Semiconductor apparatus, exposing head, and image forming apparatus

A semiconductor apparatus includes a rectangular plate-like body including a major surface. A plurality of light emitting portions formed in the major surface, and aligned in a straight line. A first terraced portion and a second terraced portion are formed in the major surface except areas in which the plurality of light emitting portions are formed.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a semiconductor apparatus, an exposing apparatus, and an image forming apparatus, and may be advantageously applied to a print head in which a plurality of light emitting element array chips are aligned on a circuit board.

2. Description of the Related Art

A conventional light emitting diode (LED) print head, which is used in an LED printer, employs a configuration in which a plurality of semiconductor light emitting element array chips are mounted on a print wiring board and are aligned in a straight line. Each array chip has a plurality of light emitting portions formed in its surface, the light emitting portions being aligned in one dimension at predetermined intervals.

This type of LED print head is assembled by first applying an adhesive to a print wiring board, and then semiconductor light emitting element array chips are pressed on the adhesive against the print wiring board, and finally allowing the adhesive to cure. In this manner, the semiconductor light emitting element array chips secured on the print wiring board.

If the space between adjacent chips is too narrow, the adhesive is drawn into the space by capillary action up to the same level as the surface of the array chips, soiling the light emitting portions and causing usable light power to decrease. The adhesive may also contaminate the wire bonding pads formed on the end portion of the surface of the array chips, reducing the mechanical strength of the wire bonded portions.

Japanese Patent Laid-Open No. 2011-131475 discloses a print head in which the adhesive is applied only to a limited surface area on the print wiring board, e.g., a middle portion of the back surface of the array chip, thereby preventing the adhesive from being drawn into the space between adjacent array chips up to the upper surface of the array chips.

However, applying the adhesive only to a middle portion of the back surface of the array chip causes end portions of the back surface to be uplifted, so that edges of the array chip may be chipped during wire bonding or the array chip may be inclined at an angle with respect to the surface of the print wiring board.

If end portions of the back surface of the array chip are uplifted from the print wiring board, heat dissipation is more difficult at the end portions than in the middle portion of the back surface, so that the temperature of the end portions is higher than the middle portion.

Although applying the adhesive only to a limited portion of the back surface is effective in preventing the adhesive from contaminating the chip surface, but may increase the chances of the chips inclining and being damaged, impairing the reliability of the LED print head.

SUMMARY OF THE INVENTION

The present invention was made to solve the aforementioned drawbacks.

An object of the present invention is to provide a semiconductor apparatus in which semiconductor array chips are protected against contamination by an adhesive, an exposing head that employs the semiconductor apparatus, and an image forming apparatus that employs the exposing head.

A semiconductor apparatus includes a rectangular plate-like body including a major surface. A plurality of light emitting portions is formed in the major surface, and aligned in a straight line. A first terraced portion and a second terraced portion are formed in the major surface except areas in which the plurality of light emitting portions are formed.

DETAILED DESCRIPTION OF THE INVENTION

The present invention will be described in detail by way of preferred embodiments with reference to the accompanying drawings.

First Embodiment

Overall Configuration of LED Printer

FIG. 1illustrates the outline of an LED printer1according to the present invention. The LED printer1includes a generally box-shaped chassis2.

The chassis2accommodates four process units3A,3B,3C, and3D for forming yellow (Y), magenta (M), cyan (C), and black (K) images, respectively, by electrophotography. The four process units3A,3B,3C, and3D are aligned along a transport path4of a recording medium P.

Each of the process units3A,3B,3C, and3D includes a photoconductive drum5as an image bearing body, a charging unit6, an exposing unit7, a developing unit9, and a cleaning unit10, which are disposed to surround the photoconductive drum5. The charging unit6uniformly charges the surface of the photoconductive drum5. The exposing unit7selectively illuminates the charged surface of the photoconductive drum5to form an electrostatic latent image on the photoconductive drum5. The developing unit9supplies toner to the electrostatic latent image to develop the electrostatic latent image into a toner image. The cleaning unit10removes the residual toner from the photoconductive drum5after transferring the toner image onto the recording medium P. The photoconductive drum5is driven by a drive source through a train of gears (not shown), and rotates in a clockwise direction inFIG. 1.

The chassis2accommodates a paper cassette11, which holds a stack of sheets of the recording medium P. A hopping roller12feeds the recording medium P into the transport path4on a sheet-by-sheet basis from the paper cassette11. Pinch rollers13and14and registration rollers15and16are disposed between the hopping roller12and the process unit3D. The registration rollers15and16cooperate with the pinch rollers13and14, respectively, to hold the sheet of the recording medium P in a sandwiched relation. The registration rollers15and16cooperate to remove skew of the recording medium P. The hopping roller12and registration rollers15and16are driven in rotation in an interlocking manner by a drive source (not shown).

A transfer roller17is disposed to face a corresponding photoconductive drum5with the transport path4sandwiched between the photoconductive drum5and the transfer roller17. The transfer roller17is formed of, for example, a semi-conductive rubber material. The potentials of the photoconductive drum5and the transfer roller17are selected so that the toner image formed on the photoconductive drum5can be reliably transferred onto the recording medium P.

A fixing unit18is located downstream of the process unit3A with respect to the transport path4. A pair of discharge rollers20and21and a pair of discharge rollers22and23are disposed downstream of the fixing unit18, and discharge the recording medium P onto a stacker19formed on the upper surface of the chassis2.

The sheet of the recording medium P is fed by the hopping roller12into the transport path4, and is further transported by the pinch rollers13and14and the registration rollers15and16. The sheet of the recording medium P then passes through the four process units3A,3B,3C, and3D, in that stated order. While the recording medium P passes through the process units3A,3B,3C, and3D, the toner images of the respective colors are transferred onto the recording medium P one over the other in registration. The recording medium P is then fed into the fixing unit18where the toner images of the respective colors on the recording medium P are fixed under heat and pressure. After fixing, the recording medium P is discharged onto the stacker19by the discharging rollers20to23.

{Configuration of LED Print Head}

A description will be given of the configuration of an LED head8mounted on the exposing unit7of each of the process units3A,3B,3C, and3D.FIG. 2Ais a perspective view of the LED print head8in its entirety and a perspective cross-sectional view.FIG. 2Bis a cross-sectional view taken along a line A-A inFIG. 2A. The LED print head8extends in a longitudinal direction thereof and includes a frame30having a generally U-shaped cross-section. The frame30is made of, for example, an aluminum block, a metal plate, or a resin (e.g., liquid crystal polymer).

The frame has a longitudinal opening31that extends in the longitudinal direction. A chip-on-board (COB) module133fits into the opening31. A narrow opening32is formed in the bottom of the U-shaped frame30, and extends in the longitudinal direction of the frame30. The COB133includes a rectangular wiring board134. A plurality of array chips135of semiconductor light emitting portions are aligned in a straight line on the print wiring board134. The COB1133is attached to the frame30such that the plurality of array chips135face the opening32. As shown inFIG. 2A, the LED print head8extends in the longitudinal direction thereof shown by arrow S, which is parallel to a main scanning direction of the printer1perpendicular to a direction in which the recording medium P is transported.

