Abstract:
An airflow measuring apparatus includes a sub-passage that takes in part of flow of fluid flowing through an intake pipe, a sensor element disposed in the sub-passage to measure the flow of fluid, a circuit part converting the flow of fluid detected by the sensor element into an electric signal, a connector part connected to the circuit part to output a signal externally, and a casing supporting the sensor element and the circuit part. The sensor element is disposed in the intake pipe, and includes a cavity disposed at a semiconductor substrate and a diaphragm including a thin film part that covers the cavity. The sensor element on a lead frame has surfaces that are mold-packaged with resin so that the diaphragm and part of the lead frame are exposed. One hole is disposed at the lead frame for communication between the cavity and exterior.

Description:
TECHNICAL FIELD 
     The present invention relates to flow measuring apparatuses to measure the flow of fluid, and particularly relates to airflow measuring apparatuses that are suitable for intake airflow of an internal combustion engine for automobile. 
     BACKGROUND ART 
     Conventionally heat-generation type airflow sensors are becoming the mainstream to measure the intake airflow, which are installed in an intake air passage of an internal combustion engine in automobile or the like, because such a type of sensor can detect mass airflow. 
     A sensor element can be formed as a thinner film partially by a semiconductor micromachining technique, whereby the airflow sensor can have high-speed responsivity. Hereinafter this thin-film part is called a diaphragm. On the diaphragm, a heating resistor and two or more thermosensitive resistors adjacent to the heating resistor are formed by patterning. The heating resistor is uniformly controlled to generate heat to be at a predetermined temperature or higher than the surrounding temperature, and the temperature distribution thereof is detected by the thermosensitive resistors. Since the temperature distribution changes with the amount of airflow passing over the sensor element, the variation in temperature distribution is detected by the thermosensitive resistors disposed upstream and downstream of the airflow direction, whereby the mass airflow can be measured. 
     As means for such a heat-generation type airflow meter using a sensor element, the sensor element and a lead frame to mount the sensor element thereon are surrounded with resin as a package by transfer molding, for example. 
     This is for reducing the number of components or the number of connections compared with the structure including a sensor element and a circuit mounted on a substrate made of ceramic or the like. 
     Such a sensor element and the thermal flow meter including such a packaged sensor element have the following problems. 
     To begin with, when stress is applied to the heating resistor and the thermosensitive resistors disposed on the diaphragm, their values of resistance change due to Piezoresistive effect, which becomes an erroneous cause of the mass airflow detected. If a pressure difference occurs between the surface and the rear face of the diaphragm part, the diaphragm part is deformed, so that stress is applied to the heating resistor and the thermosensitive resistors. To avoid this, there is a need to suppress such a pressure difference between the surface and the rear face of the diaphragm part. 
     As a method to reduce the pressure difference between the surface and the rear face of the diaphragm, Patent Literature 1 provides an opening at the surface of a diaphragm or at the rear face of a substrate to mount a sensor element thereon for communication between a cavity at the rear face of the diaphragm and the atmospheric pressure at the surface of the diaphragm. 
     CITATION LIST 
     Patent Literature 
     
         
         Patent Literature 1: JP 2008-20193 A 
       
    
     SUMMARY OF INVENTION 
     Technical Problem 
     The method described in Patent Literature 1, however, cannot avoid contaminants or droplet completely from entering through the opening at the surface of the diaphragm or on the side of the rear face of the substrate to support the diaphragm, because the opening is exposed to the interior of the intake pipe. 
     When the sensor element is mounted on a lead frame, followed by packaging by transfer molding, the cavity part at the rear face of the diaphragm will be completely cut off from the air. This means that, if the surrounding temperature of the chip package changes, the volume of the air in the cavity at the rear face of the diaphragm changes, and so a difference in pressure between the atmospheric pressure applied to the surface of the diaphragm and the air pressure at the rear face of the diaphragm deforms the diaphragm. This deformation changes the values of resistance of the heating resistor and the thermosensitive resistors on the diaphragm change due to Piezoresistive effect, thus generating an error in the mass airflow detected. 
     In this way, there is a need to establish a communication between the space at the rear face of the diaphragm part and the atmosphere to remove a difference in air pressure between the surface and the rear face of the diaphragm due to the influences from temperature. 
     On the diaphragm, a heating resistor is disposed to detect the flow, and water droplet or contaminations in the intake pipe will fly to the diaphragm part as stated above. Although an opening has to be bored to remove the difference in air pressure so as to lead the space at the rear face of the diaphragm part to any part of the thermal airflow meter, if such an opening is bored at the position that is exposed to the interior of the intake pipe, contaminations or water droplet reaching the opening may block the opening. 
     There is another problem of displacement of the mounting position of the sensor element. As stated above, the temperature distribution generated by a heater is based on the detection of the flow rate of air passing over its surface. Since the flow-rate distribution in a bypass-passage to mount a sensor element is not uniform, the displacement of the sensor element mounted causes a change in the flow detected by such a sensor element, meaning that the mass airflow cannot be measured correctly. To prevent this, there is a need to mount a sensor element precisely in the package. 
     It is an object of the present invention to provide an airflow measuring apparatus with good measurement accuracy. 
     Solution to Problem 
     To fulfill the above object, an airflow measuring apparatus of the present invention includes: a sub-passage that takes in a part of a flow of fluid flowing through an intake pipe; a sensor element that is disposed in the sub-passage to measure the flow of fluid; a circuit part that converts the flow of fluid detected by the sensor element into an electric signal; a connector part having a connector that is electrically connected to the circuit part to output a signal externally; and a casing that supports the sensor element and the circuit part, the sensor element being disposed in the intake pipe. The sensor element includes a cavity that is disposed at a semiconductor substrate, and a diaphragm including a thin film part that covers the cavity. The sensor element is mounted at a lead frame. The sensor element and the lead frame have surfaces that are mold-packaged with resin so that a diaphragm part of the sensor element and a part of the lead frame are exposed. At least one hole is disposed at the lead frame for communication between the cavity and exterior of the mold package. 
     Advantageous Effects of Invention 
     The present invention can provide an airflow measuring apparatus with good measurement accuracy. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  illustrates the state to mount a thermal airflow meter to an intake pipe. 
