Method of assembly of keyboard switch by ultrasonics

Abstract

A method of assembling a keyboard of the type used in electronic calculators in which a first Mylar sheet having openings therein for receiving switch elements is selectively ultrasonically bonded to an underlying printed circuit board, the switch elements are placed in the openings in the first sheet, and a second Mylar cover sheet is selectively ultrasonically bonded to the underlying first sheet.

Description

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates generally to a method of assemblying a keyboard of the type used in electronic calculator apparatus.

2. Description of the Prior Art

U.S. Pat. No. 3,796,843 granted Mar. 12, 1974 to Gilbert H. Durkee, Per G. Wareburg and Allen C. Yoder, assigned to the assignee of the present application, discloses a keyboard switch assembly in which a first sheet of insulating material having switch element-retaining openings therein covers one side of a printed circuit board having conductors thereon with terminal portions exposed to the first sheet openings. Switch elements are positioned in the first sheet openings in cooperative relationship with the terminal portions and a second sheet of insulating material covers the first sheet and the switch elements. Other keyboard switch assemblies are shown for example in U.S. Pat. Nos. 3,643,041 and 3,684,842, German Pats. Nos. 486,212 and 180,6241, and IBM Technical Disclosure Bulletin No. 12, Volume 7, May, 1965, Page 1168.

It has been conventional practice in such prior keyboard switch assemblies to retain the printed circuit board and several sheets of insulating material in assembled relation by the use of pressure sensitive adhesive which requires the initial application of protective cover sheets over the tacky adhesive and the subsequent removal of such cover sheets, and permits dust and other foreign particles to adhere to the tacky adhesive following removal of the cover sheets but prior to final assembly.

Ultrasonic techniques have been employed for bonding or welding one plastic article to another and for bonding plastic to metal. Ultrasonic plastic welding apparatus is commercially available from Branson Sonic Power Company (formerly Branson Instruments, Inc.) and various forms of such apparatus together with various methods employed thereby are disclosed in United States Patents assigned to that compay, referring particularly to U.S. Pat. Nos. 3,284,257 and 3,480,492. In addition, U.S. Pat. No. 2,633,894 broadly discloses the use of wax paper as a heat insulator in ultrasonic welding, the U.S. Pat. No. 3,022,814 discloses the use of a thermoplastic layer between two bodies for bonding the same ultrasonically.

SUMMARY OF THE INVENTION

The invention, in its broader aspects, provides a method of assembling a keyboard including the steps of providing an insulative, planar board having opposite surfaces, one of the surfaces having at least one switch contact thereon, and providing a planar sheet having opposite surfaces, at least one of the surfaces being coated with ultrasonically-actuatable adhesive material, the sheet having an opening therein. The adhesive-coated surface of the sheet is placed on the one surface of the insulative board with the contact exposed to the opening thereby forming a sandwich comprising the board and the sheet. The sandwich is placed in ultrasonic welding apparatus which includes a horn having a surface selectively movable into engagement with the other surface of the sheet, the horn surface having an opening therein in registry with the sheet opening. The apparatus is then energized to activate the adhesive material thereby to bond the sheet to the board and the thus bonded sandwich is then removed from the apparatus.

In the preferred embodiment of the invention, the sheet of insulating material has both of its surfaces coated with ultrasonically-actuatable adhesive material, another sheet of material which will not bond to the adhesive material being placed on the other surface of the insulative sheet prior to placing the sandwich in the welding apparatus. Upon removal of the sandwich from the apparatus, the other sheet is removed, a switch element is placed in the opening in the insulative sheet in cooperative relationship with at least one contact on the insulative board, and another sheet of insulating material is placed on the other surface of the first sheet over the switch element thereby forming a second sandwich. The second sandwich is then placed in the apparatus which is energized to activate the adhesive material on the other surface of the first insulator sheet therby to bond the same to the second insulative sheet, the resulting completely bonded sandwich being then removed from the apparatus.

It is accordingly an object of the invention to provide an improved method of assembling a keyboard.

Another object of the invention is to provide an improved method of assembling a keyboard employing ultrasonic techniques.

A further object of the invention is to provide an improved method of selectively, ultrasonically bondint the components of a keyboard.