A rod lens array36generally in the shape of a rectangular parallelepiped fits into the opening32. The rod lens array36forms an erect image of unity magnification of the light emitted from the array chips135on the charged surface of the photoconductive drum5. One of the lens surfaces,36b, of the rod lens array36is a distance L from the light emitting surface of the array chips and the other of the lens surfaces extends outwardly through the opening32, so that the image of the light emitting portions is formed on the surface of the photoconductive drum5, which is a distance L from the other lens surface of the rod lens array,36a.

Each LED print head8is assembled to a corresponding exposing unit7so that the lens surface projecting outwardly from the frame30faces the surface of the corresponding photoconductive drum5.

A further description will be given of the print wiring board134and the array chips135mounted on the print wiring board134.FIG. 3Aillustrates the appearance of the chip-on-board module133.FIG. 3Bis a partial expanded view of a portion P of the chip-on-board (COB)133shown inFIG. 3A.FIG. 4Ais a further perspective expanded view of a pertinent portion.FIG. 4Billustrates the positional relationship among two consecutive odd-numbered array chips and an even numbered array chip between the two consecutive odd-numbered array chips. For simplicity's sake,FIG. 4Adoes not show Au wires140(FIG. 3B), which connect between the print wiring board134and the array chips135.

Referring toFIGS. 3A, 3B, 4A, and 4B, a plurality of rectangular plate-like array chips135are mounted on the surface of the print wiring board134using an electrically conductive or an electrically non-conducive adhesive141, the array chips135being aligned in the main scanning direction shown by arrow S so that the first short side of an even-numbered semiconductor apparatus directly faces the second short side of an odd numbered semiconductor apparatus and the light emitting portions of adjacent array chips are in a single straight line (FIG. 4B). Light emitting portions143, which are light emitting diodes, are formed in the top surface142of each array chip135. The light emitting portions143are formed of a GaAs compound semiconductor, and are aligned in a one dimension at intervals of 42.3 μm resolution of 600 dpi) or at intervals of 21.2 μm (i.e., a resolution of 1200 dpi). Each light emitting portion143may be implemented as a light emitting diode (LED) by achieving a PN junction of a P-type semiconductor and an N type semiconductor. Alternatively, the light emitting portions143may be Thyristors that take the form of a PNPN junction or an NPNP junction.

The array chip135is rectangular, and has a longitudinal center line CL, long sides, and short sides. A straight line of the light emitting portions143extends in a direction parallel to the long sides, and is closer to one of the long sides of the array chip135than the longitudinal center line CL (FIG. 4A), while a straight line of the wire bonding pads144extends in a direction parallel to the long sides, and is closer to the other of the long sides than the longitudinal center line CL (FIG. 4A). The wire bonding pads144are aligned at predetermined intervals.

Just as in the light emitting portions143, the array chips135may be fabricated from a GaAs substrate. In addition to the light emitting portions143, the array chip135may have shift registers (not shown) that sequentially shifts a signal received from an external drive circuit. Alternatively, the array chips135may also be fabricated from an Si substrate on which IC driver circuits are fabricated for driving the light emitting portions143. In other words, thin films of light emitting layer with a thickness of less than 5 μm are grown on a GaAs substrate formed of a compound semiconductor, and are integrated on an IC driver circuit substrate using intermolecular force, and then the light emitting portions143are fabricated using thin film wiring that can be formed using photolithography and metal thin-film forming technology, and the driver circuits and light emitting portions143are integrated on the GaAs substrate through electrical interconnection. The thickness of the array chips135can be selected to be in the range of, for example, from 200 μm to 600 μm.

Referring toFIG. 4A, the array chip135has stepped portions or terraced portions146at each of the longitudinal end portions of the array chip135. At the terraced portion146, the array chip135has a transition or wall146cfrom the top surface142to a recessed surface145that is recessed from the top surface142and is substantially parallel to the top surface. In other words, the recessed surface is lower than the top surface by a predetermined distance.

More specifically, the top surface142includes a small terraced portion146aand a large terraced portion146bformed at each longitudinal end portion of the array chip135, the small and large terraced portions146aand146bdefining a peninsula-shaped portion142ain which an endmost light emitting portion143xis formed. The peninsula-shaped portion142aextends to the longitudinal end of the array chip135. The recessed surface145is lower than the top surface142by 20 μm to 200 μm depending on the thickness of the array chip, and extends at least 20 μm in the longitudinal direction of the array chip135from the longitudinal end of the array chip.

If the array chip135is formed on a GaAs substrate, the terraced portions146aand146bmay be formed as follows: A wafer is diced into individual rectangular array chips135. A photoresist material is applied to areas of the array chip135except for an area in which the terraced portion146is to be formed. The passivation film or interlayer dielectric film is removed by CF4 dry etching from the area in which the terraced portion146is to be formed, so that the GaAs substrate is exposed. The exposed GaAs substrate is subjected to wet etching using an etchant which is a mixed solution of sulfuric acid, hydrogen peroxide water, and water. The array chip135is etched to a depth of 20 μm to 200 μm. The photoresist material is then removed from the array chip135, thereby forming the terraced portions146aand146bin the array chip135.

If the array chip135takes the form of an Si IC driver circuit substrate, the terraced portions146aand146bmay be formed as follows: The driver circuit is designed such that no circuit occupies an area in which the terraced portions146aand146bare to be formed. Just as in the GaAs substrate, a photoresist material is applied to areas of the array chip except for the areas in which the terraced portions146aand146bare to be formed. The passivation film or interlayer dielectric film in the areas in which the portions146aand146bare to be formed is removed by CF4 dry etching, so that the Si substrate is exposed. The exposed Si substrate is subjected to chemical dry etching that uses a gas, for example, SF6, thereby etching the array chip to a depth of 20 μm to 200 μm. The photoresist material is then removed from the array chip135, thereby forming the terraced portions146aand146bin the array chip135.

Using an adhesive141, the array chips having the terraced portion146formed therein are mounted on the wiring board134formed of, for example, composite epoxy material (CEM3) or flame retardant 4 (FR4), being aligned in the longitudinal direction of the array chip135. The center-to-center distance D2between the respective endmost light emitting portions of adjacent array chips is equal to the center-to-center distance D1between adjacent light emitting portions143on the array chip135. When the light emitting portions143are arranged at intervals of about 42.3 μm (equivalent to 600 dpi), the distance between adjacent array chips135is selected to be about 10 μm, which is shorter than the distance D1, and when the light emitting portions are arranged at intervals of about 42.3 μm (equivalent to 1200 dpi), the distance between adjacent array chips135is selected to be about 5 μm, which is shorter than the distance D1.

The adhesive141may be transferred onto the print wiring board134using the stamp function of a die bonder (not shown). Alternatively, a dispenser (not shown) is used to form a layer of the adhesive141on predetermined areas on the print wiring board134. The layer of the adhesive141is formed so that the entire back surface of the array chip135can be in contact with the layer of the adhesive141. In this manner, the array chip135is pressed against the layer of the adhesive141. The adhesive141then cures, thereby fixedly bonding the entire back surface of the array chip135to the print wiring board134.