         FIG. 2  illustrates the structure of a thermal airflow meter and its components. 
         FIG. 3  illustrates a detection part of a sensor element. 
         FIG. 4  includes a plan view and cross sectional views of a chip package that is Embodiment 1. 
         FIG. 5  includes a plan view and a cross sectional view ( 1 ) illustrating the shapes of a cover frame, adhesive and a lead frame that are components of a lead frame assembly that is Embodiment 1. 
         FIG. 6  includes a plan view and a cross sectional view illustrating a step of Embodiment 1 in the state where a cover frame is mounted. 
         FIG. 7  includes a plan view and a cross sectional view illustrating a step of Embodiment 1 in the state where a sensor element is mounted at a lead frame assembly. 
         FIG. 8  includes a plan view and a cross sectional view illustrating a step of Embodiment 1 after transfer molding. 
         FIG. 9  includes a plan view and a cross sectional view ( 1 ) illustrating the shapes of a cover frame, adhesive and a lead frame that are components of a lead frame assembly that is Embodiment 2, which is one alternative proposal for Embodiment 1. 
         FIG. 10  includes a plan view and a cross sectional view ( 2 ) illustrating the shapes of a cover frame, adhesive and a lead frame that are components of a lead frame assembly that is Embodiment 3, which is another alternative proposal for Embodiment 1. 
         FIG. 11  illustrates Embodiment 4, illustrating a cutting part of an outer lead including a communication hole. 
         FIG. 12  is an enlarged view of a cut end of an outer lead cutting part including a communication hole. 
         FIG. 13  illustrates an alternative proposal to mount a plurality of chips and an alternative proposal to improve the connection reliability at a cutting part. 
         FIG. 14  includes a plan view and a cross sectional view illustrating the shapes of a cover frame, adhesive and a lead frame that are components of a lead frame assembly that is Embodiment 8. 
         FIG. 15  includes a plan view and a cross sectional view illustrating a step of Embodiment 8 in the state where a cover frame is mounted. 
         FIG. 16  includes a plan view and a cross sectional view illustrating a step of Embodiment 8 in the state where a sensor element is mounted at a lead frame assembly. 
         FIG. 17  includes a plan view and a cross sectional view ( 1 ) illustrating the shapes of a cover frame, adhesive and a lead frame that are components of a lead frame assembly that is Embodiment 9, which is one alternative proposal for Embodiment 1. 
         FIG. 18  includes a plan view and a cross sectional view ( 2 ) illustrating the shapes of a cover frame, adhesive and a lead frame that are components of a lead frame assembly that is Embodiment 10, which is another alternative proposal for Embodiment 1. 
         FIG. 19  illustrates an alternative proposal to form a communication groove by pressing by bending a lead frame that is Embodiment 11. 
         FIG. 20  illustrates the alternative proposal to form a communication groove by pressing by bending a lead frame that is Embodiment 11. 
         FIG. 21  illustrates another alternative proposal to form a communication groove by etching by bending a lead frame that is Embodiment 11. 
         FIG. 22  illustrates the alternative proposal to form a communication groove by etching by bending a lead frame that is Embodiment 11. 
         FIG. 23  includes a plan view and a cross sectional view illustrating the shapes of a cover frame, adhesive and a lead frame that are components of a lead frame assembly that is Embodiment 12. 
         FIG. 24  includes a plan view and a cross sectional view illustrating a step of Embodiment 12 in the state where a cover frame is mounted. 
         FIG. 25  includes a plan view and a cross sectional view illustrating a step of Embodiment 12 in the state where a sensor element is mounted at a lead frame assembly. 
         FIG. 26  illustrates a step of Embodiment 12, including a plan view after transfer molding and a cross sectional view illustrating the state where a lead frame is pressed with a mold during transfer molding. 
         FIG. 27  illustrates a method that is Embodiment 13 to form a communication hole at a lead frame by additional processing. 
         FIG. 28  illustrates a method that is Embodiment 14 to form a communication hole using a pipe-formed member. 
         FIG. 29  illustrates a die-bond material receiver at the periphery of a through hole that is Embodiment 15. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     The following describes embodiments of the present invention in details, with reference to the drawings. 
     Embodiment 1 
     The following describes Embodiment 1 that is one embodiment of the present invention. 
     As illustrated in  FIG. 1 , a thermal flow meter  100  is attached at its flange part  99  to an intake pipe  140  by mechanical fastening such as using a screw. The thermal flow meter  100  roughly includes a bypass-passage  101 , a circuit chamber  102  and a connector part  103 , and is electrically connected to an ECU that controls an engine via a connector lead  111  in the connector part  103 . Intake air  110  flowing through the intake pipe  140  enters the bypass-passage through an upstream-side opening  105  of the thermal flow meter  100  and goes out through a downstream-side opening  106 . A sensor element  701  is disposed in the bypass-passage  101  to detect the flow of air that is branched off into the bypass-passage  101  out of the intake air  110  passing through the intake pipe  140 . 
     Referring to  FIG. 2  that is a cross section taken along A-A of  FIG. 1 , the following describes components making up the thermal flow meter  100  and the structure. 
     The circuit chamber  102  and the bypass-passage  101  of the thermal flow meter  100  are surrounded with a housing member  201 , a cover member  202 , and a chip package  203  containing the sensor element  701  and its driving circuit. These members are mutually bonded at their periphery with thermosetting adhesive  104 . This can keep the interior of the circuit chamber  102  airtight perfectly, and intake air  110  passing through the sub-passage  101  does not enter the circuit chamber  102 . Such perfect airtightness of the circuit chamber, however, causes expansion of air in the circuit chamber during heating of the thermosetting adhesive  104  for curing, and so the housing member  201  and the cover member  202  may not be bonded correctly. 
     To avoid this, such expanded air has to be released from the circuit chamber  102 , and so a ventilation hole  108  is bored at the connector part  103  to communicate with the circuit chamber  102  for communication between the air inside the circuit chamber  102  and the atmosphere  109  outside the intake Pipe. 
     An outer lead  305  of the chip package  203  and the connector lead  111  inside the connector part  103  are electrically connected via aluminum wire  112 , for example. Herein as illustrated in  FIG. 2( b ) , the outer lead  305  of the chip package may double as the connector lead  111 , and in this case, the aluminum wire  112  and the circuit chamber  102  may be omitted. 
