GB2347021A

GB2347021A - Printed circuit board manufacture

Info

Publication number: GB2347021A
Application number: GB9928454A
Authority: GB
Inventors: John Scanlan
Original assignee: Dovatron Res & Dev Ltd
Current assignee: Dovatron Res & Dev Ltd
Priority date: 1998-12-01
Filing date: 1999-12-01
Publication date: 2000-08-23
Anticipated expiration: 2019-12-01
Also published as: IE981003A1; GB2347021B; GB9928454D0

Abstract

The apparatus (1) has a solder printing station (2) at which solder paste is applied through a screen printing stencil in a desired pattern on to a circuit board. An optical scanning station (10) downstream of the soldering station (2) is operable to determine the volume of solder paste deposited at selected locations on the circuit board. A component machine (25) has means to pick and place a plurality of components at associated solder paste deposits on the circuit board. A re-flow oven (27) has means for heating and circulating air in a controlled manner in a number of heating zones and delivering circuit boards at desired speed through the heating zones to affect solder joints between the components and the circuit hoard. A wave soldering machine (28) is provided for attachment of components such as connectors which have conductor leads which pass through associated holes in the printed circuit board.

Description

"A Circuit Board Manufacturing Apparatus" This invention relates to a circuit board manufacturing apparatus and to a method for manufacturing circuit boards.

According to the invention there is provided a circuit board manufacturing apparatus comprising: a solder printing station having means for applying a plurality of discrete deposits of solder paste at selected locations on an upper surface of a circuit board in a desired configuration, depositing pre-selected amounts of solder paste at each location, said means comprising a screen printing stencil, means to engage an upper surface of a circuit board with an underside of the stencil, means to deliver solder paste onto a top surface of the stencil and a squeegee movable across the top surface of the stencil to force solder paste through openings in the stencil onto the top surface of the circuit board in the desired configuration. a scanner downstream of the printing station, said scanner being operable to determine the volume of solder paste deposited at any of the deposition locations on the upper surface of the circuit board, a component placement machine downstream of the scanner, the component placement machine having means to pick and place a plurality of components at associated solder paste deposits on the upper surface of the circuit board, a solder re-flow oven downstream of the component placement machine, the solder re-flow oven having a number of heating zones, a conveyor for carriage of a circuit board at a pre-selected speed sequentially through the heating zones between an inlet and an outlet of the oven, means for heating air to a desired temperature in each heating zone, and means for circulating heated air in a controlled manner at a pre-selected flow rate through each heating zone.

In another embodiment the apparatus further includes : an adhesive printing station having means for applying a plurality of discrete deposits of adhesive at selected locations on a bottom surface of a circuit board in a desired pattem, depositing a desired volume of adhesive at each location, said means comprising a screen printing stencil, means to engage a bottom surface of a circuit board with an underside of the stencil, means to deliver adhesive onto a top surface of the stencil and a squeegee movable across the top surface of the stencil to force adhesive through the openings in the stencil onto the bottom surface of the circuit board in the desired pattern, a component placement machine downstream of the adhesive printing station, the component placement machine having means to pick and place a plurality of components at associated adhesive deposits on the bottom surface of the circuit board, . an adhesive curing oven downstream of the component placement machine, a conveyor for carriage of a circuit board between an inlet and an outlet of the adhesive curing oven, and means for heating air within the oven to a desired temperature to cure the adhesive on a circuit board which transits through the adhesive curing oven.

In a further embodiment for each stencil opening, the ratio of the cross section area of the opening to the depth of the opening is selected such that the solder or adhesive deposit has a desired height when the circuit board is withdrawn from the stencil after printing.

In another embodiment the apparatus further comprises an adhesive deposition machine intermediate the adhesive printing station and the component placement machine, the adhesive deposition machine having means to support a circuit board horizontal in a desired orientation and position with a bottom surface of the circuit board uppermost, an adhesive deposition head movable across the bottom surface of the circuit board above the circuit board and operable to deposit a desired volume of adhesive at one or more selected locations on the bottom surface of the circuit board.

