A first aspect of the present invention relates to a method for electroforming a stencil made from solid metal with printing openings which are delimited by the solid metal and define printing positions that are to be printed, in particular for use in the printing of a printed circuit board (PCB) with soldering paste.

A method of this type is known in the art. In a method of this type, insulating regions are applied to a flat electroforming die made from conductive metal at the positions of the printing openings of the stencil that are to be formed. These printing openings define the printing positions, through which a printing medium, such as soldering paste, can be applied to the substrate that is to be printed, such as a (plastic) board on which a printed circuit is provided, so that the electronic components to be attached can be fixed using soldering paste. After the insulating regions have been applied, the electroforming die is placed in an electroplating bath which contains an electroformable metal or metal alloy, and is connected as cathode. As a result, metal is deposited on the uncovered conductive regions of the electroforming die around the insulating regions until the desired thickness has been reached. Then, the stencil which has been electroformed in this way .is removed from the electroforming die.

To use the stencil obtained in this way, it is necessary for the stencil to be clamped in a (screen) printing device. By now, printing devices have been developed in which the stencil is clamped in place with the aid of profiled clamping sections. An example of a clamping system of this type is commercially available under the trade name Vector Guard™ from DEK. A profiled clamping section of this type comprises a strip in which in many cases a complex slot- shaped opening is provided, into which the edge of the stencil can be slid. However, this system is not especially suitable for electroformed stencils, since to clamp the stencil in the clamping profile the stencil needs to be bent or deformed in some other way

and/or additional clamping features have to be provided. Electroformed stencils, however, have such a high hardness and/or rigidity, that they cannot be bent or deformed in order to be accommodated in a profiled clamping section of this type without the risk of breaking. Therefore, in practice stainless steel stencils in which the printing openings are cut with the aid of a laser are generally used with profiled clamping sections of this type.

However, stainless steel stencils cut with the aid of a laser have a shorter service life than electroformed stencils, in particular made from nickel, on account of their lower hardness. Furthermore, the positioning accuracy of the printing openings cut with the aid of a laser is lower than holes formed by electroforming in a stencil. In addition, the formation of burrs, which can occur during cutting with the aid of a laser, can adversely affect the quality of the shape of the holes. This may have an adverse effect on the final print quality.

Therefore, there is a need for electroformed stencils, in particular made from nickel, one or more peripheral sides of which can be clamped directly in profiled clamping sections, while the risk of the stencil breaking is reduced and/or with which there is no need to employ any extra features .

To this end, a first aspect of the invention provides a method for electroforming a stencil made from solid metal with printing openings which are delimited by the solid metal and define printing positions that are to be printed, in particular for use in the printing of a printed circuit board (PCB) with soldering paste, which stencil can be clamped in profiled clamping sections, which method comprises the following steps : providing an electroforming die made from conducting material and having a flat die section substantially corresponding to the stencil that is to be produced and having a part-groove provided in the die surface substantially at at least one position of a peripheral side of the stencil that is to be produced; applying insulating regions to the flat die section at the positions of the printing openings that are to be formed; applying an insulating track on the outside of the part-groove

at a predetermined distance therefrom; an electroforming step of depositing metal on the electroforming die in an electroplating bath in order for the stencil with printing openings to be formed in a flat stencil part and at least one clamping edge part to be formed along the periphery of the stencil at the position of the part-groove.

In the present method, a first step involves providing an electroforming die made from conductive material, which electroforming die comprises a flat die section which substantially corresponds to the dimensions of the stencil that is to be produced. A part-groove is provided in the die surface at a position of at least one of the peripheral sides of the stencil that is to be produced. A part-groove of this type can be made in the die surface for example by means of a mechanical operation, such as milling, a chemical operation, such as etching, or physical removal of material with the aid of a laser. Then, insulating regions, the positions of which correspond to the printing openings that are to be formed, are applied to the flat die section. This is preferably done with the aid of dry resist, the height of which is greater than the desired thickness of the stencil that is to be produced. An insulating track is also arranged on the outside of the part-groove, at a certain distance therefrom, and if desired at a position around the entire periphery of the stencil that is to be produced. This track, which preferably likewise has a height that is greater than that of the stencil to be produced, defines the final dimensions of the stencil that is to be electroformed. It will be understood that the abovementioned steps can also be carried out in a different order. The end result of these first steps is the same, namely an electroforming die with a flat die section in which insulating regions corresponding to the printing openings that are to be produced are provided, and at least one part-groove provided substantially in the vicinity of the periphery of the flat die section, as well as a track of insulating material located outside the part-groove.