       FIG. 3( a )  illustrates the minimum circuit configuration of a flow detection part,  FIG. 3( b )  illustrates the configuration of the flow detection part and  FIG. 3( c )  is a cross-sectional view taken along A-A of  FIG. 3( b ) . Referring to these drawings, the following describes a typical example of the flow detection part that is formed by patterning on a detection part diaphragm  702  and its operation principle. 
     On the diaphragm  702 , a flow detection part  4  is formed by patterning. The flow detection part  4  includes a heater resistor (heating resistor)  7  and a non-thermal resistor (thermosensitive resistor)  9 , and they are connected to a driving circuit  5  that is provided separately from the flow detection part  4 . The heater resistor  7  generates heat when being energized by current fed from the driving circuit  5  described later, so as to heat the surrounding fluid (air) to be at a temperature higher than the surrounding temperature at least. The non-thermal resistor  9  detects a temperature of the fluid surrounding the flow detection part, and the heater resistor  7  is heat-controlled by the driving circuit  5  so that the temperature thereof is higher than the detected temperature by a predetermined temperature or more. 
     The flow detection part  4  further includes temperature sensors (temperature detection resistors)  11 ,  12  disposed adjacent to the downstream of the heater resistor  7  and temperature sensors (temperature detection resistors)  13 ,  14  disposed adjacent to the upstream of the heater resistor  7 , which are connected to a constant voltage source  26  that is separately provided from the flow detection part  4  and make up a bridge circuit  45 . 
     The driving circuit  5  includes fixed resistors  8 ,  10  and an operational amplifier  15  disposed therein, and so is configured as a heater control circuit to heat-control the heater resistor  7 . This driving circuit  5  allows current from the operational amplifier  15  to be fed to the heater resistor  7  so as to heat-control the heater resistor  7  based on the detection temperature of the non-thermal resistor  9  until the heating temperature of the heater resistor  7  has a predetermined value relative to the surrounding temperature (fluid). 
     In this way, a change in temperature distribution (heat quantity) of the fluid between the temperature sensors  13  and  14  disposed adjacent to the upstream of the heater resistor  7  and the temperature sensors  11  and  12  disposed adjacent to the downstream of the heater resistor  7  can be detected as the flow of the fluid (detected flow Q). When the mass airflow changes, thermal influences from the heater resistors on the temperature sensors  13  and  14  disposed adjacent to the upstream and the temperature sensors  11  and  12  disposed adjacent to the downstream of the heater resistor  7  change, and such a change is detected, whereby a voltage signal corresponding to the mass airflow and its direction can be obtained. 
     As illustrated in  FIG. 3( b ) , the heater resistor  7  has a folded pattern of a resistor to be oblong, on both sides of which (upstream and downstream sides) the temperature sensors  11  and  12  and the temperature sensors  13  and  14  are disposed. The heater resistor  7  and the temperature sensors  11 ,  12 ,  13  and  14  are disposed on the diaphragm  702  that is formed by etching from the rear face of the sensor element  701  as a silicon substrate, for example, to have a small thermal capacity. The non-thermal resistor  9  may be disposed at a place less susceptible to temperature influences from heating of the heater resistor  7 , e.g., at a place outside of the diaphragm  702 . These elements are connected for electrical connection with a circuit part by gold wire bonding, for example, from an electrode extraction part  42 . In the present embodiment, the potential at the midpoint of the bridge including the temperature sensors  11 ,  12 ,  13  and  14  is input to a characteristic adjusting circuit  6 . 
     Referring next to  FIG. 4( a )  that is a front view of a package illustrating the internal configuration with broken lines,  FIG. 4( b )  that is a cross-sectional view of  FIG. 4( a ) , and  FIG. 4( c )  that is an enlarged view of a portion of  FIG. 4( b ) , the following describes the shape of the chip package  203 . 
     The sensor element  701  typically has a rectangular shape. At the detection part of the sensor element  701 , the diaphragm  702  is disposed as described above, and this diaphragm  702  is disposed inside the sub-passage  101  illustrated in  FIG. 1 , through which air as a measurement target flows. 
     The diaphragm  702  is formed by etching from the rear-face direction of the sensor element  701  as stated above, and a cavity  703  is formed at the rear face. The diaphragm  702  is made to be a thin film mainly because a thinner film can decrease the thermal capacity, leading to advantages of improving thermal responsivity as well as lowering power consumption. 
     The cavity  703  below the diaphragm  702  and the circuit chamber  102  communicate with each other via a communication hole  705  bored at a lead frame  704 . The lead frame  704  may be made of a material such as Cu or Fe—Ni having a thickness from about 0.1 mm to 1 mm. When the diaphragm  702  and the circuit chamber  102  communicate with each other, the communication hole  705  has to be bored at the lead frame  704  or a resin part  601  of the chip package  203 . Boring of a hole at the resin part  601  or at the lead frame  704  by additional process after packaging means an increase in the number of steps compared with the conventional packaging procedure, and such a step includes micromachining, and so requires high level of difficulty for machining. 
     Then, the present invention provides the communication hole  705  inside the lead frame  704  by the following procedure for communication between the circuit chamber  102  and the cavity  703  under the diaphragm. In the following, an assembly (including a lead frame  301 , a cover frame  401  and adhesive  404  in the present embodiment) of the minimum components of the lead frame  704  to configure the communication hole  705  is called a lead frame assembly  704 . 
     Referring to  FIGS. 5 to 8 , the following describes the manufacturing procedure of the chip package  203 . 
     Firstly, the cover frame  401  and the lead frame  301  are prepared. Hereinafter, the aforementioned first lead frame and second lead frame are called the cover frame  401  and the lead frame  301 , respectively. Referring to  FIGS. 5( a )( b ) and ( c ) , the following describes the shapes of the cover frame  401 , the lead frame  301  and the adhesive  404  to bond the cover frame  401  and the lead frame  301 . 