As described above, the array chip135carries wire bonding pads144through which data can be inputted from and outputted to an external driver circuit, and the print wiring board134carries wire bonding pads147through which data can be inputted from and outputted to the array chip135. A bonding wire connects a wire bonding pad147to a corresponding wire bonding pad144. A line of wire bonding pads144and a line of wire bonding pads147extend in directions substantially parallel to the longitudinal direction or the center line CL of the array chip135, and are spaced apart by a predetermined distance.

Referring toFIG. 3B, the wire bonding pads144and the wire bonding pads147are electrically connected by means of Au wires140. A ROM, chip capacitors, connectors through which the data is communicated between the LED printer1and the LED print head8are mounted on the print wiring board134, thereby configuring the COB133for the LED print head8. This COB133is assembled to the frame30of the LED print head8.

Effects of First Embodiment

Effects of forming the terraced portions146in the array chip135will be described. The effects will be described by comparing the COB133according to the first embodiment with a comparison COB152whose array chips150have no terraced portions.

FIG. 5Ais a perspective view of the comparison COB152in which semiconductor light emitting element array chips150with no terraced portions146are mounted on a print wiring board151.FIG. 5Bis an expanded view of a portion P shown inFIG. 5A. The comparison COB152has the same configuration as the COB133except that the array chips50have not the terraced portions146formed therein.

The comparison COB152includes a plurality of array chips150bonded to the print wiring board151using an adhesive153and aligned in a straight line. The adjacent array chips150are disposed so that the lines of the light emitting portions of the adjacent array chips150are in line with each other and the distance D2between the respective endmost light emitting portions154xof the adjacent array chips152is equal to the distance D1between adjacent light emitting portions154in the array chips150. When the light emitting portions154are disposed at intervals of about 42.3 μm (equivalent to 600 dpi), the distance between adjacent array chips150is selected to be about 10 μm or less, and when the light emitting portions154xare disposed at intervals of about 42.3 μm (equivalent to 1200 dpi), the distance between adjacent array chips150is selected to be about 5 μm.

The array chip150has wire bonding pads155formed thereon and the print wiring board151has wire bonding pads156formed thereon. Au wires157connect between the wire bonding pads155and corresponding wire bonding pads156.

FIG. 6illustrates how the adhesive climbs up and flows on the surface of the adjacent array chips of the comparison COB152. The adhesive153is drawn into the space between the adjacent array chips150by capillary action up to the same level as the surface of the array chips150, soiling the light emitting portions154and causing usable light power to decrease. The adhesive153may also contaminate the wire bonding pads155formed on the end portion of the surface of the array chips150, reducing the mechanical strength of the wire bonding.

FIG. 7illustrates how the adhesive climbs up and flows on the surface of the adjacent array chips135of the COB133according to the first embodiment. While the adjacent array chips135on the COB133are aligned at the same intervals as the array chips150on the comparison COB, and the entire back surface of the array chips135are in contact with the layer of the adhesive141, the recessed surface145lower than the top surface142bypasses the adhesive141that would otherwise climb up the gap between the adjacent array chips to the top surface142.

The COB133according to the first embodiment does not place any limitation on the area on the back surface of the array chip135in which the adhesive141may be applied. In other words, the adhesive141can be applied to the entire back surface of the array chip135so that the array chip135can be bonded in its entire back surface to the print wiring board134. This prevents the end portions of the back surface of the array chip135from being uplifted, thereby reducing the chances of the array chip135inclining and the chip's end portions being damaged.

Since the entire back surface of the array chip135is bonded to the print wiring board134using the adhesive141, the heat generated by the array chip135can be evenly conducted to the COB133. In other words, heat dissipation can be uniform across the entire array chip135.

FIG. 8is a partial top view of the array chip135, illustrating the distance between the endmost light emitting portion142xand the longitudinal end of the array chip135. The terraced portion146of the COB133according to the first embodiment effectively receives or bypasses the adhesive141, which climbs up the narrow gap between the adjacent array chips135, even if the COB133employs a shorter distance D3between adjacent array chips135than the comparison COB152. Thus, employing the shorter distance D3in the first embodiment provides a longer distance D4(FIG. 8) between the endmost light emitting portion143xand the end of the array chip135shown inFIG. 8as compared to the comparison COB152. The longer distance D4increases mechanical strength of the array chip135, reducing the chances of the edge portions of the array chip135being chipped.

As described above, the COB133has a configuration in which the array chips135are mounted in a straight line on the print wiring board134using the adhesive141and the entire back surface of the array chips135may be bonded to the print wiring board134. In addition, the terraced portion146of the COB133bypasses the adhesive141which would otherwise climb up the narrow gap, thereby preventing the adhesive141from climbing onto the top surface of the array chips135.

As described above, without sacrificing the reliability, the COB133effectively prevents the array chips135from being damaged, being inclined, increasing in temperature, and being contaminated by the adhesive141.

Modification to First Embodiment

FIG. 9illustrates a modification of the first embodiment. The modification differs from the first embodiment in that the terraced portions have arrises or rounded portions160,161,62, and163.

Specifically, the wall146c, which connects the top surface142and the recessed surface145of the terraced portions146aand146b, has rounded arrises or rounded portions161that surround the peninsula-shaped portion142a. Due to the rounded portions162and163, the surfaces145of the small and large terraced portions146aand146bare wider nearer the long sides of the array chip135.

The rounded portions160and161are effective in widening the gap between the peninsula portions142aof the adjacent array chips135, reducing the chances of the adhesive141climbing up in the gap as well as guiding the adhesive141to the surfaces145.

FIG. 10illustrates how the adhesive flows on the array chips on modified array chips. The array chips135A are disposed on the print wiring board134so that one of the small and large terraced portions146aand146bof the adjacent array chips135A is a mirror image of the other. Some of the adhesive141climbs up the gap to the surfaces145, and then flows away from the peninsula portions142ato the surface of the print wiring board134, which is lower than the surfaces145.

The two rounded portions162and163of the wall146ceffectively widen the path in which the adhesive141flows to the print wiring board134, prompting the excess adhesive141to quickly flow out of the terraced portions146. Other corners of the walls146cmay also be rounded as required.

In the first embodiment, the small and large terraced portions146aand146bare disposed on both sides of the peninsula-shaped portion142a. The present invention is not limited to this. The array chip135may have only the large terraced portion146brather than the small and large terraced portions146aand146b.

FIGS. 11A and 11Billustrate a wall146cthat connects the top surface142and the recessed surface145. The wall146cmay be perpendicular to the top surface142and the recessed surface145as shown inFIG. 11A. Alternatively, the array chip135may be undercut such that the wall65bextends obliquely over the recessed surface145to form an acute angle θ with the recessed surface145as shown inFIG. 11B. In other words, the wall65bmakes an acute angle with the recessed surface145. The wall146cis more effective in preventing the adhesive141from climbing up the gap between the adjacent array chips135.