Preferably each oven has a conveyor at the outlet to deliver circuit boards away from the oven, stop means to engage and halt a circuit board downstream of the oven, and sensing means mounted adjacent the outlet of the oven to detect the presence of a circuit board at the outlet, the sensor being operatively connected through a time delay switch with an alarm which is operable in response to detection of a circuit board at the outlet for a preset period.

In another embodiment the apparatus further indues a wave solder machine downstream of the solder re-flow oven for application of solder to a bottom surface of a circuit board.

In a further embodiment the apparatus further includes stencil cleaning apparatus, the stencil cleaning apparatus comprising: a spray booth having a stencil holder within the spray booth, a bottom of the spray booth sloping to a drain, the drain communicating with a sump, a weir within the sump dividing the sump into an inlet chamber and a storage reservoir for cleaning fluid, a pump for delivery of cleaning fluid from the reservoir to a cleaning head for discharge of cleaning fluid onto a stencil.

Preferably a mesh strainer extends upwardly from a top of the weir.

10. Ideally a stencil dryer is provided. Preferably the stencil dryer comprises an air jet for connection to a compressed air supply.

In another aspect the invention provides a method for manufacturing circuit boards comprising the steps : delivering each circuit board to a solder printing station, laying a stencil over the circuit board, screen printing solder paste through openings in the stencil onto a surface of the circuit board in a desired configuration, depositing pre-selected amounts of solder paste at selected locations on the surface of the circuit board, scanning selected circuit boards to determine the volume of solder paste deposited at pre-seiected locations on the circuit board, rejecting any circuit board not having the correct amount of solder paste at a chosen location on the circuit board, placing a number of components at associated solder paste deposits on the circuit board in a component placement machine, conveying the circuit board through a solder re-flow oven downstream of the component placement machine, said solder re-flow oven having a number of heating zones, heating air within each heating zone to a desired temperature, circulating heated air in a controlled manner at a pre-sefected flow rate through each heating zone, and discharging the circuit board from the solder re-flow oven.

In another embodiment the method further includes : delivering each circuit to an adhesive printing station, overlapping a bottom surface of the circuit board with a screen printing stencil, screen printing adhesive through the stencil onto the bottom surface of the circuit board for applying a plurality of discreet deposits of adhesive at selected locations in a desired pattern on the bottom surface of the circuit board, delivering the circuit board to a component placement machine, placing a plurality of components at associated adhesive deposits on the bottom surface of the circuit board, conveying the circuit board through an adhesive--curing oven downstream of the component placement machine, heating the air within the adhesive curing oven to a desired temperature for curing the adhesive on the circuit board as it is passing through the adhesive curing oven, and passing the circuit board through a wave solder machine downstream of the solder re-flow oven for applying solder to selected locations on a bottom surface of the circuit board.

In a further embodiment the method includes the steps : delivering the stencil to a stencil cleaning apparatus, mounting the stencil in a spray booth of the stencil cleaning apparatus, spraying cleaning fluid at the stencil in the booth for cleaning the stencil, the cleaning fluid comprising a mixture of isopropyl, alcohol and de ionised water, and drying the stencil by directing an air-jet at the stencil.

The invention will be more dearly understood by the following description of some embodiments thereof, given by way of example only, with reference to the accompanying drawings, in which: Fig. 1 is a schematic flowchart illustrating a circuit board manufacturing apparatus according to the invention ; Fig. 2 is a schematic flowchart of a further apparatus according to the invention; Fig. 3 is a plan view of a stencil forming portion of the apparatus ; Fig. 4 is a plan view of another stencil ; Fig. 5 is a perspective view of a scanner forming portion of the apparatus ; Fig. 6 is a schematic illustration of a stencil cleaner forming portion of the apparatus ; and Fig. 7 is a perspective view of the stencil cleaner.