     Firstly, the configuration of the lead frame  301  is described with reference to  FIG. 5( c ) . The lead frame  301  includes an outer frame  302 , a die pad  303  to mount an electronic component such as a sensor element and the cover frame  401  thereon, a tie bar  304  to joint the outer frame  302  to the die pad so as not to cause displacement of these components due to influences from resin flow that may occur during molding by transfer molding described later, and an outer lead  305  of the chip package. 
     Next, the configuration of the cover frame  401  is described with reference to  FIG. 5( a ) . 
     The cover frame  401  includes a groove  402  (hereinafter called a communication groove  402 ) to release air from the cavity  703  below the diaphragm, which is formed by half etching or pressing, and a through hole  403  passing through the groove part, which is bored at a part immediately below the diaphragm in the area where the sensor element is to be die-bonded. Such a cover frame  401  is overlaid to the lead frame  301  with the sheet adhesive  404  illustrated in  FIG. 5( b ) . 
       FIG. 6( a )  is a front view illustrating the state where the lead frame  301  and the cover frame  401  are bonded with the adhesive  404 , and  FIG. 6( b )  is a cross sectional view thereof. Bonding of the lead frame  301  and the cover frame  401  via the adhesive  404  forms a closed space that communicates with the through hole  403 . Hereinafter this closed space defines the communication hole  705 . In some embodiments, the communication hole  705  comprises the through hole  403  and the communication groove  402 . 
       FIG. 7( a )  is a front view illustrating the state where the sensor element  701  is structurally or electrically bonded to the lead frame assembly  704 , and  FIG. 7( b )  is a cross sectional view thereof. 
     After applying a die-bond material  501  made of Ag paste or thermosetting adhesive so as to surround the through hole on the cover frame  401 , the sensor element  701  is die-bonded, and the die-bond material  501  and the adhesive  404  are heated in an oven for curing. Herein, the lead frame  301  and the cover frame  401  may be made of the same type of materials or different types of materials, between which one that is suitable for the overall shape of the chip package  203  may be selected. For instance, when the area of the lead frame  301  is sufficiently larger than that of the cover frame  401 , the cover frame  401  may be made of a material having a linear expansion coefficient closer to that of the sensor element  701  than that of the lead frame  301 , whereby stress applied to the sensor element  701  during heating for curing can be alleviated. 
     Then, the electrode extraction part  42  on the sensor element  701  and a bonding part  503  on the lead frame  301  are connected by wire bonding using Au wire  504 . 
       FIG. 8( a )  is a front view illustrating the state where molding is performed for the lead frame assembly  704  on which the sensor element  701  has been mounted, and  FIG. 8( b )  is a cross sectional view illustrating the state where the lead frame assembly is set in a mold. 
     The lead frame assembly  704  on which the sensor element  701  has been mounted, which is prepared by the procedure till  FIG. 7  as stated above, is set on a lower mold for transfer molding  1103 , which is then sandwiched with an upper mold for transfer molding  1102 . Thermosetting resin such as epoxy or polyamide that is heated to about 200° C. to 300° C. is injected into the space defined between the lower mold for transfer molding  1103  and the upper mold for transfer molding  1102  at an injection pressure of about 5 to 10 MPa, thus packaging the lead frame assembly  704 . Hereinafter the shape of such a lead frame assembly  704 , on which an electronic component such as the sensor element  701  has been mounted, just after packaging, is called a package assembly  602 . 
     At this time, if the injection pressure of the resin part  601  is too high, the Au wire  504  will be washed away by the resin part  601  and will fall, and the Au wire  504  may come into contact with the cover frame  401 . When the cover frame  401  is made of a metal material, short-circuit occurs at the Au wire  504 , and the sensor element  701  and the outer lead  305  may not be electrically connected correctly. 
     To avoid this, the cover frame  401  may be made of a material not a metal only but silicon or glass. In the case of silicon or glass used, the communication groove  402  and the through hole  403  may be formed by wet etching, dry etching or blasting. Such a configuration including silicon or glass may be applicable to all cover frames  401  in the below-described embodiments. 
     Cutting the tie bar  304  of the package assembly  602 , a part connecting the outer leads  305  of the tie bar  304  and the leading end of the outer lead  305  completes the chip package  203  of  FIGS. 4( a )-( c )  as described above. At this time, the outer lead  305  particularly has to be cut at its cutting line  1101 . The outer lead may be cut at the cutting line  1101  so as to surely include the communication groove  402 , whereby the opening  708  of the communication hole can be obtained as in  FIGS. 4( b )-( c )  as described above. 
     As stated above, the chip package  203 , the housing member  201  and the cover member  202  define the sub-passage  101  and the circuit chamber  102 , and so air inside the cavity  703  below the diaphragm flows through the communication hole  705 , the circuit chamber  102  and the ventilation hole  108  to communicate with the atmosphere  109  outside of the intake pipe through the connector part  103 . 
     Packaging of the sensor element  701  by such manufacturing procedure and to have such a configuration allows the space below the diaphragm to be cut off from the interior of the intake pipe  140 , and so concern about water droplet and contaminations to reach there can be removed. Further, the cavity  703  below the diaphragm and the circuit chamber  102  can communicate with each other without adding any step to a typical packaging technique conventionally conducted. Since the cavity  703  below the diaphragm communicates with the atmosphere, deformation of the diaphragm  702  can be reduced, which is due to a pressure difference between the surface side and the rear face side of the diaphragm, and so a change in values of resistance of resistors making up the flow detection part  4  on the diaphragm  702  due to Piezoresistive effect can be reduced, and a change in characteristics of the thermal flow meter  100  can be reduced. Clogging of the opening leading to the space at the rear face of the diaphragm also can be prevented, and so a reliable product can be manufactured. 
     A ventilation hole that is provided at the sensor element for communication between the space at the rear face of the diaphragm and the exterior of the intake pipe will not be clogged, and the sensor element can be manufactured while suppressing variations in mounting. 
     Although the present embodiment illustrates the example providing a communication hole in the lead frame, including the below-described embodiments, the present invention is intended to provide a communication hole in a member to support the sensor element. That is, the present invention is not limited to these embodiments, and a communication hole may be provided at a substrate making up a circuit pattern, for example. 
     Embodiment 2 
     Referring to  FIG. 9 , the following describes a cover frame  401 , a lead frame  301  and the shape to apply adhesive  404  that is another proposal for Embodiment 1. 
