Optical scanning device

In one embodiment, an optical scanning device includes a board and a light receiving member, the positional accuracy of which is improved. When a length in a scanning direction and a length in an intersection direction of a body portion of the light receiving member are denoted by Lx0 and Ly0, respectively, and a length in the scanning direction and a length in the intersection direction of a through-hole formed in the board are denoted by Lx1 and Ly1, respectively, the lengths Lx0, Ly0, Lx1, and Ly1 satisfy the following formula:(Lx1−Lx0)>(Ly1−Ly0).

BACKGROUND OF THE INVENTION

Field of the Invention

Aspects of the present invention generally relate to an optical scanning device equipped with a plurality of optical-system components including a light deflector, which deflects a light beam, and a detection unit, which detects timing at which a light beam passes.

Description of the Related Art

An optical scanning device, which is used for an image forming apparatus, such as a laser printer, uses a light deflector, which is composed of, for example, a rotary polygonal mirror, to perform deflection scanning with a laser light beam emitted from a light source according to an image signal. The laser light beam subjected to deflection scanning is led to a beam detection (BD) sensor (a beam detector), which serves as a light detection unit (a light receiving element), so as to control timing of the scanning start position on a surface to be scanned. The laser light beam is moved while being focused in a spot shape on a photosensitive recording medium by an imaging optical system (a scanning lens) having an fθ characteristic. The writing start timing of optical scanning is a predetermined time after the BD sensor outputs a synchronization signal.

Japanese Patent No. 4109878 discusses a configuration in which the above-mentioned BD sensor is mounted on a circuit board.

In the configuration discussed in Japanese Patent No. 4109878, the BD sensor, serving as an element, is mounted on the circuit board from a surface (first surface) of the circuit board on the side at which to receive a laser light beam. Then, the BD sensor receives a laser light beam through a through-hole provided in an optical box to which the circuit board is fixed. In recent years, it has been considered that, to reduce an apparatus size and lower costs, the BD sensor, serving as a light receiving member, is mounted on the circuit board from a surface (second surface) of the circuit board opposite to the first surface.

SUMMARY OF THE INVENTION

Aspects of the present invention are generally directed to improving the positional accuracy at which to mount a light receiving member on a circuit board from the second surface thereof.

According to an aspect of the present invention, an optical scanning device includes a light source, a deflection unit configured to deflect a light beam emitted from the light source, a board, and a light receiving member including a body portion, which includes a light receiving portion, and a first terminal portion and a second terminal portion, each of which is soldered to the board, wherein the light receiving portion receives the light beam deflected by the deflection unit and moving in a scanning direction, wherein the first terminal portion and the second terminal portion are located opposite each other across the body portion in an intersection direction that intersects with the scanning direction as viewed in a normal direction to a mounting surface of the board, wherein the board has a through-hole formed thereon, into which at least a part of the body portion is inserted, and includes a first soldering portion soldered to the first terminal portion and a second soldering portion soldered to the second terminal portion, the first soldering portion and the second soldering portion being located opposite each other across the through-hole in the intersection direction, and wherein, when a length in the scanning direction and a length in the intersection direction of the body portion are denoted by Lx0 and Ly0, respectively, and a length in the scanning direction and a length in the intersection direction of the through-hole are denoted by Lx1 and Ly1, respectively, the lengths Lx0, Ly0, Lx1, and Ly1 satisfy the following formula:
(Lx1−Lx0)>(Ly1−Ly0).

According to another aspect of the present invention, an optical scanning device includes a light source, a deflection unit configured to deflect a light beam emitted from the light source, a board having a through-hole formed thereon, and a light receiving member including a light receiving portion and mounted on the board, wherein the light receiving member receives the light beam that has entered the through-hole while being deflected by the deflection unit and moving in a scanning direction, and wherein the light receiving portion is located in a position which is downstream of the center of the light receiving member with respect to the scanning direction and which allows no light beam reflected from an inner wall of the through-hole to be incident on the light receiving portion.

DESCRIPTION OF THE EMBODIMENTS

FIG. 1illustrates an image forming apparatus101. An optical scanning device100, which is described below, is mounted on an optical bench103. The optical bench103is a part of the chassis of the image forming apparatus101. The image forming apparatus101includes, among others, a process cartridge108, which is an image forming unit, a paper feed unit104, in which transfer material P is stacked, a paper feed roller105, a transfer roller (a transfer unit)106, and a fixing device (a fixing unit)107. The process cartridge108includes a photosensitive drum (photosensitive member)8, which serves as an image bearing member, a charging roller108a, and a developing roller108b. The transfer roller106and the photosensitive drum8are in contact with each other to form a transfer nip.