Referring to the drawings, there is illustrated apparatus for manufacturing circuit boards according to the invention and indicated generally by the reference numeral 1. The apparatus 1 comprises a solder printing station 2. At the solder printing station 2, a plurality of circuit boards are stacked in amagazine. A conveyor delivers each circuit board in turn from the magazine to the screen printing stencil 5, 6, examples of which are shown in Figs. 3 and 4. Means is provided at the stencil to engage a circuit board with an underside of the stencil.

A line of solder paste is deposited at one side of the stencil. A squeegee is mounted above the stencil engaging a top surface of the stencil. Associated drive means advances the squeegee across the top surface of the stencil to force solder paste through openings 7 in the stencil 5, 6 onto a top surface of the circuit board in a desired configuration corresponding to the configuration or pattern of openings 7 in the stencil 5,6. It will be appreciated that correct alignment of the circuit board with the stencil is essential to ensure the correct deposition of solder paste on the circuit board.

The circuit board is then dropped away from the stencil and carried by a conveyor to a scanning station 10. At the scanning station 10, a scanner is operable to determine the volume of solder paste deposited at any of the deposition locations on the circuit board. If desired, each circuit board may be scanned. However, where appropriate, it may only be required to scan a representative sample of circuit boards in a batch production of circuit boards.

One type of scanning station 10 is shown in Fig. 5. The scanning station 10 has a table 11. A laser section microscope 12 is mounted on a support 13 above the table 11 for scanning a circuit board 15 mounted on the table 11 beneath the microscope 12. The microscope 12 has a pair of spaced-apart laser heads 16 generating light beams which are illustrated as parallel datum lines 18 upon an associated viewing screen 19. A controller 20 is operable to adjust the spacing of the heads 16 and thus the datum lines 18 which can be positioned at the sides or the ends of a solder paste deposit to measure the width and length of the solder paste deposit to provide a measure of the volume of solder paste in the deposit on the circuit board 15. This can then be checked against a desired volume of solder paste recorded on a job specification associated with the circuit board 15 which is illustrated on a monitor 22. In this manner, a number of solder paste deposits are checked. It has been found that by checking three solder paste deposits at spaced locations across a diagonal of the circuit board 15 gives an accurate representation of the effectiveness of the solder deposition in the screen printing process to a desired requirement.

Downstream of the scanning station 10, there is provided a component placement machine 25 having means to pick and place a plurality of components at associated solder paste deposits on the upper surface of the circuit board. A number of different types of such component placement machine are commercially available.

A re-flow oven 27 is provided downstream of the component placement machine 25. The solder re-flow oven 27 has a number of heating zones. A conveyor is operable for carriage of a circuit board with the surface mounting components at a pre-selected speed sequentially through the heating zones between an inlet and an outlet of the oven 27. The oven 27 has means for heating air to a desired temperature in each heating zone and fans for circulating heated air in a controlled manner at a pre-selected flow rate through each heating zone. The use of air rather than the more usual nitrogen is particularly advantageous from a cost point of view as the provision of a nitrogen environment within the oven is relatively expensive. It has been found that by carefully controlling the conveyor speed and thus the transit time of the circuit board, and also the temperature profile within the oven by regulating the temperatures within the different heating zones, effective solder joints between the components and the circuit board can be achieved. It is also desirable to control the air circulation within the oven to minimise air circulation, and thus minimise oxygen content which can adversely affect the joint by oxidation.

Circuit boards 15 discharged from the oven 27 are delivered by a conveyor from the outlet of the oven 27 towards an end stop which is engagable with a front end of the circuit board 15 to halt the circuit board on the conveyor. A sensor is mounted adjacent the outlet of the oven 27 to detect the presence of a circuit board at the outlet This sensor is operatively connected through a time delay switch with an alarm which is operable in response to detection of a circuit board at the outlet of the oven 27 for a preset period. Thus, if a queue of circuit boards develops on the conveyor backing into the oven 27, an alarm is generated to alert an operator to clear the backlog, otherwise the circuit boards within the oven 27 will overheat and smoulder or bum.