     Embodiment 1 requires half etching or pressing to form the communication groove  402  at the cover frame  401 . The present embodiment is a method to simply the manufacturing process of a chip package by eliminating such a step. As illustrated in  FIG. 9( b ) , paste-like adhesive  404  is applied by dispensing so as to surround the range including a through hole  403  and an outer lead  305 , or sheet-like adhesive is cut and attached, whereby a communication hole  705  can be formed. This can manufacture the chip package  203  with a smaller number of steps than that of Embodiment 1. 
     Embodiment 3 
     Referring to  FIGS. 10( a )( b ) and ( c ) , the following describes a still another proposal for a cover frame  401 , a lead frame  301  and the shape to apply adhesive  404  to mount a sensor element  701  on a lead frame assembly  704  more precisely than Embodiment 1. 
     The communication groove  402  disposed at the cover frame  401  makes the wall thickness of the cover frame  401  nonuniform, and so there is a concern to degrade flatness of the plane on which a sensor element  701  is to be mounted. The communication groove  402  disposed at the lead frame  301  then leads to a concern to degrade the flatness similarly to the case of the cover frame. The communication groove  402  may be disposed at the lead frame  301 , and degradation in flatness of the lead frame  301  may be accommodated with the adhesive  404 . 
     From the viewpoint of the accuracy in height to mount the sensor element  701 , the adhesive  404  may be applied using sheet adhesive instead of applying on the lead frame by dispensing to suppress variations in dimensions in the lamination direction. However, it is difficult to cut it into the shape surrounding the cavity as in the application area of the adhesive  404  illustrated in  FIG. 9( b ) , and so adhesive  404  that is made of a porous material that transmits not resin but air is preferably used. The present embodiment enables the lead frame assembly  704 , on which the sensor element  701  can be mounted more precisely. 
     Embodiment 4 
     Referring to  FIG. 8 , the following describes the transfer molding processing of Embodiment 1 again. The outer lead  305  and the tie bar  304  protrude from the resin part  601  of the chip package  203  to the outside, and so the upper mold for transfer molding  1102  and the lower mold for transfer molding  1103  are manufactured to avoid the outer lead  305  and the tie bar  304 . 
     As a result, if the cover frame  401  is displaced on the lead frame  301  from a predetermined shape during mounting, there occurs a gap between the outer lead  305 , which is formed by overlapping of the lead frame  301  and the cover frame  401 , and the mold, and then the resin part  601  flows out from this gap. This results in incorrect shape of the chip package  203 . In order to prevent the leakage of resin during transfer molding, the dimension of the gap has to be about 5/1,000 mm, and so very high accuracy is required to mount the cover frame  401  on the lead frame  301 . 
     Referring to  FIG. 11 , the following describes the structure and the manufacturing method to relax restrictions on such an allowable gap dimension. The basic components, structure and manufacturing steps are the same as those of Embodiments 1 to 3. 
     When the lead frame  301  and the cover frame  401  are bonded with the adhesive  404 , the communication groove  402 , which is formed in any Embodiments 1 to 3, is formed so as to define a closed space inside the cover frame  401 . Next, when the lead frame assembly  704  is molded, the package assembly  602  is formed so as to make sure that the molding range of the resin part  601  is within the range including the entire cover frame  401 , and then when the package assembly  602  is cut out from the outer frame  302 , cutting is performed at the cutting line  1101  of  FIG. 11  in the cover frame  401 . Herein, the cutting line  1101  is set so as to pass through a part of the closed space of the adhesion groove. This forms the opening  708  of the communication hole. 
     This structure prevents the leakage of resin to the outside as long as the upper mold for transfer molding  1102  and the lower mold for transfer molding  1103  cover the range including the communication groove  402  while having the width of about ±0.2 mm at the periphery of the part of the cover frame  401  making up the outer lead  305 , even when there is a displacement of about ±0.1 mm, for example, of the cover frame  401  relative to the lead frame  301  during adhesion, and so the chip package  203  formed can have a correct shape. 
     Embodiment 5 
       FIG. 12  illustrates the opening  708  of the communication hole that is obtained after cutting of the outer lead  305 . 
     In Embodiment 1 or Embodiment 3, when the outer lead  305  is cut after preparing the package assembly  602 , a punch for cutting may crush the upper side face  1201  of the communication hole when pushing the outer lead  305  for cutting, which may block the communication hole  705 . 
     Let that t denotes the wall thickness of the lead frame and w denotes the width of the communication hole, a part of the communication hole passing through the cutting line  1101  desirably has a relationship of the width of communication hole w≦the wall thickness t. 
     Embodiment 6 
       FIG. 13( a )  illustrates an alternative proposal for Embodiment 5 including the package assembly  602  without the outer frame  302 , and  FIG. 13( b )  illustrates the state of the opening  708  of the communication hole after cutting the outer lead  305  at the cutting line  1101 . 
     As illustrated in  FIG. 13( b ) , a plurality of communication grooves  402  provided can increase the total area of the opening  708  of the communication hole, whereby reliability of the connection between the cavity  703  at the rear face of the diaphragm and the opening  708  of the communication hole via the communication hole  705  can be improved. 
     Embodiment 7 
     In Embodiments 1 to 6, the chip component to be mounted on the cover frame  401  is not limited to the sensor element  701  only. The present embodiment illustrates the example where a plurality of chip components including a sensor element is mounted on the cover frame  401 . Referring again to  FIG. 13( a ) , the following describes the form to mount a plurality of chips. 
     When a chip  1301  including an arithmetic circuit, for example, in addition to the sensor element  701 , is mounted on a first lead frame, the minimum area of the first lead frame will be increased by the area of the chip  1301  at least. 
     The present embodiment can be manufactured by the same manufacturing procedure and with the structure of the components and the components used as those of Embodiment 1. However, in the case of a broader communication groove  402 , the cover frame  401  may be deformed due to the injection pressure of thermosetting resin, so that the state of the sensor element  701  and the chip  1301  mounted becomes instable and variations in dimensions to mount chip components in the lamination direction may increase. 
