GB2309338A - A circuit board production process - Google Patents

Description

"A Circuit Board Production Process" The invention relates to a process for producing circuit boards. More particularly, the invention relates to production in high volumes of circuit boards for consumer products such as televisions, video equipment, automotive components, etc.

Heretofore, such circuit boards have been produced by screen printing techniques because such techniques are much quicker and less expensive than photographic techniques. The fact that they are not as accurate and do not produce the tolerance levels achievable with photographic techniques has not been a disadvantage because the tracking gaps of such circuit boards has generally been quite large, usually in excess of O.5mm.

However, in recent years there has been an increasing tendency towards specification of higher density tracking on such circuit boards and the tracking gap specified is now often as low as 0.2mm or 0.25mm. To achieve this level of tolerance and avoid using photographic techniques because they would be prohibitively expensive, the approach has been to carry out the screen printing processes in a strictly controlled sterile environment.

This is because particles which adhere to a board or panel during the screen printing process, cause inaccuracies and short circuits or open circuits. Further, board handling is achieved by use of registry holes in the board and pick-and-place robotic equipment to deposit a board underneath the screen for screen printing. In this way, contact between the board and any other surfaces are minimised, thereby reducing the tendency for dust to adhere to the board. Further, it is generally the practice to carry out edge bevelling of the panel before screen printing in order to remove copper burrs and any dust particles at the edges. Generally speaking, the sources of particles in the production process are the exposed edges of the panels and it has been found that edge bevelling is effective at minimising this source of particles.

While these production techniques based on screen printing technology have been effective, it has been found that the costs associated with such steps as edge bevelling and pick-and-place equipment are quite high and it is very difficult to produce to the low cost demands of this particular market. For example, in an effort to reduce the cycle time per panel for screen printing when pickand-place equipment is used, the tendency has been to use large panels so that one pick-and-place operation applies to a higher number of circuit boards. However, this has in turn caused other problems due to distortion of panels of a relatively large size and there is the necessity to purchase and install very expensive equipment to handle such panels.

It is therefore an object of the invention to provide a process for high volume production of circuit boards to a minimum tracking gap of approximately 0.2mm at a relatively low cost.

According to the invention, there is provided a process for producing a printed circuit board, the process comprising the steps of: producing a panel of a copper clad laminated material by stamping from a blank, the stamping die providing an accuracy of better than +/ 0.08mm tolerance; edge brushing the panels to dislodge copper burrs; washing the panels; applying a particle immobilisation compound to edges of the panels; screen printing an etch resist by: storing panels in a stack, drawing each lowermost panel in turn from the stack using a drive dog protruding from the bed of a screen printing machine, sliding the panel until it is in edge registry until an edge stop protruding from the bed, applying a vacuum to the panel from underneath and lowering the drive dog and edge stop, and lowering a screen and printing the etch resist; curing the ink; etching the exposed copper using ammoniacal etching processes; stripping the etch resist; washing the panels; screen printing solder resist using the panel storing, drawing, sliding, edge registration, drive dog and edge stop removal vacuum application and printing steps as for screen printing of etch resist; testing the printed circuit board; and applying a protective coating to exposed copper surfaces.

In one embodiment, the panels are stamped from a preprepared strip within margins of 2 - 5mm of the strip edges.

Preferably, the panel washing steps after the edge brushing and the etch resist stripping operations comprise wet abrasive brushing followed by air-drying operations.

In another embodiment, the particle immobilisation compound comprises an alcohol and a copolymer.

Preferably the particle immobilisation compound comprises : - alcohol SD40, butane, isobutane, propane, octylacrylamide/acrylates /butylaminoethyl methacrylate copolymer, aminomethyl propanol, cetearyl octanoate, dimethicone copolyol, and lanolin alcohol.

In a still further embodiment, the etch resist is UV curable and is cured under UV light at an intensity of 80 to 120 w/cm at a speed of 3.5 to 5.5m/min for a distance of 2.5 to 3.5m.

In one embodiment, the etch resist is removed by immersion in dilute sodium hydroxide having a concentration of 1.5 to 5.0%.

Preferably, the protective coating for exposed copper surfaces is an organic soldering preservative coated by immersion for approximately 40 seconds at approximately 42"C.

In another embodiment, the process comprises the further steps of drilling through holes after testing by storing in the range of five to ten panels in a stack, applying a drill entry plate having a heat-conducting component and drilling all panels simultaneously through the drill entry plate.

Preferably, the panels are drawn from the stack for screen printing under a vacuum head.

DETAILED DESCRIPTION OF THE INVENTION The invention will be more clearly understood from 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 flow chart illustrating a production process of the invention; Fig. 2(a) is a diagrammatic front view showing a stamping operation to produce a panel, Fig. 2(b) is a plan view showing a laminate strip and the manner in which panels are stamped, Fig. 2(c) is a perspective view showing the manner in which a panel is removed from the strip, and Fig. 2(d) is a diagrammatic view showing edge brushing of a set of panels;; Fig. 3(a) is a diagrammatic front view showing the manner in which a panel is handled before and during screen printing, Fig. 3(b? is a plan view showing the screen printing and also the output stages and etch resist curing stage, and Fig. 3(c) is a detailed perspective view showing the manner in which a panel is conveyed to a printing position; Fig. 4 is a diagrammatic perspective view showing the manner in which drilling operations are performed.