The above-mentioned and other features and objects of this invention and the manner of attaining them will become more apparent and the invention itself will be best understood by reference to the following description of an embodiment of the invention taken in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a top view, partly broken away, showing a keyboard switch assembly including a keyboard which may be assembled by the method of the invention;

FIG. 2 is a cross-sectional view taken along the line 2--2 of FIG. 1;

FIG. 3 is a fragmentary, exploded perspective view showing the assembly of the first sandwich in accordance with the method of the invention;

FIG. 4 is a side view showing the ultrasonic welding appartus employed in performing the method of the invention, and further showing bonding of the first sandwich;

FIG. 5 is a bottom plan view of the horn employed in the apparatus of FIG. 4 as viewed generally along the line 5--5 of FIG. 4;

FIG. 6 is a fragmentary, exploded perspective view showing the assembly of the second sandwich in accordance with the method of the invention; and

FIG. 7 is a fragmentary view showing bonding of the second sandwich.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring now to FIGS. 1 and 2, there is shown a keyboard switch assembly of the type more fully shown and described in the aforesaid U.S. Pat. No. 3,796,843. The illustrated keyboard switch assembly, generally indicated at 10, comprises a keyboard sub-assembly 11 and a cover member and push-button sub-assembly 21. Keyboard sub-assembly 11 comprises single-sided printed circuit board 12, snap-acting dome switch members 14, and switch member retaining sheet 16 formed of suitable insulative material. Cover and push-button sub-assembly 21 comprises cover member 20 and push buttons 22. Switch members 14 and insulative sheets 16, 18 are sandwiched between printed circuit board 12 and cover member 20 and the keyboard switch assembly 10 is held in assembled relation by rivets 24. The illustrative keyboard switch assembly 10 is intended for use in an electronic calculator.

While keyboard switch assembly 10 includes a plurality of switching sub-assemblies 28 each comprising one push-button 22, one snap-acting dome switch member 14, and associated switch terminals and conductors on upper surface 26 of printed circuit board 12, each of the switching sub-assemblies are similar and functionally identical and thus description of one such sub-assembly 28 will suffice.

Printed circuit board 12 comprises a sheet of rigid plastic material having one surface 26 copper clad, the conductors and terminals on surface 26 being formed by photo-engraving techniques as is well known to those skilled in the art. Each switching subassembly 28 comprises three switch terminals 30 on surface 26 disposed in an equilateral triangle, and a center switch terminal 32 on surface 26 disposed in the center of the triangle formed by outer terminals 30. Terminals 30, 32 are connected by conductors 34, 36 on upper surface 26 of board 12 as more fully described in the aforesaid U.S. Pat. No. 3,796,843.

Snap-acting dome switch members 14 have a generally triangular configuration with apexes which are arcuately curved about the center, the apexes defining straight sides therebetween. Projections, or dimples 38 are formed downwardly from the apexes and a center projection or dimple 40 is formed projecting into the interior of the dome. Snap-acting dome switch members 14 are disposed over and facing upper surface 26 of board 12, conductors 34, 36, and center terminals 32, outer projections 38, respectively engaging outer terminals 30 and thus spacing the periphery of switch members 14 above the raised conductors and terminals. Center projections 40 are respectively in registry with and spaced from center terminals 32 in the normal positions of switch members 14. Application of a downward force on a switch member 14 in response to manual actuation of a respective push-button 22 will deflect the switch member from its normal position to a non-over center position in which its center projection 40 engages the respective center contact 32 thereby electrically connecting the center contact to the respective outer contacts 30 to complete the desired electrical circuit.

Retaining sheet 16 is formed of a sheet of relatively thin flexible insulating material, such as polyethylene terephthalate (Mylar) and has openings 42 therein which respectively receive and locate snap-acting dome switch members 14, openings 42 generally conforming to the configuration of switch members 14 with cut-out portions 44 respectively aligned with projections 38 which respectively expose portions of outer terminals 30. Cover sheet 18 is formed of an imperforate sheet of relatively thin, flexible insulative material, such as Mylar, and covers retaining sheets 16 and switch members 14. Cover members 20 covers sheet 18 and has openings 46 therein which receive push-buttons 22. Application of manual finger pressure on a particular push-button 22 results in the application of force through cover sheet 18 on a respective snap-acting dome switch member 14 thereby actuating the switch member to its non-over center deflected position to perform the desired switching function. Upon release of manual finger pressure on push-button 22, the respective switch member 14 returns to its normal position thus opening the circuit previously established and returning the respective push-button 22 it is normal, inactive position.