The surface of the photosensitive drum8is charged by the charging roller108awhile the photosensitive drum8is being rotated around the rotation shaft thereof. Then, the optical scanning device100radiates laser light for scanning onto the surface of the photosensitive drum8to form a latent image thereon. Then, the developing roller108bcauses toner to adhere to the latent image, thus forming a toner image, in which the latent image is developed with toner.

On the other hand, the transfer material P is fed from the paper feed unit104by the paper feed roller105, so that the toner image formed on the photosensitive drum8is transferred to the transfer material P by the transfer roller106. Then, the toner image on the transfer material P is fixed to the transfer material P with heat and pressure by the fixing device107. The transfer material P with the toner image fixed thereto is output to the outside of the image forming apparatus101by a discharge roller pair110.

Next, the optical scanning device100is described.FIG. 2is a schematic perspective view of the optical scanning device100.FIG. 3is a partial perspective view illustrating the vicinity of a beam detection (BD) sensor6in the optical scanning device100. A semiconductor laser unit1is the one obtained by unitizing a semiconductor laser (not illustrated), which emits a laser light beam L, and a drive circuit (not illustrated), which drives the semiconductor laser. The laser light beam L emitted from the semiconductor laser passes through a lens2, which has the collimator lens function and the cylindrical lens function, and an aperture stop3, and is then incident on one of a plurality of reflecting surfaces12, which are formed on a rotary polygonal mirror (a polygon mirror)4included in a deflection unit5. The polygon mirror4is driven by a motor included in the deflection unit5to be rotated in the direction of an arrow illustrated inFIG. 2. As the orientation of each of the reflecting surfaces12changes according to the rotation of the polygon mirror4, the direction in which the laser light beam L is reflected from each reflecting surface12consecutively changes. In this way, the laser light beam L is deflected by the polygon mirror4. When the polygon mirror4is at a certain rotational phase, the laser light beam L reflected from a corresponding one of the reflecting surface12passes through a BD lens14to be converged, and is then incident on a light receiving portion10(FIG. 3) included in the BD sensor6, which serves as a light receiving member (a light receiving element). On the other hand, when the polygon mirror4is at another rotational phase, the laser light beam L enters an fθ lens (a scanning lens)7, and is then incident on a photosensitive surface (a scanned surface), which is the surface of the photosensitive drum8. The above-mentioned optical members (the semiconductor laser unit1, the lens2, the aperture stop3, the deflection unit5, the BD sensor6, and the fθ lens7) are positioned, supported, and fixed in position within an optical box9.

[Scanning on Photosensitive Drum by Laser Light]

Next, a method for scanning the photosensitive drum8with laser light, which is performed by the optical scanning device100, is described. The laser light beam L emitted from the semiconductor laser of the semiconductor laser unit1is converted by the lens2into approximately parallel light or converged light in the main scanning direction and into converged light in the sub scanning direction. Then, the laser light beam L passes through the aperture stop3, by which the light beam width of the laser light beam L is limited, and is then focused on a corresponding reflecting surface12of the polygon mirror4in the form of a line extending long in the main scanning direction. Then, as the direction of reflection of the laser light beam L from the reflecting surface12consecutively changes according to the rotation of the polygon mirror4, the laser light beam L is deflected. When the polygon mirror4is at a predetermined rotational phase, the reflected laser light beam L is incident on a portion of the optical box9near the BD sensor6or on the surface of the BD sensor6, thus forming a circular spot S1(FIG. 3). As the polygon mirror4rotates, the spot S1of the laser light beam L moves in the direction of a dashed arrow illustrated inFIG. 3(in the X direction), and then passes through the light receiving portion10. At this time, the BD sensor6outputs a BD signal when the amount of light received at the light receiving portion10has reached a predetermined threshold value. With reference to timing at which the BD signal has been output, timing of start of light emission (start of writing of an image) performed by the light source based on image data is determined.

As the polygon mirror4further rotates a predetermined amount, the laser light beam L reflected from the polygon mirror4passes through the fθ lens7, and is then incident on the surface of the photosensitive drum8. The fθ lens7converges and focuses the laser light beam L as a spot image on the surface of the photosensitive drum8. During a period in which the polygon mirror4further rotates a predetermined amount after the laser light beam L starts entering the fθ lens7, the laser light beam L continues passing through the fθ lens7and being incident on the surface of the photosensitive drum8, and the spot image of the laser light beam L moves on the surface of the photosensitive drum8in a scanning direction corresponding to the rotational direction of the polygon mirror4. The scanning direction is parallel to the rotational axis direction of the photosensitive drum8. The fθ lens7is designed in such a way as to cause the spot image of the laser light beam L to move at a constant speed in the scanning direction (the main scanning direction) on the surface of the photosensitive drum8.

