Leaf-Spring Assembly for LED Printhead

FIG. 1 is a plan view shown in schematic of a printhead assembled in accordance with the invention;

FIG 2 is a section of the printhead taken along the line A--A of FIG. 1;

FIG. 3 is an enlarged section of a portion of the printhead illustrated in FIg. 2; and

FIG. 4 is a view similar to that of FIG. 3, but illustrates an alternative embodiment of the invention.

Because apparatus of the type described herein are well known, the present description will be directed in particular to elements forming part of or cooperating more directly with the present invention.

With reference to FIG. 1-3, an LED printhead 10 is formed using a series of tile modules 20a, b, . . . l, m. Tile module 20a is typical and has a series of gallium arsenide or gallium aluminum arsenide LED dice 30a, 30b, 30c mounted end to end to form a row of such dice on a central axis of the front face of the tile. To each side of the row of dice, there will be provided a corresponding number of integrated circuit silicon driver chips 40a,b,c and 42a,b,c so that two driver chips are associated with each LED die. Typically, an LED die 30 may have say 128 LED's arranged in say a row so that each driver chip 40, 42 drives 64 of the LED's formed within a corresponding die. Also, typically, the silicon driver chips each have 64 channels, i.e. current-generating circuits that may provide regulated driving currents to respective LED'S that are selected to be activated. Printed circuit or spreader boards 36a, 36b are mounted to the front face of the tile to provide a means for distribution of electrical signals such as data, power and clock signals to the driver chips. A specific spreader board that is preferred is described in U.S. application Ser. No. 07/455,125, filed Dec. 22, 1989. The spreader boards will be located on the tile to the outboard side of each row of driver chips.

The spreader boards each include a series of bond pads 38a, 38b along opposite edges thereof for connection of signals to adjacent spreader boards or to termination boards 50, 52 that are present at the ends of printhead 10.

As noted with more detail in U.S. application Ser. No. 07/455,125, bus bar assemblies 60a, 60b are provided for carrying power and ground signals along the length of the printhead to the various spreader boards for distribution of such signals to the various driver chips. Only tile module 20a is illustrated to show the various components mounted on the tile. In addition to the illustrated components, there will be bonding wires for connecting such components together.

As may be seen in FIG. 2, the tiles 20 are mounted upon stainless steel rails 70a, 70b which rails extend for the length of the printhead; i.e., the length of the tile assemblies, so as to support the tiles thereon. The tiles are assembled one by one onto the rails 70a, 70b using adhesive. Each tile is prepared in accordance with the method described in the cross-referenced application so that the assembly of driver chips and LED dice are accurately formed on the tile. With the spreader boards and wiring also attached to the tile, the tile module may be tested and suitably burned in. Satisfactory tile modules are then carefully assembled to the metal rails 70a, 70b to ensure that all the LED's are accurately aligned in a row. Prior to placement of the tile on the rails, the rails are each covered with two different adhesives. Each rail is a two-legged rail with the inner leg receiving a two-part structural epoxy adhesive 14 that can be cured at room temperature. The outer leg of each rail receives a UV curable epoxy 12 that is promptly cured by application of UV light after each tile has been placed in position on the rails. After all the tiles have been mounted on the rails, the assembly is placed upon a heatsink 19. The heatsink surface that is to be thermally coupled to the undersides of the tiles is coated with a thermal grease 16 or in lieu thereof, a thin sheet (0.001" thick) of aluminum foil ooated with thermally eleotrioally conductive rubber that serves as a grease replacement material may be used between the heatsink and the tiles. This construction allows for thermal expansion of the tiles relative to the heatsink by allowing sliding movement between them.

In order to ensure intimate thermal coupling between the tiles and the heatsink, the tiles are spring-urged towards the heatsink using cantilevered spring steel leaf springs 85a, 85b. The leaf springs shown in FIG. 3 extend the length of the rails and are positioned each within a slot formed within each rail. As shown in FIG. 3, a bottom cover 18 for the printhead includes a central upstanding wall on either side of the sides of the heatsink. Side extensions 64 of the heatsink extend above these side walls 66. Screw holes are provided in these upstanding walls 66 and the side extensions 64 to the heatsink to allow screws, S, to connect the side extensions 64 to the bottom cover. The leaf springs 85a, 85b are also positioned between the upstanding walls 66 and the side extensions 64 of the heatsink and include apertures through which screws, S, can pass so that when clamped together by the screws, the leaf springs 85a, 85b cantileveredly extend into the slots 75a, 75b, respectively, in the rails. An offset "d" is provided between the bottom walls of the slots 75a, 75b and the top surfaces of the upstanding walls 66 prior to clamping by the screws. Upon clamping of the heatsink to the upstanding wall 66 by tightening of the screws, S, the leaf springs are flattened between the upstanding wall 66 and the side extensions 64 of the heatsink. The cantilevered portions of the leaf springs in slots 75a, 75b distort to urge the rails downwardly in FIG. 3 thereby resiliently urging the tiles into thermal coupling with the heatsink.

While only two screws are illustrated in the Figures, it will be understood that there are a series of the screws provided and spaced along the length of the printhead beneath every other tile. As may be seen in FIGS. 1 and 2, the rails each include two depending pins, P1, P2, P3, P4, at their respective ends that engage within recesses formed in the bottom cover member. Two diagonally opposite recesses R1, R4, formed in the bottom cover are slots, one (R1) elongated in the direction of the printhead, the other (R4) elonqated in a direotion transverse to this dimension. These recesses allow the tile and rail assemblage to shift relative to the bottom cover and heatsink during operation of the printhead wherein the thermal expansion occurs.

If desired, the heatsink may be adhesively attached to the tiles using a thermally conductive adhesive. In addition, a lens such as a Selfoc lens, trademark of Nippon Sheet Glass Company, Ltd., is used to focus light from the LED's and a glass cover plate placed over the bottom of member to seal the assembly from dust.

While a preferred embodiment has been described with reference to stainless steel rails and spring steel leaf springs, other materials may be used; for example, the rails may be aluminum and the leaf springs may be formed of a beryllium copper. The materials used depend on the anticipated temperature ranges of operation.

An alternative embodiment is illustrated in FIG. 4 wherein reference numerals indicated with a prime refer to similar parts to that described for FIG. 3. In the embodiment of FIG. 4, a leaf spring 85a' is rigidly attached to a rail 70a' by screws, S'. The side extensions 64' of the heatsink engage the cantilevered end of the leaf spring to urge the rail downwardly and, thus, the tiles such as tile 20b, are urged or biased towards the heatsink. A similar arrangement is provided on the opposite side of the heatsink. In this embodiment the rail and spring could be assembled and the heatsink slid in from the end.

It will be appreciated that an improved printhead has been provided that is of relatively simple structure and provides for thermal considerations of the various parts of the assemblage.

While the invention has been described with reference to recording elements such as LED's, other recording elements such as laser diodes, ink jet, thermal, light valve, etc. may also make use of the teachings contained herein.

The invention has been described in detail with particular reference to preferred embodiments thereof, but it will be understood that variations and modifications can be effected within the spirit and scope of the invention.