GB2316360A - Computer controlled manufacturing system, stencil storage apparatus and method of operation. - Google Patents

Abstract

A computer controlled manufacturing system includes a stencil delivery system (12, 14) having an array of linked stencils, each respective stencil having a plurality of respective orifices. A computer (16) is operably coupled to the stencil delivery system (12, 14), for selecting a first stencil (22) from the array of linked stencils and for positioning the first stencil (22) in a screening position above a printed circuit board (24).

Description

COMPUTER CONTROLLED MANUFACTURING SYSTEM, STENCIL STORAGE APPARATUS AND METHOD OF OPERATION Field of the Invention This invention relates to manufacturing processes for electronic products and their operation. The invention is applicable to, but not limited to, the use of stencils in automatic screen printing systems.

Background of the Invention The manufacture of printed circuit boards is nowadays often performed using automatic component placement machines, for example a Chipshooter, to mount components onto printed circuit boards once a printed circuit board (PCB) has had solder paste applied to it. To apply the solder paste a stencil is often used. The stencil is typically laser or chemically etched to define the pad layouts for the board and then the solder paste applied. The application of solder paste to a printed circuit board is typically performed using a squeegee process to brush solder paste through the pre-designed and perforated stencil to ensure the paste is uniformly and correctly positioned on the board. The Chipshooter then positions the components and the solder paste is reflowed to complete the PCB.

Most of this process is automated on a manufacturing production line, with computer control of the squeegee operation, Chip shooter function and the oven heating to reflow the solder paste. This is typically referred to as Computer Integrated Manufacture (CIM) where the only process not typically controlled by a computer is in the changing of stencils. The significant automation of this process has allowed a number of different PCBs to be built in a particular manufacturing run. However, as mentioned, currently each PCB change requires a new stencil to be manually clamped in place using a rigid frame structure or alternatively stretching a flexible stencil over a frame and subsequently using the stencil in applying the solder paste to the PCB.

It is known in the art for a computer-controlled, stand-alone, stencil stacker to be connected to the CIM system. However, such a stand-alone stencil stacker requires a large amount of space, is very expensive and, due to the size of the stencils, typically 0.7m by 0.7m, each stacker is typically limited to accommodating approximately six stencils. In using such a prior art stand-alone stencil stacker, the manufacturing process is not totally automated as the stencil being used requires cleaning, with any excess paste needing to be removed. Furthermore the excess paste is typically not re-used unless by a manual saving of the paste by an operator.

Consequently, for an automatic manufacturing process that has a large number of products to be built on a single production run, and in particular for products that require a double sided manufacture of printed circuit boards, such a stand-alone stencil stacker, or indeed a number of stand-alone stackers, is unacceptable.

An alternative computer-controlled solder paste application system is known in the art whereby a nozzle is used to apply paste to individual solder pads on a PCB. Such a system is expensive, requires significant maintenance and is very slow. Hence, for use in current high-speed manufacturing systems this individual approach is unacceptable.

This invention seeks to provide an improved manufacturing apparatus, stencil storage mechanism and process for alleviating some of the problems associated with using such a stencil approach in applying solder paste.

Summarv of the Invention In a first aspect of the present invention, a computer controlled manufacturing system is provided. The computer controlled manufacturing system includes a stencil delivery system having an array of linked stencils, each respective stencil having a plurality of respective orifices and a computer, operably coupled to the stencil delivery system, for selecting a first stencil from the array of linked stencils and for positioning the first stencil in a screening position above a printed circuit board.

Preferably, the computer controlled manufacturing system further includes an applicator operably coupled to the computer for applying solder paste to the printed circuit board through the respective orifices of the first stencil when in the screening position and a frame for receiving and positioning the first stencil in the screening position above the printed circuit board. In the preferred embodiment of the invention, the array of linked stencils is a serial belt-like arrangement that uses chain-like flexible links for interconnecting the plurality of linked stencils. The computer controls the positioning of the belt-like arrangement such that the first stencil is positioned above the printed circuit board by movement of the belt-like arrangement.