During a period in which the spot image of the laser light beam L moves in the scanning direction on the surface of the photosensitive drum8, a driving current is supplied to the light source of the semiconductor laser unit1based on a laser driving signal (a video signal) corresponding to image data to be formed, so that the light source is turned on.

In addition to the above-mentioned rotation of the polygon mirror4, as the photosensitive drum8rotates around the rotational shaft thereof, the spot image of the laser light beam L relatively moves in a direction perpendicular to the main scanning direction (in the sub scanning direction) with respect to the surface of the photosensitive drum8. The rotation of the polygon mirror4and the rotation of the photosensitive drum8performed in the above-described way enable a two-dimensional latent image corresponding to image data to be formed on the surface of the photosensitive drum8.

The output process for the BD signal and the scanning process with the laser light beam L on the photosensitive drum8described above are performed for every reflecting surface12according to the rotation of the polygon mirror4.

Timing at which to output the BD signal is determined by the positional accuracy of a portion (a boundary portion) of the light receiving portion10on which the spot S1of the laser light beam L is first incident when moving in the X direction. In the case of a conventional configuration, a BD sensor having a relatively large light receiving portion is used, and a restriction portion that covers a part of the light receiving portion on the upstream side in the X direction and that restricts a laser light beam incident on the part of the light receiving portion is formed with a member different from a circuit board. In such a configuration, since the end of the restriction portion serves as the above-mentioned boundary portion, the acceptable range of positional errors of the BD sensor is wide as long as the position of the restriction portion is precisely determined. However, if, as in the present exemplary embodiment, no restriction portion different from a circuit board is provided for the purpose of reduction in size and cost, an end10aof the light receiving portion10on the upstream side in the X direction serves as the above-mentioned boundary portion. Therefore, it is necessary to form the end10ain the form of a straight line perpendicular to the X direction and to precisely determine the position of the light receiving portion10in the X direction.

FIGS. 4A and 4Bare perspective views illustrating a relationship between the BD sensor6and a board (a circuit board)20, on which the BD sensor6is to be mounted (fitted).FIG. 4Aillustrates a condition in which the BD sensor6is not yet mounted on the board20, andFIG. 4Billustrates a condition in which the BD sensor6has been mounted on the board20. Suppose that the direction of movement (the scanning direction) of the spot S1of the laser light beam L on the surface of the BD sensor6is the X direction and a direction perpendicular to the scanning direction X as viewed from a normal direction to a mounting surface24of the board20is the Y direction. Since the surface of the BD sensor6is parallel to the mounting surface24of the board20, the X direction and the Y direction are parallel to the mounting surface24. The laser light beam L is radiated on the reverse side (the back) of the mounting surface24from the far side toward the near side as viewed inFIG. 4B. The board has holes20H formed thereon, in which terminals of the semiconductor laser of the semiconductor laser unit1are to be inserted. The terminals of the semiconductor laser are inserted into the holes20H from the surface on the reverse side of the mounting surface24and are then connected by solder, at the mounting surface24, to a laser driving control circuit formed on the mounting surface24.

The BD sensor6includes a first terminal array (a first terminal portion)23aand a second terminal array (a second terminal portion)23b. Each of the first terminal array23aand the second terminal array23bincludes a plurality of terminals arrayed in the X direction. The first terminal array23aand the second terminal array23bare arranged to protrude from a body portion6a, having the light receiving portion10, of the BD sensor6along the Y direction. The first terminal array23aand the second terminal array23bprotrude from the body portion6ain opposite directions along the Y direction, and are located opposite each other across the body portion6awith respect to the Y direction. Furthermore, as viewed from a normal direction to the mounting surface24, the first terminal array23aand the second terminal array23bare line-symmetric with respect to a central line X1that passes thorough the center of the BD sensor6and is parallel to the X direction.