Downstream of the oven 27, a wave soldering machine 28 ils provided for attachment of components such as connectors which have conductor leads which pass through holes in the printed circuit board.

If it is desired to attach components to both sides of the circuit board and it is necessary to temporarily secure surface mounted components with adhesive on an underside of the circuit board while soldering the components on the upper surface of the circuit board.

Referring to Fig. 2, apparatus for surface mounting components on a bottom surface of a circuit board is shown. This apparatus comprises a magazine 30 for supporting a stack of circuit boards, bottom side up. A conveyor delivers each circuit board to an adhesive printing station 31. A screen printing stencil is provided at the adhesive printing station 31 with associated means for aligning and engaging an upwardly facing bottom surface of a circuit board with an underside of the stencil. A squeegee is mounted above the stencil in engagement with a top surface of the stencil, having associated drive means for movement of the squeegee across the top surface of the stencil to force adhesive through the openings in the stencil onto the bottom surface of the circuit board in the desired pattern prior to release of the circuit board for delivery through an adhesive deposition machine 32.

It will be noted that the sizing and configuration of the openings in the stencil is critical to achieve a desired adhesive deposition on the circuit board. The ratio of the cross section area of the opening to the depth of the opening is selected such that the adhesive deposit has a desired height In this way, a single stencil of a uniform thickness can be used to make adhesive deposits of different heights on the circuit board.

The adhesive deposition machine 32 may optionally be used for the application of one or more deposits of adhesive onto the circuit board. This is used where a number of components require a glue deposition of a required configuration or height which is outside the overall scope of variation possible with a stencil.

The adhesive deposition machine 32 has means to support a circuit board horizontal in a desired orientation or position with a bottom surface of the circuit board uppermost. An adhesive deposition head is movable across the bottom surface of the circuit board above the circuit board and is operable to deposit a desired volume of adhesive at one or more selected locations on the bottom surface of the circuit board.

Downstream of the adhesive deposition machine 32, the circuit board is delivered through a component placement machine 33. The component placement machine 33 has means to pick and place a plurality of components at associated adhesive deposits on the bottom surface of the circuit board.

An adhesive curing oven 34 is provided downstream of the component placement machine 33. The oven 34 has a conveyor for carriage of a circuit board between an inlet end and an outlet end of the oven 34. Means is provided for heating air within the oven to a desired temperature to cure the adhesive on a circuit board which transits through the oven 34. Cooling fans are provided above the conveyor at the outlet to assist in cooling the circuit board downstream of the oven. An alarm similar to that described previously for the re-flow oven 27 is also provided for the adhesive curing oven 34. Wth the surface mounted components adhesively secured to an underside of the circuit board, the circuit board can then be delivered through the solder line for attachment of surface mounted components to an upper surface of the circuit board by the re-flow technique as previously described prior to delivering the circuit board through the wave soldering machine 28 for soldering the components on the underside of the circuit board.