     Then, a part  1302  free from the communication hole  705  is desirably disposed at an area immediately below the sensor element  701  or the chip  1301 , such an area being disposed partially or entirely on the rear face side of the chip  1301 . 
     Embodiment 8 
     Referring again to  FIG. 2  that is a cross sectional view of the thermal flow meter, the following describes a method to improve the accuracy in position to mount the sensor element  701  to reduce variations in characteristics of the thermal flow meter  100 . 
     The positional accuracy of the sensor element  701  in the sub-passage  101  of the thermal flow meter  100  affects the variations in characteristics of the thermal flow meter  100 . The chip package  203  is bonded to the housing member  201  and the cover member  202  that make up the sub-passage  101 . This means that, in order to mount the sensor element  701  in the sub-passage  101  precisely, variations in dimensions between the surface of the resin part  601  and the surface of the sensor element  701  that is at the bonding face with the housing member  201  and the cover member  202  have to be minimized. 
     Referring next to  FIG. 8( b ) , the following considers the integration of variations in dimensions inside the chip package  203 . The positional relationship between the sensor element  701  and the resin part  601  depends on the transfer molding step. At this time, since the lead frame  301  is set to be sandwiched between the upper mold for transfer molding  1102  and the lower mold for transfer molding, the lead frame surface will serve as a reference for the dimensional tolerance. 
     This means that factors of variations in dimensions between the surface of the resin part  601  and the sensor element  701  in the lamination direction during mounting include the flatness of the face to mount the lead frame  301  thereon, variations in thickness of the adhesive  404 , variations in thickness of the cover frame  401 , flatness of the bonding face between the cover frame  401  and the lead frame  301 , flatness of the face of the cover frame  401  to mount the sensor element  701  thereon, and variations in thickness of the die-bond material  501 . 
     In the present embodiment, the cover frame  401  is bonded to the opposite side of Embodiment 1 to alleviate the factors of variations in the lamination direction of the sensor element  701  during mounting, and the die-bond material  501  is directly applied to the lead frame  301 , followed by mounting of the sensor element  701 . This can reduce the factors of variations in the lamination direction of the sensor element  701  during mounting to the flatness of the face to mount the lead frame  301  thereon and the variations in thickness of the die-bond material  501  only. The following describes the manufacturing procedure, the components included and the structure of the components with reference to  FIGS. 14 to 19 . 
     Firstly, similarly to Embodiment 1, the lead frame  301  and the cover frame  401  are prepared (in the present embodiment, the aforementioned first lead frame and second lead frame are called a lead frame and a cover frame, respectively).  FIG. 14( a )  describes the structure of the lead frame,  FIG. 14( b )  describes the shape of the lead frame and the adhesive  404  to bond the cover frame  401  and the lead frame  301 , and  FIG. 14( c )  describes the structure of the cover frame  401 . 
     The lead frame  301  includes a through hole  403  that is disposed immediately below a cavity  703  under the diaphragm of the sensor element  701 , a communication groove  402  to release air from the cavity  703  below the diaphragm, which is formed by etching or pressing, an outer frame  302 , a die pad  303  to mount an electronic component such as a sensor element  701  thereon, a tie bar  304  to joint the outer frame to the die pad  303  so as not to cause displacement of these components due to influences from resin flow during transfer molding, and an outer lead  305  to serve as a terminal of the chip package  203 . This structure is preferable because it enables a simple configuration just by cutting the outer peripheral shape surrounding the communication groove  402  to process a cover frame  401 . 
       FIG. 15  illustrates the state where the lead frame  301  and the cover frame  401  are bonded with the adhesive  404 . 
     The adhesive  404  is applied at an area between the lead frame  301  and the cover frame  401  and surrounding an adhesion groove  405 . At this time, since there is no need to provide an internal range of the application of the adhesive  404  where the adhesive  404  is not to be applied, sheet-form adhesive  404  is used preferably, whereby the step can be very simple. 
       FIG. 16  illustrates the state where the sensor element  701  is structurally or electrically bonded to the lead frame assembly  704 , where  FIG. 16( a )  is a front view and  FIG. 16( b )  is a cross sectional view. 
     A die-bond material  501  that is Ag paste or epoxy-based material is applied on the lead frame  301  so as to surround the through hole, and then sensor element  701  is die-bonded, followed by heating of the die-bond material  501  and the adhesive  404  for curing. 
     Subsequently, an electrode extraction part  42  on the sensor element  701  and a bonding part  503  on the lead frame  301  are connected by wire bonding using Au wire  504 . 
     The subsequent steps to prepare the chip package  203  are the same as those in Embodiment 1. 
     Packaging of the sensor element  701  with such structure, components included, manufacturing procedure similarly to Embodiment 1 allows the cavity  703  below the diaphragm to be cut off from the interior of the intake pipe  140 , and so concern about water droplet and contamination to reach there can be removed. Further, the space below the diaphragm and the atmosphere can communicate with each other, whereby a concern about the deformation of the diaphragm  702  can be removed, which is due to a pressure difference between the surface side and the rear face side of the diaphragm, and so a change in values of resistance due to Piezoresistive effect, i.e., a change in characteristics can be reduced. 
     Further, the sensor element  701  can be packaged precisely, which can contribute to suppress variations in characteristics of the thermal flow meter  100 . 
     Embodiments 5, 6 and 7 may be combined, whereby needless to say, a chip package can be manufactured more precisely. 
     Embodiment 9 
     Referring to  FIGS. 17( a ) ( b ) and ( c ) , the following describes a cover frame  401 , a lead frame  301  and the shape to apply adhesive  404  in Embodiment 9. Embodiment 8 requires etching or pressing at the cover frame  401  to form the components making up the lead frame assembly  704 . The present embodiment can eliminate such a step, whereby the manufacturing process of a chip package  203  can be simplified. 
     As illustrated in  FIG. 17( b ) , adhesive  404  is applied so as to surround the range including a through hole  403  and an outer lead  305 , whereby a communication hole  705  can be formed. This can manufacture the chip package  203  with a smaller number of steps than that of Embodiment 8. 
     Embodiment 10 
     Referring to  FIGS. 18( a )( b ) and ( c ) , the following describes a cover frame  401 , a lead frame  301  and the shape to apply adhesive  404  in Embodiment 10. 