Since the semiconductor laser and the BD sensor6are mounted on the board20, the board20is provided with a driving control circuit for the semiconductor laser and a BD signal output circuit. The board20has a hole21, which is a through-hole, formed thereon. On the mounting surface24, which is configured to mount electrical elements thereon, pads22aand22belectrically connected to the BD signal output circuit are arranged opposite each other across the hole21with respect to the Y direction. The pad22ais a first soldering portion to be soldered to the first terminal array23a, and the pad22bis a second soldering portion to be soldered to the second terminal array23b. When the first terminal array23aand the second terminal array23bare soldered to the pads22aand22bof the board20, respectively, with at least a part of the body portion6aof the BD sensor6inserted and fitted into the hole21, the BD sensor6is mounted on the board20. The pads22aand22bof the board20are arranged in such a manner as to be superposed on the first terminal array23aand the second terminal array23b, respectively, when the BD sensor6is fitted into the hole21. Furthermore, the pads22aand22bare arranged in a shape that is longer at the fore end thereof along the Y direction than the first terminal array23aand the second terminal array23b. Cream solder is previously applied to the pads22aand22b. When the board20with the first terminal array23aand the second terminal array23bsuperposed on the pads22aand22b, respectively, is put through a reflow furnace (not illustrated), the BD sensor6is fixed by solder to the board20.

When the board20is put through the reflow furnace, cream solder applied to the pads22aand22bis melted. Then, due to the surface tension of the melted cream solder, self-alignment occurs in which the BD sensor6is moved in the X direction in such a manner that the first terminal array23aand the second terminal array23bare superposed on the pads22aand22b, respectively. Owing to the self-alignment occurring due to the surface tension, even if the BD sensor6is deviated in the X direction, the BD sensor6is moved in the X direction to the central position of the pads22aand22band is thus positioned. Furthermore, since the pads22aand22bare arranged in a shape that is one size longer along the Y direction than the first terminal array23aand the second terminal array23b, the surface tension of solder is increased, so that self-alignment can be more stabilized. The first terminal array23aand the second terminal array23bare respectively arranged at the two sides parallel to the X direction of the body portion6aof the BD sensor6and are located line-symmetric with respect to the central line X1, which passes thorough the center of the BD sensor6and is parallel to the X direction. Therefore, a moment to rotate the BD sensor6on the surface of the board20is unlikely to occur, so that stable self-alignment can occur.

Furthermore, self-alignment is unlikely to occur with respect to the Y direction. Therefore, the position of the BD sensor6in the Y direction is determined by restricting, with the hole21, the positions of two sides parallel to the X direction of the BD sensor6and fitting the BD sensor6itself into the hole21.

With the above-described configuration, the BD sensor6is precisely mounted at the positions of the pads22aand22bof the board20with respect to the X direction. Since the X direction is a direction in which a laser light beam is thrown for scanning on the BD sensor6, mounting the BD sensor6in a precise position enables precisely determining timing of output of a BD signal and timing of writing start of an image.

Moreover, although a direction perpendicular to the scanning direction X as viewed from a normal direction to the mounting surface24of the board20has been mentioned as the Y direction, the Y direction may be an intersection direction that intersects with the scanning direction X as viewed from a normal direction to the mounting surface24of the board20.

In a second exemplary embodiment, a more adaptable shape of the hole21of the board20is described. The configurations of the image forming apparatus101and the optical scanning device100in the second exemplary embodiment are similar to those of the first exemplary embodiment. Therefore, portions similar to those of the first exemplary embodiment are assigned the respective same reference numerals and are not described again here.

FIG. 5Aillustrates the BD sensor6,FIG. 5Billustrates the board20, andFIG. 5Cillustrates a state in which the BD sensor6has been mounted on the board20, each as viewed from a normal direction to the mounting surface24of the board20. As illustrated inFIG. 5A, suppose that the dimension in the X direction of the outline of the body portion6aexcluding the first terminal array23aand the second terminal array23bof the BD sensor6is denoted by Lx0, and that dimension in the Y direction is denoted by Ly0. As illustrated inFIG. 5B, the dimension in the X direction of the hole21of the board20, in which to fit the body portion6aof the BD sensor6, is denoted by Lx1, and that dimension in the Y direction is denoted by Ly1.

As described in the first exemplary embodiment, while the BD sensor6can be positioned to the board20by self-alignment with respect to the X direction, the BD sensor6cannot be positioned by self-alignment with respect to the Y direction. Therefore, with respect to the Y direction, the board20is required to be positioned to the board20by fitting.

Furthermore, considering the workability of the board20, the angles of four corners of the hole21of the board20cannot be formed into perfect right angles. Usually, the angles of four corners of the hole21each have a rounded portion (R) with a radius r of 0.5 mm or more. Therefore, in a case where gaps between the BD sensor6and the hole21are to be equally provided both in the X direction and in the Y direction, it is necessary to make the size of the hole210.5 mm or more larger than the size of the BD sensor6both in the X direction and in the Y direction, so as to prevent the rounded portions R at four corners from interfering with the corners of the BD sensor6. Furthermore, since, considering the dimensional tolerance of the hole21or the body portion6aitself, it is necessary to additionally provide gaps with a dimension of about 0.2 mm, the size of the hole21is required to be set to a size that is 0.7 mm or more larger than the size of the body portion6aboth in the X direction and in the Y direction. Therefore, the tolerance of positional deviation becomes up to 0.7 mm both in the X direction and in the Y direction. However, in the case of the BD sensor6, the mounting positional deviation of 0.7 mm may not be acceptable to ensure the image quality in the image forming apparatus101.

