WO2001091919A1

WO2001091919A1 - Doctor system for coating a printable material onto a surface

Info

Publication number: WO2001091919A1
Application number: PCT/DE2001/002004
Authority: WO
Inventors: Ulf Oestermann; Erik Jung; Paradiso Coskina
Original assignee: Fraunhofer-Gesellschaft zur Förderung der angewandten Forschung e.V.
Priority date: 2000-05-29
Filing date: 2001-05-25
Publication date: 2001-12-06

Abstract

The invention relates to a doctor system with a doctor holder (3) and a doctor blade (1) fixed to the doctor holder (3) for coating a printable material on a surface. The doctor system is characterised in that in the direction of doctoring, a doctoring element (2) is arranged in front of the doctor blade (1), which is of such a form and connected to the doctor blade (1), in such a manner that on operation of the doctor system as prescribed, said element has a marginally greater separation from the surface with a lower boundary (2a) than the doctor blade (1) and forms a chamber (4) in co-operation with the doctor blade, sealed at the top, such that during the doctoring, some of the printable material is forced along or rolled in front of the doctoring element (2) and a part of the printable material goes under the doctoring element (2) into the chamber (4). The above doctoring system permits very good print results, even for small structures at low cost.

Description

Squeegee system for applying a printable material to a surface

Technical application area

The present invention relates to a doctor system with a doctor holder and a doctor blade attached to the doctor holder for applying a printable material, in particular a paste, to a surface.

One area of application of such a doctor blade system relates to the production of electrical contacts between electronic components and the respective carrier materials. Numerous contacting methods are known for this.

In the method most frequently encountered today, contact is made by applying solder paste using a stencil printing technique directly onto the surfaces of the carrier material to be contacted, for example a printed circuit board. Subsequent heat processing solidifies the solder paste that has been structured in this way. In addition to this mass application, the stencil printing technology can also be used to create solder deposits on semiconductor wafers. The two fields of application mentioned by way of example differ from one another in the process only in the materials used and the required process parameters. State of the art

When applying solder pastes, a general distinction can be made between sequential and simultaneous processes. The most frequently used method for printing on printed circuit boards or wafers is the stencil printing technique already mentioned, in which the contact areas are printed simultaneously. For this purpose, the solder paste is transferred through openings in the stencil to the surface to be printed during the printing process. In addition to the quality of the substrate, the solder paste used, the stencil used and the stencil printer used, an essential parameter for a good quality printing process is also the constructive design of the squeegee used.

Two different systems, the open and the closed doctor blade system, are known in the prior art.

With the open squeegee system, the solder paste is introduced into the stencil openings by rolling the paste in front of the squeegee. When the squeegee is passed over the stencil openings, the paste penetrates into these openings. The excess paste is removed from the surface of the stencil by the doctor blade. The doctor blade is attached to a doctor holder, which in turn - in automated printing techniques - is held in a doctor holder on the doctor head of the printing machine. The printing machine carries out the squeegee movement over the surface to be printed.

In the open doctor blade systems, two doctor blades are usually mounted on the doctor head, each of which and backward movement of the doctor head perform a printing cycle. These open standard squeegees are used at different angles to the stencil, different squeegee thicknesses (blade thickness) and different squeegee materials. In these systems, the entire solder paste entry into the stencil opening takes place by rolling the solder paste in front of the doctor blade and pulling it off by the same.

In contrast to this, the solder paste is not introduced in closed doctor blade systems by rolling the paste in front of a doctor blade, but by pressing the solder paste directly into the stencil openings under pressure. The closed doctor blade system consists of a chamber that is open to the surface and that is completely filled with solder paste during the printing process. A pressure is applied to the solder paste in the chamber, by means of which it is pressed into the stencil openings. The lower edge of the walls of the chamber, which can also be in the form of doctor blades, lie here on the surface of the stencil with their lower edge in order to be able to seal the chamber from the outside. It also frees the stencil surface from excess print material.

Stencil printing of solder paste is also increasingly being used to create contacts on wafers. Due to the ever progressive reduction in the size of the structures to be printed, the conventional doctor blades reach the limits of their performance due to the system. In the context of increasing miniaturization, this is also the case with solder transfer Printed circuit boards, the so-called fine pitch printing, the case.

When using open doctor blade systems for stencil printing on wafers or printed circuit boards, the problems usually lie in an uneven filling of the stencil openings, especially if the contact center distance is in a range below 300 μm. Here, there are often only partially printed contact areas on the pads to be printed. This is not acceptable for a continuous process.

