Acceleration sensor

Details Acceleration sensor Acceleration sensor

2003-04-30

2004-06-29

Kwok, Helen (Department: 2856)

Measuring and testing

Speed, velocity, or acceleration

Structural installation or mounting means

Reexamination Certificate

active

06755081

ABSTRACT:

TECHNICAL FIELD
The present invention relates to an acceleration sensor for use in a motor vehicle, an aircraft, a household electrical appliance or the like.
BACKGROUND ART
FIGS. 19 and 20
are a side sectional view and a front elevational view of a conventional acceleration sensor, respectively. The conventional acceleration sensor includes a sensor chip
101
for converting into an electrical signal an acceleration applied in a direction of a sensitive axis X parallel to a board face BF of a printed circuit board
110
, an IC chip
103
acting as a processing circuit for processing the electrical signal of the sensor chip
101
, an oblique spacer
102
for supporting the sensor chip
101
and a package
104
for accommodating the IC chip
103
and the oblique spacer
102
having the sensor chip
101
mounted thereon. The package
104
is of a shape of a rectangular box and has at its underside a mounting face MF to be mounted on the board face BF of the printed circuit board
110
. The mounting face MF of the package
104
is provided on one of outer faces of the rectangular box, which has a minimum area in the outer faces. A plurality of terminals
106
for transmitting signals between outside and the sensor chip
101
and the IC chip
103
are provided at a lower portion of each of front and rear faces of the package
104
so as to be secured to the board face BF of the printed circuit board
110
by solder
111
such that the mounting face MF of the package
104
is mounted on the board face BF of the printed circuit board
110
.
Meanwhile, an opening formed on the front face of the package
104
is covered by a cap
105
. Pads
112
of the sensor chip
101
and pads
114
of the IC chip
103
are, respectively, connected by bonding wires
109
to pads
113
formed on an inner face of the package
104
and pads
115
formed on the inner face of the package
104
. Furthermore, a wiring pattern
107
is provided for connecting the pads
113
and
115
and the terminals
106
.
As shown in
FIGS. 21 and 22
, the conventional sensor chip
101
has a so-called cantilever construction including a first substrate
120
formed by a semiconductor substrate and a second substrate
130
joined to a rear face of the first substrate
120
. As best shown in
FIG. 23
, the first substrate
120
is formed with a weight portion
123
, a deflective portion
122
which is formed on a principal surface of the first substrate
120
and has one end coupled integrally with the weight portion
123
, a support portion
121
for pivotally supporting the weight portion
123
via the deflective portion
122
, which is integrally coupled with the other end of the deflective portion
122
and a piezoresistance portion
124
disposed at the deflective portion
122
. The piezoresistance portion
124
acts as a sensing element for detecting a deformation of the deflective portion
122
so as to convert a degree of the deformation of the deflective portion
122
into an electrical signal representing an acceleration. Meanwhile, a reference numeral “
125
” in
FIGS. 21 and 22
denotes a stopper. By deflection of the deflective portion
122
upon application of an acceleration to the sensor chip
101
, the piezoresistance portion
124
is also deflected so as to change its resistance value, so that an electrical signal corresponding to the resistance value is outputted as the acceleration.
As shown in
FIG. 23
, the sensor chip
101
of the cantilever construction should be inclined at an angle &thgr; relative to a perpendicular on the board face BF of the printed circuit board
110
such that a straight line connecting a fulcrum B of deflection of the deflective portion
122
and a center C of gravity of the weight portion
123
is perpendicular to the sensitive axis X, i.e., a direction of application of the acceleration. If this angle &thgr; is not correct, the deflective portion
122
is deflected even when the acceleration is 0 G, so that off-axis sensitivity of the acceleration sensor increases, thereby resulting in an erroneous output of the acceleration sensor. The oblique spacer
102
supports the sensor chip
101
so as to define this angle &thgr; of inclination of the sensor chip
101
.
Meanwhile, the sensor chip
101
may also have a so-called fixed beam construction in which opposite ends of the weight portion
123
are, respectively, fixed by a pair of the support portions
121
through a pair of the deflective portions
122
. In this case, since the angle &thgr; in
FIG. 23
is zero, the principal surface of the sensor chip
101
is perpendicular to the sensitive axis Z.
In the conventional acceleration sensor referred to above, the sensitive axis X is substantially perpendicular to the principal surface of the sensor chip
101
. Hence, in case an acceleration parallel to the board face BF of the printed circuit board
110
is detected by the conventional acceleration sensor, the mounting face MF of the package
104
having a shape of a rectangular box should be provided on one of outer faces of the rectangular box, which has a minimum area in the outer faces, so that undesirable inclination of the package
104
due to its inaccurate mounting is likely to become large and thus, such a disadvantage may be incurred that off-axis sensitivity of the acceleration sensor becomes large due to inaccurate parallelism between the sensitive axis X and the board face BF of the printed circuit board
110
.
Meanwhile, as shown in
FIG. 19
, the package
104
is formed by a multi-layer ceramic package in which ceramic plates
104
a
to
104
e
are laminated on each other such that lamination faces of the ceramic plates
104
a
to
104
e
are perpendicular to the board face BF of the printed circuit board
110
. Since a ceramic sheet is split into the ceramic plates
104
a
to
104
e
by using a break method which is an inexpensive method for splitting the ceramic sheet into a plurality of ceramic plates each having a desired size, burrs are formed on end faces and side faces of the package
104
, which are split faces of the ceramic plates
104
a
to
104
e
, so that it is difficult to obtain accurate flatness of the package
104
and thus, undesirable inclination of the package
104
due to its inaccurate mounting is apt to become large, thereby resulting in such a problem that off-axis sensitivity of the acceleration sensor becomes large.
Furthermore, in case the package
104
is formed by the multi-layer ceramic package, a planar conductive pattern having such a size as to enable wire bonding can be formed only on faces of the package
104
parallel to the lamination faces of the ceramic plates
104
a
to
104
e
. Here, if the sensor chip
101
has a cantilever construction in which the principal surface, i.e., a wire bonding face of the sensor chip
101
is not perpendicular to the sensitive axis X as described above, the wire bonding face for the pads
112
in the sensor chip
101
and a wire bonding face for the pads
113
in the package
104
are not parallel to each other, so that wire bonding of the pads
112
and
113
should be performed between the wire bonding faces of the sensor chip
101
and the package
104
, which are not parallel to each other and thus, such an inconvenience is encountered that it is difficult to secure reliability of wire bonding of the pads
112
and
113
.
DISCLOSURE OF INVENTION
The present invention has for its object to provide, with a view to eliminating the above mentioned drawbacks of prior art, an acceleration sensor in which by lessening undesirable inclination of a sensor chip due to its inaccurate mounting, off-axis sensitivity of the acceleration sensor is reduced and reliability of wire bonding portions is secured.
In order to accomplish this object of the present invention, an acceleration sensor according to the present invention includes a sensor chip for converting into an electrical signal an acceleration applied in a direction of a sensitive axis parallel to a board face of a printed circuit board, which has a first wire bonding face. A wiring base s

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