     The communication groove  402  disposed at the lead frame  301  in the above Embodiment 8 makes the wall thickness of the lead frame  301  nonuniform as illustrated in  FIG. 16 , and so there is a concern to degrade flatness of the plane on which a sensor element  701  is to be mounted. 
     Then, as illustrated in  FIG. 18 , the communication groove  402  of the present embodiment is disposed at the cover frame  401  to accommodate the degradation in flatness of the cover frame  401  with the adhesive  404 . 
     Further, from the viewpoint of the accuracy in height to mount the sensor element  701 , the adhesive  404  may be applied using sheet adhesive instead of applying on the lead frame by dispensing to suppress variations in dimensions in the height direction. However, it is difficult to cut it into the shape surrounding the cavity as in  FIG. 14( b )  using the sheet adhesive, a shape without a hole as in  FIG. 18( b )  is preferable. 
     Then the adhesive  404  that is made of a porous material that transmits not resin but air is preferably used. 
     Embodiment 11 
     Embodiments 1 to 10 describe the lead frame assembly  704  including the lead frame  301 , the adhesive  404 , the cover frame  401 , the sensor element  701 , the die-bond material  501  and the Au wire  504  as a minimum configuration, and the following describes the present embodiment as an alternative proposal that does not include the cover frame  401  to reduce the number of components. The basic steps are the same as those in Embodiment 1 and Embodiment 8. 
       FIGS. 19 to 22  illustrate the structure of a lead frame  301  in the present embodiment. In these drawings, (a) is a front view of the lead frame  301  and (b) is a cross-sectional view including the center of the through hole  403 . 
     Firstly the lead frame  301  illustrated in  FIG. 19  is prepared. Similarly to the aforementioned lead frame  301 , the lead frame  301  includes the die pad  303 , the tie bar  304 , a dam bar  306 , the outer lead  305  and the outer frame  302 . Then, the entire lead frame  301  is divided into a main frame  2024  and a tab lead  2023  at a mountain folding line  2201  as the border, where the die pad  303 , the tie bar  304 , the through hole  403  in the range including at least a part of the cavity under the diaphragm when the sensor element  701  is mounted on the die pad  303 , and the tab lead  2023  are disposed on the main frame  2024  side. On the tab lead  2023  side, the communication groove  402  is formed as a recess by pressing performed from the face on the opposite side of the sensor-element mounting face toward the sensor-element mounting plane, and the through hole  403  and the communication groove  402  are disposed to be overlapped each other when the lead frame is folded by 180 degrees along the mountain folding line  2201 . Adhesive is then applied to the main frame  2024  side or the tab lead  2023  side so as to surround the communication groove  402  entirely, followed by bending of the lead frame along the mountain folding line  2201 , whereby the tab lead  2023  and the main frame  2024  are bonded with the adhesive  404 . The subsequent steps following the mounting of the sensor element  701  are the same as those in Embodiment 8, where the mountain folding line  2201  is used as a valley folding line, and a communication groove  402  is disposed at the main frame  2024  and a through hole  403  is bored at the tab lead  2023  similarly to Embodiment 8. 
     Considering the cover frame  401  in Embodiments 2 to 7 and Embodiment 9 to 10 as the tab lead  2023  for replacement, the members making up the communication groove  402  and the through hole  403  and the range to apply the adhesive  404  may have the same configuration, whereby the same advantageous effects from these embodiments can be achieved for the problems to be solved by the embodiments. 
       FIGS. 21 and 22  illustrate an example where the communication groove  402  of the present embodiment is formed by half etching, from which the same advantageous effects can be obtained similarly. 
     Embodiment 12 
     In Embodiments 1 to 11, when cutting out the chip package  203  and the outer lead  305  from the outer frame  302  of the package assembly  602 , the outer lead  305  making up the communication hole  705  is disconnected, whereby the opening  708  of the communication hole is formed. However, when disconnecting the outer lead  305  to make up the communication hole  705 , there is a concern to crush the communication hole  705  with a punch for disconnection, thus blocking the opening  708  of the communication hole. To avoid this concern, the present embodiment proposes another method to form the opening  708  of the communication hole by way of a typical example of the manufacturing procedure and the structure illustrated in  FIG. 1 , with reference to  FIGS. 23 to 26 . 
     Firstly a lead frame  301  and a cover frame  401  are prepared. The lead frame  301  has the same configuration as that of the lead frame in the aforementioned Embodiment 1. 
     Referring to  FIG. 23( a ) , the configuration of the cover frame  401  is described below. To release air from the cavity  703  below the diaphragm, the cover frame  401  includes a through hole  403  disposed immediately below the diaphragm, a communication groove  402  that is formed by half etching or pressing, and at least one or a plurality of lead frame openings  2301  to connect the communication groove  402  and the sensor-element mounting face. 
       FIG. 24( a )  is a front view illustrating the state where the lead frame  301  and the cover frame  401  are bonded with adhesive  404 , and  FIG. 24( b )  is a cross-sectional view thereof. When the lead frame  301  and the cover frame  401  are bonded with the adhesive  404 , the communication groove  402  leading to the through hole  403  is formed. 
       FIG. 25( a )  is a front view illustrating the state where the sensor element  701  is structurally or electrically bonded to the lead frame assembly  704 , and  FIG. 25( b )  is a cross sectional view thereof. 
     After applying a die-bond material  501  made of Ag paste or thermosetting adhesive so as to surround the through hole on the cover frame  401 , the sensor element  701  is die-bonded, and the die-bond material and the adhesive are heated in an oven for curing. 
     Then, an electrode extraction part  42  on the sensor element  701  and a bonding part  503  on the lead frame are connected by wire bonding using Au wire  504 . 
       FIG. 26  illustrates the state where molding is performed for the lead frame assembly  704  on which the sensor element  701  has been mounted. 
