Acceleration sensor

Abstract: An acceleration sensor element constitutes a piezoresistance type acceleration sensor in which an acceleration sensor chip (15) including a cantilever structure of mass (11) and beam parts (12) and a piezoresistor (13) formed on the beam part is mounted on a main lead (16) which is slanted for positioning the piezoresistor and the center of gravity (11a) of the mass (11) substantially on the same horizontal plane in mounted state of the sensor.

Description: BACKGROUND OF THE INVENTION This invention relates to an acceleration sensor device which senses the acceleration and outputs the sensed acceleration as an electric signal. DESCRIPTION OF RELATED ART Conventional semiconductor piezoresistance type acceleration sensor devices which have been employed as applied to safety air bags and so on are mostly merchandised in the form of a sensor module mounted on a circuit board performing a signal compensation for the output signal of the acceleration sensor chip in respect of offset voltage, offset temperature characteristics, sensitivity/temperature characteristics and so on, the signal compensation being performed by means of a trimming or the like. In the case of low price commodities, the acceleration sensor chip may be merchandised by itself. More specifically, the semiconductor piezoresistance type acceleration sensor device is constituted by an acceleration sensor chip which comprises a vibratory unit including a mass (weight) and a beam supporting the mass which are formed with a single crystal silicone substrate subjected to a micromachine technique applied, for example, and a piezoresistor formed on part of the beam to vary its resistance value upon application of a stress. Provided here that an acceleration is applied in a direction perpendicular to extending direction of the vibratory unit, the mass is caused to move due to its force of inertia to have the beam bent. A distortion occurred in the beam at this time causes the resistance value of the piezoresistor to vary, and the electric signal corresponding to the stress applied can be obtained. Further, the acceleration sensor device comprises a lower stopper made of a glass or silicone and supports the single crystal silicone substrate on its lower side to define a lower limit of the movement of the mass, and an upper stopper covering the upper side of the mass and beam to define an upper limit of the movement of the mass. With regard to the disposition of the mass and beam, the thickness of the beam and so on in respect of the foregoing semiconductor piezoresistor acceleration sensor device, there have been respective know-hows of some companies, but, in the case of the acceleration sensor device of cantilevered structure, it is required to have the piezoresistor and the center of gravity of the mass positioned substantially on the same horizontal plane in order to reduce the other-axis (lateral) sensitivity. With reference to styles in which the semiconductor acceleration sensor devices of the cantilevered structure have been sold hereto, on the other hand, the sensor devices have been sold with the acceleration sensor chip kept in a state of not being slanted, and they are required to be mounted in a state in which the acceleration sensor chip is slanted. In the case of such acceleration sensor devices commercialized in the state where the acceleration sensor chip is not slanted, therefore, there has been a risk that the property of commodity cannot be assured and a malfunction of the sensor is caused to occur. For the prior art, there may be enumerated U.S. Patents Nos. 4,399,692, 5,490,421 and 5,528,935, but there has been suggested no structure in which a mounting surface of the acceleration sensor chip with respect to the circuit board is slanted. An Article "MEASUREMENT TECHNIQUES, vol. 34, no. 3, 1 March 1991, New York; pages 262 to 265; L.G. Arkharova et al.: "Small-Sized Silicon Accelerometer with a Thermally Compensated Signal in the Temperature Range of 0 to 200° C" discloses an acceleration sensor device wherein an acceleration sensor chip includes a mass mounted on a beam in a cantilevered structure. Resistive elements of a bridge strain gauge circuit are mounted on the beam. The components of the acceleration sensor device are integrated into a chip which is mounted in a slanted position. The resistive elements on the beam and the center of gravity of the mass are substantially located on the same horizontal plane. A similar acceleration sensor device is disclosed in EP-A-0 481 806. In each of the known acceleration detecting devices, the slanted mounting position is defined by a wedge on which the device is mounted. The present invention as defined in claim 1 is an acceleration sensor not requiring a separate wedge to define the mounting position. Specifically, the acceleration sensor of the invention comprises: an acceleration