Therefore, according to the present exemplary embodiment, the gaps between the BD sensor6and the hole21are intentionally set to have different sizes between the X direction and the Y direction. More specifically, the dimension Lx1 of the hole21in the X direction, with respect to which the positioning action by self-alignment is expected, is set to a size 1.0 mm or more larger than the outline dimension Lx0 of the body portion6a. Accordingly, even if the dimension Ly1 of the hole21in the Y direction is equal to the outline dimension Ly0 of the body portion6, the rounded portions R of four corners of the hole21and the body portion6ahave such a relationship as not to interfere with each other. In the present exemplary embodiment, in consideration of the tolerance of the size of the hole21itself, additional gaps with a dimension of about 0.2 mm are provided both in the X direction and in the Y direction. Accordingly, the size of the hole21is set 1.2 mm larger in the X direction and 0.2 mm larger in the Y direction than the size of the BD sensor6. As described in the foregoing, with regard to the X direction, since, when cream solder is melted, self-alignment in the X direction occurs in such a manner that the first terminal array23aand the second terminal array23bare superposed on the pads22aand22b, respectively, the mounting position is precisely determined even if the dimension of the hole21is relatively large. Thus, self-alignment enables precise mounting with an error of 0.1 mm or less with respect to a predetermined position. With regard to the Y direction, since the dimension of the gap between the BD sensor6and the hole21is no larger than 0.2 mm, the mounting position is determined with a tolerance of 0.2 mm or less.

In this way, according to the present exemplary embodiment, the relationship between the outline dimension of the body portion6aand the dimension of the hole21on the board20is set to satisfy the following formula (1):
(Lx1−Lx0)≧(Ly1−Ly0)  (1)

Thus, with regard to gaps between the body portion6aand the hole21, the gap “Ly1−Ly0” in the Y direction is set smaller than the gap “Lx1−Lx0” in the X direction.

With such a dimensional relationship, the BD sensor6can be precisely positioned to the board20by fitting also in the Y direction.

Furthermore, since, in the present exemplary embodiment, the mounting position of a mounted component can be precisely determined, the present exemplary embodiment can be applied to a light receiving member, such as the BD sensor6, equipped with a light receiving portion or a light emitting member, such as a light emitting diode (LED), equipped with a light emitting portion, which needs high positional accuracy. However, the configuration of the present exemplary embodiment is not limited to such a light receiving member or light emitting member, but may be applied to positioning of an electrical element to be surface-mounted on a board.

In a third exemplary embodiment, another more adaptable shape of the hole21of the board20is described. The configurations of the image forming apparatus101and the optical scanning device100in the third exemplary embodiment are similar to those of the first exemplary embodiment. Therefore, portions similar to those of the first exemplary embodiment and the second exemplary embodiment are assigned the respective same reference numerals and are not described again here.

FIG. 6Aillustrates the BD sensor6,FIG. 6Billustrates the board20, andFIG. 6Cillustrates a state in which the BD sensor6has been mounted on the board20, each as viewed from a normal direction to the mounting surface24of the board20. As illustrated inFIG. 6B, the dimension in the X direction of the hole21of the board20, in which to fit the body portion6aof the BD sensor6, is denoted by Lx2, and that dimension in the Y direction is denoted by Ly2.

The present exemplary embodiment is characterized by providing cut portions21aat the four corners (four vertex portions) of the quadrangular hole21of the board20. As described in the second exemplary embodiment, considering the workability of the board20, the vertices of four corners of the hole21of the board20cannot be formed into perfect right angles. Therefore, in the present exemplary embodiment, the cut portions21aare provided at the four corner portions of the hole21, so that the hole21is enlarged. More specifically, as viewed from a normal direction to the mounting surface24, the body portion6aof the BD sensor6is of an approximately quadrangular shape having four sides61,62,63, and64and four corner portions65, at which the respective two adjacent sides of the four sides meet. The sides62and64are parallel to the X direction, and the sides61and63are parallel to the Y direction. Moreover, the hole21has four sides211,212,213, and214, which respectively face the four sides61to64of the body portion6a, and four cut portions21a, which are located at the positions respectively facing the four corner portions65. Owing to the provision of the cut portions21a, the edges of the cut portions21aare further away from the four sides61to64and the four corner portions65of the body portion6than the four sides211to214.