Second Embodiment

Configuration of COB

A second embodiment differs from the first embodiment in the configuration of semiconductor light emitting element array chip. The second embodiment will be described mainly in terms of chip-on-board (COB).

FIGS. 12A, 12B and 13Aillustrate the appearance of a COB200according to a second embodiment.FIG. 12Ais a partial perspective view of the COB200.FIG. 12Bis an expanded view of a relevant portion P of the COB200.FIG. 13Ais another expanded view of the relevant part of the COB200.FIG. 13Billustrates the positional relationship among two consecutive odd-numbered array chips and an even numbered array chip between the two consecutive odd-numbered array chips. For simplicity's sake, Au wires203, which connect between a print wiring board201and semiconductor light emitting element array chips202, are omitted fromFIG. 13A.

A plurality rectangular plate-shaped array chips202of light emitting portions are aligned in a main scanning direction S of an LED printer1so that a line of even-numbered array chips202extends parallel to a line of odd-numbered array chips202, the even-numbered array chips202are staggered with respect to the odd-numbered array chips202, and long sides of adjacent array chips face each other in an overlapped relation.

Just as in the first embodiment, each array chip202can be fabricated on a GaAs substrate or an IC driver circuit substrate. The array chip202has a thickness of, for example, 200 μm to 600 μm. A straight line of a plurality of light emitting portions206is fabricated in the top surface205of the array chip202at intervals D1of about 42.3 μm (600 dpi) or about 21.2 μm (1200 dpi). The light emitting portions206are formed mainly of, for example, a GaAs compound semiconductor material.

The light emitting portions206are located closer to one of the long sides than a longitudinal center line CL (FIG. 13) and wire bonding pads208are arranged closer to the other of the long sides than the longitudinal center line CL. Adjacent array chips202are arranged so that the lines of even-numbered and odd numbered light emitting portions206are parallel to each other and the endmost light emitting portions206xin the two lines are spaced apart by the distance D2. Each array chip202includes stepped portions or terraced portions210(FIG. 13) at each of the longitudinal end portions of the array chip202, the terraced portions110defining a peninsula-shaped portion207(FIG. 13) in which no light emitting portion206is formed. The terraced portion210extends in the line of the light emitting portions206further than the endmost light emitting portion106xto the longitudinal end of the array chip202. The terraced portion210has a transition or an L-shaped wall211that connects the top surface205and a recessed surface209, which is recessed from the top surface205and is substantially parallel to the top surface205. In other words, the recessed surface209is lower than the top surface205by a predetermined distance. The recessed surface209is lower than the top surface205by, for example, 20 μm to 200 μm, and extends over 20 μm or longer in a direction substantially perpendicular to the longitudinal center line CL. The terraced portion210and the peninsula-shaped portion207are aligned side by side in a direction perpendicular to the longitudinal center line CL. The peninsula-shaped portion207extends a distance of 100 μm to 500 μm from the top surface205to the end of the array chip102. The peninsula-shaped portion207has a plurality of wire bonding pads108formed therein and aligned in a direction parallel to the longitudinal center line CL with an interval of a predetermined distance. Since the wire bonding pads208are formed in the peninsula-shaped portion214, thereby providing more efficient utilization of space so that a larger number of array chips202can be diced from a single wafer.

If the array chip202is formed on a GaAs substrate, the terraced portion210may be formed as follows: A wafer is diced into individual rectangular array chips202. A photoresist material is applied to areas of the array chip202except for an area in which the terraced portion210is to be formed. The passivation film or interlayer dielectric film in the area, which will be the terraced portion210, is removed by, for example, CF4 dry etching, so that the GaAs substrate is exposed. The exposed GaAs substrate is subjected to chemical dry etching that uses, for example, chlorinated gas, or wet etching that uses an etchant which is a mixed solution of sulfuric acid, hydrogen peroxide water, and water. The array chip202is etched to a depth of 20 μm to 200 μm. The photoresist material is then removed from the array chip202, thereby forming the terraced portion210in the array chip202.

If the array chip202takes the form of an Si IC driver circuit substrate, the terraced portion210may be formed as follows: The driver circuit is designed such that no circuit occupies an area in which the terraced portion210is to be formed. Just as in the GaAs substrate, a photoresist material is applied to areas of the array chip202except for the area in which the terraced portion210is to be formed. The passivation film or interlayer dielectric film in the area, which will be the terraced portion210, is removed by CF4 dry etching, so that the Si substrate is exposed. The exposed Si substrate is subjected to chemical dry etching that uses a gas, for example, SF6, thereby etching the array chip to a depth of 20 μm to 200 μm. The photoresist material is then removed from the array chip202, thereby forming the terraced portion210in the array chip202.

As shown inFIG. 13A, the wall211, which connects the top surface205and the recessed surface209of the terraced portion210, has rounded arrises or rounded portions211aat the longitudinal end of the array chip202and rounded portions211bnear the endmost light emitting portion206xsuch that the surface205is wider nearer the short side of the array chip202and is wider nearer the long side of the array chip202.

Referring toFIG. 13B, the array chips202are aligned generally in two directions parallel to the center line CL of the array chips, so that even-numbered array chips202lie in one of two directions and odd-numbered array chips202lie in the other of the two directions. The even-numbered array chips202are staggered with respect to the odd-numbered array chips202, so that the even-numbered array chips202and the odd-numbered array chips202are overlapped with each other. One of two parallel lines perpendicular to the center line CL passes through the center of the endmost light emitting portion206xof one of the adjacent array chips202, and the other of the two parallel lines passes through the center of the endmost light emitting portion206xof the other of the adjacent array chips202such that the distance D2between the two parallel lines is equal to a center-to-center distance D1between adjacent light emitting portions206in each array chip202.

It is preferable that the adjacent array chips202are mounted on the print wiring board201so that the distance D5between the long side of one of the adjacent array chips202and the long side of the other of the adjacent array chips202is as short as possible. Since the array chips202mounted on the print wiring board201includes the even-numbered array chips202and the odd-numbered array chips, the even-numbered array chips202being staggered with respect to the odd-numbered array chips202, if the distance D5can be sufficiently short, the amount of light emitted from the even-numbered array chips202and incident on a rod lens36can be substantially equal to the amount of light emitted from the odd-numbered array chips202and incident on the rod lens36.

The adhesive204is applied to a predetermined area of the print wiring board201so that the entire back surface of the array chip202may be in contact with the adhesive204. The array chip202is pressed against the layer of the adhesive204formed on the print wiring board201. The adhesive204is then cured, thereby securely bonding the array chip202to the print wiring board201across the entire back surface of the array chip202.

As described above, the array chip202has wire bonding pads208formed therein for communicating data with external driver circuits. Likewise, wire bonding pads213are formed on the print wiring board201in correspondence with the wire bonding pads208. Each wire bonding pad208and a corresponding wire bonding pad213are spaced apart by a predetermined distance.