The use of closed doctor systems in such application fields leads to higher quality results, but closed doctor systems are far more demanding in their use and operation. They require a number of installations and in some cases even a retrofit of the existing machine park. Their handling is also a lot more complex than that of the open doctor blade systems. Furthermore, a relatively large amount of solder paste has to be used to fill the pressure chambers of these systems, which can have a negative effect, particularly with regard to the number of pieces to be printed, and requires an increased financial outlay. The time required to prepare and rework closed systems is an additional cost factor that makes them seem unattractive to most end users.

The object of the present invention is to provide a doctor blade system for applying printable material to a surface, which is is easy to handle and produces high-quality print quality even for the printing tasks at wafer level.

Presentation of the invention

The object is achieved with the doctor system according to claim 1. Advantageous embodiments of the doctor system are the subject of the dependent claims.

The present squeegee system with a squeegee holder and at least one squeegee blade attached to the squeegee holder for applying a printable material, in particular a paste, to a surface is characterized in that at least one additional squeegee element is arranged in the squeegee direction in front of the squeegee blade and is connected to the doctor blade in such a way that it forms an upwardly closed chamber - or groove open to the surface - with the doctor blade and, when the doctor system is used as intended, has a lower boundary with a slightly greater distance from the surface than the lower edge of the doctor blade, so that, on the one hand, printable material is rolled or pushed in front of the doctor element and, on the other hand, part of the printable material can penetrate into the chamber under the doctor element.

The present invention provides a squeegee system that can be handled like an open squeegee system, but additional advantageous ones

Features of a closed doctor system. The arrangement of the doctor element in the specified embodiment in front of the doctor blade will Rolls of the printable material - hereinafter referred to as solder paste by way of example - in front of the bow of this doctor element. Since the lower limit of the squeegee element is at a defined distance from the surface of the stencil - while the squeegee blade lies on the stencil during the printing process - the chamber behind it is also referred to below as the pressure chamber due to its function during the traversing movement of the squeegee system filled with solder paste. This configuration, on the one hand, provides a larger area for filling the stencil openings compared to an open doctor blade system and, on the other hand, exerts a higher pressure on the solder paste located in the chamber than is the case with conventional rolling in front of a doctor blade.

The combination of these properties leads to a significantly improved solder paste entry into the stencil openings, comparable to a solder paste entry that can be produced with closed doctor blade systems. However, the present doctor system is easier and quicker to handle than such a closed system.

When using the present squeegee system, no structural changes need to be made to the stencil printing machine used. The volume of solder paste provided in the pressure chamber is only slightly larger than the volume of solder paste that must be used in conventional open doctor blade systems. The resulting printed image is characterized by a homogeneous filling of the stencil openings. The achievable wet layer thickness is in Area of the known systems, but combines the advantages of an open and closed system in terms of robustness, manageability, print quality to be achieved and the costs involved. In addition, different stencil opening geometries can be filled uniformly and homogeneously at the same time.

The doctor system, in particular its doctor holder and / or doctor blade and / or doctor element, can be formed from different, preferably easily structured, materials. Examples of such materials are metals, plastics, composite materials or ceramics. The doctor blade system is suitable for the structured application of a printable material, such as solder pastes, adhesives, polymers or materials for thick-film technology, to printed materials such as semiconductors, wafers, ceramics, organic substrates, inorganic substrates, rigid or flexible

Substrates. Of course, the squeegee system can not only be used for stencil printing technology, but also for other printing techniques, for example screen printing technology. In principle, any suitable material can be used with the doctor system

Layer can be applied to a substrate, for example for producing conductor tracks, as a dielectric layer, etc. The doctor system itself is usually mounted with the doctor holder in a doctor holder of a printing machine. The doctor blade holder only has to be adapted to the shape of the doctor blade holder. When using conventional doctor holders, such as those of open doctor systems are known, both the open and the present doctor system can be used without changes to the same printing press.

The doctor blade and the at least one doctor element can be fastened to the doctor holder in a conventional manner, for example by screwing on. Of course, other fastening mechanisms are also possible. For example, the squeegee holder can also fix the squeegee blade and the one or more squeegee elements via a clamping mechanism. The individual components, doctor blade and doctor element, can be formed in one piece with one another or can be present as separate components, which are either directly connected to one another or held together by the doctor holder.

The doctor blade and the doctor element or elements preferably consist of one or more structured blocks of material, in particular of plastic.

In a further embodiment of the present doctor system, not only one but several doctor elements are provided in front of the doctor blade. These doctor elements are connected in series, have a defined distance from the surface of the stencil during the printing process and each form individual, upwardly closed chambers between the individual doctor elements. In this way, several pressure chambers lying one behind the other as well as an even larger area for the paste entry are provided. The known rolling of the paste takes place in front of the foremost doctor element. The present doctor system is preferably used for applying a paste to a surface. The distance between the lower boundary of the doctor element and the surface of the stencil or screen is set so that it corresponds to at least four times the diameter of the average particle size of the paste.