Then, in the method according to the invention, the electroforming die which has been prepared in this way is placed in an electroplating bath which contains an electroformable metal or metal

alloy, and is connected as cathode. As current is passed through the electroplating bath, metal is deposited on the uncovered parts of the electroforming die, including the part-groove, so as to produce a stencil having a flat stencil part with printing openings, and the metal which is deposited in the part-groove forming a clamping edge part along the associated peripheral part of the stencil.

By suitably dimensioning the groove and by setting appropriate electroforming conditions, it is possible to define the shape of the associated clamping edge part. It is thus possible to slide a profiled clamping section onto the clamping edge part formed, and in this way it is possible to employ a stencil having all the advantages of an electroformed stencil for the printing of substrates, such as printed circuit boards.

It is advantageous for part-grooves to be provided along all the peripheral sides of the stencil that is to be produced. More particularly, the part-grooves are not connected to one another, in order to make it easier to slide on profiled clamping sections. In this way, the method can be used for the production of a stencil having a flat stencil part, in which the printing openings are provided, and clamping edge parts, which are not connected to one another, along all the peripheral sides.

In a preferred embodiment of the method according to the invention, the stencil, including the printing openings and a clamping edge part, is electroformed in a single step. The advantage of electroforming the entire stencil in one step is that the attachment of the clamping edge parts to the flat stencil part is better than if a stencil of this type were to be built up in a plurality of steps. The final strength of an integrally formed stencil with printing openings and clamping edge part(s) according to the invention, in particular at the location of this/these clamping edge part(s), is sufficient to absorb the clamping forces.

In a further advantageous embodiment of the invention, a part-groove has a substantially rectangular cross section.- A rectangular cross section of this type makes it possible to adapt the shape of the clamping edge parts to the desired strength and thickness of the

stencil. In a further expedient embodiment thereof, which is intended in particular for relatively thin stencils, the electroforming step is carried out for a predetermined period of time, in such a manner that a clamping edge part is substantially U-shaped. A U-shape of this type is stronger than just an L-shaped hook part in the case of a low thickness, and also the limbs of the U-shape can be bent towards one another slightly, which can facilitate clamping in a profiled clamping section. It is preferable for the electroforming step to be carried out for a predetermined period of time, in such a manner that the thickness of a clamping edge part which is formed, as seen in a direction perpendicular to the local die surface, is greater than the thickness of the flat stencil part. This greater thickness of the clamping edge parts, in particular the U-shape, can be obtained by increasing the current density compared to standard levels. The higher the current density, the greater the differences in thickness of the deposition of metal in the part-grooves compared to the flat stencil part. Furthermore, the thickness of a clamping edge part can be controlled by covering a relatively large area with insulating material along the at least one part-groove. In other words, the width of the insulating track can be utilized to set the thickness of a clamping edge part.

For thicker stencils, the electroforming step of deposition of metal by electroplating is advantageously carried out for a predetermined period of time, in such a manner that a clamping edge part which is formed has a substantially rectangular cross section. In the case of these thicker stencils, it is advantageous for the electroforming step to be carried out until the growth front of the deposition of metal in the part-grooves is ultimately level with the growth front of the deposition of metal on the flat die part of the electroforming die.

The thickness of the stencil obtained is preferably in the range from 50 to 300 micrometres.

Nickel is a particularly preferred electroformable metal, since it has a high hardness, with the result that the stencils have a longer service life than the stainless steel stencils which are cut with the aid of a laser. For this purpose, the electroforming step is

advantageously carried out in a nickel-containing bath. Suitable examples of such baths include what are known as Watts baths

(comprising nickel chloride, nickel sulphate, boric acid and a brightener of the first or second class) or a sulphonate bath (comprising nickel chloride, nickel sulphonate, boric acid and a brightener of the first or second class) .

The width of a part-groove is advantageously in the range from a few tens of micrometres to a thousand micrometres, but is substantially determined by the shape and dimensions of the slot-shaped opening in the profiled clamping section with which the stencil produced needs to be used.

A second aspect of the invention relates to an electroformed stencil made from solid metal, comprising a flat stencil part, in which there are printing openings which are delimited by the solid metal and define printing positions that are to be printed, in particular for use for the printing of a printed circuit board (PCB) with soldering paste, which stencil is provided along the periphery, at a distance therefrom, with at least one clamping edge part. A stencil of this type can be obtained with the aid of the above-described method according to the invention and has the same advantages . The other preferred embodiments of the stencil as described above also apply to this aspect of the invention. In particular, the ratio of the thickness of a clamping edge part to the thickness of the flat stencil part of the stencil is in the range from 4:1 to 1:1.