In this way, providing the cut portions21aat the respective four corners enables preventing the body portion6afrom being affected by rounded portions R which might be formed at the vertices of four corners of the hole21, and also enables designing the dimension Lx2 in the X direction of the hole21with a smaller dimension as compared with that in the second exemplary embodiment. Therefore, the area occupied by the hole21on the board20can be reduced, so that the degree of freedom of designing, such as layout, of the board20can be improved. more specifically, in consideration of the tolerance of the size of the hole21itself, gaps with a dimension of about 0.2 mm added to the size of the hole21in the X direction and the Y direction can be provided with respect to the outline size of the body portion6a. Since self-alignment occurs in the X direction, the BD sensor6can be mounted almost with an error of 0.1 mm or less relative to a predetermined position. With regard to the Y direction, since the dimension of the gap between the BD sensor6and the hole21is no larger than 0.2 mm, the mounting position is determined with a tolerance of 0.2 mm or less.

With the above-described configuration, the BD sensor6can be precisely positioned by self-alignment with respect to the X direction and can also be precisely positioned relative to the board20by fitting with respect to the Y direction. Furthermore, since the cut portions21aare provided at the respective four corner portions of the hole21, the dimension Lx2 in the X direction of the hole21can be reduced, so that the area occupied by the hole21on the board20can be reduced and the degree of freedom of designing, such as layout, of the board20can be increased.

Moreover, since, in the present exemplary embodiment, the mounting position of a mounted component can be precisely determined, the present exemplary embodiment can be applied to a light receiving member, such as the BD sensor6, equipped with a light receiving portion or a light emitting member, such as a light emitting diode (LED), equipped with a light emitting portion, which needs high positional accuracy. However, the configuration of the present exemplary embodiment is not limited to such a light receiving member or light emitting member, but may be applied to positioning of an electrical element to be surface-mounted on a board.

In a fourth exemplary embodiment, yet another more adaptable shape of the hole21of the board20is described. The configurations of the image forming apparatus101and the optical scanning device100in the fourth exemplary embodiment are similar to those of the first exemplary embodiment. Therefore, portions similar to those of the first exemplary embodiment and the other exemplary embodiments are assigned the respective same reference numerals and are not described again here.

FIG. 7Aillustrates the BD sensor6,FIG. 7Billustrates the board20, andFIG. 7Cillustrates a state in which the BD sensor6has been mounted on the board20, each as viewed from a normal direction to the mounting surface24of the board20. As illustrated inFIG. 7B, the dimension in the X direction of the hole21of the board20, in which to fit the BD sensor6, is denoted by Lx3, and the largest dimension and the smallest dimension in the Y direction of the hole21of the board20are denoted by Ly3a and Ly3b, respectively.

In the present exemplary embodiment, the relationship between the outline dimension of the body portion6aand the dimension of the hole21on the board20is set to satisfy “Lx3≧Lx0” and “Ly3a>Ly3b≧Ly0”. In this way, the hole21is of a shape having different lengths in the Y direction depending on the position in the X direction.

At the time of reflow mounting, the BD sensor6moves in the X direction toward the adjusted position, such as that illustrated inFIG. 7C, in which the first terminal array23aand the second terminal array23bare superposed on the pads22aand22b, respectively, by self-alignment due to the surface tension of solder. Therefore, in the present exemplary embodiment, when the BD sensor6moves in the X direction toward the adjusted position by self-alignment, the width in the Y direction of the hole21becomes gradually narrower. The length Ly3b at the position where the width in the Y direction of the hole21is smallest is approximately equal to the dimension Ly0. Therefore, the position of the BD sensor6in the Y direction is restricted along with the movement in the X direction by self-alignment, so that the position in the Y direction of the BD sensor6can also be precisely determined.

In this way, in the present exemplary embodiment, a difference is set in the dimension of the hole21in the Y direction between the position where the BD sensor6is fitted (inserted into the hole21) and the adjusted position where the BD sensor6is positioned by self-alignment. This enables designing, with a minimum numerical value, the dimension in the Y direction of the hole21at the position where the BD sensor6is fixed by solder while assuring the ease of fitting (ease of mounting) during the fitting of the BD sensor6into the hole21, so that the BD sensor6can be precisely mounted. For example, when the dimension of the gap (Ly3b−Ly0) in the Y direction at the adjusted position is set to 0.1 mm, since the BD sensor6bumps into the hole21to determine the position thereof, a mounting error in the Y direction can be kept to 0.1 mm.