In this manner, the selection and storage of the stencils for each production run of a printed circuit board is totally automated. This is achieved by automating the paste application, cleaning and paste removal functions in the manner described. Furthermore, the belt-like arrangement of the stencil storage apparatus is such that a large number of stencils can be easily stored and speedily and efficiently retrieved when changing production runs.

In a second aspect of the present invention, a stencil storage apparatus is provided. The stencil storage apparatus includes at least one motorised drum having a plurality of linked stencils circumscribely mounted thereon.

Preferably, the stencil storage apparatus has two motorised drums, operably coupled together via chain-like flexible links where each motorised drum has a plurality of linked stencils circumscribely mounted thereon.

In this manner, the selection, storage and positioning of the stencils, for each production run of a printed circuit board, is automatically performed. The speed and efficiency of the retrieval system is greatly improved and the number of stencils that can be stored on an apparatus per cubic area of space is significantly improved.

In a third aspect of the present invention, a method of computer controlling a manufacturing system is provided. The computer controlled manufacturing system includes a stencil delivery system having an array of linked stencils, each respective stencil having a plurality of respective orifices. A computer is operably coupled to the stencil delivery system and an applicator. The method includes the step of selecting a first stencil from the array of linked stencils, positioning the first stencil in a screening position above a printed circuit board and applying solder paste, via the applicator, to the printed circuit board through respective orifices of the first stencil of the array of linked stencils.

In a preferred embodiment of the third aspect of the present invention, the stencil delivery system further includes two motorised drums, operably coupled together via chain-like flexible links. Each motorised drum has the plurality of linked stencils circumscribely mounted thereon and the step of positioning the first stencil in a screening position includes rotating at least one motorised drum.

In this manner, the computer controlled rotation of at least one of the motorised drums enables a speedy and efficient positioning of a desired stencil to a screening position above the printed circuit thereby facilitating the application of solder paste to the printed circuit board, by the applicator and through the desired stencil.

A preferred embodiment of the invention will now be described, by way of example only, with reference to the drawings.

Brief Description of the Drawings FIG. 1 is an elevational view of an automatic stencil change apparatus according to a preferred embodiment of the invention.

FIG. 2 is a plan view of an automatic stencil change apparatus according to the preferred embodiment of the invention.

FIG. 3 is a flow chart of a computer-controlled stencil change operation according to the preferred embodiment of the invention.

Detailed Description of the Drawings Referring first to FIG. 1, an elevational view of an automatic stencil change apparatus 10 according to a preferred embodiment of the invention, is shown. The automatic stencil change apparatus 10 includes two motorised drums 12, each motorised drums having a drum cog 14 controlled by a computer 16. The motorised drums 12 are connected by a chain-like arrangement 19 containing, and interconnecting, a number of stencils 18. Each stencil has respective orifices dependent upon the layout of the associated printed circuit board for use with the respective stencil.

The chain-like arrangement 19 is wound around the drums in an audio tape-to-tape arrangement. The chain-like arrangement 19 includes a number of symmetrical links 17 for connecting stencils to the chain-like arrangement 19. The symmetrical nature of the symmetrical links 17 allow the stencils to be positioned in either orientation. The two motorised drums 12 are separated by an applicator, for example a squeegee 28, having a vertical plane of motion 30. A replenishment element may also be used in order to replace the solder paste used by the squeegee, from time to time. The squeegee 28 is positioned above a chosen stencil 22 with an associated printed circuit board 24 positioned below the chosen stencil 22.

There is a leaf 29 mounted on either side of the squeegee 28 to catch the roll of paste. The leaf 29 is preferably a ramp-type arrangement for catching the roll of paste when scraped across the stencil and work area. The printed circuit board 24 is also positioned via a vertical plane of motion 26.

In the preferred embodiment of the invention, a vision recognition system is used for aligning the stencils to the PCB. On both sides of the squeegee 28, there is positioned a stencil wash unit 20, preferably controlled by the computer 16, for cleaning the chosen stencil 22, after use.