The invention will be explained below with reference to the accompanying drawing, in which: Fig. 1 shows a perspective view of an embodiment of a flat stencil according to the invention;

Fig. 2 shows an example of a, profiled clamping section; Fig. 3 shows a stencil according to the invention produced in the profiled clamping section shown in Fig. 2; and Figs 4-7 show various shapes of cross sections of clamping edge parts.

Fig. 1 shows en embodiment of a stencil 10 according to the invention which comprises a substantially flat, thin nickel plate.

The stencil 10 comprises a flat stencil part 12 in which there are printing openings 14 which correspond to the printing positions for printing a substrate. Clamping edge parts 18, which in the embodiment illustrated are substantially rectangular in cross section, are provided along the peripheral sides 16 at a certain distance therefrom. It will be understood that the dimensions of the printing openings 14 and of the clamping edge parts 16 are illustrated on a very exaggerated scale. If desired, the corners of the stencil 10 can be removed if required by the shaping of the opening in the profiled clamping section.

Fig. 2 shows an example of a profiled clamping section 20 which is used with stainless steel, laser-cut stencils. In this case, a stencil is received and fixed in the clamping slot 22.

Fig. 3 shows a combination of a stencil 10 according to the invention and a profiled clamping section 20, with the stencil 10 having been slid into the clamping slot 22. A clamping edge part 18 hooks behind a bend or corner 24 which is provided in the wall 26 delimiting clamping slot 22.

Figs 4-7 show various forms of the cross section of a clamping edge part 18 as can be produced with the aid of the method according to the invention. Fig. 4 shows part of an electroforming die 30 having a flat die section 32 and a part-groove 34 which is rectangular in cross section. Insulating regions 36 are provided on the section 32 at positions of the printing openings 14 that are to be formed in the stencil 10 to be produced. These printing openings 14 are in this way delimited by metal in the finished stencil 10. An insulating track 38 is provided along the outside of the part-groove 34, at a certain distance therefrom. During the electroforming step, the stencil 10, including flat stencil part 12 and clamping edge part 18, is formed in one step on this die 30 by deposition of metal from an electroforming bath on the flat die section 32 around the insulating regions 36 and in the part-groove 34 all the way to the insulating track 38. Figs 5-7 show only the clamping edge part 18 which is formed. In all cases, the groove width is 600 micrometres. In Fig. 4, the cross section of clamping edge part 18 is U-shaped, with the thickness of the limbs 40 and the base 42 being

approximately three times the thickness of the stencil 10 (approximately 50 micrometres) . This thickness is controlled with the aid of the current density. As has already been stated above, the higher the current density, the greater the differences in thickness between the clamping edge part 18 and the flat stencil part 12. On the outer side of the U-shaped clamping edge part 18 there is also a short projection 50. This is the result of the fact that the insulating track 38 is not provided directly adjacent to the part-groove 34, since otherwise the adjoining limb 40 would not grow to a sufficient extent during the electroforming step. By suitable selection of the distance from the insulating track 38 to the outer side of the part-groove 34 and the width of the track 38 in combination with the current density used, it is possible to vary the thickness of the U-shaped clamping edge part 18 from the required minimum to completely solid. Figs 5 and 6 show other examples of a substantially U-shaped clamping edge 18, in which the difference in thickness of the limbs 40 and base 42 of the U-shape compared to the thickness of the flat stencil part 12 is less. Finally, Fig. 7 shows a clamping edge part 18 which is completely solid and is preferably used for stencils with a thickness towards the upper end of the range described.

The electroforming bath used was a Watts bath, comprising nickel chloride=200 g/1, nickel sulphate=100 g/1, boric acid=40 g/1 and brightener of the first class 10 ml/1. The pH of the bath was 4 and the temperature was 50 ⁰ C.

The clamping edge part 18 illustrated in Fig. 4 was produced at a current density of 8-10 A/dm ² , while the parts illustrated in Fig. 5 were made at 4 A/dm ² . The width of the resist track was varied from approx. 1 mm (Fig. 4) to approximately 50 micrometres (Fig. 7) . It is advantageous to use a wide track and for the thickness of a clamping edge part with respect to the flat stencil part to be controlled by selecting the current density that is to be used.