WO2017182777A1

WO2017182777A1 - Printing screen

Info

Publication number: WO2017182777A1
Application number: PCT/GB2017/051031
Authority: WO
Inventors: Graham James HALL
Original assignee: Tannlin Technology Limited
Priority date: 2016-04-20
Filing date: 2017-04-12
Publication date: 2017-10-26

Abstract

A method of making a printing screen (10) is described. The printing screen (10) includes a substrate (12), wherein the substrate (12) includes one or more areas of thickness (16, 18) different than the thickness of the substrate (12), wherein the method comprises the steps of: creating an aperture (14) through the substrate (12); inserting a first patch of material (15) into the aperture; fusing the first patch to the substrate (12); creating an aperture (14) through the first patch (15); inserting a subsequent patch of material (15) into the aperture(14) through the first patch (15); fusing the subsequent patch (15) to the first patch (15); and repeating the steps of creating an aperture (14), inserting a subsequent patch of material (15) and fusing the subsequent patch (15) to the previous patch (15) until cumulatively the patches (15) and the substrate (12) provide a predetermined final thickness.

Description

Printing Screen

Field of Invention

The invention relates to a method of making a printing screen for application of solder paste on printed circuit boards and films. In particular the invention relates to the preparation of a printing screen, which comprises varying thicknesses representing high or low areas on the surface of the printed circuit board.

Background to the Invention

A printing screen, also known as a stencil or mask typically includes apertures via which solder paste is applied as a printed image on a printed circuit board.

A printing screen represents a template for the required solder paste areas on a printed circuit board and facilitates the application of solder paste thereto. The volume of solder paste applied to the printed circuit board is determined by the volume of the apertures created through the printing screen.

Solder paste is applied to the printed circuit board by laying the printing screen on top of a substrate material and applying solder paste to the surface of the printing screen and using a squeegee to spread the paste evenly across the surface of the printing screen. As it is spread, the solder paste flows into and fills any apertures through the printing screen and adheres to the substrate material situated below. Upon removal of the printing screen from the substrate a pattern of solder paste remains. This pattern of solder paste can have electronic devices attached thereto by means of melting the solder paste and adding electronic components. The resulting circuit board can then be used to operate/control electronic components and electronic devices.

The volume of solder paste that is applied to the substrate material is determined by the size of the aperture, which includes the area of the opening and also the depth of the opening. Insufficient solder paste can lead to the rejection of printed circuit boards because electronic components cannot be easily attached.

There is increasing demand for printed circuit boards to include printed images that include areas of solder paste, which are smaller, larger, higher and shallower to accommodate attaching different sizes of electronic devices thereto.

The printing screen material is typically a sheet of metal, for example stainless steel, which is of uniform thickness. As described above the desire is to produce a stencil comprising non-uniform depths of solder paste. As such the printing screen must include non-uniform apertures, where some of the apertures are deeper than others. As described above the process of applying solder paste to the circuit board is by the process of applying solder paste to the printing screen and pressing the solder paste through the apertures to produce a printed circuit board. To produce thicker deposits of solder paste the printing material needs to be thicker. Therefore, holes are cut through the stencil material, and inserts of different thicknesses are inserted and welded in place. By applying this technique the thickness of the aperture and therefore the volume of solder paste to be applied to the circuit board can be controlled. Inserting thicker material and fixing it into a hole through thin sheet material can be problematic. When thin sheet materials are welded together localised heat may cause distortion of the sheet material. Frequently when two metals to be welded are of substantially different thicknesses, the thinner material may become distorted. Any distortion is problematic in the printing process because solder paste may bleed at the areas of distortion. Bleeding of solder paste may lead to short circuiting in the finished component. Moreover, localised damage to the sheet material, in the area of the inserts, may also occur due to the heat required in welding a thicker material to a thinner material. It is desirable to provide an improved printing screen that includes areas of increased or decreased thickness such that the application of solder paste is efficiently controlled.