An Au bonding wire203(FIG. 12B) connects each wire bonding pad208and a corresponding wire bonding pad213. The print wiring board201has a ROM, chip capacitors, and connectors for communicating data with the LED printer1, all being not shown, thereby completing the COB200for an LED print head8. The COB200is assembled to a frame30of the LED print head8just as in the first embodiment.

Effects of Second Embodiment

A description will be given of the effects of forming the terraced portion210in the array chips202arranged in a staggered relation. The effects will be described by comparing the COB200according to the second embodiment with a comparison COB222whose array chips202have no terraced portions.

FIG. 14Aillustrates the comparison COB222in which light emitting element array chips220are mounted on a print wiring board221.FIG. 14Bis an expanded view of a portion P shown inFIG. 14A. The comparison COB222differs from the comparison COB200according to the second embodiment in that the array chips220have no terraced portions210.

The comparison COB222includes a plurality of array chips220bonded to the print wiring board221using an adhesive223and aligned in such a way that even-numbered array chips220are staggered with respect to odd-numbered array chips220. The array chips220are aligned generally in two directions parallel to the center line CL of the array chips220, so that even-numbered array chips220lie in one of two directions and odd-numbered array chips220lie in the other of the two directions. The even-numbered array chips220are staggered with respect to the odd-numbered array chips220, so that the even-numbered array chips220and the odd-numbered array chips220are overlapped with each other′. One of two parallel lines perpendicular to the center line CL passes through the center of the endmost light emitting portion224xof one of the adjacent array chips220, and the other of the two parallel lines passes through the center of the endmost light emitting portion224xof the other of the adjacent array chips202such that the distance D2between the two parallel lines is equal to a center-to-center distance D1between adjacent light emitting portions224in each array chip220. Thus, the array chips220are mounted on the print wiring board221so that the distance D2is equal to the center-to-center distance D1.

It is preferable that the distance D5between the two parallel lines is as short as possible. For this reason, it is preferable that the distance D6between the long side of one of the adjacent array chips220and the long side of the other of the adjacent array chips220is as short as possible, that is, the even-numbered array chips220and the odd-numbered array chips220should be disposed as close to each other as possible. For example, the distance D6between the array chips220is selected to be in the range of 10 to 50 μm.

Wire bonding pads226are formed on longitudinal end portions225of the array chip220where no light emitting portions224are formed, and wire bonding pads227are formed on the print wiring board221. Au wires228connect between the wire bonding pads226and corresponding wire bonding pads227.

FIG. 15illustrates how the adhesive climbs up the gap between adjacent array chips of the comparison COB and flows on the array chips.

The adhesive223climbs up the gap between the adjacent array chips220to the top surface of the adjacent array chips220, contaminating the light emitting portions224and/or the wire bonding pads226. Contamination of the light emitting portions224decreases available light power and contamination of the bonding wire pads226impairs mechanical strength of the wire bonding portion to decrease.

FIG. 16illustrates how the adhesive223climbs up and flows on the surface of the adjacent array chips202of the COB220. The distance D6between the adjacent array chips202is equal to that between the adjacent array chips202of the comparison COB222, and the adhesive204is applied to the entire back surface of the array chip. However, as shown inFIG. 16, the adhesive204climbs up the gap between the adjacent array chips202due to capillary action, and is guided to flow to the recessed surface209of the terraced portion210, thereby preventing the adhesive204from climbing onto the top surface205.

The COB200of the second embodiment does not place any limitation on an area in the back surface of the array chip202to which the adhesive may be applied, but allows the adhesive204to be applied to the entire back surface of the array chip202so that the entire back surface of the array chip202can be securely bonded to the print wire board201. This prevents the end portion of the back surface of the array chip202from being uplifted from the print wiring board201. The COB200also prevents the corners and edges of the array chip202being chipped or the array chip202being inclined relative to the print wiring board201.

The fact that the back surface of the array chip202can be securely bonded to the print wire board201is advantageous in that the heat generated by the array chip202can be uniformly conducted to the print wiring board201, hence uniform heat dissipation across the array chip202.

For the COB200according to the second embodiment, the distance D6between adjacent array chips202is shorter than the comparison COB222but the adhesive204that climbs up the gap between the adjacent array chips202can be bypassed to the terraced portion210. Thus, the distance D7between the long side of the array chip202and the line of light emitting portions close to the long side may be longer than the comparison COB200, providing more freedom in arranging the light emitting portions on the array chip202.

The terraced portion210has the L-shaped wall211that partially surrounds the recessed surface209. The L-shaped wall211has a rounded portion211b, which is further effective in providing as wide a gap between adjacent array chips202as possible in the vicinity of the longitudinal end of the array chips202, thereby promoting the adhesive204to flow to the recessed surface209.

The L-shaped wall211also has a rounded portion211a, which is further effective in providing as wide a gap between adjacent array chips202as possible in the vicinity of the endmost light emitting portions, thereby promoting the adhesive to flow to the recessed surface209.

When the array chips202have been mounted on the print wiring board201, the terraced portion210of one of adjacent array chips202faces the light emitting portions206closest to the endmost light emitting portion206xof the other of the adjacent array chips202. Thus, the adhesive204climbs up the gap between the adjacent array chips202, then flows on the recessed surface209, and finally flows down to the print wiring board201from the end of the terraced portion210at the longitudinal end of the array chip202. The rounded portion211aof the L-shaped wall211is effective in widening the area of the recessed surface209at the longitudinal end of the array chip202, permitting the adhesive204to smoothly flow onto the print wiring board201.

As described above, the second embodiment still provides the same advantages as the first embodiment when the array chips are aligned in the main scanning direction S such that even-numbered array chips202are staggered with respect to odd-numbered array chips202and long sides of adjacent array chips face each other and are overlapped. In other words, the second embodiment prevents contamination of array chips202due to an adhesive without sacrificing reliable operation of the array chips202, and provides more freedom in arranging the light emitting portions206on the array chip202.

Third Embodiment

A third embodiment differs from the second embodiment in that an additional terraced portion is formed in a light emitting element array chip. The basic configuration of an LED print head8according to the third embodiment is the same as that of the first and second embodiments, and their detailed description is omitted. A description will given of only a chip-on-board (COB)300according to the third embodiment.

FIG. 17Ais a partial perspective view of the COB300.FIG. 17Bis a cross-sectional view taken along a line B-B inFIG. 17A.FIG. 17Cillustrates the positional relationship among two consecutive odd-numbered array chips and an even numbered array chip between the two consecutive odd-numbered array chips. A plurality of rectangular plate-like array chips303are aligned in a main scanning direction S of an LED printer1or in a longitudinal direction of the LED print head8, and securely bonded to a print wiring board301. The array chips303are aligned in the main scanning direction S so that a line of even-numbered array chips303extends parallel to a line of odd-numbered array chips303, the even-numbered array chips are staggered with respect to the odd-numbered array chips303, and long sides of adjacent array chips face each other in an overlapped relation just as in the second embodiment.

Just as in the second embodiment, the array chip303may be formed on a GaAs substrate or an IC driver circuit substrate. The array chip303may have a thickness in the range of 300 to 600 μm. A plurality of light emitting portions305are aligned in a straight line on a top surface304of the array chip303, and are arranged at intervals of about 42.3 μm (600 dpi) or about 21.2 μm (1200 dpi).