sensor chip including a mass, a beam supporting the mass in a cantilevered structure and an acceleration detecting means provided on the beam; a main lead on which the acceleration sensor chip is mounted; a second-pad side lead electrically connected to the acceleration sensor chip; and a premold package supporting the acceleration sensor chip as well as the main and second-pad side leads; the premold package being provided with a mounting surface for mounting the premold package with respect to a substrate; a portion of the main lead whereon the acceleration sensor chip is mounted being bent to be in a slanted posture with respect to the mounting surface of the premold package so as to position the acceleration detecting means as well as the center of gravity of the mass of the acceleration sensor chip substantially on the same horizontal plane in mounted state of the premold package to the substrate; and the acceleration sensor chip as well as the main lead and second-pad side lead being simultaneously molded. Other objects and advantages of the present invention shall be made clear in the following description of the invention detailed with reference to preferred embodiments shown in accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS FIGURE 1 is a sectioned view of an acceleration sensor chip; FIG. 2 is a schematic sectioned view of the acceleration sensor device in an embodiment according to the present invention; FIGS. 3(a) to 3(e) show in schematic sectioned views steps of a method for manufacturing the acceleration sensor device in the embodiment of FIG. 2 of the present invention; FIG. 4 is a schematic sectioned view of the acceleration sensor device in another embodiment according to the present invention; FIGS. 5(a) to 5(e) show in schematic sectioned views steps in one working aspect of the method for manufacturing the acceleration sensor device of the embodiment of FIG. 8 of the present invention; FIG. 6 is a schematic sectioned view of the acceleration sensor device in another embodiment according to the present invention; FIGS. 7(a) to 7(e) show in schematic sectioned views steps in one working aspect of the method for manufacturing the acceleration sensor device in the embodiment of FIG. 6 of the present invention; FIGS. 8(a) to 8(d) show in schematic sectioned views steps in another working aspect of the method for manufacturing the acceleration sensor device in the embodiment of FIG. 6; FIG. 9A is a schematic sectioned view of the acceleration sensor device in another embodiment according to the present invention; FIG. 9B is a plan view of the acceleration sensor device of FIG. 9A; and FIGS. 10 to 12 are schematic sectioned views of the acceleration sensor device in other embodiments respectively of the present invention. While the present invention shall be described in the followings with reference to the preferred embodiments shown in the drawings, it should be appreciated that the intention is not to limit the subject invention only to these embodiments shown but rather to include all alterations, modifications and equivalent arrangement possible within the scope of appended claims. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT The acceleration sensor device in the various embodiments preferred as shown in the drawings according to the present invention shall now be described. Referring to FIG. 1, there is shown an embodiment of the acceleration sensor device 10 according to the present invention, which includes an acceleration sensor chip 15 having a cantilevered structure of a mass 11 and a beam 12, as well as a piezoresistor 13 formed on the beam 12, respectively formed by means of a semiconductor etching technique, as shown in FIG. 1. With reference to Fig. 2, the acceleration sensor device 10B includes a main lead 16B on which the acceleration sensor chip 15B is mounted, and a second-pad side lead 17B of the main lead 16B and to be electrically connected to the sensor chip 15B by means of a bonding wire 19B bonded across the sensor chip 15B and second-pad side lad 17B. In FIG. 2, the main lead 16B on which the acceleration sensor chip 15B is die-bonded is bent downward to be slanted to have the piezoresistor 13 and the center lla of gravity of the mass 11 positioned substantially on the same horizontal plane, as shown in FIG. 1. A mounting surface PMF of a premold package PM is formed to be substantially parallel to the plane of the main lead 16B, so that the acceleration sensor chip 15B can be mounted for positioning the piezoresistor 13 and the center lla of gravity of the mass 11 substantially on the same horizontal plane by mounting the device 10B with the surface PMF engaged to the board. The manufacture of such acceleration sensor device 10B shown in FIG. 2 may be performed in such manner as shown in FIG. 3, for example. More specifically, the acceleration sensor chip 15B is die-bonded