Referring to the drawings, Fig. 1 illustrates the full production process of the invention and Figs. 2 to 4 illustrate the manner in which the more important operations are performed. The process is indicated generally by the numeral 1 and the first step is step 10 which is stamping of a laminate strip, illustrated in detail in Figs. 2(a), 2(b) and 2(c). To perform this operation, a press 30 is used having a press head (31) carrying a stamping die, the edges of which are capable of providing a tolerance of +/- 0.05mm in the panel which is stamped. The press head 31 stamps a strip 33 of a copper clad laminate over a bed 32. The copper clad laminate may have a substrate of a composite material, or it may be of a woven glass material with paper or possibly a non-woven substrate. The press head 31 is operated to stamp panels 34 with repeatability in size of +/- 0.05mm. The panel sizes may be 250mm x 150mm, 400mm x 500mm, or 250mum x 330mm. The panel thickness is in the range of 0.5mm to 0.7mm. The panels 34 are stamped from the strip 35 leaving a margin of 2mm to 5mm, as shown most clearly in Figs. 2(b) and 2(c). An important point that the stamping operation is performed to provide a very fine cutting line between the panel and the margin whereby the panel 34 must be tapped away from the strip 33 as shown in Fig. 2(c).

A very important aspect is the very fine tolerance of the stamping operation, which is generally better than +/0.08mm and in this example above is +/- 0.05mm.

In step 11, a number of the panels 34 are stacked together in a vertical stack 40 shown in Fig. 2(d) and are together wire-brushed to dislodge some of the loose material in the edges. It must be borne in mind that the edges of the panels are the major source of contamination for the remainder of the process and the manner in which this is controlled is essential to the process.

In step 12 the panels are conveyed through a wet abrasive brushing machine with an air drying stage.

In step 13 the panels are stacked in sets much like the stack 40 and a particle immobilisation compound is applied by spraying. In this embodiment, the composition of the immobilisation compound is as follows: alcohol SD40, butane, isobutane, propane, octylacrylamide/acrylates /butylaminoethyl methacrylate copolymer, aminomethyl propanol, cetearyl octanoate, dimethicone copolyol, and lanolin alcohol.

It has been found that the acrylamide and acrylate components are particularly effective at immobilising particles at the edges of the panels throughout the duration of the process by virtue of their gelling and resinous characteristics.

In steps 14 to 17 an etch resist is applied using screen printing techniques as illustrated in Figs. 3(a), 3(b) and 3(c). The panels 34 are formed into a horizontal stack 50. The lowermost panel 34 in the stack 50 is drawn from the stack 50 under a vacuum head 51 by drive dogs 52 connected to a pneumatic piston 53. This drive operation is shown most clearly in Fig. 3(c). The panel 34 is drawn past a limit switch which causes edge stops 54 to move upwardly under solenoid control. The edge stops 54 are mounted underneath the bed of the screen printing machine and rise through an aperture in the bed, again as shown most clearly in Fig. 3(c).When the panel 34 is in position in edge registration with the edge stops 54 a vacuum is applied through apertures 55 in the bed of the machine to retain the board in position and subsequently the edge stop 50, stops 54, and the drive dugs 52 are retracted. A screen printing frame 57 is then lowered and a squeegee 56 is operated to screen print the panel 34 in this position. It has been found that because the panel has been produced to a very fine accuracy by a stamping operation, edge registration techniques may be used for location of the panel for screen printing. Heretofore, edge registration has not been regarded as being applicable. In the invention, problems of inaccuracy have been overcome by stamping the panel.A further reason why edge registration has not been used is because the edges of the panels tend to cause particles to become airborne during the handling operations to place the panel under the screen. As is clear from the diagrams of Fig. 3(a), 3(b) and 3(c) it is necessary to slide the panel up against the edge stops for registration. This problem has been overcome in the invention by application of the particle immobilisation compound which prevents dust and particles rising from the edges and therefore prevents inaccuracies arising in the screen printing operation.

This is also of course helped by use of the vacuum head 51 as the panel 34 is drawn from the stack 50. A very important aspect of the these steps are that they are extremely simple. For example, the stamping operation is a relatively simple and relatively inexpensive equipment is required. Further, application of the particle immobilisation compound is also an inexpensive step.

These steps help to ensure that the screen printing of the etch resist may be carried out using edge registration techniques for a very fast throughput by avoiding the necessity for pick-and-place panel handling operations.

In this embodiment the cycle time for printing of a panel is less than 10 seconds. It will also be apparent that the equipment required for screen printing is relatively inexpensive and simple to maintain.

The steps of forming the panels 34 into a stack 50 is indicated by the step 14 in Fig. 1, of conveying under a vacuum by step 15, and of edge registration by step 16.