Referring to Figs. 6 and 7, there is illustrated a cleaning apparatus 40 for the stencils 5,6 used in both the solder printing station and the adhesive printing station. The cleaning apparatus 40 comprises an open fronted spray booth 41 having an inclined bottom panel 42 with an upstanding rear panel 43 and end panels 44. A cover panel 45 extends between upper ends of the rear panel 43 and end panels 44. Channel section mounting brackets 46 are provided on the bottom panel 42 and rear panel 43 for supporting a stencil 47 in an upright position. A drain 48 is provided at a lower end of the bottom panel 42. A grating 49 extends across an inlet of the drain 48. The drain 48 communicates with a sump 50. The sump 50 has a base 51 with an upstanding front wall 52, rear wall 53 and end walls 54. A removable cover 55 sits on top of the walls 52, 53,54. A weir 58 within the sump 50 divides the sump 50 into an inlet chamber 59 and a storage reservoir 60 for cleaning fluid. A mesh strainer 61 extends upwardly from a top of the weir 58. A pump 62 is mounted within the reservoir 60 for delivery of cleaning fluid from the reservoir 60 to a cleaning brush 64 at the end of a flexible pipe 65. This can be used to spray water onto the stencil 47 while cleaning the stencil 47 with the brush 64. A dryer 68 comprises an air gun 69 at the end of a flexible air line 70 which communicates with a compressed air supply. The air gun 69 is operable to deliver a jet of air at the stencil 47 for drying the stencil 47 after cleaning.

The cleaning fluid used is a mixture of isopropyl alcohol and de-ionised water.

In use, the pump 62 delivers cleaning fluid from the reservoir 60 to the brush 64 for deaning the stencil 47. Cleaning fluid run off is collecte in the chamber 59 with solid material settling at the bottom of the chamber 59 and cleaning fluid overflowing the weir 58 into the reservoir 60 for re-use. After cleaning, the air gun 69 is used to dry the stencil 47.

The invention is not limited to the embodiments hereinbefore described which may be varied in both construction and detail.