In operation, a stencil is chosen, by the computer 16, according to the associated printed circuit boards of the current production run. The stencil is manoeuvred into position above the printed circuit board by control of one of the two motorised drums 12. The drum is rotated under control of the computer 16, thereby causing a lateral movement of the chain-like arrangement 19 across the work area. The chosen stencil 22 has, after its previous use, passed through a stencil wash unit 20, thereby ensuring a cleaned stencil is fixed in the working (screening) position.

When the chosen stencil 22 is in position, it is fixed in a frame. The squeegee 28 is applied to the stencil, in a vertical plane of motion 30, and then applied across the stencil in a horizontal motion, perpendicular to the vertical plane and to the lateral movement of the chain-like arrangement 19, in order to squeeze solder paste through respective holes in the chosen stencil 22, and onto the printed circuit board. There is a leaf 29 mounted on either side of the squeegee 28, which is used to catch the roll of paste when the squeegee 28 finishes its stroke. The next printed circuit board to be applied with solder paste is then automatically positioned under the chosen stencil 22.

In this manner, the application of solder paste to a number of PCBs on a number of production runs is achieved. The manufacturing system is totally computer-controlled and is very fast. There is no need for large and expensive frames to hold each stencil and no need to store stencils off-line from the production process. The introduction of computer controlled stencil wash units removes the need for an operator to clean each stencil after, or prior to, being used. Furthermore, with the introduction of a leaf 29, the roll of paste is not wasted and facilitates the whole process being totally computer-controlled; the only totally computer-controlled alternative being the nozzle arrangement previously mentioned that delivers a particular amount of paste to a particular solder pad, but is unacceptably slow. When another production run is to commence, with a new printed circuit board arrangement, a new stencil is selected and positioned via computer-controlled movement of the chain-like arrangement 19, as previously described.

Referring to FIG. 2, a plan view of the automatic stencil change apparatus of FIG. 1, in accordance with the preferred embodiment of the invention, is shown. The set-up and operation is as previously described with reference to FIG. 1, with the chain like arrangement 19, shown circumscribing the two motorised drums 12, having stencils 18 interconnected by the chain like arrangement 19. The horizontal movement of the squeegee 28, across the chosen stencil 22, is shown, with each leaf 29 collecting the roll of solder paste.

Advantageously, the selection and positioning of the chosen stencil is performed automatically by the computer 16. Furthermore, the arrangement for storing the stencils, circumscribely mounted on the drums, enables a large number of stencils to be stored and subsequently computer-controlled for positioning above the associated PCBs. This process is speedily and efficiently performed, without any need for human intervention in physically performing a tooling change, and hence negates the possibility of introducing human error. In this manner, the number of production runs, capable of total and automatic control by a computer, is greatly increased. As there is no longer a requirement to have frames to hold each stencil, the operation of the motorised drums is controlled in such a manner as to stretch the stencil so that it is drum-tight and therefore there is no "slack" in the stencil being used. Each stencil is typically made from stainless steel or brass and is approximately 0.125 mm in thickness. Preferably each stencil has a mesh material (silk screen) around the circumference of the stainless steel work area.

Referring now to FIG. 3, a flow chart of a computer controlled stencil change operation according to the preferred embodiment of the invention, is shown. The stencils are linked in a chain-like manner to form a chain with the stencils being separated from each other, as shown in step 102. In the preferred embodiment of the invention, the chain-like arrangement is circumscribely mounted on two drums, as in step 104, where the two drums are separated by an applicator, in a physical arrangement similar to an audio cassette tape.

Preferably there are blank spaces at the end of the chains to accommodate stencil positions that, given the dimensions and design of the stencil delivery system, can not be positioned in a screening position above a printed circuit board, for example, at the extremities of the chain that are physically connected to the drums. One of the drums is rotated in order to select the desired stencil and position it over the printed circuit board. This rotation can be performed manually, as in step 106, or, in the preferred embodiment of the invention, via computer controlled commands, as shown in steps 108 to 112.