The rectangular array chip303has long sides and short sides. A line of the light emitting portions305is disposed closer to one of the long sides than a longitudinally extending center line CL of the array chip303, and extends in a direction parallel to the center line CL. The array chip303has a small terraced portion308and a large terraced portion310formed in the top surface304at each longitudinal end portion of the array chip303where no light emitting portion is formed, the small and large terraced portions308and310defining a peninsula-shaped portion311. The small terraced portion308and the large terraced portion310extend in directions parallel to the center line CL further than the endmost light emitting portion305xto the longitudinal end of the array chip303. The peninsula-shaped portion311extends in a direction parallel to the center line CL. A rectangular recessed surface310ais recessed from the top surface304, and is substantially parallel to the top surface304. In other words, the recessed surface is lower than the top surface304by a predetermined distance. A surface308ais also recessed from the top surface304.

Just as in the first and second embodiments, the terraced portions308and310can be formed by partially etching away the surface304simultaneously, thereby defining the peninsular-shaped portion311.

A wall310bslopes down from the top surface304to the recessed surface310a, thereby connecting the top surface304and the recessed surface310a. In other words, the wall310makes an obtuse angle with the recessed surface310a. Thin film wiring patterns (not shown) are formed on the sloped wall310b, and electrically connect the wire bonding pads313to the light emitting portions305. The sloped wall310bis formed of an organic insulating film, which can be formed by photolithography. Alternatively, if the array chip303is fabricated of a GaAs substrate, the etching rate in a vertical direction and in a horizontal direction may be independently adjusted to form a sloped surface. If the array chip303is fabricated from an Si substrate, the etching rate in a vertical direction and in a horizontal direction may be independently adjusted to form a sloped surface.

FIG. 17Dis an expanded view of a projection. FIG.17E is an expanded view of rounded corners.

Just as in the second embodiment, an L-shaped wall has rounded arrises or corners, and connects the top surface304and the surface308a. The array chip303may have a projection314at the longitudinal ends of the array chip303, the projection314projecting from the peninsula-shape portion311in a direction perpendicular to the center line CL. This projection314effectively prevents the adhesive302from climbing onto the surface310a, and prevents the wire bonding pads313from being contaminated.

As described above, the array chips303are mounted on the print wiring board301so that the even-numbered array chips303are staggered with respect to the odd-numbered array chips303and the terraced portion308of one of adjacent array chips303faces the light emitting portions305on the other of the adjacent array chips303.

The adjacent array chips305should be arranged so that the distance D5between a line passing through the center of the light emitting portions305formed on one of adjacent array and a line passing through the center the light emitting portions305formed on the other of the adjacent array chips.303is as short as possible. The distance D6between the long sides of the adjacent array chips305which directly face each other should also be as short as possible. In practice, the distance D6is selected to be in the range of, for example, 10 to 50 μm.

Just as in the first and second embodiments, the adhesive302may be transferred onto the print wiring board301using the stamp function of a die bonder (not shown). Alternatively, a dispenser (not shown) may be used to form a layer of the adhesive302on predetermined areas on the print wiring board301. The layer of the adhesive302is formed so that the entire back surface of the array chip303is in contact with the layer of the adhesive302.

As described above, the array chip303carries wire bonding pads313formed on the recessed surface310athrough which data can be inputted from and outputted to external driver circuits. The print wiring board301carries wire bonding pads315through which data can be inputted from and outputted to the array chips303. A bonding wire316connects a wire bonding pad313to a corresponding wire bonding pad315. A line of wire bonding pads313and a line of wire bonding pads315extend in directions substantially parallel to the longitudinal center line CL or the longitudinal direction of the array chip303, and are spaced apart by a predetermined distance.

The wire bonding pads313formed on the array chip303and the wire bonding pads315formed on the print wiring board301are electrically connected by means of Au wires316. Balls317are formed on the wire bonding pads313and315, so that the top of the balls317after wire bonding should be lower than the top surface of the peninsula-shaped portion311.

In practice, the surface310ais lower than the top surface304by the sum of the thickness of the bonding pad313and the height of the ball317.

A ROM, chip capacitors, and connectors through which the data is communicated between the LED printer1and the LED print head8, which are not shown, are mounted on the print wiring board301, thereby configuring the COB300for the LED print head8. Just as in the first and second embodiment, the COB300is assembled to the frame30of the LED print head8.

Effects of Third Embodiment

Just as in the second embodiment, the third embodiment also provides effects of forming the terraced portion308on a side of the peninsula-shaped portion opposite the terraced portion310, and its detailed description is omitted. Effects of the third embodiment will be described.

The third embodiment has the terraced portions308and310, which defines the peninsula-shaped portion311formed between the terraced portions308and310. The wire bonding pads313are formed on the recessed surface310a.

The terraced portion308of one of adjacent array chips303faces the light emitting portions305closest to the endmost light emitting portion305xof the other of the adjacent array chips303.

FIG. 18illustrates that the ball317is out of an angular range R in which the light emitting portions emit light. If the ball317reflects the light emitted from the light emitting portions305, the reflected light may illuminate the charged surface of the photoconductive drum5, which causes streaks and lines in the print results leading to poor print quality. To avoid such adverse effects, the Au bonding wires316on one of adjacent array chips303extend from the wire bonding pads313in such a direction as to be away from the light emitting portions305on the other of the adjacent array chips303. As a result, the ball317is out of the angular range R in which light emitting portions305emits light as shown inFIG. 18, so that there is no chance of the ball317reflecting the light emitted from the light emitting portions305.

A recessed surface (not shown) may be formed in the longitudinal end portions of the array chip303in which no light emitting portions are fabricated, and the balls317may be formed in the recessed surface, thereby hiding the balls317. The recessed surface may have a depth lager than the sum of the height of the balls317and the bonding pad213. This configuration is also effective in preventing the ball317from reflecting the light emitted from the light emitting portions305. Each recessed surface304may accommodate a corresponding ball317. Alternatively, a larger recessed surface may be formed in which a plurality of wire bonding pads313and corresponding balls317are accommodated.

In the third embodiment, the Au wires316are connected to the wire bonding pads313by ball bonding. Instead, stitch bonding may be employed. The wire bonding pads313require to be larger in stitch bonding than in ball bonding. Therefore, the width of the array chips303also requires to be larger in stitch bonding than in ball bonding. On the other hand, the height of stitched portions may be lower than that of the balls317, which is effective in preventing the ball317from reflecting light emitted from the light emitting portions305. In addition, the depth of the recessed surface from the top surface304may be as shallow as, for example, 10 μm.

The first, second, and third embodiments have been described in terms of the LED print head8mounted on the LED printer1. The invention is not limited to the LED print head8, and may also be applied to other exposing heads that use light emitting portions other than LEDs, for example, contact image sensors (CIS), which are used as reading heads used for, for example, scanners.