The above-described configuration enables, while assuring the ease of fitting (ease of mounting) during the fitting of the BD sensor6into the hole21, precisely positioning the BD sensor6by self-alignment with respect to the X direction and also precisely positioning the BD sensor6by fitting to the board with respect to the Y direction using the movement in the X direction by self-alignment.

Moreover, since, in the present exemplary embodiment, the mounting position of a mounted component can be precisely determined, the present exemplary embodiment can be applied to a light receiving member, such as the BD sensor6, equipped with a light receiving portion or a light emitting member, such as a light emitting diode (LED), equipped with a light emitting portion, which needs high positional accuracy. However, the configuration of the present exemplary embodiment is not limited to such a light receiving member or light emitting member, but may be applied to positioning of an electrical element to be surface-mounted on a board.

FIG. 8is a schematic perspective view of an optical scanning device100according to a fifth exemplary embodiment. The fifth exemplary embodiment differs from the first to fourth exemplary embodiments in that the optical scanning device100includes no BD lens14. When the polygon mirror4is at a certain rotational phase, the laser light beam L reflected from a corresponding one of the reflecting surfaces12is incident on a light receiving portion of the BD sensor6without passing through any lens or the like. The above-described first to fourth exemplary embodiments can also be applied to such an optical scanning device100.

Next, as a sixth exemplary embodiment, an example is described in which a false detection by the BD sensor6caused by a laser light beam reflected from the inner wall surface of a through-hole, which is formed on a board and in which the BD sensor6is mounted, incident on the light receiving portion10of the BD sensor6can be prevented.

FIGS. 9A and 9Bare perspective views illustrating a relationship between the BD sensor6and the board20, on which the BD sensor6is to be mounted (fitted).FIG. 9Aillustrates a condition in which the BD sensor6is not yet mounted on the board20, andFIG. 9Billustrates a condition in which the BD sensor6has been mounted on the board20. Suppose that the direction of movement (the scanning direction) of the spot S1of the laser light beam L on the surface of the BD sensor6is the X direction and a direction perpendicular to the scanning direction X as viewed from a normal direction to the mounting surface24of the board20is the Y direction.

As described in the first exemplary embodiment, since the semiconductor laser and the BD sensor6are mounted on the board20, the board20is provided with a driving control circuit for the semiconductor laser and a BD signal output circuit. The board20has a hole21, which is a through-hole, formed thereon. Terminal arrays23of the BD sensor6are soldered to pads22of the board20, so that the BD sensor6is mounted on the board20. The terminal arrays23may be soldered to the pads22with at least a part of the BD sensor6inserted and fitted into the hole21. The pads22of the board20are arranged in such a manner that the terminal arrays23are superposed on the pads22when the BD sensor6is fitted in the hole21. Furthermore, the pads22are arranged in a shape that is longer at the fore end thereof along the Y direction than the terminal arrays23. Cream solder is previously applied to the pads22. When the board20with the terminal arrays23superposed on the pads22is put through a reflow furnace, the BD sensor6is fixed by solder to the board20.

When the board20is put through the reflow furnace, cream solder applied to the pads22is melted. Then, due to the surface tension of the melted cream solder, self-alignment occurs in which the BD sensor6is moved in the X direction in such a manner that the terminal arrays23are superposed on the pads22. Owing to the self-alignment occurring due to the surface tension, even if the BD sensor6is deviated in the X direction, the BD sensor6is moved in the X direction to the central position of the pads22and is thus positioned. Furthermore, since the pad22are arranged in a shape that is one size longer along the Y direction than the terminal arrays23, the surface tension of solder is increased, so that self-alignment can be more stabilized. The terminal arrays23are arranged at the two sides parallel to the X direction of the BD sensor6and are located line-symmetric with respect to a central line which passes thorough the center of the BD sensor6and is parallel to the X direction. Therefore, a moment to rotate the BD sensor6on the surface of the board20is unlikely to occur, so that stable self-alignment can occur.

With the above-described configuration, the BD sensor6is precisely mounted at the positions of the pads22of the board20with respect to the X direction. Since the X direction is a direction in which a laser light beam is thrown for scanning on the BD sensor6, mounting the BD sensor6in a precise position enables precisely determining timing of output of a BD signal and timing of writing start of an image. Furthermore, self-alignment is unlikely to occur with respect to the Y direction. Therefore, the position of the BD sensor6in the Y direction is determined by restricting, with the hole21, the positions of two sides parallel to the X direction of the BD sensor6and fitting the BD sensor6itself into the hole21.