In such a computer controlled operation, the position of each stencil is programmed into the computer, as shown in step 108, the program is executed to select a particular stencil, as in step 110 and the computer controls the rotation of the drum to select and position the desired stencil, as shown in step 112. The desired stencil is then locked in place, as in step 114, and an applicator program initiated to apply solder paste to a printed circuit board by screening the solder paste through holes in the stencil.

It is within the contemplation of the invention that the manufacturing system herein described is the preferred embodiment of the invention and that the scope of the invention applies to any variants of the elements and processes, or indeed the orientation or number of said elements and processes.

Thus a manufacturing apparatus and method of operation are provided that are an improvement over, and more flexible than, prior art manufacturing systems and processes. Furthermore a stencil storage mechanism is provided that alleviates some of the problems associated with using stencils in manufacturing processes in the application of solder paste.

Claims (14)

Claims

1. A computer controlled manufacturing system, wherein the computer controlled manufacturing system comprises: a stencil delivery system having an array of linked stencils, each respective stencil having a plurality of respective orifices; and a computer, operably coupled to the stencil delivery system, for selecting a first stencil from the array of linked stencils and for positioning the first stencil in a screening position above a printed circuit board.

2. The computer controlled manufacturing system in accordance with claim 1, further comprising an applicator operably coupled to the computer for applying solder paste to the printed circuit board through the respective orifices of the first stencil when in the screening position.

3. The computer controlled manufacturing system in accordance with any one of claims 1 or 2, wherein the array of linked stencils is a serial beltlike arrangement and wherein the computer controls the positioning of the belt-like arrangement such that the first stencil is positioned above the printed circuit board by movement of the belt-like arrangement.

4. The computer controlled manufacturing system in accordance with claim 3, wherein the belt-like arrangement is achieved using chain-like flexible links for interconnecting the plurality of linked stencils.

5. The computer controlled manufacturing system in accordance with any one of the preceding claims, further comprising at least one computercontrolled wash unit, operably coupled to the computer, for automatically cleaning the first stencil after use.

6. The computer controlled manufacturing system in accordance with any one of the preceding claims, further comprising at least one leaf, operably coupled to the applicator, for catching a roll of paste applied to the first stencil by the applicator.

7. The computer controlled manufacturing system in accordance with any one of the preceding claims, wherein the stencil delivery system comprises at least one motorised drum operably coupled to the computer and having the plurality of linked stencils circumscribely mounted on the drum and where the computer controls a rotational operation of the motorised drum for positioning the first stencil in the screening position.

8. The computer controlled manufacturing system in accordance with claim 7, the stencil delivery system comprising two motorised drums, operably coupled together via the chain-like flexible links and separated at least by the applicator, wherein the positioning of the first stencil in the screening position is performed by rotation of at least one of the motorised drums thereby winding further stencils around at least one of the motorised drums.

9. A stencil storage apparatus comprising: at least one motorised drum having a plurality of linked stencils circumscribely mounted thereon.

10. The stencil storage apparatus in accordance with claim 9, the stencil storage apparatus having two motorised drums, operably coupled together via chain-like flexible links and each motorised drum having a plurality of linked stencils circumscribely mounted thereon.

11. A method for computer controlling a manufacturing system, wherein the computer controlled manufacturing system includes a stencil delivery system having an array of linked stencils, each respective stencil having a plurality of respective orifices, and wherein a computer is operably coupled to the stencil delivery system and an applicator, the method comprising the step of: selecting a first stencil from the array of linked stencils; positioning the first stencil in a screening position above a printed circuit board; and applying solder paste, via the applicator, to the printed circuit board through respective orifices of the first stencil of the array of linked stencils.

12. The method for computer controlling a manufacturing system in accordance with claim 11, the stencil delivery system further comprising two motorised drums, operably coupled together via chain-like flexible links and each motorised drum having the plurality of linked stencils circumscribely mounted thereon, wherein the step of positioning the first stencil in a screening position includes: rotating at least one motorised drum.

13. A computer controlled manufacturing system substantially as described herein with respect to FIG. 1 or FIG. 2 of the drawings.

14. A method for computer controlling a manufacturing system substantially as described herein with respect to FIG. 3 of the drawings.