When a CIS is employed, the reading head may have a configuration in which light receiving element array chips, each having a plurality of light receiving portions aligned in one dimension, are securely bonded using an adhesive. The present invention may also be applicable to this type of CIS.

The first, second, and third embodiments have been described in terms of COB133, COB200, and COB300, respectively. The invention is not limited to these, and may also be applicable to apparatus having a configuration in which semiconductor chips are securely bonded to a circuit board using an adhesive. The semiconductor chip may be any type in which a plurality of semiconductor portions are fabricated in its surface.

The first, second, and third embodiments have been described in terms of the LED printer1. The invention may also be applied to printers and image forming apparatus that employ exposing heads whose light emitting portions are other than LEDs. The image forming apparatus include scanners, facsimile machines, multi-function printers (MFPs), and copying machines. The scanners employ, for example, reading heads such as compact image sensors. The first, second, and third embodiment have been described as using Au wires140,203, and316, respectively. However, any electrically conductive wires including a Cu wire can be used.

The invention is not limited to the first, second, and third embodiments but modifications may be made by combining these embodiments or using a part of these embodiments.

Fourth Embodiment

A fourth embodiment differs from the first to third embodiments in the configuration of semiconductor light emitting element array chip. The fourth embodiment will be described mainly in terms of chip-on-board (COB).

A description will be given of the print wiring board434and the array chips435mounted on the print wiring board434.FIG. 19Ais a partial perspective view of the chip-on-board module433.FIG. 19Bis a cross-sectional view taken along a line C-C inFIG. 19A.

Referring toFIGS. 19A and 19B, a plurality of rectangular plate-like array chips435are mounted on the surface of the print wiring board434using an electrically conductive or an electrically non-conducive adhesive440. The array chips435are aligned in the main scanning direction shown by arrow S. Light emitting portions442, which are light emitting diodes, are formed in the surface441of each array chip435.

The light emitting portions442are formed of a GaAs compound semiconductor, and are aligned in a one dimension at intervals of 42.3 μm (i.e., a resolution of 600 dpi) or at intervals of 21.2 μm (i.e., a resolution of 1200 dpi). Each light emitting portion442may be implemented as a light emitting diode (LED) by forming a PN junction of a P-type semiconductor and an N type semiconductor. Alternatively, the light emitting portions442may be Thyristors that take the form of a PNPN junction or an NPNP junction.

The array chip435is rectangular, and has a longitudinal center line CL, long sides, and short sides. A straight line of the light emitting portions442extends in a direction parallel to the long sides and is closer to one of the long sides of the array chip435than the longitudinal center line CL, while a straight line of the wire bonding pads447extends in a direction parallel to the long sides and is closer to the other of the long sides than the longitudinal center line CL. The wire bonding pads447are aligned at predetermined intervals.

Referring toFIG. 19A, the array chip435has an extended portion443at each of the longitudinal end portions of the array chip435, the extended portion443extending a distance D8of 100 to 500 μm from the endmost light emitting portions442x. The array chip435also has a stepped portion or terraced portion445formed in the extended portion443. The terraced portion445has a transition or wall445bthat extends from the top surface441to a recessed surface445athat is recessed from the top surface441. For example, the recessed surface445ais lower than the top surface441by a distance of 20 to 200 μm. The recessed surface445aextends from the wall445bto the longitudinal end of the array chip435and from the center line CL to one of the longitudinal sides of the array chip435.

An insulating film446, which is formed of, for example, Si or SiO2, is formed on the recessed surface445a, and a plurality of (e.g., two) wire bonding pads447are formed on the insulating film446at predetermined intervals. Forming the terraced portion445in the extended portion443leaves a peninsula-shaped portion443a. The terraced portion445and the peninsula-shaped portion443aare aligned side by side in a direction perpendicular to the main scanning direction S of the LED printer1. The peninsula-shaped portion442aextends to the longitudinal end of the array chip435. Forming the terraced portion445and forming the wire bonding pads447on the recessed surface445ais advantageous in that the short sides of the array chip can be shorter and a larger number of array chips435can be diced from a single wafer.

If the array chip435is formed on a GaAs substrate, the terraced portion445may be formed as follows: A wafer is diced into individual rectangular array chips435. A photoresist material is applied to areas of the array chip435except for an area in which the terraced portion445is to be formed. The passivation film or interlayer dielectric film is removed by CF4 dry etching from the area in which the terraced portion445is to be formed, so that the GaAs substrate is exposed. The exposed GaAs substrate is subjected to wet etching using an etchant, which is a mixed solution of sulfuric acid, hydrogen peroxide water, and water. The array chip435is etched to a depth of 20 μm to 200 μm. The photoresist material is then removed from the array chip435, thereby forming the terraced portions445in the array chip435.

If the array chip435takes the form of an Si IC driver circuit substrate, the terraced portions445may be formed as follows: The driver circuit is designed such that no circuit occupies areas in which the terraced portions445are to be formed. Just as in the GaAs substrate, a photoresist material is applied to areas of the array chip435except for the areas in which the terraced portions445are to be formed. The passivation film or interlayer dielectric film in the areas in which the terraced portions445are to be formed is removed by CF4 dry etching, so that the Si substrate is exposed. The exposed Si substrate is then subjected to chemical dry etching that uses a gas, for example, SF6, thereby etching the array chip435to a depth of 20 μm to 200 μm. The photoresist material is then removed from the array chip435, thereby forming the terraced portions445in the array chip435.

A wall445bslopes down from the top surface441to the recessed surface445a, thereby connecting the top surface441and the recessed surface445a. Thin film wiring patterns (not shown) are formed on the sloped wall445b, and electrically connect the wire bonding pads447to the light emitting portions442.

The sloped wall445bis formed of an organic insulating film, which can be formed by photolithography. Alternatively, if the array chip435is fabricated of a GaAs substrate, the etching rate in a vertical direction and in a horizontal direction may be independently adjusted to form the sloped wall445b. If the array chip435is fabricated from an Si substrate, the etching rate in a vertical direction and in a horizontal direction may be independently adjusted to form the sloped wall445b.

Using an adhesive440, the array chips435, which have the terraced portions445formed thereon, are mounted on the wiring board434formed of, for example, composite epoxy material (CEM3) or flame retardant 4 (FR4), and are aligned in the longitudinal direction parallel to the center line CL of the array chip435.

FIG. 19Cillustrates the positional relationship among two adjacent odd-numbered array chis and an even numbered array chip between the two adjacent odd-numbered array chips. The array chips435mounted on the print wiring board434include a line of even-numbered array chips402and a line of odd-numbered array chips435, the even-numbered array chips435being staggered with respect to the odd-numbered array chips435, so that the even-numbered array chips435and the odd-numbered array chips435are overlapped with each other.

When the array chips435have been mounted on the print wiring board434, the peninsula-shaped portion443aof one of adjacent array chips435faces the light emitting portions442closest to the endmost light emitting portion442xof the other of the adjacent array chips435.