to the main lead 16B of a metal-made, mostly by an iron series or copper series alloy, then the wire-bonding is performed as shown in FIG. 3(b), thereafter an external force is given to part of the main lead 16B at which the chip 15B is die-bonded, i.e., to the die-bond pad part, so as to deform the main lead 16B in a direction of falling the wire-bond pad side of the acceleration sensor chip 15B down as shown in FIG. 3(c), a resin part of the premold package PM with a resin-sealing performed, and terminals are formed by cutting and bending exposed end parts of the main and second-pad side leads 16B and 17B as shown in FIG. 3(e). According to the embodiment of FIG. 2, the acceleration sensor chip and leads can be simultaneously molded, to be effectively contributive to the improvement in the productivity. In the embodiment of FIGS. 2 and 3, other arrangements than those in the above are the same as those in the embodiment of FIG. 1, and the same constituents as those in FIG. 1 are denoted by the same reference numerals but with a suffix "B" added in FIGS. 2 and 3. When the second pad of the acceleration sensor chip 15C to be connected through the bonding wire 19C to the lead is disposed adjacent to the bent part of the main lead 16C, as shown in FIG. 4, the main lead 16C may be bent in a direction in which a part of the acceleration sensor chip 15C on a side opposite to the side having the wire bond pad is raised upward. The acceleration sensor device 10C shown in FIG. 4 may be manufactured in such manner as shown, for example, in FIGS. 5(a) to 5(e). In this case, the only difference from the steps of FIGS. 3(a) to 3(e) is that, as shown in FIG. 5(c), the main lead 16C is bent so as to raise the side of the acceleration sensor chip 15C opposite to the side provided with the wire bond pad. All other arrangements in FIGS. 4 and 5 than those referred to above are the same as those in the embodiments of FIGS. 1 and 2, and the same constituents as those in FIGS. 1 and 2 are denoted in FIGS. 4 and 5 by the same reference numerals but with a suffix "C" added. In another embodiment shown in FIG. 6 of the present invention, a spacer 20D is disposed as held between the main lead 16D bent upward for tilting the acceleration sensor chip 15D and the lead 17D on the second-pad side, in the acceleration sensor device 10D. In this respect, the spacer 20D may be employed even in the event where the main lead 16D is bent in the direction shown in FIG. 2. The acceleration sensor device shown in FIG. 6 may be manufactured through such steps as shown in FIGS. 7(a) to 7(e), which steps differ from the steps shown in FIGS. 3(a) to 3(e) only in a step shown in FIG. 7(c) where the main lead 16D is bent with the spacer 20D held between part of the main lead 16D being bent and the other lead 17D being not bent. Other constituents and steps than those referred to in the above in the embodiment of FIGS. 6 and 7 are the same as those in the embodiments of FIGS. 1 and 2 and are denoted by the same reference numerals but with a suffix "D" added. In FIGS. 8(a) to 8(d), another embodiment of the method for manufacturing the acceleration sensor device according to the present invention is shown, in which the acceleration sensor chip 15E is die-bonded to the main lead 16E made of a metal (mostly iron series or copper series alloy) as shown in FIG. 8(a), the wire-bonding 19E is then performed as shown in FIG. 8(b), thereafter the resin sealing is performed to form the resin part PM acting also as the casing in a state where a projection 21E formed in a lower one of the transfer molding dies is engaged to a lower side of the bonding wire 19E of the acceleration sensor chip 15E on the main lead 16E to have the lead bent in a direction in which part of the main lead 16E on the other side than a side of the acceleration sensor chip 15E having the bonding wire 19E is raised upward as shown in FIG. 8(c), and then the exposed end parts of the leads 16E and 17E are cut and bent to form the terminals as shown in FIG. 8(d). Thereafter, preferably, a hole formed in the resin part PM after removal of the dies may be filled with a resin, metal or the like. Further, as shown in a sectioned view of FIG. 9A and a top plan view of FIG. 9B, the projection 22F may be provided in upper die of the transfer molding dies to have the main lead 16F bent down as has been shown in FIG. 2. In this case, the manufacturing method is the same as that has been described with reference to FIG. 8. It may be also possible to form both of the upper and lower dies of the transfer molding dies to have the projections for bending the main lead 17F as has been shown in FIG. 4. In FIG. 10, an embodiment of the acceleration sensor device 10G prepared in such manner is shown, in which a hole is formed with the projection 23G of the upper molding die while a hole is formed by the projection 24G of the lower molding