Screen printing is indicated by the step 17 and as shown in Fig. 3(b) a screen printed panel 60 is driven by a further set of drive dogs away from the screen 57 to an ultra-violet curing station 65 having ultra-violet lamps 66. At the curing station 65 ultra-violet lamps having an intensity of 80 to 120 watts per centimetre are used and the panels 60 are conveyed at a speed in the range of 3.5m to 5.5m per minute for a total of 2.5m to 3.5m. It has been found that this has been particularly effective at curing the etch resist.

In step 19 the copper is etched using ammoniacal etching techniques and in step 20 the etch resist is stripped using dilute sodium hydroxide at a concentration of 1.5% to 5.0%. In step 21 there is wet abrasive brushing with an air-drying stage, much like the operation 12. This cleans the copper after the etching operations.

In step 22 a solder resist is applied over the copper which is not to be exposed. The resist is applied using a screen printing operation much like that shown in Figs.

3(a) to 3(c). Again, the fact that edge registration is used provides for a very high throughput and there is very little particle adherence to the panel for high accuracy screen printing. An important point to note is that where in the prior art registry holes are used for placement of panels, the holes tend to become worn towards the end of the production process. This can lead to inaccuracies.

In the invention, this problem is avoided by edge registration.

Although not shown in Fig. 1, various data may be printed onto the board such as ident data or component print data.

Further, carbon may be printed for bridging of tracks and these are cured using infra-red heating.

As indicated by step 23 the board is then tested using a test station of the bed-of-nails type.

Many boards which are produced will require mechanical operations, as indicated by step 24. If these involve drilling, the boards 70 as shown in Fig. 4 are placed in a horizontal stack, on top of which there is a drill entry plate 71 having a copper layer for heat conduction away from the drill head. The boards 70 are placed on a bed 73 and CNC controlled drill heads 72 are then used to drill at the required locations. It has been found that use of the drill entry plate 71 helps to improve the quality of the drilling operation because heat is removed and all boards are more consistently drilled.

When all mechanical operations have been carried out, a protective coating is applied to the exposed copper. In this embodiment the protective coating is an organic soldering preservative which is applied by immersion for 40 seconds at a temperature of 42"C. An ultra-violet spectrometer is used to measure and control flow of the active ingredients to the immersion tank.

It will be appreciated that the invention provides extremely simple and inexpensive steps which allow screen printing to be carried out very quickly with low cycle times, while at the same time producing circuit boards having a relatively small tracking gap. It has been found that a tracking gap as low as 0.2mm to 0.25mm is achievable with a very high throughput in this manner.

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

Claims (12)

1. A process for producing a printed circuit board, the process comprising the steps of: producing a panel of a copper clad laminated material by stamping from a blank, the stamping die providing an accuracy of better than +/- 0.08mm tolerance; edge brushing the panels to dislodge copper burrs; washing the panels; applying a particle immobilisation compound to edges of the panels; screen printing an etch resist by:: storing panels in a stack, drawing each lowermost panel in turn from the stack using a drive dog protruding from the bed of a screen printing machine, sliding the panel until it is in edge registry until an edge stop protruding from the bed, applying a vacuum to the panel from underneath and lowering the drive dog and edge stop, and lowering a screen and printing the etch resist; curing the ink; etching the exposed copper using ammoniacal etching processes; stripping the etch resist; washing the panels; screen printing solder resist using the panel storing, drawing, sliding, edge registration, drive dog and edge stop removal vacuum application and printing steps as for screen printing of etch resist; testing the printed circuit board; and applying a protective coating to exposed copper surfaces.

2. A process as claimed in claim 1, wherein the panels are stamped from a pre-prepared strip within margins of 2 - 5mm of the strip edges.

3. A process as claimed in claim 1 or 2, wherein the panel washing steps after the edge brushing and the etch resist stripping operations comprise wet abrasive brushing followed by air-drying operations.

4. A process as claimed in any preceding claim, wherein the particle immobilisation compound comprises an alcohol and a copolymer.

5. A process as claimed in claim 4, wherein the particle immobilisation compound comprises: alcohol SD40, butane, isobutane, propane, octylacrylamide/acrylates /butylaminoethyl methacrylate copolymer, aminomethyl propanol, cetearyl octanoate, dimethicone copolyol, and lanolin alcohol.

6. A process as claimed in any preceding claim, wherein the etch resist is UV curable and is cured under UV light at an intensity of 80 to 120 w/cm at a speed of 3.5 to 5.5m/min for a distance of

2.5 to 3.5m.

7. A process as claimed in any preceding claim, wherein the etch resist is removed by immersion in dilute sodium hydroxide having a concentration of

1.5 to 5.0%.

8. A process as claimed in any preceding claim, wherein the protective coating for exposed copper surfaces is an organic soldering preservative coated by immersion for approximately 40 seconds at approximately 42"C.

9. A process as claimed in any preceding claim, wherein the process comprises the further steps of drilling through holes after testing by storing in the range of five to ten panels in a stack, applying a drill entry plate having a heat conducting component and drilling all panels simultaneously through the drill entry plate.

10. A process as claimed in any preceding claim, wherein the panels are drawn from the stack for screen printing under a vacuum head.

11. A process substantially as hereinbefore described with reference to and as illustrated in the accompanying drawings.

12. A printed circuit board whenever produced by a process as claimed in any preceding claim.