Summary of the Invention

A first aspect of the present invention provides a method of making a printing screen comprising a substrate, wherein the substrate comprises one or more areas of thickness substantially different than the thickness of the substrate, wherein the method comprises the steps of:

creating an aperture through the substrate;

inserting into the aperture a first patch of material, which is fractionally thicker or thinner that the substrate material such that the difference in thickness between the substrate and the patch is part way to the required final change in thickness;

fusing the first patch to the substrate;

creating an aperture through the first patch;

inserting into the aperture through the first patch a subsequent patch, which is fractionally thicker or thinner than the first patch such that the difference in thickness between the substrate and the patch is further part way to the required final change in thickness;

fusing the subsequent patch to the first patch; and

repeating the steps of creating an aperture, inserting a subsequent patch of material, which is fractionally thicker or thinner that the previous patch and fusing the subsequent patch to the previous patch until cumulatively the patches and the substrate provide a predetermined final thickness. Introducing a change of thickness in stages as defined above means that the localised thickness of the printing screen is increased or decreased as a gradual process and creates a desirable ramp change from the top of the substrate to the top or bottom of the inserted final patch rather than a step change, which would occur if the increase or decrease in thickness was done with a single patch. A step change can adversely affect the desired smooth application and distribution of solder paste.

In addition, by fusing/welding the materials together after the addition of each patch the occurrence of distortion due to thermal affects of welding two different thicknesses of material is reduced because the difference in material thickness between each patch and a previous patch or the substrate material is less than the conventional manner of inserting a full depth patch in one stage. Each step of increasing or decreasing thickness to obtain the predetermined final thickness may be by a uniform increase or decrease in patch thickness, wherein each patch is consistently thicker or thinner than the substrate or a previous patch.

Alternatively, each step of increasing or decreasing thickness to obtain the predetermined final thickness may be by a non-uniform increase or decrease in patch thickness, wherein each patch is inconsistently thicker or thinner than the substrate or a previous patch..

The step of fusing a subsequent patch to a previous patch or substrate may comprise laser welding or electron beam welding.

The laser beam providing the weld energy may be in the region of 0.05mm in diameter. This may reduce the risk of excessive heat damage to the substrate material because the substrate material (being very thin) is easily distorted to become non-flat due to thermally induced residual stress.

The method may further comprise using a high resolution, high magnification camera to aid the welding process. For example in laser welding, the camera may be operable to place the laser beam with high precision. The high resolution camera may be used to ensure that the weld accurately penetrates the two levels of material (patch and substrate or patch and patch) and uniformly melts a consistent volume of material from both the patch and substrate or patch and patch to form a fusion zone. Advantageously, the camera allows the path of the weld to be tracked. This means that the path of the weld is controlled such that the weld cannot and does not stray more into the substrate or the patch. As such uneven welding is avoided.

The camera may be operable to place the laser beam within +0 to 0.5 micron of its target weld path.

The method may further comprise repeating the welding process on a second side of the substrate after each patch insertion. Alternatively, the method may further comprise repeating the welding process on a second side of the substrate after insertion of the final patch. Therefore, a suitable surface finish is maintained on both faces of the substrate material.

A ramp change in level from the top of the substrate to top of the final patch insert is preferred to avoid damage to the print squeegee or print mechanism , which is used to apply solder paste to the printed circuit board. In addition, the ramped structure provided between substrate and patch or patch and patch reduces the risk of distortion of the substrate and therefore ensures that the solder paste is applied substantially uniformly to the printed circuit board. A printing screen produced by the method according to a first aspect of the present invention is used for printing where flatness of the printing screen is extremely important; this helps to avoid bleeding of the solder paste as it is applied.

A further aspect of the control process is that laser parameters are automated to ensure that that the rate of energy is proportional to the speed of travel. Creating the apertures may comprise laser cutting the substrate or patch material, wherein the laser cutting process is controlled by a camera and motion control system. As such the material removal and patch insertion is automated. Therefore, the size of the aperture and the size of the patch may match to within a tightly set tolerance. For example, the size of the aperture and the size of the patch may match with a tolerance of +0 to 0.005mm.