die. In the case of this manufacturing method, too, the arrangement is the same as that has been described with reference to FIG. 8. In another embodiment of the acceleration sensor device 10H of the present invention as shown in FIG. 11, the main lead 16H bent downward as in FIG. 9 with the acceleration sensor chip 15H mounted thereto is formed to have a height difference from the wire-bonded second-pad side lead 17H so that even the wire-bonded second pad side of the chip 15H will be higher, while the wire-bonded second pad side lead 17H is formed to be disposed substantially in the same plane (the same height) as the surface of the chip 15H to which the bonding wire is connected. In this case, the transfer molding dies are designed so that the configuration of the resin part PM (i.e., the shape of the transfer molding dies) will have the mounting surface PMF on the lower side of the resin part PM which defines a predetermined angle with respect to the frame surface of the main lead 16H. With this arrangement, the acceleration sensor chip 15H is put in the state of being slanted by a predetermined angle with respect to the horizontal plane of the circuit board, when the acceleration sensor device 10H is mounted to the circuit board. Further, as a height difference between the wire-bonded second pad and the pad of the acceleration sensor chip 15H is minimized, it is possible to improve the reliability of the wire bonding. In the respective embodiments shown in FIGS. 6 to 11 of the present invention, other arrangements than the above described are the same as those in FIGS. 1 and 2, and the same constituents as those in FIGS. 1 and 2 are denoted by the same reference numerals but sequentially with each of suffixes "D", "E", "F", "G" and "H" added. In another embodiment of the acceleration sensor device 10I of the present invention as shown in FIG. 12, the acceleration sensor chip 15I is mounted on a planar main lead 16I, while the mounting surface PMF with respect to the board on the bottom side of the resin part PM is formed as slanted to define a predetermined angle with respect to the surface of the main lead 16I. In the arrangement of FIG. 12, other constituents than those described above are the same as those in the embodiments of FIGS. 1 and 2, and are denoted by the same reference numerals as those in FIGS. 1 and 2 but with a suffix "I" added.

Claims: An acceleration sensor comprising: an acceleration sensor chip (15B-15I) including a mass (11), a beam (12) supporting said mass (11) in a cantilevered structure and an acceleration detecting means (13) provided on the beam; a main lead (16B-16I) on which the acceleration sensor chip (15B-15I) is mounted; a second-pad side lead (17B-17I) electrically connected to the acceleration sensor chip (15B-15I); and a premold package (PM) supporting the acceleration sensor chip (15B-15I) as well as the main and second-pad side leads (16B-16I; 17B-17I); the premold package (PM) being provided with a mounting surface (PMF) for mounting the premold package (PM) with respect to a substrate; a portion of the main lead (16B-16I) whereon the acceleration sensor chip (15B-15I) is mounted being bent to be in a slanted posture with respect to the mounting surface (PMF) of the premold package (PM) so as to position the acceleration detecting means (13) as well as the center of gravity of the mass (11) of the acceleration sensor chip (15B-15I) substantially on the same horizontal plane in mounted state of the premold package (PM) to the substrate; and the acceleration sensor chip (15B-15I) as well as the main lead (16B-16I) and second-pad side lead (17B-17I) being simultaneously molded. The sensor device according to claim 1, wherein the bent portion of the main lead (16B; 16G; 16H; 16I) is bent down toward the mounting surface (PMF) of the premold package (PM). The sensor device according to claim 2, wherein the second-pad side lead (17H) is disposed substantially in the same plane as a surface of the acceleration sensor chip (15H) to which a bonding wire (19H) is connected. The sensor device according to claim 1, wherein the bent portion of the main lead (16C; 16D; 16E) is bent in a direction in which a part of the acceleration sensor chip (15C; 15D; 15E) is raised upward on the side opposite to the side having a wire-bond pad.

Inventors: KASANO FUMIHIRO, NOHARA KAZUYA, SUMI SADAYUKI, TAKAMI SHIGENARI, TANIGUCHI NAOHIRO, KASANO, FUMIHIRO, NOHARA, KAZUYA, SUMI, SADAYUKI, TAKAMI, SHIGENARI, TANIGUCHI, NAOHIRO, Kasano, Fumihiro c/o Matsushita Elec. Works, Ltd, Nohara, Kazuya c/o Matsushita Elec. Works, Ltd, Sumi, Sadayuki c/o Matsushita Elec. Works, Ltd, Takami, Shigenari c/o Matsushita Elec. Works, Ltd, Taniguchi, Naohiro c/o Matsushita Elec. Works, Ltd
Applicants: MATSUSHITA ELECTRIC WORKS LTD, MATSUSHITA ELECTRIC WORKS, LTD.
Application year: 1996
Main classification: G01P1
IPC codes: G01P1, G01P15