG. 6 illustrates one possible configuration of airflow cavity 88, which is defined, in part, by a first interior surface 94 of unitary housing member 12 that extends from major exterior surface 16 toward mass flow sensor die 46. In some embodiments, such a configuration of first interior surface 94 may be utilized to affect a sensing range of mass flow sensor die 46. In particular, by reducing a cross-sectional area of airflow cavity 88 proximate to mass flow sensor die 46, a velocity of fluid flow may be increased. This may result in greater sensitivity for mass flow sensor die 46, because an increase in velocity of fluid flow may increase an amount of heat transferred in microbridge mass airflow sensor 60, which will be discussed more fully below. In this way, by controlling the shape of first interior surface 94 and the cross-sectional area of airflow cavity 88, a sensitivity of mass flow sensor die 46 may be controlled such that the sensitivity is appropriate for the mass flow rates with which mass flow sensor apparatus 10 will be used. For example, in relatively low fluid flow rate applications, first interior surface 94 may extend further into airflow cavity 88 and reduce a cross-sectional area of airflow cavity 88. By contrast, in relatively higher fluid flow rate applications, first interior surface 94 may not extend as far into airflow cavity 88, resulting in a larger cross-sectional area of airflow cavity 88. [0040] In some embodiments, mass flow sensor die 46 may be a

microelectromechanical system (MEMS), which includes both a sensor and associated circuitry. For example, mass flow sensor die 46 may be a MEMS-based thermal anemometer, such as a bridge, microbridge, brick, microbrick, or diaphragm mass flow sensor. In some cases, sensing portion 48 of mass flow sensor die 46 may include a microbridge mass airflow sensor 60. As shown in FIG. 7, microbridge mass airflow sensor 60 may include a cavity 82 defined in a silicon substrate 62. In some embodiments, cavity 82 may be etched in the silicon substrate 62 using known semiconductor processing techniques. Microbridge mass airflow sensor 60 includes an upstream bridge 108 and a downstream bridge 110, which each span cavity 82. Upstream bridge 108 and downstream bridge 110 are separated by a gap 120, and are substantially thermally isolated from silicon substrate 62. Microbridge mass airflow sensor 60 further includes a resistive heating element 112 formed on upstream bridge 108 and downstream bridge 110. Upstream temperature sense resistors 114 are formed on upstream bridge 108 and downstream temperature sense resistors 116 are formed on downstream bridge 110, such that resistive heating element 112 is located between upstream temperature sense resistors 114 and downstream temperature resistors 116. Upstream bridge 108 and downstream bridge 110 may be Wheatstone bridge sense elements according to some implementations.

[0041] Microbridge mass airflow sensor 60 senses a mass flow rate of the fluid flowing past sensor 60 based on a temperature differential between upstream temperature sense resistors 114 and downstream temperature sense resistors 116. In particular, a current is conducted through resistive heating element 112. This causes resistive heating element 112 to be heated above an ambient temperature of microbridge mass airflow sensor 60. At steady state (i.e., in the absence of substantial fluid flow through airflow cavity 88), a temperature distribution within upstream bridge 108 may be substantially symmetrical (e.g., a mirror image) with the temperature distribution within downstream bridge 110.

[0042] As the fluid flows past sensor 60 in the direction indicated by arrow 118, upstream temperature sense resistors 114 are cooled by convection, while downstream temperature sense resistors 116 are heated by convection due to the heat flow from resistive heating element 112 to the flowing fluid. When upstream temperature sense resistors 114 and downstream temperature sense resistors 116 are connected in a Wheatstone bridge circuit, the temperature difference between upstream temperature sense resistors 114 and downstream temperature sense resistors 116 results in a voltage difference between the resistors 114 and 116, which can be utilized to determine a mass flow rate of the fluid.

[0043] In other implementations, mass flow sensor die 46 may include a brick or microbrick mass flow sensor. A brick or microbrick mass flow sensor may generally be similar to microbridge mass flow sensor 60, but may not include cavity 82 etched in silicon substrate 62. Because of this, a brick or microbrick mass flow sensor may have a higher thermal mass and respond more slowly to thermal influences.

[0044] In other examples, mass flow sensor die 46 may include a diaphragm mass flow sensor. A diaphragm mass flow sensor may include a thin diaphragm of silicon covering an etched cavity. The temperature sense resistors (e.g., resistors 114 and 116) and resistive heating element (e.g., resistive heating element 112) may be formed on the diaphragm. In some examples, a diaphragm mass flow sensor may have a thermal mass between the thermal mass of a brick or microbrick mass flow sensor and the thermal mass of a microbridge mass airflow sensor 60. Because of this, a diaphragm mass flow sensor may have a responsiveness to temperature changes between the responsiveness of a brick or microbrick mass flow sensor and the responsiveness of microbridge mass airflow sensor 60.