The step of inserting the first patch into an aperture may comprise activation of a substantially flat vacuum or a magnetic fixture such that when the first patch is inserted into the substrate, the patch and substrate are securely held substantially flat whilst the first weld takes place.

In welding thin material, especially welding two materials of different thicknesses there can be a significant problem in attaining suitable weld penetration in the thick material without causing excessive damage to the thinner of the two materials; particularly, concerning butt welds.

According to an embodiment of the present invention material thickness is changed incrementally, where the insertion of each patch increases the localised thickness of the substrate material by a fraction of the thickness of the substrate material. For example, each patch is fractionally thicker or thinner than the substrate or a previous patch. The step of inserting patches is done incrementally.

A further aspect of the present invention provides a printing screen comprising:

a substrate comprising one or more areas of thickness greater or less than the thickness of the substrate, wherein the one or more areas of different thickness comprise the addition of a plurality of patches of material in an aperture created through the substrate, wherein the addition of patches is such that the thickness of material in the one or more areas is increased or decreased in stages by creating an aperture in the substrate, inserting into the aperture a first patch of material and fusing the first patch to the substrate and subsequent to fusing the first patch to the substrate creating an aperture through the first patch and inserting a subsequent patch into the aperture through the first patch and fusing the subsequent patch to the first patch, and repeating the steps of creating an aperture, inserting a subsequent patch of material and fusing the subsequent patch to the previous patch until cumulatively the patches and/or the substrate provide a final predetermined thickness, wherein each patch is fractionally thicker or thinner than the substrate or previous patch.

A printing screen according to embodiments of the present invention demonstrates how to maintain a suitable surface finish such that the application of solder paste is not adversely affected by a change in height or depth from the substrate material to the peak (or trough) of the patch material. Therefore the printing screen is created in stages by incrementally increasing or reducing the thickness of the printing screen and by efficient fusion/welding of a substrate material to patch material in stages.

According to an embodiment of the present invention the change in material thickness is done in steps, where each step is a fraction thicker or thinner than the substrate or patch material.

Brief description of the Drawings

Embodiments of the present invention will now be described, by way of example only, with reference to the accompanying drawings in which: [

Figure 1 illustrates an example of a printing screen according to an embodiment of the present invention;

Figure 2 illustrates a single stage, localised increase in thickness of the printing screen; Figure 3 illustrates a two-stage localised increase in thickness of the printing screen; and Figure 4 illustrates a two-stage localised decrease in thickness of the printing screen. Description of an Embodiment

Figure 1 shows an example of a printing screen 10 according to an embodiment of the present invention. The printing screen 10 includes a substrate material 12 through which is provided a plurality of apertures 14. In the present example the apertures are cut from the substrate using a laser cutter (not shown).

The substrate material may be metal, for example stainless steel or nickel. Alternatively, the substrate material may be plastic.

Figure 1 also illustrates areas/zones 16 of increased thickness, where material 18 has been added to increase the depth of an aperture 14 in a particular area of the printing screen 10. In the illustrated example, the printing screen 10 also includes an area of reduced thickness 20.

The areas of increased thickness 16 and reduced thickness 20 are created by first creating an aperture 14 through the substrate and then applying a patch 15 of material that matches, within tight tolerances, the dimension of the aperture 14. The patch 15 is fractionally thicker or thinner than the substrate material 12.

Figure 2 illustrates the process of applying patches 15 to increase the thickness of the printing screen 10 in a localised area.