     The lead frame assembly  704  on which the sensor element  701  has been mounted, which is prepared by the procedure till  FIG. 26  as stated above, is set in a mold for transfer molding, and resin such as epoxy or polyamide is poured into the mold by transfer molding, thus forming a package assembly  602 . At this time, an opening  2301  of the lead frame is covered with a pin  2602  that is larger than the opening  2301 . This can prevent the transfer molding resin from flowing into the communication hole  705 , and the place covered with the pin  2602  becomes a package opening  2601  after releasing of the mold for transfer molding, and the combination of the opening  2301  of the lead frame and the package opening  2601  forms an opening  708  of the communication hole of the package assembly  602 . The subsequent manufacturing procedure to prepare the chip package  203  is the same as those in Embodiment 1. 
     Such manufacturing procedure and configuration can form the opening  708  of the communication hole without cutting the outer lead  305  making up the communication hole  705 , thereby removing a concern to block the communication hole during disconnection of the outer lead  305 . 
     The present embodiment can be applied to Embodiments 2 and 3 as well as Embodiments 7 to 11, from each of which the same advantageous effects can be obtained. 
     Embodiment 13 
     Referring to  FIG. 27 , Embodiment 13 is described below. The present embodiment includes additional processing performed to the lead frame  301  so as to form a communication hole  705  and a through hole  403 . When the wall thickness of a material for the lead frame is sufficiently thick of 2 mm or more, for example, a hole can be bored there in the thickness direction using a drill of about φ 1 mm. 
     A horizontal hole vertical to the face on which the sensor element  701  is to be mounted is bored at a die pad to be a through hole  403 . Another horizontal hole is bored from the outside of the outer frame  302  so as to intersect the through hole  403  and penetrate through the outer lead  305  in the direction parallel to the sensor-element  701  mounting face to be a communication hole  705 . The thus prepared lead frame  301  is a lead frame  704 , and a chip package  203  is manufactured by the same steps as those in Embodiment 1. 
     The present embodiment can reduce the number of components as compared with Embodiments 1, 8 and 11, and the lead frame assembly can be formed of materials having minimum sizes. As compared with the case of bonding lead frames or separate members to form a communication hole as in Embodiments 1 to 11, there is no concern to block the communication hole  705  during transfer molding or during cutting of the outer lead, and so reliability of the connection between the cavity  703  below the diaphragm and the opening  708  of the communication hole can be improved. 
     Embodiment 14 
       FIG. 28  illustrates another proposal to improve the reliability of connection between the cavity  703  below the diaphragm and the opening  708  of the communication hole, referring to  FIG. 27 . Embodiment 8 illustrates the configuration of disposing the cover frame  401  at the rear face of the communication hole of the lead frame  301 . The present embodiment includes, instead of the cover frame  401 , a pipe-formed member  2701  under the through hole that is bonded with adhesion or by welding. The pipe-formed member may be made of soft metal such as copper or a resin material having a melting point from about 100° C. to 200° C. or higher that is a temperature during injection for transfer molding, for example. After bonding to the lead frame  301 , the pipe-formed member  2701  is bent toward the direction where the circuit chamber of the thermal flow meter  100  is disposed, which then becomes a lead frame assembly  704 . The subsequent steps to manufacture the thermal flow meter are the same as those in Embodiment 8. 
     This can avoid a concern about the adhesive  404  protruding over the communication hole  705 , which is due to the communication hole  705  made up of two members, and so the cavity  703  below the diaphragm and the opening  708  of the communication hole can be connected more reliably. 
     Embodiment 15 
       FIG. 29  is an enlarged cross-sectional view of a plane that passes through the center line of the through hole  403 . In the case of Embodiments 1 to 13, if the amount of the die-bond material  501  applied is not appropriate when the sensor element  701  is die-bonded on the lead frame  301 , the cover frame  401  or the tab lead  2023 , the die-bond material  501  will flow out to the through hole  403  as in  FIG. 29( a )  and may block the through hole  403 . To prevent this, a die-bond material receiver  2801  is disposed so as to surround the through hole  403  as in  FIG. 29( b ) . The die-bond material receiver  2801  is a recess that is lower than the applied face of the die-bond material  501  (dam structure), and so variations in the amount of the die-bond material  501  applied can be accommodated with the volume of the recess. 
     The present embodiment alleviates a concern about the die-bond material  501  to stick out over the through hole  403 , and so the cavity  703  below the diaphragm and the opening  708  of the communication hole  705  can be connected more reliably. 
     REFERENCE SIGNS LIST 
     
         
           4 ,  360  Flow detection part 
           5  Driving circuit 
           6  Characteristic adjusting circuit 
           7  Heater resistor 
           8 ,  10  Fixed resistor 
           9  Non-thermal resistor 
           11  to  14  Temperature sensor (temperature detection resistor) 
           15  Operational amplifier 
           26  Constant voltage source 
           42  Electrode extraction part 
           99  Flange part 
           100  Thermal flow meter 
           101  Sub-passage 
           102  Circuit chamber 
           103  Connector part 
           104  Thermosetting adhesive 
           105  Upstream side opening 
           106  Downstream side opening 
           107 ,  701  Sensor element 
           108  Ventilation hole 
           109  Atmosphere outside intake pipe 
           110  Intake air 
           111  Connector lead 
           112  Aluminum wire 
           140  Intake pipe 
           201  Housing member 
           202  Cover member 
           203  Chip package 
           301  Lead frame 
           302  Outer frame 
           303  Die pad 
           304  Tie bar 
           305  Outer lead 
           306  Dam bar 
           331  Heater 
           332  Upstream side thermosensitive resistor 
           333  Downstream side thermosensitive resistor 
           401  Cover frame 
           402  Communication groove 
           403  Through hole 
           404  Adhesive 
           501  Die-bond material 
           503  Bonding part 
           504  Au wire 
           601  Resin part 
           602  Package assembly 
           702  Diaphragm 
           703  Cavity 
           704  Lead frame (lead frame assembly) 
           705  Communication hole 
           708  Opening of communication hole 
           1101  Cutting line of outer lead making up communication hole 
           1102  Upper mold for transfer molding 
           1103  Lower mold for transfer molding 
           1201  Upper side face of communication groove 
           1301  Chip 
           1302  Part without communication hole  705   
           2023  Tab lead 
           2024  Main frame 
           2201  Mountain folding line 
           2202  Position to mount sensor element 
           2301  Lead frame opening 
           2601  Package opening 
           2602  Pin 
           2701  Pipe-formed member 
           2801  Die-bond material receiver