Claims

CLAIMS 1. A circuit board manufacturing apparatus comprising: a solder printing station having means for applying a plurality of discrete deposits of solder paste at selected locations on an upper surface of a circuit board in a desired configuration, depositing pre-selected amounts of solder paste at each location, said means comprising a screen printing stencil, means to engage an upper surface of a circuit board with an underside of the stencil, means to deliver solder paste onto a top surface of the stencil and a squeegee movable across the top surface of the stencil to force solder paste through openings in the stencil onto the top surface of the circuit board in the desired configuration, a scanner downstream of the printing station, said scanner being operable to determine the volume of solder paste deposited at any of the deposition locations on the upper surface of the circuit board, a component placement machine downstream of the scanner, the component placement machine having means to pick and place a plurality of components at associated solder paste deposits on the upper surface of the circuit board, a solder re-flow oven downstream of the component placement machine, the solder re-flow oven having a number of heating zones, a conveyor for carriage of a circuit board at a pre-selected speed sequentially through the heating zones between an inlet and an outlet of the oven, means for heating air to a desired temperature in each heating zone, and means for circulating heated air in a controlled manner at a pre- selected flow rate through each heating zone.
2. Apparatus as claimed in claim 1, which further includes : an adhesive printing station having means for applying a plurality of discrete deposits of adhesive at selected locations on a bottom surface of a circuit board in a desired pattern, depositing a desired volume of adhesive at each location, said means comprising a screen printing stencil, means to engage a bottom surface of a circuit board with an underside of the stencil, means to deliver adhesive onto a top surface of the stencil and a squeegee movable across the top surface of the stencil to force adhesive through the openings in the stencil onto the bottom surface of the circuit board in the desired pattern, a component placement machine downstream of the adhesive printing station, the component placement machine having means to pick and place a plurality of components at associated adhesive deposits on the bottom surface of the circuit board, an adhesive curing oven downstream of the component placement machine, a conveyor for carriage of a circuit board between an inlet and an outlet of the adhesive curing oven, and means for heating air within the oven to a desired temperature to cure the adhesive on a circuit board which transits through the adhesive curing oven.
3. Apparatus as claimed in claim 1 or 2, wherein for each stencil opening, the ratio of the cross sectional area of the opening to the depth of the opening is selected such that the solder or adhesive deposit has a desired height when the circuit board is withdrawn from the stencil after printing.
4. Apparatus as claimed in claim 2 or 3, further comprising an adhesive deposition machine intermediate the adhesive printing station and the component placement machine, the adhesive deposition machine having means to support a circuit board horizontal in a desired orientation and position with a bottom surface of the circuit board uppermost, an adhesive deposition head movable across the bottom surface of the circuit board above the circuit board and operable to deposit a desired volume of adhesive at one or more selected locations on the bottom surface of the circuit board.
5. Apparatus as claimed in any preceding claim, wherein each oven has a conveyor at the outlet to deliver circuit boards away from the oven, stop means to engage and halt a circuit board downstream of the oven, and sensing means mounted adjacent the outlet of the oven to detect the presence of a circuit board at the outlet, the sensor being operatively connected through a time delay switch with an alarm which is operable in response to detection of a circuit board at the outlet for a preset period.
6. Apparatus as daimed in any preceding claim, wherein the apparatus further includes a wave solder machine downstream of the solder re-flow oven for application of solder to a bottom surface of a circuit board.
7. Apparatus as daimed in any preceding claim, wherein the apparatus further inclues stencil cleaning apparatus, the stencil cleaning apparatus comprising: a spray booth having a stencil. holder within the spray booth, a bottom of the spray booth sloping to a drain, the drain communicating with a sump, a weir within the sump dividing the sump into an inlet chamber and a storage. reservoir for cleaning fluid, a pump for delivery of cleaning fluid from the reservoir to a cleaning head for discharge of cleaning fluid onto a stencil.
8. Apparatus as claimed in claim 7, wherein a mesh strainer extends upwardly from a top of the weir.
9. Apparatus as claimed in claim 7 or 8, wherein a stencil dryer is provided.
10-Apparatus as claimed in claim 9, wherein the stencil dryer comprises an air jet for connection to a compressed air supply.
A method for manufacturing circuit boards comprising the steps: delivering each circuit board to a solder printing station, laying a stencil over the circuit board, screen printing solder paste through openings in the stencil onto a surface of the circuit board in a desired configuration, depositing pre-selected amounts of solder paste at selected locations on the surface of the circuit board, scanning selected circuit boards to determine the volume of solder paste deposited at pre-selected locations on the circuit board, rejecting any circuit board not having the correct amount of solder paste at a chosen location on the circuit board, placing a number of components at associated solder paste deposits on the circuit board in a component placement machine, conveying the circuit board through a solder re-flow oven downstream of the component placement machine, said solder re-flow oven having a number of heating zones, heating air within each heating zone to a desired temperature, circulating heated air in a controlled manner at a pre-selected flow rate through each heating zone, and discharging the circuit board from the solder re-flow oven.
12. A method as claimed in claim 11 wherein the method further inclues : delivering each circuit to an adhesive printing station, overlapping a bottom surface of the circuit board with a screen printingstencil, screen printing adhesive through the stencil onto the bottom surface of the circuit board for applying a plurality of discreet deposits of adhesive at selected locations in a desired pattern on the bottom surface of the circuit board, delivering the circuit board to a component placement machine, placing a plurality of components at associated adhesive deposits on the bottom surface of the circuit board, conveying the circuit board through an adhesive curing oven downstream of the component placement machine, heating the air within the adhesive curing oven to a desired temperature for curing the adhesive on the circuit board as it is passing through the adhesive curing oven, and passing the circuit board through a wave solder machine downstream of the solder re-flow oven for applying solder to selected locations on a bottom surface of the circuit board.
13. A method as claimed in claim 11 or 12 wherein the method includes the steps of: delivering the stencil to a stencil cleaning apparatus, mounting the stencil in a spray booth of the stencil cleaning apparatus, spraying cleaning fluid at the stencil in the booth for cleaning the stencil, the cleaning fluid comprising a mixture of isopropyl, alcohol and de-ionised water, and drying the stencil by directing an air-jet at the stencil.

4 method for manufacturing circuit boards substantially as hereinbefore described with reference to the drawings.

15 Circuit boards whenever produced with the apparatus of claims 1 to 10 or by the method of daims 11 14