[0045] In other embodiments, mass flow sensor die 46 may include another type of mass flow sensor, such as a membrane sensor, in which a cavity is formed in a silicon substrate and the cavity is covered with a silicon or glass film. A membrane sensor may detect a mass flow rate by sensing a differential pressure across the membrane. Other mass flow sensors also may be utilized in mass flow sensor apparatus 10.

[0046] FIG. 8 is a flow diagram illustrating an example technique for

manufacturing mass flow sensor apparatus 10. Initially, the method includes forming unitary housing member 12 (122). In some embodiments, unitary housing member 12 includes a polymer or plastic, and may be formed by, for example, injection molding, compression molding, or the like. As described above, the unitary nature of unitary housing member 12 may simplify reduce costs associated with manufacture compared to a housing member that includes multiple portions.

[0047] In a separate step, the technique includes forming electrical substrate 14 (124). In some embodiments, forming electrical substrate 14 (124) includes forming a single-layer electrical substrate 14, while in other embodiments, forming electrical substrate 14 (124) includes forming a multi-layer electrical substrate 14. Whether electrical substrate 14 includes a single layer or multiple layers may depend on, for example, the extent and complexity of the circuitry attached to electrical substrate 14, including, for example, mass flow sensor die 46, another sensor, signal conditioning circuitry such as ASIC 106 or another processing unit, or passive components such as resistors, capacitors, and inductors. In some embodiments, electrical substrate 14 may include a single layer, which may further reduce material and manufacturing costs associated with mass flow sensor apparatus 10 compared to a mass flow sensor including a more complex design (e.g., a multilayer electrical substrate).

[0048] Once electrical substrate 14 is formed, the technique continues with attaching components to electrical substrate 14 (126). Components attached to electrical substrate 14 include mass flow sensor die 46, any other sensors utilized in mass flow sensor apparatus, such as a humidity sensor, temperature sensor, or the like, and signal conditioning circuitry, such as ASIC 106 and/or passive components. As described above, mass flow sensor die 46 may be physically attached to electrical substrate 14 using an adhesive, and may be electrically connected to electrical substrate 14 using bond wires 80. In some embodiments, the adhesive may include a thermal cure adhesive. Both physical and electrical connection of components to electrical substrate 14 are included within the step of attaching components to electrical substrate 14 (126). In some embodiments, electrically connecting components to electrical traces on electrical substrate 14 may include soldering electrically conductive portions of the components to the electrical traces on electrical substrate 14. For example, bond wires 80 may be reflow soldered to die wire bond pads 52 on mass flow sensor die 46 and electrical substrate wire bond pads 64 on electrical substrate 14. In other examples, bond wires 80 may be thermosonically bonded to die wire bond pads 52 on mass flow sensor die 46 and electrical substrate wire bond pads 64 on electrical substrate 14.

[0049] Once the predetermined components are attached to electrical substrate 14, the technique continues with dispensing adhesive on electrical substrate 14 along a predetermined path (128). As illustrated in FIGS. 3 and 4, the predetermined path may include first adhesive channel 54, second adhesive channel 68, and third adhesive channel 70. As described above, the adhesive couples projections 100, 102, 104 of unitary housing member 12 to electrical substrate 14 at predetermined points or along a predetermined path. In some embodiments, the adhesive contributes to substantial fluid isolation among airflow cavity 88, first substantially enclosed cavity 90, and second substantially enclosed cavity 92. The adhesive may include any adhesive suitable for coupling electrical substrate 14 to the material from which unitary housing member 12 is formed. For example, the adhesive may include an epoxy or the like.

[0050] In other examples, instead of or in addition to dispensing adhesive on electrical substrate 14 along the predetermined path (128), the technique may include dispensing adhesive on at least a portion of first projection 100, second projection 102, and/or third projection 104 of unitary housing member 12.

[0051] After the adhesive is dispensed on electrical substrate 14 along the predetermined path (128) (or on at least a portion of one or more of the projections 100, 102, 104), unitary housing member 12 is coupled to electrical substrate 14 (130). In some embodiments, coupling unitary housing member 12 to electrical substrate 14 (130) includes aligning first clip 32 and second clip 34 with first notch 42 and second notch 44, respectively, and inserting first clip 32 in first notch 42 and second clip 34 in second notch 44. In other embodiments, unitary housing member 12 may not include first clip 32 and second clip 34, and coupling unitary housing member 12 to electrical substrate 14 (130) does not include fitting first clip 32 and second clip 34 in first notch 42 and second notch 44.