Referring now to FIG. 3, in accordance with the method of the invention, opposite surfaces 48, 50 of retaining sheet 16 are coated with a thin layer of an ultrasonically-actuatable adhesive material. A heat activated, thermoplastic polyester film adhesive sold by USM Corporation, Girder Chemical Division, identified as Bostik 10-302 has been found to be suitable. The adhesive material described in the aforesaid U.S. Pat. No. 3,480,492 is also suitable. Adhesive-coated surface 50 of sheet 16 is placed on surface 26 of circuit board 12 covering conductors 34, 36 with center contacts 32 respectively exposed to openings 42 and outer contacts 30 respectively exposed to cut-out portions 44. A relatively thin, flexible sheet 52 of material which will not bond to the adhesive material is then placed on adhesive-coated surface 48 of a sheet 16 thus forming a first sandwich 54. Wax paper has been found to be a suitable material for sheet 52.

Referring now to FIGS. 4 and 5, first sandwich 54 is then placed in ultrasonic welding apparatus 56 with bottom surface 58 of circuit board 12 engaging bed 60 of the apparatus. Apparatus 56 includes horn 62 which may be manually selectively raised and lowered by level 64 so that its flat bottom surface 66 may be moved into engagement with the upper surface of sheet 52 of sandwich 54, as indicated in dashed lines in FIG. 4. A Branson Ultrasonic Welder, Model 4120 which generates mechanical vibratory energy in horn 62 in the range of 20kHz has been used in perfoming the method of the invention.

In accordance with an important aspect of the invention, bottom surface 66 of horn 62 has a plurality of shallow openings 68 formed therein arranged in the same pattern as openings 42 in sheet 16, each opening 68 in horn 62 having generally the same diameter as the corresponding opening 42 in sheet 16. Sandwich 54 is placed on bed 60 of welding apparatus 56 so that openings 42 in sheet 16 are respectively in registry with openings 68 in horn 62. Openings 68 in working surface 66 of horn 62 serve to concentrate the ultrasonic energy in the areas of sheet 16 where bonding is desired, i.e., in the areas intermediate and surrounding openings 42. In the case of the specific keyboard sub-assembly 11 shown in FIG. 1, openings 68 having a depth of one-sixteenth inch were found to be suitable.

Upon placing sandwich 54 on bed 60 of ultrasonic welding apparatus 56 and properly orienting the same, which may be accomplished by a suitable fixture, horn 62 is lowered by lever 64 so that bottom surface 66 engages sandwich 54 with sufficient pressure to maintain the sandwich tightly compacted, approximately 50 pounds per square inch pressure having been found to be suitable. Apparatus 56 is then energized for a time sufficient ultrasonically to actuate the adhesive on bottom surface 50 of sheet 16 so as to bond sheet 16 to upper surface 26 of circuit board 12, approximately 11/2 seconds having been found to be suitable. Energization of apparatus 56 may be under the control of a suitable timer.

Horn 62 of apparatus 56 is then raised to the upper position shown in solid lines in FIG. 4, sandwich 54 is removed from apparatus 56, and sheet 52 is removed thus leaving circuit board 12 with sheet 16 bonded thereto.

Referring now to FIG. 6, switch elements 14 are then inserted in openings 42 of sheet 16 and cover sheet 18 is placed on adhesive-coated upper surface 50 of sheet 16 covering the same and switch elements 14 thus forming a second sandwich 70.

Referring now additionally to FIG. 7, second sandwich 70 is placed in welding apparatus 56 with bottom surface 58 of circuit board 12 engaging bed 62, openings 42 in sheet 16 again being respectively positioned in registry with openings 68 in bottom surface 66 of horn 62. Horn 62 is then lowered into engagement with the upper surface of sheet 18 and sufficient pressure, such as about fifty pounds per square inch, is applied tightly to compact sandwich 70. Welding apparatus 56 is then again energized for a length of time sufficient ultrasonically to activate adhesive on upper surface 48 of sheet 16 thereby to bond the same to cover sheet 18, approximately 6/10th second having been found to be suitable. Horn 62 is then raised to its upper position and the thus bonded keyboard sub-assembly 11 removed from apparatus 56.