For common applications, with the doctor blade resting on it, a distance of the lower limit of the doctor element to the surface of the stencil is selected, which is in the range between 1 mm and 5 mm. The volume of the chamber formed between the doctor element and doctor blade preferably takes up a maximum of 20 ml of the printable material per cm of doctor blade width.

Brief description of the drawings

The present squeegee system is described below using exemplary embodiments in conjunction with the drawings without restricting the general

Invention concept briefly explained again. Here show:

Fig. 1 shows an example of the solder paste when stencil printing with the help of an open

Squeegee;

2 shows a schematic phase representation of the functioning of the present doctor system; 3 shows an exemplary embodiment of a doctor blade system (without doctor blade holder) according to the present invention; 4 shows another example of a doctor blade system according to the present invention;

5 shows the doctor system of FIG. 3 in an exploded view; 6 shows a comparison of a known open doctor blade system with an embodiment of the doctor blade system of the present invention;

7 shows an exemplary print image generated with a standard doctor blade;

8 shows a print image generated with the present doctor blade.

WAYS OF CARRYING OUT THE INVENTION FIG. 1 shows, by way of example, the solder paste entry in the stencil printing technique using a doctor blade, as is known from the prior art. When producing contact structures on wafers, a template 10 with correspondingly structured openings 11 is placed on the surface of the wafer 12. The template openings 11 are provided at locations where, for example, a bond pad 14 is to be produced on the wafer. To insert the solder paste 6 into the stencil openings 11, the solder paste is drawn over the stencil with a squeegee 13. In this case, the openings 11 of the template 10 fill with the solder paste 6. A tempering step is then carried out to solidify the applied solder paste. However, especially when using such a stencil printing technique on wafers

Use of an open doctor blade, such as that of Figure 1, from problems in terms of print quality in a specific area of application (<300 μm pitch), as will be shown later with reference to FIG. 7.

The present doctor system avoids this

Problem and can be used very advantageously for wafers as well as for other application fields with microstructures.

Figure 2 shows schematically an example of a

Design of the present doctor system and three doctor phases to illustrate the functioning of this doctor system. The present doctor system is composed in the usual way of a doctor holder 3 with a doctor blade 1. In addition, before

Doctor blade 1 a doctor element 2 arranged and connected to the doctor blade 1. The connection can be made by direct attachment to the doctor blade 1 or via the doctor holder 3. The doctor element 2 is designed and arranged opposite the doctor blade 1 and inserted in a doctor holder in such a way that during the printing process or doctoring, its lower boundary 2a is at a greater distance from the surface 5 of the printing stencil or the surface to be printed than that Doctor blade 1, more precisely its lower edge. Furthermore, the connection of the doctor element 2 with the doctor blade 1 forms an upwardly closed chamber 4 in the form of a groove open to the surface. This configuration can be seen in the three phases shown in FIG. 2. When this squeegee system is used as intended, solder paste 6 is applied to the surface 5 of the stencil 10 in front of the squeegee and the squeegee is moved in the direction of the solder paste (phase 1). Because of this movement and the distance of the lower ones

Boundary 2a of the doctor element 2 to the surface 5, the chamber 4 fills with the paste 6 (phase 2). As soon as the chamber 4 is completely filled, the excess paste rolls in front of the bow 2b of the doctor element 2 (phase 3). This rolling of the paste corresponds to the functioning of a conventional open doctor system. At the same time, the paste in the chamber 4 is under pressure due to the constantly pressing material 6, so that the solder paste is introduced from this chamber 4 into the stencil openings (not shown) under increased pressure, as is the case with a closed doctor system. The chamber 4 thus acts as a pressure chamber and the doctor element 2 as a pressure booster - by means of which the pressure on the solder paste is increased.

In contrast to a closed squeegee system, the present squeegee system does not require a further pressure-generating mechanism or further sealing elements and, like an open squeegee system, is easy to handle. To use this multi-stage doctor blade, no structural changes need to be made to the stencil printing machine used.

The shape of the pressure chamber 4 and the doctor element 2 is not limited to the shapes disclosed in the present exemplary embodiments. Rather, the geometric shape of this Vary components of the doctor system as long as the mode of operation described above is still achieved.

3 shows an example of a possible embodiment of the present doctor system, in which the doctor blade 1 is formed in one piece together with the doctor element 2. The squeegee holder is not shown in this figure. Rather, this squeegee holder receives the zigzag-shaped clamping edge 7 of the unit consisting of squeegee blade 1 and squeegee element 2. In the figure, the rear doctor blade 1 and the doctor element 2 connected to it can be seen, on the bow 2b of which the paste or the printable material is rolled. In the illustrated embodiment, the inner boundary of the pressure chamber 4 with the adjoining lower boundary 2a and the bow 2b of the doctor element 2 is formed in a wave form. Such an embodiment can be easily produced from a deformable metal or plastic material.