[0052] In any case, coupling unitary housing member 12 to electrical substrate 14 (130) includes positioning projections 100, 102, 104 in substantial alignment with the adhesive dispensed along first adhesive channel 54, second adhesive channel 68, and third adhesive channel 70, and contacting the projections 100, 102, 104 to the adhesive. Additionally, in some embodiments, coupling unitary housing member 12 to electrical substrate 14 (130) may include curing the adhesive, e.g., via heat, radiation, or the like, or allowing the adhesive to set. In this way, the adhesive adheres unitary housing member 12 to electrical substrate 14 and contributes to substantial fluid isolation among airflow cavity 88, first substantially enclosed cavity 90, and second substantially enclosed cavity 92, along with electrical substrate 14 and unitary housing member 12.

[0053] The technique illustrated in FIG. 8 further includes dispensing encapsulant 86 in first substantially enclosed cavity 90 (132). As described above, first aperture 18 and a second aperture 20 permit access to first substantially enclosed cavity 90 after unitary housing member 12 and electrical substrate 14 are coupled together. Encapsulant 86 may be dispensed through one of first aperture 18 or second aperture 20 in liquid form via a nozzle. Once dispensed through first aperture 18 or second aperture 20, encapsulant 86 may flow around bond wires 80, die wire bond pads 52 and electrical substrate wire bond pads 64 to substantially encapsulate wires 80 and pads 52 and 64. Encapsulant 86 may then set (i.e., increase in viscosity), crosslink, or cure to form a substantially non-flowing material. In some embodiments, encapsulant 86 may substantially fill cavity 90. In other embodiments, encapsulant 86 may cover bond wires 80, die wire bond pads 52, and electrical substrate wire bond pads 64, but may not substantially fill cavity 90. As described above, in some embodiments, encapsulant 86 includes silicone, such as a fluorosilicone or a dimethyl silicone. Encapsulant 86 may reduce or substantially prevent contact between the fluid flowing through airflow cavity 88 and bond wires 80, die bond pads 52 and electrical substrate die pads 64. This may reduce or substantially eliminate risk of corrosion or oxidation of bond wires 80, die bond pads 52 and electrical substrate die pads 64, which may otherwise occur due to presence of oxygen, water vapor, or another chemical species in the fluid flowing through airflow cavity 88.

[0054] Various embodiments have been described. These and other embodiments are within the scope of the following claims.

Claims

CLAIMS:

1. A mass flow sensor apparatus, comprising:

an electrical substrate that comprises a major surface having a first portion, a second portion and a third portion, and a plurality of electrical traces;

a unitary housing member physically coupled to the major surface, wherein the unitary housing member comprises an interior surface oriented to face the major surface, and a plurality of projections that extend from the interior surface toward the major surface, wherein the major surface, the interior surface, and the plurality of projections define a first substantially enclosed cavity bounded by the first portion, a second substantially enclosed cavity that is fluidly isolated from the first substantially enclosed cavity and bounded by the second portion, and an airflow cavity that is fluidly isolated from the first substantially enclosed cavity and from the second substantially enclosed cavity and bounded by the third portion;

a mass flow sensor die physically coupled to the major surface, wherein the mass flow sensor die comprises (i) a sensing portion that is disposed within the airflow cavity and configured to sense air flow through the airflow cavity, and (ii) a wire bonding portion that is disposed within the second substantially enclosed cavity and electrically connected to the plurality of electrical traces by a plurality of wire bonds, and wherein the plurality of wire bonds are encapsulated by an encapsulant; and

an integrated circuit that is disposed within the first substantially enclosed cavity and electrically coupled to the mass flow sensor die via at least one of the plurality of electrical traces.

2. The mass flow sensor apparatus of claim 1, wherein at least a portion of at least one of the plurality of projections is adhered to the major surface of the electrical substrate by an adhesive.

3. The mass flow sensor apparatus of claim 1, wherein the unitary housing member defines an aperture extending from an exterior surface of the unitary housing member to the interior surface of the unitary housing member, and wherein the encapsulant is introduced into the second substantially enclosed cavity via the aperture.

4. The mass flow sensor apparatus of claim 1, wherein a second major surface of the electrical substrate and an exterior surface of the unitary housing member define outer surfaces of the mass flow sensor apparatus.

5. The mass flow sensor apparatus of claim 1, wherein the unitary housing member further defines an inlet orifice fiuidly connected to the airflow cavity and an outlet orifice fiuidly connected to the airflow cavity.

6. The mass flow sensor apparatus of claim 1 , wherein the plurality of projections comprises a first projection, a second projection, and a third projection, and wherein the first projection defines walls of the airflow cavity, the second projection, the third projection, and a first portion of the first projection define walls of the first substantially enclosed cavity, and the second projection and a second portion of the first projection define walls of the second substantially enclosed cavity.

7. The mass flow sensor apparatus of claim 6, wherein a portion of the first projection extends toward a surface of the mass flow sensor die between the wire bonding portion and the sensing portion, and wherein the portion of the first projection does not contact the surface of the mass flow sensor die.

8. The mass flow sensor apparatus of claim 1, wherein the plurality of electrical traces run between the first portion of the major surface of the electrical substrate and the second portion of the major surface of the electrical substrate.