It will readily be seen that the selective ultrasonic bonding method of the invention facilitates the handling of the several components of the keyboard since the ultrasonically bondable adhesive material is not tacky. Further, the need initially to apply and subsequentially to peel-off protective cover sheets is eliminated and there is no tendency for dust or other foreign material to collect on the adhesive. Further, the tendency of pressure-sensitive adhesive material to migrate or spread upon the application of pressure is eliminated. Finally, the invention provides a relatively fast, uniform and inexpensive method of assembling a keyboard in production.

While there have been described above the principles of this invention in connection with specific apparatus, it is to be clearly understood that this description is made only by way of example and not as a limitation to the scope of the invention.

Claims

What is claimed is:

1. The method of assembling a keyboard comprising the steps of:

a. providing an insulative, planar board having opposite surfaces, one of said surfaces having at least one switch contact thereon,

b. providing a first sheet of insulating material having opposite surfaces respectively coated with ultrasonically actuatable adhesive material, said first sheet having an opening therein adapted to receive a switch element,

c. placing one surface of said first sheet on said one surface of said board with said contact exposed to said opening,

d. placing a second sheet of material which will not bond to said adhesive material on the other surface of said first sheet thereby forming a first sandwich comprising said board and said first and second sheets,

e. placing said first sandwich in ultrasonic welding apparatus and energizing the same thereby to activate said adhesive material on said one surface of said first sheet to bond the same to said one surface of said board,

f. removing said first sandwich from said apparatus and removing said second sheet from said first sheet,

g. placing a switch element in said opening in cooperative relationship with said contact,

h. placing a third sheet of insulating material on said other surface of said first sheet over said switch element thereby forming a second sandwich comprising said board and said first and third sheets,

i. placing said second sandwich in said apparatus and energizing the same thereby to actuate said adhesive material on said other surface of said first sheet to bond the same to said third sheet, and

j. removing the thus bonded second sandwich from said apparatus.

2. The method of claim 1 wherein said apparatus includes a horn having a surface selectively movable into engagement with said second and third sheets respectively, said horn surface having an opening therein of generally the same size as said first sheet opening, said placing steps (e) and (i) including orienting said first and second sandwiches, respectively, so that said first sheet opening is in registry with said horn surface opening.

3. The method of claim 2 wherein there are a plurality of said first sheet openings arranged in a predetermined pattern, a corresponding plurality of contacts and switch elements, and a corresponding plurality of horn openings arranged in the same pattern, said horn openings being respectively in registry with said first sheet openings in said placing steps (e) and (i).

4. The method of claim 3 wherein said adhesive material is a heat-activated, thermoplastic polyester film.

5. The method of claim 4 wherein said sheets of insulating material are polyethylene terephthalate.

6. The method of claim 5 wherein said second sheet is wax paper.

7. The method of claim 1 wherein said apparatus produces mechanical vibrations in said horn in the region of 20kHz.

8. The method of claim 2 wherein said horn is moved into engagement with said first and second sandwiches, respectively, during said placing steps (e) and (i), and approximately 50 pounds per square inch pressure is applied thereby to the respective sandwich.

9. The method of claim 1 wherein said appratus is energized for approximately 11/2 seconds during said placing step (e), and for approximately 6/10 second during said placing step (i).

10. In a method of assembling a keyboard, the steps comprising:

a. providing an insulative, planar board having opposite surfaces, one of said surfaces having at least one switch contact thereon,

b. providing a planar sheet having opposite surfaces, at least one of said surfaces being coated with ultrasonically actuatable adhesive material, said sheet having an opening therein,

c. placing said one surface of said sheet on said one surface of said board with said contact exposed to said opening thereby forming a sandwich comprising said board and sheet,

d. placing said sandwich in ultrasonic welding apparatus including a horn having a surface selectively movable into engagement with the other surface of said sheet, said horn surface having an opening therein in registry with said sheet opening,

e. energizing said apparatus thereby to actuate said adhesive material thereby to bond said sheet to said board, and

f. removing the thus-bonded sandwich.

11. The method of claim 10 wherein there are a plurality of said sheet openings arranged in a predetermined pattern and a corresponding plurality of horn openings arranged in the same pattern and respectively in registry in said placing step (d).

12. The method of claim 11 wherein each of said horn openings is generally the same size as the respective sheet opening.