As described above the first step to creating the zone 16 of increased thickness requires removal of material from the substrate material 12. An aperture 14 is cut by a laser cutting machine, for example a Tannlin T11 laser machine. Subsequently a patch 15 is inserted into the aperture 14 and is welded in place. This process is repeated until the desired thickness is obtained. The Tannlin T11 machine uses a high resolution, high magnification camera that can place the laser beam within 0.5 micron of its target weld path. The camera is used to ensure that the welding material accurately and uniformly penetrates the substrate 12 and the patch 15 material such that a consistent volume of material is melted from both the substrate 12 and the patch 15 to form a weld 17, which secures the substrate 12 and the patch 15 together.

In the illustrated example (see figure 2) the patch 15 is no more than 0.05mm thicker than the substrate 12. To make a thicker zone, i.e. greater than 0.05mm thicker the process of cutting an aperture and applying a patch, which is fractionally thicker than the previous patch, is repeated until the desired thickness is achieved (see figure 3).

By way of illustration, the thickness of zones/areas of the printing screen 10 may be increased or decreased in increments of 0.05mm. Therefore if the required increase in thickness is 0.1 mm the process will be repeated twice (as illustrated in figure 3) such that a first aperture 14 is created through the substrate 12 and a first patch 15, 0.05mm thicker than the substrate material, is inserted into the aperture 14 and welded in place. Subsequently, a second aperture 14 is created through the first patch 15 and a second patch 15, again 0.05mm thicker than the first patch, is inserted in the second aperture and welded in place. The resulting surface finish of the printing screen is a ramp between the upper face of the substrate 12 and the upper edge 19 of the second/final patch. It will be appreciated that the process of creating an aperture and inserting a patch can be repeated until the desired thickness is required. It will also be appreciated that the specific dimensions given are by way of illustration only and that the increase or decrease in thickness of the substrate material is by inserting patches that are fractionally thicker or thinner than the previous patch to achieve a ramped transition zone. Similarly, material can be removed incrementally where thinner patches are inserted and welded to the aperture as illustrated in figure 4. Again, by way of illustration, if the decrease in thickness is 0.1 mm the process will be repeated twice such that a first aperture 14 is created through the substrate 12 and a first patch 15, 0.05mm thinner than the substrate material, is inserted into the aperture 14 and welded in place. Subsequently, a second aperture 14 is created through the first patch 15 and a second patch 15, again 0.05mm thinner than the first patch, is inserted in the second aperture and welded in place. The resulting surface finish of the printing screen is a ramp between the substrate 12 and the upper edge 19 of the second patch.

In the illustrated example, the laser beam providing the weld energy may be as small as 0.05mm in diameter. This avoids excessive heat damage to the substrate material and therefore reduces the effects of thermal residual stresses which may cause the substrate material to become distorted, i.e. non-flat which is not desired because any distortion may result in bleeding of solder paste during the printing process.

In the illustrated example, the camera used to track the weld path in the present example is a high precision camera, high magnification camera such that the path of the weld can be tracked efficiently. By tracking the weld path the weld is controlled such that it does not stray more into the substrate or the patch. As such a uniform weld is assured.

The Tannlin T11 machine, as used in the present example, allows accurate preparation of printing screens anywhere within a 600mmx600mm operational window of the machine in a single operation. As such multiple inserts of reduced thickness or increased thickness can be added without requiring repositioning of the substrate material. Whilst producing an improved printing screen this operation also reduces the production time of complex printing screens/stencils. The present example provides an automated process where the laser operation is controlled such that the rate of energy is proportional to the speed of travel such that a highly uniform material melt can be maintained during unavoidable accelerations and decelerations. This arrangement ensures that the weld penetration is suitable for the materials being joined together to form thicker or thinner zones on the printing screen.

Above, the process of increasing and reducing thickness in particular areas of a printing screen have been described in respect of cutting the aperture and securing the patch. It will be appreciated that a weld 21 will be applied to the opposite side of the substrate also (as illustrated in figures 2, 3 and 4).

In the zones of increased or reduced thickness it will be appreciated that the first aperture will be greater in size than the final aperture at a given zone because the first aperture is sized to receive a patch which subsequent to welding will be cut to produce an aperture, which may be the final aperture or an aperture sized to receive a further patch.