One of two parallel lines perpendicular to the center line CL passes through the center of the endmost light emitting portion442xof one of the adjacent array chips435, and the other of the two parallel lines passes through the center of the endmost light emitting portion442xof the other of the adjacent array chips435such that the distance D2between the two parallel lines is equal to a center-to-center distance D1between adjacent light emitting portions442in each array chip435.

It is preferable that the adjacent array chips435are mounted on the print wiring board434so that the distance D3between a line parallel to the center line CL and passing through the light emitting portions442of one of the adjacent array chips435and a line parallel to the center line CL and passing through the light emitting portions442of the other of the adjacent array chips is as short as possible. If the distance D3can be sufficiently short, the amount of light emitted from the even-numbered array chips435and incident on a rod lens36can be substantially equal to the amount of light emitted from the odd-numbered array chips435and incident on the rod lens36.

Thus, it is preferable that the distance D3between the directly facing long sides of the adjacent array chips435is as short as possible. Thus, achieving the shorter distance D3in the fourth embodiment provides a shorter distance D4. As a result, the wire bonding pads447of one of the adjacent array chips435may be closer to the light emitting portions442of the other of the adjacent array chip435. The distance between a line passing through the center of the wire bonding pads447and a line passing through the center of the light emitting portions442is selected to be about 100 μm.

The wire bonding pads448are formed on the print wiring board434, and the wire bonding pads447are formed on the array chips435in correspondence with the wire bonding pads448. The wire bonding pads447and448are spaced apart by a predetermined distance in directions perpendicular to the main scanning direction S.

Au wires449connect the wire bonding pads447and corresponding bonding pads448. The Au wire449is bonded to the bonding pad447by ball bonding or stitch bonding. The bonding pads447can be smaller when ball bonding is employed than when stitch bonding is employed. In fact, a 50 μm square bonding pads447can be used in ball bonding. The ball450of the Au wire449is a substantially spherical ball450having a diameter of 50 μm and a height greater than 20 μm. The top surface441is above the top of the ball450. The depth of the recessed surface445afrom the top surface441is selected to be larger than the sum of the height of the ball450and the thickness of the bonding pads447.

If stitch bonding is employed, the size of the wire bonding pads447should be more than 80 μm square. The height of stitched portion of the Au wire449can be lower than the diameter of the Au wire449, and well below the top surface441.

The print wiring board434includes a ROM, chip capacitances, and connectors through which the data is communicated between the LED printer1and the LED print head8, thus configuring the COB433for the LED print head8. This COB433is assembled to a frame30of the LED print head8.

Effects of Fourth Embodiment

Effects of forming the terraced portions445in the array chip435will be described. The effects will be described by comparing the COB433according to the fourth embodiment with a comparison COB462whose array chips460have no terraced portions.

FIG. 20Ais a perspective view of the comparison COB462on which semiconductor light emitting element array chips460are mounted.FIG. 20Bis across-sectional view taken along a line D-D inFIG. 20A. The comparison COB462has the same configuration as the COB433except that the array chips460have no terraced portion formed therein.

The comparison COB462includes a plurality of array chips460bonded to the print wiring board461using an adhesive463and aligned generally in a line. The array chips460are aligned generally in two directions parallel to the center line CL of the array chips460, so that even-numbered array chips460lie in one of two directions and odd-numbered array chips460lie in the other of the two directions. The even-numbered array chips460are staggered with respect to the odd-numbered array chips460, so that the even-numbered array chips460and the odd-numbered array chips460are overlapped with each other. One of two parallel lines perpendicular to the center line CL passes through the center of the endmost light emitting portion464xof one of the adjacent array chips460, and the other of the two parallel lines passes through the center of the endmost light emitting portion464xof the other of the adjacent array chips460. The adjacent array chips460are arranged such that the distance D2between the two parallel lines is equal to a center-to-center distance D1between adjacent light emitting portions464xin each array chip460.

The distance D3between the line of light emitting portions464of the even-numbered array chips460and the line of light emitting portions464of the odd-numbered array chips460should be as short as possible, and is selected to be in the range of 50 to 100 μm. Accordingly, the distance D4between the long sides of the adjacent array chips460should be as short as possible, and is selected to be in the range of 10 to 50 μm.

Thus, the wire bonding pads466formed on an extended portion465of one of the adjacent array chips460may also be closer to the light emitting portions on the other of the adjacent array chips460. The extended portion465extends a distance of D9. Likewise, the Au wire467on one of the adjacent array chips460may also be closer to the light emitting portions464on the other of the adjacent array chips460.

A plurality of bonding pads468are formed on the print wiring board61in correspondence with the wire bonding pads466formed on the array chips460. The Au wire467connects each wire bonding pad466to a corresponding wire bonding pad468. As described above, the Au wire467should be bonded by ball bonding so that the wire bonding pad466occupies as small an area as possible, in which case the height of the ball469is usually higher than 20 μm.

When the array chips460of the comparison COB462have been mounted on the print wiring board461, the extended portion465of one of adjacent array chips460faces the light emitting portions464closest to the endmost light emitting portion464xof the other of the adjacent array chips460.

The balls469sit on the bonding pads466formed on the top surface470of the array chip460. Thus, the light emitted from the light emitting portions464reaches the balls469, which in turn reflects the light.

FIG. 21illustrates an angular range R in which the light emitting portions442emit light. In contrast, the COB433according to the fourth embodiment has the following configuration. The distance D3and the distance D4are the same as the comparison COB462. The wire bonding pads447are formed on the recessed surface445asuch that the top of the balls450is below the top surface441as shown inFIG. 21, i.e., the balls450are out of an angular range R in which the light emitting portions442emit light. The difference in height between the top surface441and the recessed surface445ais selected to be in the range of 20 to 200 μm.

Therefore, there is no chance of the balls450reflecting the light emitted from the light emitting portions442.

As described above, a light-blocking wall is not formed on the top surface441of the array chip435, which is intended to prevent the light emitted from the light emitting portions442from reaching the balls450. Instead, the recessed surface445ais formed below the top surface441to prevent the light emitted from the light emitting portions442from reaching the balls450. This simple configuration ensures that the balls450are out of the angular range R in which the light emitting portions442emit light, and is effective in preventing the balls450from reflecting the light emitted from the light emitting portions442.

With the comparison COB462, the balls469reflect the light from the light emitting portions464, and the reflected light may illuminate the charged surface of a photoconductive drum5(not shown) of the LED printer1, causing streaks and lines in the print results, thus leading to poor print quality. In contrast, the COB433according to the fourth embodiment minimizes the chance of the balls450reflecting the light emitted from the light emitting portions442, and therefore minimizes streaks and lines in the print results and not leading to poor print quality.

In the fourth embodiment, the Au wires449are connected to the wire bonding pads447by ball bonding. Instead, stitch bonding may be employed. The wire bonding pads447require to be larger in stitch bonding than in ball bonding. Therefore, the array chips435requires to have a larger width (i.e., short sides) in stitch bonding than in ball bonding. However, the height of stitched portions may be lower than that of the balls450, which is effective in preventing the balls450from reflecting light emitted from the light emitting portions442. In addition, the depth of the recessed surface445afrom the top surface441may be as shallow as, for example, 10 μm.