Figures 4 and 5 show another example of an embodiment of the present doctor system. In this embodiment, a standard doctor blade holder 3 is used to hold the doctor blade 1 and the doctor element 2. It can be seen that the doctor blade system in question can be combined with a standard doctor blade holder and can therefore be used without problems in conventional doctor blade holders for open doctor blade systems. FIG. 4 shows the composite doctor system with doctor holder 3, doctor blade 1 and doctor element 2 and that between doctor blade 1 and Recognizing doctor element 2 formed chamber 4. In this example, the doctor blade 1 and the doctor element 2 are designed as separate components which are held together by the doctor holder 3, which is likewise made in two parts. This can be seen from FIG. 5, which represents an exploded view of the individual parts of the squeegee system before assembly.

FIG. 6 finally shows a comparison between an open doctor blade system with doctor blade holder 3 and screwed-on doctor blade 1 (left side of the figure) and an embodiment of the present doctor system with doctor blade holder 3 and screwed-on unit comprising doctor blade 1 and doctor element 2 (right side). From this comparison it can be seen that the present doctor system differs from a known open doctor system only in the design of the area of the doctor blade. The squeegee holder can be identical.

Finally, FIG. 7 shows an example of a printing result that is lower for structures when using a conventional open doctor blade system in connection with a stencil printing technique

Dimensions was achieved. In the figure, the inhomogeneously printed pad 8 clearly shows that the stencil openings are filled with paste 6 only very irregularly.

In contrast, when using a doctor blade system according to the present invention, significantly better printing results are achieved, as shown in FIG. 8 is recognizable. This figure shows the printing result when using the present doctor system with the same stencil as was used in the example of FIG. 7. The improved application behavior of the paste with the present doctor blade system improves the degree of filling of the stencil openings, as can be seen from the homogeneously printed out pad 9.

LIST OF REFERENCE NUMBERS

doctor blade

Doctor element a lower limit of the doctor element b bow of the doctor element

Rake1ha1terung

Chamber or pressure chamber

surface

paste

Clamping edge inhomogeneously printed pad homogeneously printed pad 0 stencil 1 stencil openings 2 wafers 3 open squeegee 4 bond pad

Claims

claims

1. Doctor blade system for applying a printable material, in particular a paste (6), to a surface, with a doctor blade holder (3) and at least one doctor blade blade (1) attached to the doctor blade holder (3), characterized in that in the doctor blade direction in front of the doctor blade (1) at least one additional squeegee element (2) is arranged and connected to the squeegee blade, which forms an upwardly closed chamber (4) with the squeegee blade (1), the squeegee element (2) having a lower limit when the squeegee system is used as intended (2a) has a slightly greater distance from the surface than the doctor blade (1), so that on the one hand printable material is rolled or pushed in front of the doctor element (2) during the doctoring and on the other hand a part of the printable material passes under the doctor element (2) the chamber (4) can penetrate.

2. Doctor system according to claim 1, characterized in that the chamber (4) is designed with a volume for receiving about 1 to 50 ml of printable material per cm of doctor blade width.

3. Doctor system according to one of claims 1 or 2, characterized in that the lower boundary (2a) of the doctor element (2) to the lower edge of the doctor blade (1) has a distance of 1 to 5 mm.

4. Doctor system according to one of claims 1 to 3, characterized in that the doctor blade (1) together with the doctor element (2) is integrally formed.

5. Doctor system according to one of claims 1 to 3, characterized in that the doctor blade (1) and the doctor element (2) are separate components which are held together by the doctor holder (3).

6. Doctor system according to one of claims 1 to 5, characterized in that the doctor blade (1) and the doctor element (2) from one or more structured material blocks, in particular made of plastic, are formed.

7. Doctor system according to one of claims 1 to 6, characterized in that the lower boundary (2a) of the doctor element (2) is rounded in the doctor direction.

8. Doctor system according to claim 7, characterized in that the inner boundary surface of the chamber (4) together with the subsequent lower boundary (2a) of the doctor element (2) in Squeegee direction has a wavy course.

9. Doctor system according to one of claims 1 to 8, characterized in that a plurality of additional doctor elements (2) are arranged one behind the other in front of the doctor blade (1), that several successive chambers (4) are formed, in the printable material during doctoring can penetrate under the doctor elements (2).

10. Use of a doctor blade system according to one or more of the preceding claims for applying a printable material by means of screen or stencil printing technology to a surface.

11. Use of a doctor blade system according to one or more of the preceding claims for applying a paste by means of screen or

Stencil printing technique on a surface, the distance of the lower boundary (2a) of the doctor element (2) from the surface of the stencil or screen used when squeegeeing being at least 4 times the average particle diameter of the paste.