[Prevention of Stray Light to BD Sensor6]

FIG. 10illustrates a laser light beam L that is incident on the BD sensor6in a sectional view taken along line A-A illustrated inFIG. 9Bserving as a scanning line corresponding to the laser light beam L deflected by the polygon mirror4, as viewed from above. As described in the foregoing, since the BD sensor6is mounted on the mounting surface24of the board20, on which a semiconductor laser is mounted, the laser light beam L is incident on the mounting surface24at an angle of θ1 to a normal direction to the light receiving portion10, which is a light receiving surface parallel to the mounting surface24. Since the hole21of the board20is formed by press working with a die from the side of the mounting surface24, the inner wall surface of the hole21is formed to have an inclination of angle θ2 with respect to a normal direction to the mounting surface24.

The laser light beam L moves for scanning in the X direction (from right to left inFIG. 10), and the hole21is widely open at the side through which the laser light beam L passes after passing through the light receiving portion10(at the downstream side in the X direction of the light receiving portion10). The magnitude of a portion of the hole21at the downstream side in the X direction of the light receiving portion10is described. Supposing that a portion at the most downstream side in the scanning direction of the inner wall surface of the hole21is referred to as an end face25, light reflected from the end face25after the laser light beam L passes through the light receiving portion10may be incident on the light receiving portion10, and the BD sensor6may output a BD signal based on the light received by the light receiving portion10. In particular, since the light receiving portion10in the present exemplary embodiment is located at the more downstream side in the X direction than the center of the BD sensor6and is thus near the end face25, light reflected from the end face25is likely to be incident on the light receiving portion10. To prevent such a false detection by the BD sensor6, the hole21is formed in such a shape as to prevent light reflected from the end face25from being incident on the light receiving portion10.

A point closest to the polygon mirror4on the end face25is referred to as point B. The point B is a reflecting point from which the reflected laser light beam L passes in a direction most toward the light receiving portion10. Furthermore, the length of a perpendicular line drawn from the light receiving portion10to a back side26of the board20, which is a plane containing the point B and parallel to the mounting surface24, is referred to as “t”.

FIG. 11illustrates a laser light beam L being reflected from the point B in a sectional view taken along line A-A illustrated inFIG. 9Bserving as a scanning line corresponding to the laser light beam L deflected by the polygon mirror4, as viewed from above, as withFIG. 10. Similar to a light beam incident on the light receiving portion10, a laser beam incident on the point B at an angle of incidence of θ1 relative to the perpendicular direction of the light receiving portion10is reflected from the point B at an angle of “θ1+2·θ2” in consideration of the angle θ2 of the inner wall surface of the hole21. Therefore, a distance D1with respect to the X direction from the point B to a point at which the laser beam reflected from the point B reaches the same height as that of the light receiving portion10is obtained as “t·tan (θ1+2·θ2)”.

Accordingly, if a distance between the light receiving portion10and the point B with respect to the X direction satisfies the following formula (2), a laser beam reflected from the point B is not incident on the light receiving portion10.
D>t·tan(θ1+2·θ2)  (2)

When the angle of the light beam reflected from the point B with respect to a direction perpendicular to the X direction is denoted by θ (=θ1+2·θ2), formula (2) can be substituted with the following formula (2)′.
D>t·tan θ  (2)′

In this way, if the distance D satisfies formula (2), since the laser light beam L reflected from the point B is not incident on the light receiving portion10, no laser light beam L reflected from the end face25of the hole21of the board20is incident on the light receiving portion10. In the present exemplary embodiment, the position of the BD sensor6in the X direction is set to such a position that the distance D satisfies formula (2). This enables preventing light reflected from the inner wall surface of the hole21from being incident on the light receiving portion10of the BD sensor6.

A seventh exemplary embodiment differs from the sixth exemplary embodiment in that the optical scanning device100includes no BD lens14, as with the configuration described in the fifth exemplary embodiment. When the polygon mirror4is at a certain rotational phase, the laser light beam L reflected from a corresponding one of the reflecting surfaces12is incident on a light receiving portion of the BD sensor6without passing through any lens or the like. In the case of the present exemplary embodiment, in which no BD lens14is included, the spot S1of the laser light beam L in the position of the BD sensor6is larger than that in the configuration of the first exemplary embodiment in which the BD lens14is included. Therefore, if the laser light beam L is reflected from the inner wall surface of the hole21and is then incident on the light receiving portion10, the laser light beam L may be detected by the BD sensor6as a relatively large amount of light. Accordingly, such a configuration as to satisfy equation (2) is more effective.

This application claims the benefit of Japanese Patent Applications No. 2015-028944 filed Feb. 17, 2015 and No. 2015-028945 filed Feb. 17, 2015, which are hereby incorporated by reference herein in their entirety.