9. The mass flow sensor apparatus of claim 1 , wherein the sensing portion of the mass flow sensor die includes a mass flow sensor that comprises a first Wheatstone bridge sense element and a second Wheatstone bridge sense element.

10. The mass flow sensor apparatus of claim 9, wherein the mass flow sensor senses a temperature differential between the first Wheatstone bridge sense element and the second Wheatstone bridge sense element, the temperature differential caused at least in part by air flow through the airflow cavity and across the first Wheatstone bridge sense element and the second Wheatstone bridge sense element.

11. The mass flow sensor apparatus of claim 1 , wherein the electrical substrate comprises at least one of a printed circuit board, a ceramic, or a flexible circuit.

12. A method comprising:

attaching a mass flow sensor die to a major surface of an electrical substrate, wherein the mass flow sensor die comprises a sensing portion and a wire bonding portion, wherein the wire bonding portion comprises a plurality of die wire bond pads, and wherein the plurality of die wire bond pads are electrically coupled to respective ones of a plurality of electrical substrate wire bond pads on the major surface of the electrical substrate by respective wire bonds;

dispensing an adhesive along at least a portion of a plurality of projections extending from an interior surface of a unitary housing member;

coupling the unitary housing member to the electrical substrate by:

positioning in substantial alignment with the plurality of projections extending from the interior surface of the unitary housing member a predetermined path on the major surface of the electrical substrate, wherein the predetermined path defines a first portion, a second portion and a third portion of the major surface of the electrical substrate, and wherein the sensing portion of the mass flow sensor die is disposed on the first portion of the major surface of the electrical substrate, and the wire bonding portion of the mass flow sensor die and the electrical substrate wire bond pads are disposed on the second portion of the major surface of the electrical substrate, and

contacting the predetermined path and the adhesive, wherein the interior surface of the unitary housing member, the plurality of projections, and the major surface of the electrical substrate define:

a first substantially enclosed cavity, wherein an integrated circuit is disposed within the first substantially enclosed cavity and electrically connected to at least one electrical trace of the electrical substrate, wherein the at least one electrical trace and at least one of the bond wires electrically connect the mass flow sensor die and the integrated circuit, and wherein the third portion of the major surface of the electrical substrate bounds the first substantially enclosed cavity, a second substantially enclosed cavity that is fluidly isolated from the first substantially enclosed cavity, wherein the wire bonding portion of the mass flow sensor die is disposed within the second substantially enclosed cavity, and wherein the second portion of the major surface of the electrical substrate bounds the second substantially enclosed cavity, and

an airflow cavity, wherein the sensing portion of the mass flow sensor die is disposed within the airflow cavity and is configured to sense air flow through the airflow cavity, wherein the first portion of the major surface of the electrical substrate bounds the airflow cavity, and wherein the airflow cavity is fluidly isolated from the first substantially enclosed cavity and from the second substantially enclosed cavity.

13. The method of claim 12, wherein coupling the unitary housing member to the electrical substrate further comprising:

aligning a first clip and a second clip extending from the unitary housing member with a first notch and a second notch, respectively, defined by the electrical substrate; and

inserting the first clip in the first notch and the second clip in the second notch, wherein the first clip engages with a first portion of the electrical substrate proximate to the first notch and the second clip engages with a second portion of the electrical substrate proximate to the second notch.

14. The method of claim 12, further comprising:

dispensing an encapsulant through a first aperture defined in the unitary housing member and into the second substantially enclosed cavity, wherein the first aperture extends from an exterior surface of the unitary housing member to the interior surface of the unitary housing member, and wherein the encapsulant encapsulates the plurality of die wire bond pads, the plurality of wire bonds, and the plurality of electrical substrate wire bond pads.

15. The method of claim 14, further comprising curing the encapsulant.

16. The method of claim 12, wherein the major surface of the electrical substrate comprises a first major surface of the electrical substrate, and wherein a second major surface of the electrical substrate and an exterior surface of the unitary housing member define outer surfaces of the mass flow sensor apparatus.

17. The method of claim 12, wherein the unitary housing member further defines an inlet orifice that is fluidly connected to the airflow cavity, and further defines an outlet orifice that is fluidly connected to the airflow cavity.

18. The method of claim 12, wherein a portion of the first projection extends toward a surface of the mass flow sensor die between the wire bonding portion and the sensing portion, and wherein the portion of the first projection does not contact the surface of the mass flow sensor die.

19. The method of claim 12, wherein the plurality of electrical traces run between the first portion of the major surface of the electrical substrate and the second portion of the major surface of the electrical substrate.

20. The method of claim 12, wherein the sensing portion of the mass flow sensor die includes a mass flow sensor that comprises a first Wheatstone bridge sense element and a second Wheatstone bridge sense element.