In the present example the patch is secured by laser welding, but other suitable welding processes may be employed, for example electron beam welding. To complete the printing process the printing screen 10 is placed on a substrate material before applying solder paste. The solder paste is spread using a squeegee such that the solder paste fills the apertures and upon removal of the printing screen 10 a printed image of solder paste is formed on the underlying substrate material.

Whilst specific embodiments of the present invention have been described above, it will be appreciated that departures from the described embodiments may still fall within the scope of the present invention.

Claims

1. A method of making a printing screen comprising a substrate, wherein the substrate comprises one or more areas of thickness substantially different than the thickness of the substrate, wherein the method comprises the steps of:

creating an aperture through the substrate;

fusing the first patch to the substrate;

creating an aperture through the first patch;

fusing the subsequent patch to the first patch; and

repeating the steps of creating an aperture, inserting a subsequent patch of material, which is fractionally thicker or thinner that the previous patch and fusing the subsequent patch to the previous patch until cumulatively the patches and the substrate provide a predetermined final thickness.

2. A method as claimed in claim 1 , wherein each step of increasing or decreasing thickness to obtain predetermined final thickness is by uniform increase or decrease in patch thickness, wherein each patch is consistently thicker or thinner than the substrate or a previous patch.

3. A method as claimed in claim 1 , wherein each step of increasing or decreasing thickness to obtain the predetermined final thickness is by non-uniform increase or decrease in patch thickness, wherein each patch is inconsistently thicker or thinner than the substrate or a previous patch..

4. A method as claimed in claim 1 , 2 or 3, wherein the step of fusing a subsequent patch to a previous patch or substrate comprises laser welding or electron beam welding.

5. A method as claimed in claim 4, wherein a laser beam providing weld energy may be in the region of 0.05mm in diameter.

6. A method as claimed in claim 5, further comprising using a high resolution, high magnification camera to aid laser welding.

7. A method as claimed in claim 6, wherein the camera is be operable to place the laser beam within +0 to 0.5 micron of its target weld path.

8. A method as claimed in any preceding claim further comprising repeating the welding process on a second side of the substrate after each patch insertion.

9. A method as claimed in any preceding claim, further comprising repeating the welding process on a second side of the substrate after insertion of a final patch.

10. A method as claimed in any preceding claim wherein rate of laser energy supplied is proportional to speed of travel.

1 1. A method as claimed in any preceding claim, wherein creating each aperture comprises laser cutting the substrate or patch material, wherein the laser cutting process is controlled by a camera and motion control system.

12. A method as claimed in any preceding claim, wherein the step of inserting the first patch into an aperture comprises activation of a substantially flat vacuum fixture such that when the first patch is inserted into the substrate, the patch and substrate are securely held substantially flat whilst the first weld takes place.

13. A method as claimed in any of claims 1 to 11 , wherein the step of inserting the first patch into an aperture comprises activation of a magnetic fixture such that when the first patch is inserted into the substrate, the patch and substrate are securely held substantially flat whilst the first weld takes place.

14. A printing screen comprising:

a substrate comprising one or more areas of thickness greater or less than the thickness of the substrate, wherein the one or more areas of different thickness comprise the addition of a plurality of patches of material in an aperture created through the substrate, wherein the addition of the patches is such that the thickness of material in the one or more areas is increased or decreased in stages by creating an aperture in the substrate, inserting into the aperture a first patch of material and fusing the first patch to the substrate and subsequent to fusing the first patch to the substrate creating an aperture through the first patch and inserting a subsequent patch into the aperture through the first patch and fusing the subsequent patch to the first patch, and repeating the steps of creating an aperture, inserting a subsequent patch of material and fusing the subsequent patch to the previous patch until cumulatively the patches and/or the substrate provide a predetermined thickness, wherein each patch is fractionally thicker or thinner than the substrate or previous patch.

15. A method as described herein with reference to the drawings.

16. A printing screen as described herein with reference to the drawings.